Hi! So I got this used reverb thank with it's circuit attached to it's back, to be used as an effect or added to an amp. The thing is, I have no idea on how to decipher the pin out, as it has no marks besides that ELKA 1910 on the back, and I can't find the schematic anywhere. My guess is that the white plug should have pins (there are 8 of them) for power, in & out, control... How would you go about finding out which is which, without burning anything? Thanks!
(https://i.postimg.cc/2Vv6kYQv/1.jpg) (https://postimg.cc/2Vv6kYQv)
(https://i.postimg.cc/w7dzkmNy/2.jpg) (https://postimg.cc/w7dzkmNy)
(https://i.postimg.cc/yDYwfX8d/3.jpg) (https://postimg.cc/yDYwfX8d)
If you can't find any documentation online, you'll have to trace it out. It's single-sided with big traces, so should be fairly easy. The only IC that I can positively identify in your picture is an LM308CN, the ProCo Rat opamp. I'm curious about the 16-pin chip with only one side connected.
Its a TDA1904 a 4w amplifier. The side which looks unconnected are all connected to common (ground) and act as a heatsink.
The bigger chip is a well-known small audio power amp (Tony's TDA1904). Find its pinout. Trace power + and - pins to connector. These should also come to the smaller chips, possibly through an R-C smoothing filter.
You can also trace signal input from big chip back to connector. I can see how reverb drive comes from power chip to spring. The other end of the spring must go to the smaller chips, set up as some basic audio amplifier. I do not know what there is a third chip.
Thanks for the reply. I assumed so, that it needs tracing, but I'm not really knowledgeable in the area... I've just made few vero-board effects, following drawings and, as it worked out well several times, I got interested.
Here are the datasheets of the ICs:
https://www.digchip.com/datasheets/parts/datasheet/456/TDA1904-pdf.php?fbclid=IwAR1wA0Dr4gV1mVb-s_iBDtgmdXMJV7-0U_k3XgRqNABIcYDd8w3MSGFaU7s
https://www.ti.com/lit/ds/symlink/tl082-n.pdf?fbclid=IwAR1wA0Dr4gV1mVb-s_iBDtgmdXMJV7-0U_k3XgRqNABIcYDd8w3MSGFaU7s
https://www.digchip.com/datasheets/parts/datasheet/321/LM3080-pdf.php?fbclid=IwAR1wA0Dr4gV1mVb-s_iBDtgmdXMJV7-0U_k3XgRqNABIcYDd8w3MSGFaU7s
Ok, so what I can figure out at the moment is that the 1st pin of the connector is probably +V, as it has power filter cap there.
The next three pins are all soldered together and are connected to the ground.
5th pin goes, via mylar cap, to that 308 chip that's connected to one end of the spring, so I guess it's IN, or should I say SEND, I don't know...
6th is also the ground.
7th goes to the 082. I assume that this is to be connected to some kind of a pot, to control the mix.
8th is connected to the TDA1904 amp and I guess its the OUT, or RETURN (still not sure about the therms here)
How does this sound to you? Since I have 4 ground pins there, I assume that one goes for the power, one for IN, one for OUT and one for the control pot...
QuoteOk, so what I can figure out at the moment is that the 1st pin of the connector is probably +V, as it has power filter cap there.
The next three pins are all soldered together and are connected to the ground.
5th pin goes, via mylar cap, to that 308 chip that's connected to one end of the spring, so I guess it's IN, or should I say SEND, I don't know...
6th is also the ground.
7th goes to the 082. I assume that this is to be connected to some kind of a pot, to control the mix.
8th is connected to the TDA1904 amp and I guess its the OUT, or RETURN (still not sure about the therms here)
How does this sound to you? Since I have 4 ground pins there, I assume that one goes for the power, one for IN, one for OUT and one for the control pot...
I came to quite a different conclusion.
Using your numbering:
1 DC control for LM3080. Don't know what this is used for.
2, 3, 4 grounds
5 Audio Out
6 ground for audio in
7 Audio In
8 +V Power
As far as the circuit goes, it's
Audio IN --> Opamp ---> TDA1904 ---> Reverb Tank In ---> Reverb Tank Out ---> Recover Amp (Opamp)
As far as I could see, and I didn't trace the whole thing, there doesn't seem to be any mixing on the board to combine the Reverb signal back with the Dry signal.
There's also an OTA. The second 8-pin device appears to be an LM3080 OTA not a LM308 opamp.
Connector Pin 1 seems be some sort of control input, it passes through the transistor then to the LM3080 control pin.
The output of the OTA is wired to the Audio -Out.
I don't know what input source is for the LM3080. It's possible it doesn't have an input. It could be used for disabling the Reverb signal; It could be related for Tremolo or Vibrato, but the lack of dry signal mixing means it would an odd implementation.
The fact there's no obvious mixing and the presence of the OTA means the circuit is a little weird and you will really need to trace it out if there's any hope of making sense of it.
Quote from: Rob Strand on September 07, 2020, 06:36:08 PM
QuoteOk, so what I can figure out at the moment is that the 1st pin of the connector is probably +V, as it has power filter cap there.
The next three pins are all soldered together and are connected to the ground.
5th pin goes, via mylar cap, to that 308 chip that's connected to one end of the spring, so I guess it's IN, or should I say SEND, I don't know...
6th is also the ground.
7th goes to the 082. I assume that this is to be connected to some kind of a pot, to control the mix.
8th is connected to the TDA1904 amp and I guess its the OUT, or RETURN (still not sure about the therms here)
How does this sound to you? Since I have 4 ground pins there, I assume that one goes for the power, one for IN, one for OUT and one for the control pot...
I came to quite a different conclusion.
Using your numbering:
1 DC control for LM3080. Don't know what this is used for.
2, 3, 4 grounds
5 Audio Out
6 ground for audio in
7 Audio In
8 +V Power
As far as the circuit goes, it's
Audio IN --> Opamp ---> TDA1904 ---> Reverb Tank In ---> Reverb Tank Out ---> Recover Amp (Opamp)
As far as I could see, and I didn't trace the whole thing, there doesn't seem to be any mixing on the board to combine the Reverb signal back with the Dry signal.
There's also an OTA. The second 8-pin device appears to be an LM3080 OTA not a LM308 opamp.
Connector Pin 1 seems be some sort of control input, it passes through the transistor then to the LM3080 control pin.
The output of the OTA is wired to the Audio -Out.
I don't know what input source is for the LM3080. It's possible it doesn't have an input. It could be used for disabling the Reverb signal; It could be related for Tremolo or Vibrato, but the lack of dry signal mixing means it would an odd implementation.
The fact there's no obvious mixing and the presence of the OTA means the circuit is a little weird and you will really need to trace it out if there's any hope of making sense of it.
Thanks for a great insight! I have no real knowledge in circuitry, so I just used some sort of logic, but what you're saying seems way more probable and makes sense. So I guess it's probably from an old amp that had reverb/tremolo effect with this tank and circuit and mixing was happening on the main board - for it to function I'd have to add these myself somehow...
The transistor is BC415B... Don't really know what it does...
Guess I have to learn how to trace and draw a diagram to make some sense out of it all and I hope I'll manage to do so, as I like how this plate and tank look and I'd hate for it to go to waste. If not, perhaps I could just use the tank and make one of the vero-board drivers for it...
Actually, since that reverb tank has I/O amplifiers, you can just solder come jacks to the input and output....
But I don't think it would work...
QuoteThanks for a great insight! I have no real knowledge in circuitry, so I just used some sort of logic, but what you're saying seems way more probable and makes sense.
I only did a quick scan. Checking stuff like power pins on the datasheet and general flow of the signal.
QuoteSo I guess it's probably from an old amp that had reverb/tremolo effect with this tank and circuit and mixing was happening on the main board - for it to function I'd have to add these myself somehow...
I was thinking it's was from an old keyboard. Mainly because styling of the separate board. Nonetheless a guitar amps is just as good a guess.
QuoteThe transistor is BC415B... Don't really know what it does...
It's pretty common for a transistor to drive the control pin of an OTA. I'm assuming it's a circuit like that. It's such an unusual configuration it could be anything.
I think if you had the datasheets in front of you, you could probably do reasonable job of tracing it in about 2hrs.
The TDA1904 circuit will largely follow the example in the datasheet. The opamp before the TDA1904 is wired
as an inverting amp (many examples on the web). The other half of the opamp which is used for the recovery
stage is a non-inverting gains stage [Actually it's inverting] ; very common configuration for reverb circuits and it will match circuits on the web.
So most of the circuit you can just copy the shape and fill-in the different values. The only one which needs careful attention is the LM3080 OTA. Without tracing some of it, it's hard to give an example to copy from.
Thanks again, and again you're most probably right -
Quote from: Rob Strand on September 08, 2020, 07:18:45 AM
QuoteThanks for a great insight! I have no real knowledge in circuitry, so I just used some sort of logic, but what you're saying seems way more probable and makes sense.
I only did a quick scan. Checking stuff like power pins on the datasheet and general flow of the signal.
QuoteSo I guess it's probably from an old amp that had reverb/tremolo effect with this tank and circuit and mixing was happening on the main board - for it to function I'd have to add these myself somehow...
I was thinking it's was from an old keyboard. Mainly because styling of the separate board. Nonetheless a guitar amps is just as good a guess.
QuoteThe transistor is BC415B... Don't really know what it does...
It's pretty common for a transistor to drive the control pin of an OTA. I'm assuming it's a circuit like that. It's such an unusual configuration it could be anything.
I think if you had the datasheets in front of you, you could probably do reasonable job of tracing it in about 2hrs.
The TDA1904 circuit will largely follow the example in the datasheet. The opamp before the TDA1904 is wired
as an inverting amp (many examples on the web). The other half of the opamp which is used for the recovery
stage is a non-inverting gains stage; very common configuration for reverb circuits and it will match circuits on the web.
So most of the circuit you can just copy the shape and fill-in the different values. The only one which needs careful attention is the LM3080 OTA. Without tracing some of it, it's hard to give an example to copy from.
the thing's almost definitely from an old keyboard, as ELKA is the organ brand (it did come up when I first googled it, but I didn't make the connection). There are lots of pictures of the insides of the organs, but I can seem to spot this unit anywhere. Also there's this thingy:
(https://i.postimg.cc/Z0fkwsqp/Elka-Reverb-III.jpg) (https://postimg.cc/Z0fkwsqp)
but I don't think it has anything to do with the unit I got...
How would you go about testing this thing, making it usable? I have one small amp that runs on 12v battery and it as loop effect, so I was thinking of adding this reverb to it. Even if I managed to put together a diagram, I'm not sure what would be the next step...
Quotethe thing's almost definitely from an old keyboard, as ELKA is the organ brand (it did come up when I first googled it, but I didn't make the connection). There are lots of pictures of the insides of the organs, but I can seem to spot this unit anywhere. Also there's this thingy:
I found a video of an Elka E19 1983 organ. I had a similar style reverb board and connector to yours but the electronics looked older. Your IC's are maybe 1988/1990 assuming they are original. By 1990 the synth models seem to appear. So maybe yours is mid to late 80's. No idea what Elka organs fit that era.
Quotebut I don't think it has anything to do with the unit I got...
Well, you never know. I also came across an Elka amp RM 140 which has a reverb.
QuoteHow would you go about testing this thing, making it usable? I have one small amp that runs on 12v battery and it as loop effect, so I was thinking of adding this reverb to it. Even if I managed to put together a diagram, I'm not sure what would be the next step...
Well the main issues the how and were to are it into your amp and also mixing in the reverb and dry signal. The Fender style resistive mixer might work. You will need a reverb pot somewhere as well. Another thing is you might need to adjust the signal gain on the reverb board so that the reverb springs are driven at the right level.
The simplest test would be to connect power to the PCB and connect the output to an amp. Then scratch on the springs and see if you can hear something. If the test doesn't work it doesn't mean the unit is broken as the LM3080 OTA circuit could be affecting things - we don't know what that does. If you do hear something then that only checks the reverb spring recovery amp. So the next step would be to leave it connect to an amp and see if you can get a signal through the unit. If you do it will be *all* reverb.
It's alive! Yup, you were right again - I've connected it as per your instructions, powering the plate with a 9v battery and I got sound of springs when I poke them, very low, as I turned the amp all the way up, but it's there. I also have connected the audio IN and could hear my guitar as well, but also very quiet and I think a bit distorted as well, but it seems to me that it lacks the "reverbness".
Then I though, ok, it probably needs more power and I just "knowledgeably" hooked it to a 12v power adapter I had laying around. Oh, the smell of burning electronics, and a fair bit of smoke too! It turns out that I only burnt the 10 Ohms resistor that leads to the TDA1904s' power pin. Replaced it and got the same results as before. The adapter I used is rated 1.5A, so it might be it's way too much current... I've read somewhere that these tanks would use a lot of volts, so some circuits need a 18v charge pump. It says in the datasheet of the TDA1904 that the supply should be from 4 to 20v...
The thing is, I have this TX30 Crate Taxi amp that runs on a 12v battery. It's just like a car battery and it can be recharged and the amp is pretty ok for it's purpose - performing at locations with no power, street performance... But it kind of lacks some of the benefit that a reverb can give, less clear, more atmosphere... Thus the reverb tank and all.
(https://i.postimg.cc/Y4G8x1xH/tx30-2.jpg) (https://postimg.cc/Y4G8x1xH)
(https://i.postimg.cc/CZ37rzvf/tx30-3.jpg) (https://postimg.cc/CZ37rzvf)
I thought I could place it inside, though it's a bit tight and magnetic pull from the speaker could be the problem, then supply it with the same battery and tap into the amps effect loop jack from the inside. I understand that the original plate would need a way to mix dry and wet, but I'd probably just set it at one position and leave it like that, adding just a toggle switch on the outside, to turn the effect's power on.
Adjusting the signal gain on the board... Could it be that trim pot's function?
Anyway, a lot of cool stuff turned out while searching about this - I've seen the Elka E19 video you mentioned
Quote from: Rob Strand on September 08, 2020, 09:14:36 PM
I found a video of an Elka E19 1983 organ.
- good stuff, I even like the rhythm section. Also, I've read about the guy finding an old Elka organ in a dumpster and it turned out that it only had couple of fuses burnt, he replaced them and it's fully functional and nice sounding too! I also came across another amp that Elka made, a Rig 200 that has a reverb too.
I don't really know what I'll do next with this one - I thought, since the photos I found of some different units suggested that they had a fast/slow switch, I was thinking that maybe that mysterious 3080 is responsible for giving it a sort of short, slap back and a sustaining, long reverb... So I might experiment with connecting it somehow and see what happens. Also, I hope I'll manage somehow to give it more power, as like this it's barely audible.
Thank you for your great insight and for your help! I hope I get it to work fully somehow.
QuoteIt's alive!
OK cool, that's a start.
Quote
but also very quiet and I think a bit distorted as well, but it seems to me that it lacks the "reverbness".
So I think the quietness is due to the LM3080, see below.
QuoteThen I though, ok, it probably needs more power and I just "knowledgeably" hooked it to a 12v power adapter I had laying around. Oh, the smell of burning electronics, and a fair bit of smoke too! It turns out that I only burnt the 10 Ohms resistor that leads to the TDA1904s' power pin. Replaced it and got the same results as before. The adapter I used is rated 1.5A, so it might be it's way too much current... I've read somewhere that these tanks would use a lot of volts, so some circuits need a 18v charge pump. It says in the datasheet of the TDA1904 that the supply should be from 4 to 20v...
The smoke isn't normal. My expectations of handling 12V is exactly what you have written. At this I'd be thinking the TDA1904 could be fried. Another explanation is the TDA1904 has been replaced with a chinese fake, these often blow up at higher voltages. If you have a multimeter you could power-up on 9V and measure some voltages on the TDA1904 pins.
A dodgy TDA1904 might explain the distortion.
QuoteThe thing is, I have this TX30 Crate Taxi amp that runs on a 12v battery.
hus the reverb tank and all.
If you like reverb it's going be be worthwhile for sure. You could get an el-cheapo reverb pedal. They are good enough for a touch of reverb. It would save a lot of head-aches.
Quote
I thought I could place it inside, though it's a bit tight and magnetic pull from the speaker could be the problem, then supply it with the same battery and tap into the amps effect loop jack from the inside. I understand that the original plate would need a way to mix dry and wet, but I'd probably just set it at one position and leave it like that, adding just a toggle switch on the outside, to turn the effect's power o
it's all very doable. I wouldn't worry about the speaker magnetic pull.
QuoteAdjusting the signal gain on the board... Could it be that trim pot's function?
The pot does adjust the level of the reverb. BUT I have a very good reason to suspect the LM3080 is reason for the low level, not the trim pot adjustment.
I noticed the trimpot when I eye-balled the circuit. I had another look and I noticed the reverb recover amp is inverting - I think I said it was non-inverting above.
Quotegood stuff, I even like the rhythm section. Also, I've read about the guy finding an old Elka organ in a dumpster and it turned out that it only had couple of fuses burnt, he replaced them and it's fully functional and nice sounding too! I also came across another amp that Elka made, a Rig 200 that has a reverb too.
There's some cool stuff on the web. Always someone tinkering around with some obscure stuff. Often it turns out to be a big help to people fixing units. One small piece of info is enough to get them over the line.
QuoteI don't really know what I'll do next with this one -
I think the most important thing at this point is to work out the state of the TDA1904. I don't have a good feeling about it. If it's fried you have the option of replacing it, but at the risk of buying some chinese fake which will fail again. Another option is to build your own reverb on a vero board.
If the TDA1904 is fried as it turns out you aren't using many parts on the PCB anyway! Like you could build a minimal reverb using using a common LM386 amplifier instead of the TDA1904.
QuoteI thought, since the photos I found of some different units suggested that they had a fast/slow switch, I was thinking that maybe that mysterious 3080 is responsible for giving it a sort of short, slap back and a sustaining, long reverb... So I might experiment with connecting it somehow and see what happens. Also, I hope I'll manage somehow to give it more power, as like this it's barely audible.
Something along those lines came to me but I'm not a keyboard player or keyboard electronics person so I don't know what to expect. As an engineer it's something the designers might have done. The way I see that working is the reverb level would be artificially shaped for long and short. The envelope would have to come from the key activations.
I scanned over the circuit around the LM3080. I can't see all the parts or the values but I can make some conclusions.
- the audio passes entirely through the LM3080. (no signal from the opamp like I originally thought).
- If control signal on pin 1 of the connector is not present then I'm fairly certain the audio with be cut-off.
That might explain the low audio level.
You want to enable the audio permanently.
One option is to do a mod on the board an completely bypass the LM3080, you don't need any of the functions is provides.
As far as testing the board it is possible to re-enable the audio. I can't make out the resistor in the top right of you picture. It looks like red red red which is 2.2k; maybe you could check. That's a low value. So to get the audio working you would need to connect pin 1 of the connector to a 10k resistor then the other end of the 10k resistor to positive (+V). I wouldn't recommend connecting pin 1 directly to +V as could damage the LM3080.
Rob, I can't thank you enough for taking time and trying to figure this out, as I'm most of the time clueless of what I'm doing with this, but your insights are making things a lot clearer.
I've put together a drawing of the circuit:
(https://i.postimg.cc/47LWW1S4/ELKA1910board.jpg) (https://postimg.cc/47LWW1S4)
As I'm not experienced in tracing this is all I could do - just to put it as is with all the values. Some of the capacitors are marked the way I couldn't make the value precisely and I don't have measuring equipment (thinking of getting some real soon). I hope this helps in making things out...
Quote from: Rob Strand on September 09, 2020, 08:08:22 PM
As far as testing the board it is possible to re-enable the audio. I can't make out the resistor in the top right of you picture. It looks like red red red which is 2.2k; maybe you could check. That's a low value. So to get the audio working you would need to connect pin 1 of the connector to a 10k resistor then the other end of the 10k resistor to positive (+V). I wouldn't recommend connecting pin 1 directly to +V as could damage the LM3080.
I'm not sure if I understand this correctly - that no power should go to the TDA amp, but instead to power the board via first pin, to the 3080 with a 10k resistor? What would be the result of this?
Also, I remembered that when I was making some pedals using vero boards, the schematic often shown that the output of the board should be connected to the first pin of a pot, then another connection would be made from the board and the output would go from the third pin to the out jack. So, I was thinking that out pin and the pin from the 3080 part of the circuit could maybe go together like that - I know that what I'm saying might be silly, but I'm shooting in the dark about this, with that little experience in building that I have...
Oh, and the 1904 visually seems fine, the burning and the smoke seems that have came only from that resistor, which turned all black in the middle. I don't know, maybe I've reversed polarity of the adapter or something...
Drawing a schem.. may I ask of the value for this resistor? You didn't write it's value down in your trace...
(https://i.postimg.cc/KRBJP0gf/Screen-Shot-2020-09-10-at-8-20-43-PM.png) (https://postimg.cc/KRBJP0gf)
Also, what's the value of the cap labelled "1047"?
But that's one good first trance you got there... 8)
EDIT: here's the schem...
There are still some components without definite value, (like the orange drop caps, highlighted orange) and there is 1 other cap and 1 resistor that I queried about...
(https://i.postimg.cc/TpYQSFC3/DC21-ED44-18-AA-4159-992-E-6-F787-A8-B754-F.jpg) (https://postimg.cc/TpYQSFC3)
Hey that's so cool! I mean the schematic you pulled out of the drawing and good of you to spot that resistor value.
I also have one more mistake that I found out - there's the link missing and I corrected it in the updated drawing, marekd with red arrow.
(https://i.postimg.cc/4KzWxDs7/ELKA1910board.jpg) (https://postimg.cc/4KzWxDs7)
I still don't really know what to make of those orange caps, as they suggest the simplest solution - that they are of values .1, .022, .22, but searching the web I found different statements, so I left it like that.
Also that 1047 cap is marked like that since I can't really say what's it's value and I still don't have tools to measure it.
But thanks for the schematic - I've already said that I'm not knowledgeable about all of this (yet), but I'm trying and your input is certainly helpful! I'm ok with graphics, as I do illustration most of the time, but music is my love and I am also very intrigued with all of the gear around playing electric, made some pedals for myself... I believe that there's always something poetic inside of it all, the physics and the behavior of the waves. That's probably why I'm sorry to trow away this circuit and simply build a new driver for the tank. Hope I'll hook it up somehow, with help from you guys!
Another update - I remembered to mark the connector pins the way Rob suggested earlier, as I did make a first test like this - the +V, IN, OUT, DC control for LM3080
(https://i.postimg.cc/Y4NFJkz9/ELKA1910board.jpg) (https://postimg.cc/Y4NFJkz9)
1047: The 7 is misleading (is it a Z?). Probably 104 or 0.1uF aka 100nF. Not critical, it's the power supply high-frequency noise bypass/suppressor.
Vref, bias reference voltage for the TL082 opamp - you're missing a ground trace for the 47uF & 4k7 resistor at the bottom (may be outside the image border?).
The orange film cap values 10-90-an gives look right. I would trust them. Any film capacitors can replace those if necessary.
if this board is from an organ, it might be a swell pedal output to the LM3080 is the missing control input thingy.
Quote from: anotherjim on September 10, 2020, 10:44:40 AM
1047: The 7 is misleading (is it a Z?). Probably 104 or 0.1uF aka 100nF. Not critical, it's the power supply high-frequency noise bypass/suppressor.
Vref, bias reference voltage for the TL082 opamp - you're missing a ground trace for the 47uF & 4k7 resistor at the bottom (may be outside the image border?).
The orange film cap values 10-90-an gives look right. I would trust them. Any film capacitors can replace those if necessary.
You're right! It is a "Z", so I'll input 0.1uF, as you said. Also, you're right about the grounds - I missed them drawing the circuit, I'll add those too.
(https://i.postimg.cc/3d1t562r/ELKA1910board.jpg) (https://postimg.cc/3d1t562r)
Thank you for your input - I'm not sure about the orange caps, what you meant, but I'll have a look at it.
Quote from: duck_arse on September 10, 2020, 11:36:21 AM
if this board is from an organ, it might be a swell pedal output to the LM3080 is the missing control input thingy.
Sounds possible to me! Any idea if this could be useful in any way with adding the unit to a guitar amp? Thanks!
Just that there's nothing very special about them being orange, they are film type caps very suitable for audio. The important thing is that they are not ceramic disc types. The capacitance values will be correct (they read in uF).
The one in series with 1ohm on the power amp output is commonly found to be .22uF AKA 220nF. The one in the input to the power amp is .022uF (AKA22nF) and is I think a deliberately low value to reduce bass since bass doesn't work so well through a spring line.
QuoteI'm not sure if I understand this correctly - that no power should go to the TDA amp, but instead to power the board via first pin, to the 3080 with a 10k resistor? What would be the result of this?
Not quite. So you need the 10k from connector pin 1 to +V to enable the audio through the LM3080. In the default open pin state the LM3080 is cut-off. You still need to connect power to run the circuit, as you did in your tests.
QuoteDrawing a schem.. may I ask of the value for this resistor? You didn't write it's value down in your trace...
Excellent job on the schematic.
Few small points:
- The transistor is pnp, so flip the arrow in the emitter
- The 10k + 220k gain trimpot should wire the trimpot to the input and the 10k to the output.
I think that missing value is 10k.
- The Vref network is missing. 4k7 + 4k7 + 47uF
- Output should come from pin 7 on TL082
So as far as using the circuit goes:
- The 2k2 on audio-in is likely to produce way too much gain, specially in combination with
the gain of the TDA1904.
- Don't really need LM3080, could take the output after the 0.1uF cap on pin 7 of the TL082.
Lift input side of the following 68k to reuse PCB holes.
- Tweaks to a few caps to roll-off low end to reverb.
Ahh this schematic drawing is fun... :icon_biggrin:
Anyway, i was thinking, if the lm3080 was for reverb swells, OP could add a expression pedal with one end to either VCC or VREF and the other end to the "control input thingy"...
Updated schem with many wonderful colors.... 8)
(https://i.postimg.cc/1nPGj3Zj/image.jpg) (https://postimg.cc/1nPGj3Zj)
QuoteUpdated schem with many wonderful colors....
Looks nice.
Maybe the 47k resistor from the tank is 4k7.
I was looking at how to merge the reverb into the amp. It looks very easy since the amp is full of inverting circuits,
that makes mixing simpler.
https://elektrotanya.com/crate_tx-30_07s259_sch.pdf/download.html
However, the amp has a number of inputs: guitar, mic and external. Also an audio out.
You have to ask yourself what signal do you want reverb to be on and do you want reverb to be on the audio out.
Based on how most amps work I'd say:
- reverb on guitar
- reverb on mic
- no reverb on external
- reverb on audio out
That combination might not be easiest to do but we need to start somewhere.
It would be helpful if you can measure the DC resistance of the coils on the reverb.
It's not clear if the reverb drive coil is a low or high impedance type. My guess is low.
I can see a few things which might make the reverb tone poor for guitar,
- TDA1904 is wired as voltage drive, not current drive. This can make the sound muddy.
A common way to compensate for this is to make the input cap to the drive circuit small.
- The output of the reverb tank drives 4.7k (I think). This can cause the highs to roll-off and sound muddy.
- The inverting recovery amp for the reverb tank isn't the best choice for noise.
You can probably get away with it though.
I suspect the first two issues need to be tweaked as they both contribute to a muddy reverb.
Circling back around to this thread, so I was wrong about the LM308. I thought I saw CN but it was 0N. Oops.
Quote from: Rob Strand on September 10, 2020, 09:46:10 PM
https://elektrotanya.com/crate_tx-30_07s259_sch.pdf/download.html
However, the amp has a number of inputs: guitar, mic and external. Also an audio out.
You have to ask yourself what signal do you want reverb to be on and do you want reverb to be on the audio out.
Based on how most amps work I'd say:
- reverb on guitar
- reverb on mic
- no reverb on external
- reverb on audio out
That combination might not be easiest to do but we need to start somewhere.
It would be helpful if you can measure the DC resistance of the coils on the reverb.
It's not clear if the reverb drive coil is a low or high impedance type. My guess is low.
I can see a few things which might make the reverb tone poor for guitar,
- TDA1904 is wired as voltage drive, not current drive. This can make the sound muddy.
A common way to compensate for this is to make the input cap to the drive circuit small.
- The output of the reverb tank drives 4.7k (I think). This can cause the highs to roll-off and sound muddy.
- The inverting recovery amp for the reverb tank isn't the best choice for noise.
You can probably get away with it though.
I suspect the first two issues need to be tweaked as they both contribute to a muddy reverb.
Why not just place the reverb after the summing amp? just makes everything simpler... maybe a switch for on/off reverb and a pot for wet/dry mix?
Maybe OP should test putting the tank first, then change the resistor values when it's audibly muddy... something bad might happen along the way.. :P
QuoteWhy not just place the reverb after the summing amp? just makes everything simpler... maybe a switch for on/off reverb and a pot for wet/dry mix?
Maybe OP should test putting the tank first, then change the resistor values when it's audibly muddy... something bad might happen along the way.. :P
Not sure which summing amp?
We need the reverb board before at least one summing amp so we can combine the Dry and Wet. The amp has more than one summing point we can use.
The switch idea will work, perhaps two options. Could even tweak them with an internal trim pot (s).
As for something bad happening, that's possible for sure. I guess you can always fix any unintended damage.
(https://i.postimg.cc/8sHQkTTd/Screen-Shot-2020-09-11-at-12-03-07-PM.png) (https://postimg.cc/8sHQkTTd)
So it's actually a power amp?... (Don't really know... :icon_redface:)
So it must be this one...
(https://i.postimg.cc/rRphwX9g/Screen-Shot-2020-09-11-at-12-06-31-PM.png) (https://postimg.cc/rRphwX9g)
Something like this?
(https://i.postimg.cc/qNZpJ4XD/Screen-Shot-2020-09-11-at-12-12-43-PM.png) (https://postimg.cc/qNZpJ4XD)
Of course, "Reverb Tank" means all associated components... Including an wet/dry simple op-amp mixer that can be demonstrated by the other forumites... ;) ;)
If you look closely, you can see a vactrol which controls the gain of that op-amp... That's somewhat like a limiter, i suppose... :icon_biggrin:
QuoteOf course, "Reverb Tank" means all associated components... Including an wet/dry simple op-amp mixer that can be demonstrated by the other forumites... ;) ;)
If you look closely, you can see a vactrol which controls the gain of that op-amp... That's somewhat like a limiter, i suppose...
That way will work. So you resistively mix the dry and reverb, a but like R20 R21 on this circuit,
https://elektrotanya.com/PREVIEWS/63463243/23432455/marshall/marshall_valvestate20_20w_8020.pdf_1.png
Yes the Taxi is supposed to have a limiter, it stops those small amps farting out.
Quote from: willienillie on September 10, 2020, 09:57:04 PM
Circling back around to this thread, so I was wrong about the LM308. I thought I saw CN but it was 0N. Oops.
I thought it might be QN which I'm sure was a code on some NS chips, but no - there is no Q version of the 308.
Really good reverse engineering work here though. I don't really have the stomach for it myself - only do it when I must!
Wow! Great input from everyone - much appreciated.
I have to say that I can understand most of what you're suggesting, but since I'm a newbie in understanding circuitry I'll have to refer to a drawing I've made to make those applications - just so I can know where I'm at, before I learn and understand the language. So, here's the drawing with those details:
-enabling the audio 1+8 via 10k, for test purpose
-2k2 possibly needs to be of higher value, I don't know - 10k?
-mod to bypass the 3080
-as for tweaking the caps, I guess this would come when we have something audible to work with
(https://i.postimg.cc/3ydHy8Nf/ELKA1910board.jpg) (https://postimg.cc/3ydHy8Nf)
Thanks for getting into connecting with the amp as well - I would need to decipher the suggestions on site, and to see what's what and how to approach this. Before, I was thinking this could be done by just soldering the IN/OUT/GROUND of the effect unit on the inside of the INSERT jack, as it acts as a loop effect and it's a stereo jack with mapping of the usage given in the amp's manual:
(https://i.postimg.cc/9RsT3QR6/tx30-insert.jpg) (https://postimg.cc/9RsT3QR6)
I was thinking that no more than a output volume pot would be necessary between the two, to control the mix, but I understand that a reverb pot would be needed to control the level of the reverberation.
Also, the amp has it's own power plate circuit that's separate from the rest and I don't know if powering the effect would be better to run from there, than to just tap into the battery, with an OFF/ON switch.
Uhh... sorry to rain on your parade, but there isn't any reverb pot that controls reverbation...
If we would add a "reverb" pot, it would just be the wet/dry mix... :icon_biggrin:
Are you sure you don't want an expression pedal controlling reverb swells? :icon_mrgreen:
Yes, that INSERT jack would be much better... (would avoid ruining your amp..)
If I may ask.... do you have any electronic components at hand? Say, any op-amps?
I don't mind the rain ;D , as long as my feet are dry and insulated by a thick rubber sole :D
Guess I was thinking more like a dwell control or something, if that makes sense, or to have a choice of quick, slap back and long, deep one...
The use of swells does intrigue me and I'd be glad to try it out, but maybe more like a auto controlled, like auto wah or something... Rather than complicate the thing with an expression pedal.
I have some stuff from previous builds and I could always get some in a local store. Right now I have a TL072 and TL082, KA358 as well, but that's just another 072...
I don't think you can change the time constants on a reverb tank... not really sure though, maybe adjusting the "pre-gain" before entering the spring-tank..
In reality, the expression pedal will be about 5x easier to implement than the envelope follower... ;)
Good to see that you have some op-amps, and even a local store that's (presumably) open.. :icon_eek:
In traditional gear, "Dwell" is really a drive level control into the spring. Easy to do, it's just a volume control before the power amp.
It's alive! Again, thanks to Rob's suggestion - I've connected the 10k to the pins 1 and 8 and got the sound of it. The sound of the springs when touched got immediately much louder than before and I also did a little test with the guitar. Here's the clean (LP Jr straight into the Reaper):
https://drive.google.com/file/d/18bKUMfEZzLxyDr-XNgeDB3iQvUNzfz4f/view?usp=sharing
And here's the 'verb:
https://drive.google.com/file/d/17JFoRakT7A4yh5ai4lo4VvgkvMnLYrd-/view?usp=sharing
I'm not sure on how to improve the sound of it, what exactly does it lack. It certainly distorts at times and that should be taken care of. I did roll back the trim pot a bit so it doesn't get nasty - all the way it's just too much distorting, but the signal is more audible.
Also, the question remains about the mixing and how to apply that passive mix - I'd probably need to build a sister board of the mixer and I'd need a vero diagram for it, as I'm not skilled in making it out a schematic. I'll try to find a stereo jack and check if I can hook it up via INSERT jack.
Now that I think about it, I think the LM3080 could be used as a makeshift compressor to limit gain...
@razabiri does the distortion come from the reverb tank ckt or from the amp being too loud?
Passive mix you say... ::)
Nooo, I meant resistive mix like Rob said, but of course, I have no idea what I'm saying half of the time so there you go :)
Quote from: Rob Strand on September 11, 2020, 01:25:33 AM
That way will work. So you resistively mix the dry and reverb, a but like R20 R21 on this circuit,
https://elektrotanya.com/PREVIEWS/63463243/23432455/marshall/marshall_valvestate20_20w_8020.pdf_1.png
Yes the Taxi is supposed to have a limiter, it stops those small amps farting out.
The distortion is from the tank, as I didn't record it via amp at all, just stuck it to the DAW directly, so that's the Reaper I'm referring to. Guess I'll try to replace 2k2 resistor, give it a higher value, as Rob suggested as like this it has too much gain - and there I made a mistake on the drawing - I marked a different 2k2 as an input resistor, here's the corrected drawing:
(https://i.postimg.cc/JtxjBCfB/ELKA1910board.jpg) (https://postimg.cc/JtxjBCfB)
Another update:
I pulled out the 2k2 input resistor, 4k7 tank one and the input cap - slotted the holes and I've been trying different values. I don't have much to choose from, but higher value on the input does reduce distortion, different cap makes somewhat difference and the tank resistor between few different values makes huge differences. Right now I have listened to it and tweaking it for too long - I don't know if I'm getting anywhere or going away from the right combo... Maybe I should somehow place some trim pots instead of resistors and roll the right values.
Also, I've been testing it with the 9v battery and I don't know if switching to 12v will make audible difference. Haven't tried the 3080 bypass mod yet - I'm also not sure if this will reflect on the tone.
I did some calculations.
So the 4k7 on the tank output is *just* OK. I wouldn't worry about it at this point.
The biggest problem is the 2.2k input resistor. It needs to be increased to about 1Meg.
Yes! it's *way* off.
The 100nF (0.1uF) cap that goes connects to the 2.2k is OK for now.
Try just changing the resistor!
So a couple of things:
- Have you tried running with the LM3080 bypassed with wires?
- On your "wire" pic, you have show the 68k lifted or cut on one end, that's OK
However, I don't understand the wire that goes from that point to pin 3 of the opamp.
That wire should not go to pin 3, the wire becomes the new output wire.
We can clean-up a few things easily but for now just try the above.
You might need to adjust the reverb level trimpot. I'm expecting you might need to crank
that up full.
When you bypass the LM3080 there might be a bit of bass cut, because of the small output cap - we'll will deal with that later.
The result you should expect from that is
- a clean reverb, no distortion
- the reverb level is about equal to the original signal
@Rob what do you think would be more suitable for this application? passive mix? or active mixer?
Quote@Rob what do you think would be more suitable for this application? passive mix? or active mixer?
Both can be made to work.
I do have a couple of concerns about the passive mixer. The first option is at the output of IC4 on the TX 30 schematic. The passive mixer might affect the clipper. So to fix that we could move the passive mixer to after the clipper. Also, we would prefer not to have the 10k R72 loading down the passive mixer. So that means maybe put the passive mixer between R72 and C30. Now we still have a problem the passive mixer is going to cause a signal loss of 0.5 (-6dB). The limiter level is tuned to match the power amp. If we put attenuation after the limiter the signal getting to the power is going to be too low. To get around that we would have to play with the limiter level.
For the active mixer we would put it before IC4 and simply use IC4 to mix in the reverb. We would resistively mix the inputs to the reverb from each channel then take the output of the reverb and mixed that with the original signal at IC4. I think this method is a lot less intrusive. That was my original plan to graft in the reverb circuit.
Quote from: Rob Strand on September 11, 2020, 11:04:25 PM
I did some calculations.
So the 4k7 on the tank output is *just* OK. I wouldn't worry about it at this point.
The biggest problem is the 2.2k input resistor. It needs to be increased to about 1Meg.
Yes! it's *way* off.
The 100nF (0.1uF) cap that goes connects to the 2.2k is OK for now.
Try just changing the resistor!
So a couple of things:
- Have you tried running with the LM3080 bypassed with wires?
- On your "wire" pic, you have show the 68k lifted or cut on one end, that's OK
However, I don't understand the wire that goes from that point to pin 3 of the opamp.
That wire should not go to pin 3, the wire becomes the new output wire.
We can clean-up a few things easily but for now just try the above.
You might need to adjust the reverb level trimpot. I'm expecting you might need to crank
that up full.
When you bypass the LM3080 there might be a bit of bass cut, because of the small output cap - we'll will deal with that later.
The result you should expect from that is
- a clean reverb, no distortion
- the reverb level is about equal to the original signal
I was coming to the same conclusion, as for the values of those components. Found this one on the web:
(https://i.postimg.cc/dh0tcvhw/v3EIw5A.png) (https://postimg.cc/dh0tcvhw)
And I've looked at it's values - the 4k7 is the same as with the ELKA, so I left it at peace and it really just sounds best from the bunch. There's that 1M you mentioned too and the cap of 0.1uF too.
I haven't found any of the 1M, but I did try out 220k and 560k and with value increased it does sound much better, but it's fainting - guess that with 1M it would be hardly audible. And I did crank it up all the way. Maybe we'd need to make a return output higher somehow, as it's not really equal this way to the clean signal.
About the 3080 bypass mod - I made a mistake of connecting that pin, as I thought it was the pin 7, as you suggested before - the IC is upside down.
pull out the lm3080 and save it for later... (If I'm not mistaken it's socketed...) It will be very useful later on...
QuoteAnd I've looked at it's values - the 4k7 is the same as with the ELKA, so I left it at peace and it really just sounds best from the bunch. There's that 1M you mentioned too and the cap of 0.1uF too.
It's hard to draw parallel without a schematic. The reason you need 1M is because the gain of the TDA1904, and the fact the signal before the power amp is relatively strong.
The reverb units which use only use opamps, like the link you posted, tend to be for 600 ohm reverb tanks. Some use parallel opamps to drive 150ohm tanks. However, for 8 ohm tanks you need a power amp (TDA1904, LM380, LM386), or, an OPAMP with added transistors. No many circuits have the "extra" opamp before the power amp.
Do you have a multimeter? You can check what you tank DC resistance is. I'm pretty sure it will be an 8 ohm type, they typically have DC resistances on the drive coil of 0.8 ohms.
Still no multimeter here, waiting for the next paycheck... ::)
You're probably right, as you have been all the way and we'll check that value soon.
I did try to jump that hole from 68k to 7th pin of the 082, but there's no signal at all - have tried it with and without the jumping 10k from 1 to 8 - no luck.
However, once I put it back as it were and with the 560k as input resistor, I tried to connect it with the 12v adapter again - it worked like a charm and I got a much better output, close to the clean signal. I realized that I must have reversed the polarity before and burned that resistor, as the circuit has no polarity protection diodes.
About the mix, how would you feel about this, rather than tapping into the amp's circuitry:
(https://i.postimg.cc/WhZK2mp5/2chan-mixer-rev1.png) (https://postimg.cc/WhZK2mp5)
I might be able to build this mixer and add clean to one and reverb to the other channel and adjust the two - I'd probably use trimpots. It does run on 9v, but I hope that 12v would be ok too for it. Than just go into that INSERT jack.
If the solutions you've mentioned are better/simpler, I'd go ahead with those too...
Here's another drawing, flipped correctly so that I don't confuse myself again about the ICs orientation.
(https://i.postimg.cc/N2YjD64b/ELKA1910board2.jpg) (https://postimg.cc/N2YjD64b)
QuoteI did try to jump that hole from 68k to 7th pin of the 082, but there's no signal at all - have tried it with and without the jumping 10k from 1 to 8 - no luck.
Hmm, something not right. I has to work ;D.
The bridge wire you added from pin 7 of the opamp isn't required.
Pin 7 goes to the 0.01uF cap.
You want to take the reverb out from the *other side* of the 0.01uF cap,
basically that's the side of the 0.01uF cap that used to go to the 68k.
The 0.01uF cap is quite a bit too small. It needs to be increased to something like
47n to 220nF, depending on how you want the low-end of the reverb to sound.
QuoteHowever, once I put it back as it were and with the 560k as input resistor, I tried to connect it with the 12v adapter again - it worked like a charm and I got a much better output, close to the clean signal. I realized that I must have reversed the polarity before and burned that resistor, as the circuit has no polarity protection diodes.
The LM3080 could add some distortion. That's a good reason to bypass it.
As for 560k, that's good news. I honestly think you need to got to 1M. If you find the reverb signal too weak we need to increase the recover amp gain. The first step would be to adjust the trimpot for maximum output. If that's not enough we need to increase the gain by changing a resistor.
QuoteAbout the mix, how would you feel about this, rather than tapping into the amp's circuitry:
I'm pretty sure you don't need to add a mixer if you use the external audio out/audio in jack.
The way that jack works is it takes the guitar and mic channels, sends them to the audio out.
*However* while the jack is in the guitar and mic signal still pass through to the internal mixer and power amp.
The audio in is mixed back with those existing signals.
So the way that jack work already doesn't the mixing. I don't know why the amp is designed like that but it's
exactly what you want for the reverb. You just use the reverb board as is and splice it between
the audio out and audio in on the external jack.
QuoteI might be able to build this mixer and add clean to one and reverb to the other channel and adjust the two - I'd probably use trimpots. It does run on 9v, but I hope that 12v would be ok too for it. Than just go into that INSERT jack.
Since the audio out contains both guitar and mix mixed aleady you actually won't gain anything but adding that extra board.
QuoteIf the solutions you've mentioned are better/simpler, I'd go ahead with those too...
If you can get the LM3080 bypass working and maybe change that 0.01uF cap you are pretty much there.
Seems I didn't understand the 3080 bypass mod, but I think I got it now. There are two holes that are just like made for bypassing it and didn't have any component there, so I placed jumper over there and disconnected the powering 10k from pins 1 and 8. Got he signal through!
(https://i.postimg.cc/LqcY1Skj/ELKA1910board3.jpg) (https://postimg.cc/LqcY1Skj)
I'm not sure about that output cap, I seem to don't have 47n, nor 220n at the moment, so I placed that 100n (.1uF) and it's ok - increasing should give more or less low-end?
About the mixing - I think I understand what you're saying, but I'm not sure how will I adjust the level of the effect, as the mixing INSERT jack doesn't have it's own control pot or anything - should I add a volume pot at the effect board? S that I can lower it thus letting the clean signal through, as I like it somewhere half/half...
Quote[have any component there, so I placed jumper over there and disconnected the powering 10k from pins 1 and 8. Got he signal through!
That's pretty much it. You should also remove the 100nF (0.1uF) capacitor to the right of the link. Otherwise the signal feeds back into the *output* of the LM3080.
Instead of a link a 100ohm to 1k ohm resistor would be better. This is for technical reasons to avoid oscillations when the opamp drives a cable.
Quote'm not sure about that output cap, I seem to don't have 47n, nor 220n at the moment, so I placed that 100n (.1uF) and it's ok - increasing should give more or less low-end?
Larger is more low end.
You could re-use the 100nF cap removed in the previously.
QuoteAbout the mixing - I think I understand what you're saying, but I'm not sure how will I adjust the level of the effect, as the mixing INSERT jack doesn't have it's own control pot or anything - should I add a volume pot at the effect board? S that I can lower it thus letting the clean signal through, as I like it somewhere half/half...
It's best to just put a pot at the output of the Reverb. That will let you adjust the reverb level from 0 to full.
You won't be able to get *only* the reverb signal. It's not much use have only reverb anyway. The reason is the amp mixes the mic and guitar signal internally. There's no way to remove those signals via the external connector [I shouldn't say no way as there is an evil way.]
If you want a switch to bypass the 3080, have the 100nF output cap always in the feed to the output and with an SPDT switch (changeover) to select the feed into the output cap from either the 3080 output pin6 (as it is) or the TL082 output pin7. So the switch lug 2 to the 100nF lower leg and lug 1 & 3 wired from each of those two outputs.
Quote from: Rob Strand on September 12, 2020, 10:15:45 AM
Quote[have any component there, so I placed jumper over there and disconnected the powering 10k from pins 1 and 8. Got he signal through!
That's pretty much it. You should also remove the 100nF (0.1uF) capacitor to the right of the link. Otherwise the signal feeds back into the *output* of the LM3080.
Instead of a link a 100ohm to 1k ohm resistor would be better. This is for technical reasons to avoid oscillations when the opamp drives a cable.
Quote'm not sure about that output cap, I seem to don't have 47n, nor 220n at the moment, so I placed that 100n (.1uF) and it's ok - increasing should give more or less low-end?
Larger is more low end.
You could re-use the 100nF cap removed in the previously.
QuoteAbout the mixing - I think I understand what you're saying, but I'm not sure how will I adjust the level of the effect, as the mixing INSERT jack doesn't have it's own control pot or anything - should I add a volume pot at the effect board? S that I can lower it thus letting the clean signal through, as I like it somewhere half/half...
It's best to just put a pot at the output of the Reverb. That will let you adjust the reverb level from 0 to full.
You won't be able to get *only* the reverb signal. It's not much use have only reverb anyway. The reason is the amp mixes the mic and guitar signal internally. There's no way to remove those signals via the external connector [I shouldn't say no way as there is an evil way.]
Evil way! Hahahaaa, that's a good one Rob, pretty devlish. Well, I've tried that connector, as the plan was to connect the thing from the inside - to solder on the tip, sleeve and ring of the jack itself, and it works pretty well (at least now, from the outside of it). The mix is alright by my ear and it's kind of well balanced between clean and reverb, but I should roll the effect back a bit, I think.
I did everything you soggested - the link is now replaced with a 1k resistor, the output cap is now 0.1uF and I removed the one that was connecting the 3080. I'm not sure if I'll keep the value of the output cap like so, as it might need a bit more bass to the reverb, we'll see.
Still haven't found a 1M resistor for the input, but I did found a 1.5M one and tried it like that. The effect is pretty much lowered and the 1M is probably a better match. I did push the driver all the way and it would need more on the return amp to get something usable with the guitar amp - through the PC it does sound well, but into the Taxi it would need to be stronger to equal the mix with the dry signal. I'll probably go and buy some 1Ms', as there's no use of pushing it like this. On the other hand, when the driver is rolled all the way up, the device is more prone to picking up electric noise from the surrounding, but the signal has less chance of being distorted, as with a lower value of the resistor, 560k, or even 220k it's quieter, as I don't need to push the driver, but it's more prone to distort the sound, as you hit the strings harder - guess I'll need to find the sweet spot ???
Thanks for following all of it, I couldn't get anywhere without you and the guys - the rest of it, when I set the sound right, is to mount it and I was thinking to power it via the amp's circuit, so the effect and the amp can be started with the same switch.
Quote from: anotherjim on September 12, 2020, 04:07:02 PM
If you want a switch to bypass the 3080, have the 100nF output cap always in the feed to the output and with an SPDT switch (changeover) to select the feed into the output cap from either the 3080 output pin6 (as it is) or the TL082 output pin7. So the switch lug 2 to the 100nF lower leg and lug 1 & 3 wired from each of those two outputs.
Good suggestion, I'm not sure if I were to have any use of the 3080 part of the circuit and I've bypassed it for now, but it's good to know and perhaps I'll tweak with it too sometime. Thanks!
I've found that maybe I can adjust the effect level to the fixed spot, before mounting it, with the use of it's own trimpot. Perhaps I don't even need a volume pot... But if so, how would I connect it and what value would be best?
Is it alright to use this diagram as a guitar wiring standard?
(https://i.postimg.cc/1fLPqH2k/volume-pot.jpg) (https://postimg.cc/1fLPqH2k)
Quote from: razabri on September 13, 2020, 07:11:27 AM
I've found that maybe I can adjust the effect level to the fixed spot, before mounting it, with the use of it's own trimpot. Perhaps I don't even need a volume pot... But if so, how would I connect it and what value would be best?
Is it alright to use this diagram as a guitar wiring standard?
(https://i.postimg.cc/1fLPqH2k/volume-pot.jpg) (https://postimg.cc/1fLPqH2k)
Yes, that can be used
A 100k pot would be ok.
QuoteWell, I've tried that connector, as the plan was to connect the thing from the inside - to solder on the tip, sleeve and ring of the jack itself, and it works pretty well (at least now, from the outside of it). The mix is alright by my ear and it's kind of well balanced between clean and reverb, but I should roll the effect back a bit, I think.
Sounds good. Adding the reverb pot will be the icing on the cake.
QuoteI did everything you soggested - the link is now replaced with a 1k resistor, the output cap is now 0.1uF and I removed the one that was connecting the 3080. I'm not sure if I'll keep the value of the output cap like so, as it might need a bit more bass to the reverb, we'll see.
OK cool. You might get a bit more bass but on the whole there's not a lot of low-end filtering as it is now.
I think anything larger 1uF isn't going to produce any noticeable change. The 100nF is at the point where is cuts some bass, probably less than a fender amp does.
QuoteOn the other hand, when the driver is rolled all the way up, the device is more prone to picking up electric noise from the surrounding, but the signal has less chance of being distorted, as with a lower value of the resistor, 560k, or even 220k it's quieter, as I don't need to push the driver, but it's more prone to distort the sound, as you hit the strings harder - guess I'll need to find the sweet spot ???
Exactly! It's a delicate balancing act. I always try to drive the reverb tanks hard to reduce noise etc. As it is now, it's probably close to the ball-park. Tuning of small details can only be done by playing the unit. I have a feeling 560k is going to overload the reverb drive amp when you crank the amp.
If you need more gain in the recover amp you can increase the 100k in series with the trimpot with say 220k.
Quote
A 100k pot would be ok.
I'm in favour of a 10k pot. The reason is the amp input impedance is about 9.1k so a 100k pot will cause the load to vary on the reverb tank and that's going to cause the LF roll-off to shift depending on the reverb pot setting. The low pot value reduces the filter shift.
When the reverb pot is dialed down. The opamp sees about 100k so the that forms a high pass filter with the 100nF output cap of 1/(2*pi*100k*100n) = 15.9Hz. When the reverb pot is set to full the opamp sees 100k // 9.1k = 8.3k ohm so the high-pass filter moves up to 1/(2*pi*8.3k*100n) = 192Hz ; a big change.
With the 10k pot the load varies from 10k to 10k//9.1k = 4.67k and the cut-off varies from 159Hz to 341Hz much less change, still not great perhaps.
We simply need to choose the cap to set the frequency. A 1uF cap will drop the cut-off to 15.9Hz and 34.1Hz. Which is pretty much doing nothing. Maybe 220nF to 470nF is more on the money.
perhaps that "electric noise from your surroundings" is because the board itself isn't in a shielded enclosure yet...
Quote from: Rob Strand on September 13, 2020, 09:44:03 PM
QuoteWell, I've tried that connector, as the plan was to connect the thing from the inside - to solder on the tip, sleeve and ring of the jack itself, and it works pretty well (at least now, from the outside of it). The mix is alright by my ear and it's kind of well balanced between clean and reverb, but I should roll the effect back a bit, I think.
Sounds good. Adding the reverb pot will be the icing on the cake.
QuoteI did everything you soggested - the link is now replaced with a 1k resistor, the output cap is now 0.1uF and I removed the one that was connecting the 3080. I'm not sure if I'll keep the value of the output cap like so, as it might need a bit more bass to the reverb, we'll see.
OK cool. You might get a bit more bass but on the whole there's not a lot of low-end filtering as it is now.
I think anything larger 1uF isn't going to produce any noticeable change. The 100nF is at the point where is cuts some bass, probably less than a fender amp does.
QuoteOn the other hand, when the driver is rolled all the way up, the device is more prone to picking up electric noise from the surrounding, but the signal has less chance of being distorted, as with a lower value of the resistor, 560k, or even 220k it's quieter, as I don't need to push the driver, but it's more prone to distort the sound, as you hit the strings harder - guess I'll need to find the sweet spot ???
Exactly! It's a delicate balancing act. I always try to drive the reverb tanks hard to reduce noise etc. As it is now, it's probably close to the ball-park. Tuning of small details can only be done by playing the unit. I have a feeling 560k is going to overload the reverb drive amp when you crank the amp.
If you need more gain in the recover amp you can increase the 100k in series with the trimpot with say 220k.
Quote
A 100k pot would be ok.
I'm in favour of a 10k pot. The reason is the amp input impedance is about 9.1k so a 100k pot will cause the load to vary on the reverb tank and that's going to cause the LF roll-off to shift depending on the reverb pot setting. The low pot value reduces the filter shift.
When the reverb pot is dialed down. The opamp sees about 100k so the that forms a high pass filter with the 100nF output cap of 1/(2*pi*100k*100n) = 15.9Hz. When the reverb pot is set to full the opamp sees 100k // 9.1k = 8.3k ohm so the high-pass filter moves up to 1/(2*pi*8.3k*100n) = 192Hz ; a big change.
With the 10k pot the load varies from 10k to 10k//9.1k = 4.67k and the cut-off varies from 159Hz to 341Hz much less change, still not great perhaps.
We simply need to choose the cap to set the frequency. A 1uF cap will drop the cut-off to 15.9Hz and 34.1Hz. Which is pretty much doing nothing. Maybe 220nF to 470nF is more on the money.
That's great, Rob, that you did the calculations and all - I think I have a 22k pot that's close enough to try out how it works. I'm not sure about the cap you're mentioning - is that the output one? Now it's 100n and I should go from 220n to 470n, as I understand.
Also, I'm not sure about what are you saying about the 100ks' in series and trimpot of 220k ???
There's that noise issue too... I don't really know if it's lacking shielding or is it the circuit itself, as when the trimpot is rolled down, there's no noise and no effect, but cranked full up it is a bit noisy. I get that reverb tanks can be a bit noisy, and I'm not sure if the noise here is coming from the fact that it's, as you said, power driver and not a current driver... I lack the knowledge to understand what's the difference and how maybe that 4w 1904 amp is itself a bit noisy, or it's that not-so-good of a choice TL082 as the recovery. Maybe it's because my power adapter is not giving very clean power and it'll pass once I switch to the battery. When I was recording some tests, I could clearly hear that it's picking up noise that's coming from my laptop's hard drive and rolling the trimpot increased this noise. There was some more noise that was from the lack of grounding and I'm not sure but it seems like it happened once we bypassed that 3080, but then I just connected the ground of the circuit to the chassis of the springs and it stopped.
Anyway, I think that it's time to try and mount it, as things can vary on the position and power and I'll try and do so in a way that I can still easily adjust the details.
Quote from: 11-90-an on September 13, 2020, 10:32:18 PM
perhaps that "electric noise from your surroundings" is because the board itself isn't in a shielded enclosure yet...
Guess that's very possible too - I really don't know how this 1904 is going to behave, once when it's all mounted. I know that lm386 can pick up various noises and even radio signal (I had this issue with an Acapulco Gold build). Perhaps I could make a sort of a cover out of tin for the circuit, if necessary.
>Also, I'm not sure about what are you saying about the 100ks' in series and trimpot of 220k
Rob means changing the value of this resistor to 220k (circled in red)
(https://i.postimg.cc/jWMw2hns/ELKA1910board2.jpg) (https://postimg.cc/jWMw2hns)
QuoteGuess that's very possible too - I really don't know how this 1904 is going to behave, once when it's all mounted. I know that lm386 can pick up various noises and even radio signal
If you get radio and some forms of crackle adding judicious place of some ceramic caps can help. For the reverb spring you can try adding say 1nF across the input of the recovery amp. Maybe best done by soldering a cap on the underside of the PCB.
One thing really asking for trouble are those flying leads on the output side of the reverb tank. At minimum you should twist these fairly firmly.
Accutronics reverbs have very short wires. They then go to RCA jacks, some are insulated RCA's. From the RCA's shielded wire is used to connect to the electronics. It's not so important for the drive side but the recovery side is very important. You can see the short leads in this pic,
(https://sound-au.com/articles/rvb-f1.jpg)
In addition to that, output side of the tank which connects to the circuit ground at the recover amp is normally connected to the outer metal casing of the tank. That lets the metal casing act as a shield. The way that is done is by using a non-isolated RCA jack. For tanks without RCAs it is done with a small wire link or folded metal tab which goes to the chassis.
IMHO these measures are important to fend off noise on reverbs. The signals are very small so you need to put in as many counter measures as possible.
As for noise in the form of hiss. You would need to redesign the recovery amp to do any better. It shouldn't be too bad. The inverting op amp used on the ELKA isn't the best choice for low noise. You can shave off a small amount of noise by low-pass filtering the output of the recovery amp. That's done by selecting adjusting the cap across the outside of the 220k trimpot + 100k resistor; essentially across pin 6 and 7 of the opamp. Current that value is 47pF which sets the low-pass filter frequency to be
flp = 1/(2*pi * 47pF *(220k+100k)) =10.6kHz to 1/(2*pi * 47pF *(100k)) =34kHz depending on the pot setting. Given we gain is set high at the moment it's probably more like 10.6kHz. If you wanted to shave a small amount of noise you could use say 100pF.
(Keep in mind if you increase the 100k resistor to get more recover gain you need to redo the calcs and that will require a smaller cap for the same filter cut-off.
Quote
Rob means changing the value of this resistor to 220k (circled in red)
That's it, thanks.
A big thanks to 11-90an for doing that schematic. There's *no way* we would have got this far without it.
>A big thanks to 11-90an for doing that schematic. There's *no way* we would have got this far without it.
It was fun to do, to say the least... :icon_biggrin:
what if we also add a simple R/C LPF after the output?
Quotewhat if we also add a simple R/C LPF after the output?
No problems doing that. The RC LPF and the adding the feedback cap on the recovery amp both achieve the same result.
Normally the feedback cap has the advantage that you don't need to add a series resistor to the output
or the opamp. However, in our case we have already added a 1k resistor. The LPF at the output avoids
the filter cut-off changing with different trimpot settings so the LPF might be a better solution.
I have now successfully mounted the thing inside and it is fully functional! Tapped into the INSERT jack from the inside and took the power at the small cable connecting the power board with the amp board and it lit up nicely! Not to mention that a placing of the effect was as if it's built for it - just right above the speaker, with two screws through already made holes on the chassis. 8)
No more of that noise either, and yes, it was a hiss sound before, but I think it's due to the adapter used (from an old modem). Now, I do have some noise in the reference to the clean sound, but it seems to me that it's kind of normal, as I remember that when you turn the reverb knob on amps that actually have them, it gets a bit hissy. The amp itself is not that quiet and I can't seem to find its signal to noise ratio, but my guess is that it's somewhere close to >50dB for the overdrive, >65dB for the clean, as that's how they rated it with the other Crate amp that I have, and there's pretty much the same amount of noise when turned on.
Also, I did that for the shielding/grounding - a ground pin is connected to the metal chassis of the effect and it does prevent additional noise that was happening.
I have tried to connect 22k pot as the output, but it didn't wok well and I also tried 1k, as I had it at hand, but that too didn't make a good match - I guess it's that 10k would be just right. Oh, and I don't think we really need to amplify the return signal, it sound good to my ear like it is. I'll just get that 1M input resistor and set the trimpot right, I guess.
Another thing is, there's a big reverb splash when the amp is turned on. I mean, I know it's sort of normal, but maybe it could be quieter. I thought that I was doing it as I'd hit the switch and maybe disturb the springs, but no, it's still loud even if you do it real gentle.
Oh, and I'll be saving that circuit drawing, of course, and the whole discussion, as when I understand it better, I'll surely get back to this one and tweak it some more, maybe even include that 3080 somehow ::)
Thank you guys, you've been so great about it all and the amp now sounds way more inspiring to play, as before it was just too dry. Cheers!
Cool! As I said before, try removing the LM3080, as you can use it on other projects, like an Engineer's Thumb compressor... :icon_cool:
Quote from: 11-90-an on September 15, 2020, 04:10:40 AM
Cool! As I said before, try removing the LM3080, as you can use it on other projects, like an Engineer's Thumb compressor... :icon_cool:
Hey, that's exactly the one compressor I've considered building! Perhaps I'll give it a shot one of these days. Love the guy who's demoing it embedded at tagboard effects, great insight and he made me want to build it, although I've never really used one.
I just got me some 1M resistors, so I'll stick one in there, see how it goes...
QuoteI have now successfully mounted the thing inside and it is fully functional! Tapped into the INSERT jack from the inside and took the power at the small cable connecting the power board with the amp board and it lit up nicely! Not to mention that a placing of the effect was as if it's built for it - just right above the speaker, with two screws through already made holes on the chassis.
Excellent!
QuoteNo more of that noise either, and yes, it was a hiss sound before, but I think it's due to the adapter used (from an old modem). Now, I do have some noise in the reference to the clean sound, but it seems to me that it's kind of normal, as I remember that when you turn the reverb knob on amps that actually have them, it gets a bit hissy.
If you trim the low-pass filter cap to have a cut-off off 6kHz or so you can easily gain 2dB in signal to noise. There's virtually no effect on signal quality. It's good value IMHO.
QuoteI have tried to connect 22k pot as the output, but it didn't wok well and I also tried 1k, as I had it at hand, but that too didn't make a good match - I guess it's that 10k would be just right. Oh, and I don't think we really need to amplify the return signal, it sound good to my ear like it is. I'll just get that 1M input resistor and set the trimpot right, I guess.
Not sure why the 22k would affect things so much. All I can suggest is once you pick the reverb pot 22k or 10k you need to adjust to 100nF cap at the output of the opamp to trim-up how much low-end you want from the reverb. Perhaps you should try a 220nF, or add another 100nF in parallel with the existing 100nF (giving 200n in total). It's easy to do A/B testing with the parallel cap.
With a 1k pot you are going to cause the filter cut-off to go way-up and start cutting considerable bass. f3 = 1/(2*pi*2k*100n) = 800Hz; it's 2k because of the 1k at the output of the opamp. Also, the 1k pot starts to load-down the 1k in series with the opamp output so it cuts the signal by half. The combination of the 10k pot and 1k resistor on the opamp output doesn't attenuate much at all.
QuoteAnother thing is, there's a big reverb splash when the amp is turned on. I mean, I know it's sort of normal, but maybe it could be quieter. I thought that I was doing it as I'd hit the switch and maybe disturb the springs, but no, it's still loud even if you do it real gentle.
That happens because the TDA1904 is a single supply amplifier. Some modern power amp chips have a "soft start"/power-on mute feature. It's hard to fix without adding more circuitry.
If you look at the schematic there is a 47uF cap on pin 2 of the TDA1904. Basically it's across the power rails. If you increase the value of that cap to say 470uF or 1000uF it might soften some of the "bang" when you power-up the circuit. You can only try it.
Tried that 1M at input today, didn't really like how it was going - it was as if something changed as of yesterday and the device started to easily make a feedback, the whistling sound that would go forever and that didn't work well with adding that 1M, as I had to increase the output on trimpot, which made it worse, so I got it back to the 560k and to my ear, with the trimpot set to half (as it was before I even touched it), sounds good. Still clueless about why would it make a feedback like that, too much of something... When I'd touch the springs it would stop, as if they were reacting to something and vibrating, producing the whistle....
QuoteIf you trim the low-pass filter cap to have a cut-off off 6kHz or so you can easily gain 2dB in signal to noise. There's virtually no effect on signal quality. It's good value IMHO.
This sounds good to me, but still my brain struggles with what it actually mean in praxis. :P
QuoteNot sure why the 22k would affect things so much. All I can suggest is once you pick the reverb pot 22k or 10k you need to adjust to 100nF cap at the output of the opamp to trim-up how much low-end you want from the reverb. Perhaps you should try a 220nF, or add another 100nF in parallel with the existing 100nF (giving 200n in total). It's easy to do A/B testing with the parallel cap.
With a 1k pot you are going to cause the filter cut-off to go way-up and start cutting considerable bass. f3 = 1/(2*pi*2k*100n) = 800Hz; it's 2k because of the 1k at the output of the opamp. Also, the 1k pot starts to load-down the 1k in series with the opamp output so it cuts the signal by half. The combination of the 10k pot and 1k resistor on the opamp output doesn't attenuate much at all.
I'll try and get the 10k then - as I understand you'd recommend the use of the pot to set the effect output, rather than adjusting the input and trimpot onboard, and just do the combo of the 1M input resistor and crank the trimpot all the way?
QuoteThat happens because the TDA1904 is a single supply amplifier. Some modern power amp chips have a "soft start"/power-on mute feature. It's hard to fix without adding more circuitry.
If you look at the schematic there is a 47uF cap on pin 2 of the TDA1904. Basically it's across the power rails. If you increase the value of that cap to say 470uF or 1000uF it might soften some of the "bang" when you power-up the circuit. You can only try it.
This worked like a charm! I had 470uF at hand and with it the splash is far more tolerable! Thanks!
Just another thought - since 11-90-an mentioned the Engineer's Thumb compressor that uses 3080, could it be possible that that part of the circuit was made to compress the signal? Maybe it did, but I didn't really think of it, so I didn't notice... ???
QuoteTried that 1M at input today, didn't really like how it was going - it was as if something changed as of yesterday and the device started to easily make a feedback, the whistling sound that would go forever and that didn't work well with adding that 1M, as I had to increase the output on trimpot, which made it worse, so I got it back to the 560k and to my ear, with the trimpot set to half (as it was before I even touched it), sounds good. Still clueless about why would it make a feedback like that, too much of something... When I'd touch the springs it would stop, as if they were reacting to something and vibrating, producing the whistle....
The 1M shouldn't affect that. So me it sounds like some of your wires sitting on top of each other, or near each other, and it's causing oscillations. Try to keep all the signal wires away from each other, including the reverb wires. You need to fix it because those types of problems mess with the sound and send you on a false path when adjusting stuff.
QuoteThis sounds good to me, but still my brain struggles with what it actually mean in praxis.
All it means is the reverb sound is unaffected but the hiss is reduced. The signal out of the reverb starts to drop around 4kHz to 7kHz but the hiss (at all frequencies) still comes through. The low-pass filter removes the hiss part.
Quote
I'll try and get the 10k then - as I understand you'd recommend the use of the pot to set the effect output, rather than adjusting the input and trimpot onboard, and just do the combo of the 1M input resistor and crank the trimpot all the way?
I might have misunderstood your motives here. The 10k pot is a Reverb put that you would put on a panel with a knob. If you are only going to have one reverb level locked-up inside the unit then there's no real point having it since you can just tweak the reverb level with the trimpot and leave it. Earlier in the thread we also spoke about having a switch instead of a pot so you could have two reverb levels but that still needs the switch to be accessible to the outside world.
Quote
This worked like a charm!
Cool.
QuoteJust another thought - since 11-90-an mentioned the Engineer's Thumb compressor that uses 3080, could it be possible that that part of the circuit was made to compress the signal? Maybe it did, but I didn't really think of it, so I didn't notice...
I'm more inclined to accept "fake" reverb time theory, Makes sense if ELKA did that type of thing. It could also be used as a noise gate for the reverb. The thing about the reverb is you need to add it back to the original signal with a mixer. The keyboard the ELKA board came from would have had a mixer somewhere. A compressor needs to operate on the whole signal so the position of the LM3080 is in the wrong place in the signal chain.
For your amp, getting a compressor working on the ELKA has the same problem. The raw signals are passed through to the power amp regardless of what happens to the signal through the jack. To get a compressor to work you would need to break the signal chain. You would also need a place to put the controls for the compressors. IMHO, I think it's easier to steal the LM3080 from the ELKA board and build a separate Engineers thumb. That way you can use it elsewhere as well.
+1 on the stealing of the LM3080
Quote from: Rob Strand on September 15, 2020, 07:17:51 PM
The 1M shouldn't affect that. So me it sounds like some of your wires sitting on top of each other, or near each other, and it's causing oscillations. Try to keep all the signal wires away from each other, including the reverb wires. You need to fix it because those types of problems mess with the sound and send you on a false path when adjusting stuff.
So, I've "refreshed" my wiring, as at first I used an audio cable for the IN/OUT/GND where all the wires are insulated together - it seemed like it would be a cleaner job, as I have speaker, power, switch wires in there too. Now I've used only those colored breadboard wires, so they are all sitting away from each other. The power wiring too is done the same way. I'm now using only those first 4 connectors - +V, IN, GND, OUT and I'm connecting only +V from the amp's board as it's getting ground from the INSERT jack.
Also, I've placed the 1M resistor back in there, but I still don't really like what is happening - it's not that the resistor itself is causing any trouble, but with IN signal lowered, I do need to amplify that output via trimpot and that's when I'm getting the noise.
Of course, I still don't have any clear idea on what's happening, but I'm beginning to think that maybe with the 3080 still connected, the noise was less of a problem and I think maybe to reverse it's bypassing, but I don't know if that's making any sense, or it's just me...
I think it's not the shielding either, but I'm willing to try making some sort of metal case around the circuit. Still it seems to me that the circuit itself could be quieter.
Quote
All it means is the reverb sound is unaffected but the hiss is reduced. The signal out of the reverb starts to drop around 4kHz to 7kHz but the hiss (at all frequencies) still comes through. The low-pass filter removes the hiss part.
What I meant to say is - should I apply something more in a sense of LPF? How to do so?
QuoteI might have misunderstood your motives here. The 10k pot is a Reverb put that you would put on a panel with a knob. If you are only going to have one reverb level locked-up inside the unit then there's no real point having it since you can just tweak the reverb level with the trimpot and leave it. Earlier in the thread we also spoke about having a switch instead of a pot so you could have two reverb levels but that still needs the switch to be accessible to the outside world.
Yeah, I kind of thought that some permanent intervention on the amp would be necessary, but then I realized that I like the mod to be not so invasive and to still be able to put things back as they were. So, no drilling, etc.
Quote
I'm more inclined to accept "fake" reverb time theory, Makes sense if ELKA did that type of thing. It could also be used as a noise gate for the reverb. The thing about the reverb is you need to add it back to the original signal with a mixer. The keyboard the ELKA board came from would have had a mixer somewhere. A compressor needs to operate on the whole signal so the position of the LM3080 is in the wrong place in the signal chain.
For your amp, getting a compressor working on the ELKA has the same problem. The raw signals are passed through to the power amp regardless of what happens to the signal through the jack. To get a compressor to work you would need to break the signal chain. You would also need a place to put the controls for the compressors. IMHO, I think it's easier to steal the LM3080 from the ELKA board and build a separate Engineers thumb. That way you can use it elsewhere as well.
My thoughts were more like - did that part of the circuitry had an effect already? Was it noise-gating? And since I already seem to feel like the set-up could be more "clean", I thought it might be the right thing somehow... Not that I don't want the Thumb and I'll surely build it ;D
If I may ask, where did you get that? From a shop somewhere? Or from an attic or garage? Surely there must be some clue to where it came from... :o
Ask away :D Well, there's no much to say - bought it off some guy at our local online trade platform, he has no idea on it's origin, although I think he's into some sort of folk music show biz and that over here has lot to do with accordions, keyboards... Only clue is that ELKA 1910 printed on the board that we've already discussed here and there is no precise instrument, since it's clear that ELKA was keyboard manufacturer, that we could relate this piece to. It's also logical since they made these keyboards with slotting a lot of boards, one for each note, so the board may as well be out of the keyboard unit, although they did make some guitar amps as well, with reverb... ::)
QuoteAlso, I've placed the 1M resistor back in there, but I still don't really like what is happening - it's not that the resistor itself is causing any trouble, but with IN signal lowered, I do need to amplify that output via trimpot and that's when I'm getting the noise.
At some point reducing the drive level (ie. increasing the 560k/1M resistor) and increasing the gain (ie. increasing the gain with the trimpot) will cause the hiss to become noticeable. At the other end of the scale reducing the resistor and decreasing the gain will overload the reverb drive amp or the reverb spring itself, however, when the signal isn't high this scenario will have lower hiss. There is a game of finding a happy medium. If the reverb recovery circuit isn't designed well and is noisey that happy medium turns into a very fine line.
So the question is, if your problem is *only* hiss, then it's probably the above scenario. And maybe the 560k is as good as you can do.
You can always reduce the hiss with a low-pass filter. For example add a 33nF cap from the output at the connector to ground. The 1k output resistor and the cap form a low -pass filter. It's very common to filter the highs on a reverb spring.
If you problem isn't just hiss it could be something more sinister like oscillation. That can seem like hiss as well but you might also get whistles, weird fizzes in the signal, even sometimes even radio. A tell-tale sign might be the unit suddenly acts funny as you slowly increase the gain trimpot. Like is breaks into oscillation. It's likely to be caused by the trimpot setting regardless of 560k or 1M value. To make life difficult these problem can run on a thin edge and sometimes the 560k to 1M can tip the balance. An on top of that adding the 33n cap I mentioned about usually helps reduce these problems!
QuoteOf course, I still don't have any clear idea on what's happening, but I'm beginning to think that maybe with the 3080 still connected, the noise was less of a problem and I think maybe to reverse it's bypassing, but I don't know if that's making any sense, or it's just me...
Not impossible but not very likely.
Quote
At some point reducing the drive level (ie. increasing the 560k/1M resistor) and increasing the gain (ie. increasing the gain with the trimpot) will cause the hiss to become noticeable. At the other end of the scale reducing the resistor and decreasing the gain will overload the reverb drive amp or the reverb spring itself, however, when the signal isn't high this scenario will have lower hiss. There is a game of finding a happy medium. If the reverb recovery circuit isn't designed well and is noisey that happy medium turns into a very fine line.
So the question is, if your problem is *only* hiss, then it's probably the above scenario. And maybe the 560k is as good as you can do.
You can always reduce the hiss with a low-pass filter. For example add a 33nF cap from the output at the connector to ground. The 1k output resistor and the cap form a low -pass filter. It's very common to filter the highs on a reverb spring.
If you problem isn't just hiss it could be something more sinister like oscillation. That can seem like hiss as well but you might also get whistles, weird fizzes in the signal, even sometimes even radio. A tell-tale sign might be the unit suddenly acts funny as you slowly increase the gain trimpot. Like is breaks into oscillation. It's likely to be caused by the trimpot setting regardless of 560k or 1M value. To make life difficult these problem can run on a thin edge and sometimes the 560k to 1M can tip the balance. An on top of that adding the 33n cap I mentioned about usually helps reduce these problems!
I hear you Rob - well, after a lot of tweaking and listening to it, I've rolled back to that 560k, as it does let me go easy on the trimpot thus reducing noise. The design is probably like you described - not so great and the use of TL082 probably doesn't help. However, I'm pretty happy on how it sounds and given that I had doubts if this is even possible to set up - everything turned out great, of course, with the huge and essential help and advises from you and the guys. Here's the "last" view at the set-up before I close it for whole eternity:
(https://i.postimg.cc/JtsQnCxm/DSCN4114.jpg) (https://postimg.cc/JtsQnCxm)
Oh, and I did place the cap at output, in similar way that we did that 1-8 pin resistor, under the board - just connected the out from underside and one of the ground pins. I didn't have 33n, but placed 47n instead and it did shave of some of the hiss, but 33n could be a better match - I'll try that when I get one. Also, I gave up on reversing the process and returning the 3080 part as it was, as it probably wouldn't make difference.
Anyway, the whole thing sounds pretty good and I can't stop playing - thank you guys so much!
QuoteI hear you Rob - well, after a lot of tweaking and listening to it, I've rolled back to that 560k, as it does let me go easy on the trimpot thus reducing noise. The design is probably like you described - not so great and the use of TL082 probably doesn't help.
It's sounds like that's as good as you can do.
QuoteHowever, I'm pretty happy on how it sounds and given that I had doubts if this is even possible to set up - everything turned out great, of course, with the huge and essential help and advises from you and the guys.
Here's the "last" view at the set-up before I close it for whole eternity:
Looks great.
One thing, when you push it all together be very careful that the speaker wires don't get close to the wiring to the reverb connector, or the reverb tank wires - especially the reverb tank output to the recover amp. That could cause all sorts of weird oscillation problems.
QuoteOh, and I did place the cap at output, in similar way that we did that 1-8 pin resistor, under the board - just connected the out from underside and one of the ground pins. I didn't have 33n, but placed 47n instead and it did shave of some of the hiss, but 33n could be a better match - I'll try that when I get one. Also, I gave up on reversing the process and returning the 3080 part as it was, as it probably wouldn't make difference.
Honestly the 47n is probably fine and that's going to knock out the hiss as much as possible. There's plenty of other places the design could be modified/tweaked to get a bit more twang out of the reverb.
QuoteAnyway, the whole thing sounds pretty good and I can't stop playing - thank you guys so much!
Yeah, if you like reverb, putting reverb in an amp is a big plus. I've done this for my friends in the past.
Now I want a reverb tank... ::)
Well, guys, I've spoken too soon about wrapping up as I have now entered the feedback hell. Something's not right and the amp became too prone to just getting into this continuous feedback that seems to me is coming from a bad combo between reverb tank and the speaker, as it works just fine through the headphones. Some sort of oscillation is at work and I can't seem to remove it.
I've tried many things to solve this, to give it more grounding, shield the wires of send and return, roll the set 180 degrees so that the springs are further away from the speaker (and that only made it worse, maybe it's the closeness of the circuit...), get all the wires as far away from each other as I could, but noting really helped. Only thing that works and that I don't really like is to mute one of the springs with a piece of felt, so there's less possibility for feedback, but it's kind of still there, lurking...
All I can think of now is to separate the circuit and the tank and place them further, but also shield them in some sort of metal casing.
Also, I'm not sure if anything I've done so far would make it more sensitive, as I don't really recall having the issue earlier, but I did made first test with longer cables and everything was not so tight back then...
Quote from: 11-90-an on September 18, 2020, 10:45:29 PM
Now I want a reverb tank... ::)
Besides this nerve wrecking issues, it is a lot of fun ;)
Quote from: Rob Strand on September 18, 2020, 05:42:35 PM
One thing, when you push it all together be very careful that the speaker wires don't get close to the wiring to the reverb connector, or the reverb tank wires - especially the reverb tank output to the recover amp. That could cause all sorts of weird oscillation problems.
It seems like it's happening...
QuoteHonestly the 47n is probably fine and that's going to knock out the hiss as much as possible. There's plenty of other places the design could be modified/tweaked to get a bit more twang out of the reverb.
I did found me an old 39n paper cap and it went straight in there - I may be wrong, but it seems like a better match. What tweaks did you have in mind, once I somehow solve the issue of the feedback?
From what @Rob said, try moving the speaker wires further...
Or...
Wrap the speaker wires in aluminium foil (especially the ones near the tank!), then ground that foil....
Things have now gone from bad to worse, as I lost power somehow and the amp's not turning on. Guess one of the fuses blew on the power supply board, as I don't have any power at switch no at a cable that runs to the main amp board.
(https://i.postimg.cc/MvNSGdsv/tx30-power.jpg) (https://postimg.cc/MvNSGdsv)
My guess is that it wasn't such a good idea to patch that power-board-to-main-board cable and take the power for the reverb from there, as it might reflect badly on what was happening within the amps circuit - only reason was to use the same switch and get the effect powered together with the amp and not go directly from the battery. Who knows, maybe that's the reason I got all those oscillations ???
That also might be a possibility...
So no magic smoke...? (Hopefully not)
Replace the fuse and try powering the reverb tank with a separate PSU..
QuoteI've tried many things to solve this, to give it more grounding, shield the wires of send and return, roll the set 180 degrees so that the springs are further away from the speaker (and that only made it worse, maybe it's the closeness of the circuit...),
get all the wires as far away from each other as I could, but noting really helped. Only thing that works and that I don't really like is to mute one of the springs with a piece of felt, so there's less possibility for feedback, but it's kind of still there, lurking...
You have done the right things here.
Separating the wires reduces electronic feedback. Typically that is very high frequency. You don't always hear the main oscillation but you hear a lot of weird side effects.
Another type of feedback is acoustic feedback. The vibrations of the speaker transmit through the cabinet structure and get coupled into the reverb tank. This type of feedback is similar to microphone feedback. However you might experience much lower frequencies.
The fact you can stop the oscillation by touching the springs could imply acoustic feedback. The way you would prove the point is to remove the Reverb tank from the enclosure the sit it on a pillow outside of the unit.
One way around that is to mount the Reverb tank on dampers. The simplest form is to have the Reverb tank *loosely* mounted to the base of the amp with rubber grommets under. Some units have rubber mats or foam pads. Foam decays after a while so I don't recommend it.
Another solution to the feedback problem is to cut the bass on the reverb. Say by decreasing the cap in series with the output of the recovery opamp. Sometimes the feedback comes from very low frequencies outside of the frequencies of the guitar. In this case you can also decrease the cap in series with the 560k on the input side to say 4.7n. The two caps together doubly attenuates the vibration frequencies.
QuoteThings have now gone from bad to worse, as I lost power somehow and the amp's not turning on. Guess one of the fuses blew on the power supply board, as I don't have any power at switch no at a cable that runs to the main amp board.
You need to find at what point you lose supply. Check all the fuses. Also you need to check for shorts on the power. A short may have developed somewhere. A new problem completely independent of the mods. These things happen.
If you wired you reverb board before the fuses and reverb wiring has a short it will kill the power even though the fuses are good, for example where the wires connect to the reverb board.
QuoteMy guess is that it wasn't such a good idea to patch that power-board-to-main-board cable and take the power for the reverb from there, as it might reflect badly on what was happening within the amps circuit - only reason was to use the same switch and get the effect powered together with the amp and not go directly from the battery.Who knows, maybe that's the reason I got all those oscillations
Normally it doesn't cause big problems like that. The shared power and audio grounds can cause oscillations.
In the end you need to work out the nature of the oscillations, or if it's an unrelated problem.
Step one would be get the amp up and running. Perhaps temporarily remove the power going to the Reverb board.
Ok, got it running again - it was one of the pico fuses on the powering board. Didn't have the same one to replace, but temporarily used higher rated one (original was 4A, this one's at 5). I'm not even sure if I'll be able to find the same one, as it seems to me that there's limited offer here at our stores, will see...
Anyway, I'm not sure what caused it - perhaps I kind of exhausted it by switching the amp on and off all the time, as I couldn't really tell if there's a short anywhere and the reverb was connected after these, as the power from it was coming from the cable that sends power to the amp's board.
Quote
Separating the wires reduces electronic feedback. Typically that is very high frequency. You don't always hear the main oscillation but you hear a lot of weird side effects.
Another type of feedback is acoustic feedback. The vibrations of the speaker transmit through the cabinet structure and get coupled into the reverb tank. This type of feedback is similar to microphone feedback. However you might experience much lower frequencies.
The fact you can stop the oscillation by touching the springs could imply acoustic feedback. The way you would prove the point is to remove the Reverb tank from the enclosure the sit it on a pillow outside of the unit.
One way around that is to mount the Reverb tank on dampers. The simplest form is to have the Reverb tank *loosely* mounted to the base of the amp with rubber grommets under. Some units have rubber mats or foam pads. Foam decays after a while so I don't recommend it.
Another solution to the feedback problem is to cut the bass on the reverb. Say by decreasing the cap in series with the output of the recovery opamp. Sometimes the feedback comes from very low frequencies outside of the frequencies of the guitar. In this case you can also decrease the cap in series with the 560k on the input side to say 4.7n. The two caps together doubly attenuates the vibration frequencies.
Normally it doesn't cause big problems like that. The shared power and audio grounds can cause oscillations.
In the end you need to work out the nature of the oscillations, or if it's an unrelated problem.
Step one would be get the amp up and running. Perhaps temporarily remove the power going to the Reverb board.
Next, I'll try and use these details to see how to deal with this feedback issue. Guess I'll try and power it differently and see what happens, as it sounds most probable to me that that's the cause. Perhaps I can patch the unit directly to the battery and use a small switch to turn the effect on and off...
Got it all running up again - tried setting the powering differently, directly from the battery, but the feedback was still present, starting right behind the reverb splash. Also, it got sort of noisier, as maybe the unit needs certain amount of mA set for it to run correctly and it's maybe getting a "better" power via that patch cable between power and amp boards. I have then reversed it all as before, making the wiring even more tidy than earlier and I've managed to set it running pretty good - the feedback would only appear if the trimpot would be at highest value, but I don't even need it to run that high, at half I can get a decent amount of effect.
I was also thinking to place a switch for the effect, and have tried a simple on/off one, but I'm getting huge pop with it. Guess the simple switch cutting the effect's power won't do and I'd need a DPDT one to avoid popping.
If you want to use a switch, it *would* make more sense to use it to turn the output of the reverb on and off... ;)
Thanks Nathan, I didn't even think of that one, since I got myself all worked up around powering... :icon_redface:
To me it seems the oscillation is acoustic.
It's possible your reverb spring hasn't been engineered as well as the Accutronics or OC electronics tanks and it is more sensitive to vibrations. If you look at the unit the internal part floats in springs. Even those springs can affect things.
The power-ups might sometimes trigger the oscillation but that doesn't mean it's the cause. A strong pluck might start things oscillating.
It's fine to knock the reverb level down to prevent oscillation.
Adding damping to the mounting points has to work but it's more a question if the practicalities of that are enough. Decreasing the cap in series with 560k input resistor is also worth trying. It's all about shaving off the problems.
You aren't the only one to have these problems,
https://www.amplifiedparts.com/products/grommet-rubber-reverb-tank-vibration
Driving the springs harder and knock the recovery gain down helps. However, I'm pretty sure you are close to the drive limit on your unit. You might be able to squeeze a small factor out of it by dropping the 560k to 470k or 390k. However, when you push things this close you need to check the reverb spring isn't being overdriven with an oscilloscope.
Quote from: Rob Strand on September 22, 2020, 06:50:21 PM
To me it seems the oscillation is acoustic.
It's possible your reverb spring hasn't been engineered as well as the Accutronics or OC electronics tanks and it is more sensitive to vibrations. If you look at the unit the internal part floats in springs. Even those springs can affect things.
The power-ups might sometimes trigger the oscillation but that doesn't mean it's the cause. A strong pluck might start things oscillating.
It's fine to knock the reverb level down to prevent oscillation.
Adding damping to the mounting points has to work but it's more a question if the practicalities of that are enough. Decreasing the cap in series with 560k input resistor is also worth trying. It's all about shaving off the problems.
You aren't the only one to have these problems,
https://www.amplifiedparts.com/products/grommet-rubber-reverb-tank-vibration
Driving the springs harder and knock the recovery gain down helps. However, I'm pretty sure you are close to the drive limit on your unit. You might be able to squeeze a small factor out of it by dropping the 560k to 470k or 390k. However, when you push things this close you need to check the reverb spring isn't being overdriven with an oscilloscope.
Did the thing - went for 470k input resistor and the cap in series is now at 47n - this is a great combo! I do drive the springs more this way, but it seems like it's just enough so I can get more effect and reduce trimpot, thus canceling some more noise.
The feedback is no longer an issue and I think so too - that it was acoustic problem rather than electrical - I can get it with all the parts out and if I turn the tank in a certain position, but it doesn't appear by itself anymore. Now I only have something like tape saturation noise and it's probably just the way this was designed. I did place a switch cutting the output on the effect, so that I can have a clean tone without opening the amp, but it does still have a huge pop on the first hit. :P
To me, it still remains a mystery of what that 3080 part of this circuit did and I'd kind of wish that there could be a way of finding out about it and using it too, instead of just bypassing it... ???
QuoteTo me, it still remains a mystery of what that 3080 part of this circuit did and I'd kind of wish that there could be a way of finding out about it and using it too, instead of just bypassing it... ???
sW
ELLs
i think... ???
Quoteso that I can have a clean tone without opening the amp, but it does still have a huge pop on the first hit. :P
Good to see it's all coming together.
You need to add a 1M resistor from the output terminal of the reverb to ground. (at the PCB connector) This is permanently connected. The switch then connects between the reverb output terminal and the amp input.
The reason you get the bang is the output cap on the reverb side isn't charged-up when the switch is off. When you close the switch it charges up through the amp input and that charging current is the bang you hear. The addition of the 1M resistor provides a path for the cap to charge when the switch is open. Additional resistors to stop bangs are common place for audio.
Quote from: Rob Strand on September 23, 2020, 07:42:23 PM
Quoteso that I can have a clean tone without opening the amp, but it does still have a huge pop on the first hit. :P
Good to see it's all coming together.
You need to add a 1M resistor from the output terminal of the reverb to ground. (at the PCB connector) This is permanently connected. The switch then connects between the reverb output terminal and the amp input.
The reason you get the bang is the output cap on the reverb side isn't charged-up when the switch is off. When you close the switch it charges up through the amp input and that charging current is the bang you hear. The addition of the 1M resistor provides a path for the cap to charge when the switch is open. Additional resistors to stop bangs are common place for audio.
Thanks Rob, but I've tried this and it didn't really work. At the reverb's circuit PCB I've made a connection between the out and the ground, sort of like when we placed that filtering cap of 39n over there, but the effect's intensity got way cut off and the pop was still there. I'm not sure if I understood you correctly... I'm using this simple switch, perhaps a DPDT would be better...
(https://i.postimg.cc/ThH3Myb4/tx30-switch.jpg) (https://postimg.cc/ThH3Myb4)
If you place that 1M pot and there's a difference, you might be putting the resistor wrongly...
Mind i ask for a pic of how you did it...?
Quote from: 11-90-an on September 23, 2020, 08:34:40 AM
QuoteTo me, it still remains a mystery of what that 3080 part of this circuit did and I'd kind of wish that there could be a way of finding out about it and using it too, instead of just bypassing it... ???
sWELLs
i think... ???
You're probably right - I suppose, since Rob did say that one of the pins that was related to this part of the circuit is a DC control for the 3080 that depending on the voltage let through that there would be swells, as in playing keyboard there would be some sort of charge released upon pressing the keys, and depending on how long you're holding the pressure, the swells would begin and stop upon releasing... Not sure how this would apply with playing the guitar, if I'm on a right track with this at all ::)
Quote from: 11-90-an on September 24, 2020, 05:56:51 AM
If you place that 1M pot and there's a difference, you might be putting the resistor wrongly...
Mind i ask for a pic of how you did it...?
There's no pot, I've only did what Rob suggested - tied the 1M resistor at output on the reverb's PCB to the ground at the same connector
(https://i.postimg.cc/62Z5vn5b/DSCN4141.jpg) (https://postimg.cc/62Z5vn5b)
This is just a temporary connection, to try it out. You can see the paper 39n cap that's filtering some noise at the back, tied to the same posts.
QuoteThere's no pot, I've only did what Rob suggested - tied the 1M resistor at output on the reverb's PCB to the ground at the same connector
Maybe it's not connecting properly. It happens a lot when you poke the legs down, even though it looks good to the eye.
Other than that I can't see why it shouldn't work. That *is* the way you fix this type of problem.
Wiggle it around? ::)
QuoteWiggle it around? ::)
Yes!
Unfortunately as soon as you get the cap charged up the pop isn't there anymore.
Maybe this would be more reproducable:
- close switch (so the cap discharges into the amp's input resistor)
- turn off amp
- wait
- open switch (so the cap is hanging at power-up)
- turn on amp
- close switch.
The cap would need to charge up.
If it didn't charge-up through the 1M resistor then you should get a bang when the switch is closed.
I'm embarrassed to say that the issue was that I've read my resistor colors backwards, so I placed the 240Ohm resistor in stead of 1M and it was "sucking" the signal. Once I got the value right the thing actually works! The pop is almost entirely gone, but I do need to wait a bit for the cap to charge before I turn on the effect. This is not a problem at all, as to get rid of the popping was just not to stress the speaker when switching between clean and reverb.
So, got it all sorted! Oh, and the issue about the feedback was indeed due to acoustic issue - I think that it's vibrating was caused by the position of one of those four short springs on which the tank floats, as wiggling it around made it disappear. I assume that those would need to be a bit more loose, a bit longer, for the tank to hang more freely and so that there's no acoustic effect, passing the vibrations from the speaker to it.
Here are some pics...
(https://i.postimg.cc/9wzqNrQZ/tx30-1.jpg) (https://postimg.cc/9wzqNrQZ)
(https://i.postimg.cc/06xK1tN8/tx30-2.jpg) (https://postimg.cc/06xK1tN8)
(https://i.postimg.cc/sM5G8Fs6/tx30-3.jpg) (https://postimg.cc/sM5G8Fs6)
Glad to see it working now. :icon_biggrin:
Hope you stick around too in this forum... :icon_biggrin:
QuoteSo, got it all sorted!
You did really well.
This project was pretty cool. You started from very humble and uncertain beginnings and ended up with something that works and is bug free.
Quote from: 11-90-an on September 26, 2020, 03:24:54 AM
Glad to see it working now. :icon_biggrin:
Hope you stick around too in this forum... :icon_biggrin:
Thank you! I surely will hang around here, as I've got so much help from you guys and read other interesting stuff - this kind of commitment is nowhere to be found, to follow the thread all the way till the mystery gets solved and everything works out.
Quote from: Rob Strand on September 26, 2020, 03:56:50 AM
QuoteSo, got it all sorted!
You did really well.
This project was pretty cool. You started from very humble and uncertain beginnings and ended up with something that works and is bug free.
Never couldn't get anywhere without you guys - thanks so much!
Now I'm thinking about a follow up - since the powering of the effect worked well using the same power source, could I maybe set up a pedal power mod to this amp? ??? As I'm only using few pedals, perhaps I can instal a jack for daisy chain cable on the amp, try to reduce voltage from 12v to 9v... I've seen some circuits that do so, I'm not sure which would work best for pedals.
(https://i.postimg.cc/H8tZgHRp/12to9-1.jpg) (https://postimg.cc/H8tZgHRp)
(https://i.postimg.cc/9zjcCpKS/12to9-2.jpg) (https://postimg.cc/9zjcCpKS)
Maybe it would be best to create another thread for this one, calling it Taxi amp mod - pedal power supply, or something like that...
Well... your pedalboard would habe to be really close to your amp, don't you think? ;)
Yes it's possible, but I don't see it as quite practical... my opinion, of course.
Quote from: 11-90-an on September 27, 2020, 06:54:55 AM
Well... your pedalboard would habe to be really close to your amp, don't you think? ;)
Yes it's possible, but I don't see it as quite practical... my opinion, of course.
Haha, yeah, it would be a "tight" set up. Well, I'm sure I can piece together a "custom" cord for such powering and it's purpose would be, well, the same as having a battery powered amp such as the Taxi - go off the grid, if you like ;)
For performing outside, I'd have to worry about 9v batteries as well, but if I manage to "steal" some power from the amp...