The info here is applicable to the Acoustic 360 project
http://www.lynx.net/~jc/pedals360.html
-------------------------
The other day in this thread by rankot,
http://www.diystompboxes.com/smfforum/index.php?topic=118943.0
I realized the Acoustic corp amp circuits rely to some degree on the inductor's DC resistance.
The DC resistance is not specified.
When you see an inductor it looks like an ideal inductor in series with the coil resistance. The coils
resistance is often not on the schematic but in reality it is there and affects the circuit's behaviour.
No one bothers measuring it because it is a "hidden" parameter. So when you need to know what it
is there is little info out there.
The same problem exists for the Mesa Boogie equalizer. It is a common problem with circuits containing inductors. Just look at the number of Wah threads discussing inductors.
After much detective work (which I'm not going to clutter the post with) there is very little concrete info
indicating measured values of DC resistance for the inductors.
After combining the info from all the sources, including correlating manufactures specs with spice simulations. I've managed to reduce the window of uncertainty.
From photos the inductors appear to be wound on P14/8 pot-cores which have a screw in the center.
1) Estimated inductors and DC resistance (DCR)
I made these reasonable assumptions:
Potcore: P14/8
Material: N48
ui: ~ 2300 (not ue)
type: gapless core, screw hole in centre leg
AL 2100
All coils to be wound with same wire.
All wound with 42AWG wire (**** The 6H inductor will not fit on bobbin.)
L [H] 6.0 3.0 1.75 1.5 0.75 0.375 0.187 0.093
DCR [ohms] 280 184 134 123 83 57 34 27
Turns 1690 1195 913 845 598 423 298 210
All wound with 44AWG wire
L [H] 6.0 3.0 1.750 1.500 0.750 0.375 0.187 0.093
DCR [ohms] 399 268 198 182 125 87 60 42
Turns 1690 1195 913 845 598 423 298 210
The 44AWG wire seems to be the overall best match to the info I gathered. The 6.0H inductor
fits on the bobbin with 44AWG but not 42AWG.
The aim here is not to wind an optimum inductor.
The aim is to second guess what the manufacturer has done.
As a side note, gapless core inductors are likely to have +/- 20% to +/- 30% tolerance. The DC resistance is probably going to be +/- 10%.
2) Gyrators
Obviously winding such inductors is a pain so it is desirable to replace them with a Gyrators.
Once trick with LCR based equalizers is the ordering of the L, C and R can be changed from what
is on the schematic. The first step to replacing an inductor with a gyrator is to order them as,
Rs + C + DCR + L where Rs is a real resistor in the circuit and DCR is the inductor's DC resistance.
The reason for doing this is to make one end of the gyrator grounded. This simplifies the type of gyrator circuits and avoids using more complicated floating gyrators circuits such as that used here, in
JC Maillet's Acoustic 360 project,
http://www.lynx.net/~jc/pedals360.html
3) Gyrator Example
(http://www.muzique.com/news/images/gyrator.gif)
As an example, take the 1.5H inductor,
R2 = DCR = 180 ohms
R1 = 100k (high enough not to affect circuit but low enough not to cause noise)
C2 = 82nF
L = R1 * R2 * C2 = 180 * 100k * 82nF = 1.48H
C1 = capacitor in series with the inductor in the circuit.
If there isn't one then, without knowing the specifics of the circuit, it is probably a good idea to put in say 10uF.
4) FYI: Acoustic schematic bugs
I noticed there's bugs on the Acoustic schematics.
Some people mentioned there were parts on their boards which weren't on the schematics.
eg.
the Acoustic 220 and 320 Equalizer schematics have missing resistors.
Most schematics contain "Rev B, Feb 1978, uses Equalizer 170084".
which shows
Band Resistors
2000Hz 390R
820Hz 270R
350Hz 220R
125Hz s/c } bug on this schematic.
70Hz s/c }
However the Rev, E, July 1978, 170084 EQ Schematic
specifically says "R27 to R29 Corrected" and has the parts
people see on their PCB's.
2000Hz 390R
820Hz 270R
350Hz 220R
125Hz 390R (R27) } the bug is fixed
70Hz 560R (R28) } schematic represents actual board
The coil series resistance is knowable by tank Q and L-C values.
In the example circuit, the Q goes up with frequency.
Winding coils is not a major pain. But the gauge-math may be.
With modern TL072 opamps, the gyrator series resistance can be a little lower than the calculated "coil resistance". This leaves room for the additional loss of Q due to the large value resistor.
QuoteThe coil series resistance is knowable by tank Q and L-C values.
For a circuit given as a schematic the Q is not documented and cannot be derived.
QuoteIn the example circuit, the Q goes up with frequency.
Depends what is kept constant and what is allowed to vary.
QuoteWith modern TL072 opamps, the gyrator series resistance can be a little lower than the calculated "coil resistance". This leaves room for the additional loss of Q due to the large value resistor.
Sure, but usually the series value is so much smaller than the parallel one you don't have to worry. The only time you would worry is if you dropped the parallel resistance to reduce noise and the series resistance is relatively high. Even if you compensate for the Q you can't get rid of the parallel resistance, which can affect the circuit at high frequencies.
One thing about those equalizers, if we adjust the output to be a specific dB boost or cut, and we keep L and C the same, then the response is essentially independent of the DC resistance. The boost/cut adjustment effectively hides the difference in DC resistance. The only time you see a difference is when you set the pot to one extreme then the one with the higher DC resistance runs out of adjustment range!
There is also an element of parasitic parallel capacitance to be considered. You can estimate this using a similar method used to estimate coil resistance. You have both coupling from windings-core-windings along with winding-winding capacitance. A more advanced model includes series impedance to account for coupling paths.
In practice you can get a good idea for whether the parasitic capacitance makes a material difference at the selected center frequencies of the resonant tank.
> For a circuit given as a schematic the Q is not documented and cannot be derived.
Of course.
For a "nice broad" bump assume Q~~1.
There is little use of Q>10 in audio.
Musician's EQs may fall anywhere in there, just like all other musical effects. Some guesswork needed.
> Depends what is kept constant and what is allowed to vary.
If they don't give you a tapped inductor, then the only "vary" is the C; L and R are constant. (Unless they switch an R with each C.) This gives the increasingly narrow curves shown in JC's article.
QuoteMusician's EQs may fall anywhere in there, just like all other musical effects. Some guesswork needed.
In this case I used the +/- dB specs to narrow down the R somewhat. I don't trust the specs 100% but they are a good guide to squash down the region of uncertainty. Especially if you take enough samples over the different models.
It wasn't as easy as it sounds as there were cases where different bands on the same equalizer had the same valued inductor but different added series resistors. So there's no way the equalizer could produce the spec'd +/- dB on both of those bands.
I noted the narrow bandwidth bands tended to have the lower resistors, and hence more boost. So I suspect they tried to make the perceived level volume increase in each band the same as opposed to the same maximum +/- dB in each band (as the spec. implied).
Using that and the fact the wire had to fit onto the bobbin, I'm fairly confident the estimates narrowed below a 2 AWG step in wire. If for some reason they used an odd number AWG that would be within +/- 20% for resistance. Not enough to worry about, especially on that type of equalizer.
Rob, I happened to buy one not working Acoustic 220, and AFAIK, it has 1.5H inductors inside. I will soon have time to take care of it, and I promise to measure that inductor and post resistance here (in case it's working)!
QuoteRob, I happened to buy one not working Acoustic 220, and AFAIK, it has 1.5H inductors inside. I will soon have time to take care of it, and I promise to measure that inductor and post resistance here (in case it's working)!
Good to know. The other thing that would be nice to know is the DC resistances of the inductors. I struggled to back-engineer those, it's pretty much impossible to do with any confidence.
Glad to see you back online!
Quote from: Rob Strand on May 16, 2019, 06:02:16 AM
The other thing that would be nice to know is the DC resistances of the inductors. I struggled to back-engineer those, it's pretty much impossible to do with any confidence.
Rob, those Acoustic 1.5Hy coils have about 240 ohms of parasitic resistance
QuoteRob, those Acoustic 1.5Hy coils have about 240 ohms of parasitic resistance
Thanks! That's a lot higher than I expected. The tables above only get to 182 ohms with reasonable assumptions of a P14/8 core, 44AWG wire, N27 material. The 3.0H hits 268ohms. So maybe the cores have very small gaps? Philips offered small gaps but perhaps not in that era.
> very small gaps?
Cheap wire.
Most coil uses want low resistance. But audio chokes are usually low-Q. You can use a high-Q choke and add a resistor. If you custom order them, it makes sense to go thinner on the wire, save a penny there and another 11 pennies on a resistor.
You can do the math and add a resistor.
QuoteCheap wire.
Perhaps. That's a far easier explanation.
I was assuming 44AWG was about as thin as you would like to go before it is a pain to wind (which can indirectly push up the costs). Also 44AWG is near the edge of basic wire gauges. The problem is it's an assumption. Those miniature transformers could very well use smaller wire than 44AWG and they make those all the time, and cheap too.
EDIT: Actually, 46AWG will give higher resistance than 240ohm, so we don't need much thinner than 44AWG. That might explain it.
Hi Rob, did you build a preamp clone, or just tried to fix an original one? I am struggling with my clone, it's working really fine, but it's noisy. Used BC550B transistors instead of unobtanium originals, but they're low noise. Tried both inductor and gyrator versions and they're almost the same, although inductor based seems to be a little bit trebly.
QuoteHi Rob, did you build a preamp clone, or just tried to fix an original one? I am struggling with my clone, it's working really fine, but it's noisy. Used BC550B transistors instead of unobtanium originals, but they're low noise. Tried both inductor and gyrator versions and they're almost the same, although inductor based seems to be a little bit trebly.
I didn't build a clone I was just trying to work out what these things *really* are.
Gyrators can be a little noisier (hiss-wise) than real inductors but since you aren't seeing a big difference the problem is elsewhere. The EQ is at the end of the signal chain so while the EQ can *affect* the noise it's probably not the thing *causing* the noise. The noise could originate from previous stages. In many circuit changing the transistors won't change the noise because the noise is determined by the circuit design itself and the physics of the transistor. Noise often comes from first stage.
One way to get idea of a noisy stage is to short the input of the stage. Then, disconnect that stage from next and short the input cap of that stage to ground. Keep moving up stages until you find where the noise stops.
Quoteh inductor based seems to be a little bit trebly.
I would only expect that to be the case if the inductor and the gyrator aren't really the same value, or have the different equivalent series resistances. Imagine if the real inductor version had more boost and/or the peak frequency was higher, it would seem more trebly. You should be able to see the differences by measuring the frequency response.
Thanks for advice, I'll try to isolate noise in stages.
I typically use a slightly different topology to simulate series LCR. This does away with the parallel resistance, and allows you to have a fairly high Q pretty easily. The minimum resistance is now the sum of the two resistors. Here's an example (log-log) plot of impedance vs frequency. An op amp can be used as the unity gain buffer. You do however get a phase shift of pi (rather than zero) at resonance, but this doesn't really matter too much in most of the circuits I use it for. It works just fine for a frequency boost (or cut) in a graphic EQ circuit, for example.
(https://i.postimg.cc/jLxZ4xZH/gyr.png) (https://postimg.cc/jLxZ4xZH)
QuoteI typically use a slightly different topology to simulate series LCR. This does away with the parallel resistance, and allows you to have a fairly high Q pretty easily. The minimum resistance is now the sum of the two resistors.
Those work best when the series resistance of the inductor is high-ish (say over 1k to 2k).
IIRC with transistor buffers and series resistances on the low side the parallel resistance starts to re-appear.
At this point the problem looks elsewhere.
Quote from: Rob Strand on February 12, 2020, 05:00:06 PM
Gyrators can be a little noisier (hiss-wise) than real inductors but since you aren't seeing a big difference the problem is elsewhere. The EQ is at the end of the signal chain so while the EQ can *affect* the noise it's probably not the thing *causing* the noise. The noise could originate from previous stages. In many circuit changing the transistors won't change the noise because the noise is determined by the circuit design itself and the physics of the transistor. Noise often comes from first stage.
One way to get idea of a noisy stage is to short the input of the stage. Then, disconnect that stage from next and short the input cap of that stage to ground. Keep moving up stages until you find where the noise stops.
The first stage is a simple buffer, so I don't think it adds noise. I will start from the second stage, because my o-scope is not sensitive enough to see noise that is less than 2mV, and, according to that, there's no noise in the first stage.
This is my PCB (for gyrator version, inductor based one is very similar). Maybe the layout is a problem?
(https://i.postimg.cc/vch4KcLY/Acoustic360-PCB.jpg) (https://postimg.cc/vch4KcLY)
The best plan is to narrow down the cause.
Quote
The first stage is a simple buffer, so I don't think it adds noise. I will start from the second stage, because my o-scope is not sensitive enough to see noise that is less than 2mV, and, according to that, there's no noise in the first stage.
Tracking down noise (hiss) with a CRO can be a bit unreliable. Some CROs produce lot of noise themselves. If you have a low bandwidth or filter setting on your CRO enabling that can reduce the CRO noise. Also you can pick-up a lot of noise above audio which makes the audio noise hard to see. Ideally you need to have a x10 or more preamp and a 10kHz to 20kHz 2nd order filter between the probe and the CRO. To see noise, turn off averaging if you have that setting.
Power supply noise could affect the buffer. Especially with the bias divider on the input.
JC Maillet's page mentions he added an active filter on the power rails in his later version.
With the input shorting trick the noise is probably best judged by ear. Narrowing down the noise to a preamp stage (or stages) is a very helpful moving forward.
Quote
This is my PCB (for gyrator version, inductor based one is very similar). Maybe the layout is a problem?
What stands out to me is the switcher on the supply.
I doubt the layout is causing noise in the audio circuits themselves. However, with the switcher present, the layout could have an impact. The layout of the switcher part could also have an impact. The switcher can create noise on the power rails and also couple noise into the audio circuits.
To completely remove any causes related the switcher it's probably best to:
- remove the switcher inductor; so when the switcher is disabled it doesn't short across the supply
- disable the 555 from oscillating
- power the unit from a clean external supply.
If you get a significant change doing this them the cause of the noise is related to the switcher.
If the noise is still present it's probably a wise move to debug the noise problem with the clean external supply in place.
I'll try that all in the morning. In the meantime, this is what Mr Maillet wrote to me when I asked him about transistor choice:
QuoteHi Ranko,
if you look at the overall structure
you'll notice something very "unique"
which everybody seems to miss btw ...
it's not so much about the style of circuit design which is very primitive by today's stds
now ...
THAT very idea can be ported into newer design practices which I did with my "360 MDRN"
same "OVERALL" idea, radically different circuit implementation ...
best,
~jcm
Well, I'm electronics apprentice, so the only unusual thing I notice is the Volume (gain actually) pot which is connected as variable negative feedback, if I'm right. Am I?
QuoteWell, I'm electronics apprentice, so the only unusual thing I notice is the Volume (gain actually) pot which is connected as variable negative feedback, if I'm right. Am I?
To me most aspects have appeared somewhere before. There are some uncommon aspects to the design and back when that thing was made they would be even less common.
- The variable gain volume. Inverting amp with variable feedback. Not so common.
Lets you get near zero gain. Can keep noise down, depending is specifics.
- They didn't hold back on the buffers, in particular the one in the middle - that would help keep noise down.
- The mid control circuit like that wasn't so common for guitar/bass amps.
Some possibly 'hidden' behaviours would be how the transistor stage or the mid control loads down the tone-control. It looks like they have made an effort to prevent interactions by scaling down the tone control values.
BTW, is your complaint about noise on the clean signal, the distorted signal or both?
The gain in the distortion section is going to bump-up the noise. On the good side it's
only got the buffer before so most of the noise will be from itself.
That distortion structure appears in a few pedal of the 60's/early 70's.
On the whole I always thought the guys at Acoustic done a pretty good job for their time.
Clean signal, I've built this without fuzz section (not enough space in 1590BB).
I tried to measure noise using my o-scope. Those are switcher noise before filtering (blue), output noise (yellow) and A-B (pink):
(https://i.postimg.cc/B8jVV97J/diff.png) (https://postimg.cc/B8jVV97J)
Don't seem to be too related to me, do they? There are some coincident peaks, but they appear at ~100kHz, which is my switching frequency.
This is Q1c (filtered B+ at 25V) compared to unfiltered switcher noise (yellow), also don't seem to be too related. Filtering is done with 1uH inductor and 220p+100u capacitors.
(https://i.postimg.cc/mhKCYxCQ/Q1-C-PWR-noise.png) (https://postimg.cc/mhKCYxCQ)
This is the signal on all transistors (except Q2e, which I missed), with grounded input:
(https://i.postimg.cc/p9Yp3TGH/Q-CBE.png) (https://postimg.cc/p9Yp3TGH)
Quote from: rankot on February 14, 2020, 05:23:33 PM
This is the signal on all transistors (except Q2e, which I missed), with grounded input ...
Ranko,
May I first suggest you try powering up the circuit using three 9v batteries wired in series, at least for a bit at a time just for noise testing ... this will rule out any psu issues you might be having
I would then consider replacing all your transistors with new ones ... start from the back end and move backwards one device at a time ... test for noise improvement/change after each swap ... you might also want to consider using better noise devices like 2n5210 which I've used in many of my builds going back to early 90's ... I've found them to be reliable and fairly quiet
post voltages if you keep having trouble ...
I agree with Eb7+9. Probably making life hard for yourself using the CRO. It's not always easy to correlate what you see with what you hear. The external power supply or batteries will wipe out the PSU altogether. It's a good idea to disable the SMPS like I mentioned before to be 100% sure the switcher is out of the picture.
FWIW, the small pips on both the PSU and audio could imply the PSU noise is getting into the audio. It could also be caused by a noisy CRO grounding point.
I will try to follow advices from both of you, they are very nice! What's CRO? :-[
QuoteWhat's CRO?
It's just an abbreviation for an Oscilloscope.
https://en.wikipedia.org/wiki/Oscilloscope
Quote from: Rob Strand on February 15, 2020, 01:19:50 AM
FWIW, the small pips on both the PSU and audio could imply the PSU noise is getting into the audio. It could also be caused by a noisy CRO grounding point.
I've connected CRO ground to input or output jack, if I remember well.
But could it be the problem with noisy pots? It seems that noise is starting to go wild at Q4 collector, which is connected to Treble/Bass tone stack. But to stop guessing, I will try with another power supply or 3 batteries.
Hm, I was a little bit lazy to desolder TLC555, so I tried another approach - simply tried to run SMPS at higher frequency. Instead of 120kHz, I tried 450kHz and the most of the noise is gone now. So it is definitely SMPS problem, but I must find out how to fix it. Maybe just leave it as is - but run it at 1.2MHz, for example.
QuoteI tried another approach- simply tried to run SMPS at higher frequency.
Good idea.
Quotemost of the noise is gone now. So it is definitely SMPS problem, but I must find out how to fix it.
Noise can get in through the power-rails or coupled electromagnetically into the audio (or both).
Assuming it's coming through the power rails, a simple RC filter between the switcher output and the circuit
is good thing to try and not too hard to retrofit.
If it's coupling electromagnetically then it's not so easy. You can only use a better inductor, better layout, or some judicious shielding between the switcher and the audio circuits.
Well, I already have LC filter which seems to kill all the noise on power supply - 1uH + 100uF || 220pF. Power is very noisy before that, and after that almost nothing (see one of previous posts), so it is probably being coupled somewhere else. I may try to add another capacitor pair at the end of B+ line, near Q4 and see what happens then.
My apologies I've somehow edited your post. I've tried to restore it
Quote
Well, I already have LC filter which seems to kill all the noise on power supply - 1uH + 100uF || 220pF. Power is very noisy before that, and after that almost nothing (see one of previous posts), so it is probably being coupled somewhere else. I may try to add another capacitor pair at the end of B+ line, near Q4 and see what happens then.
OK I see it now. Is that inductor 1uH or 100uH? The layout shows 100uH.
As far as the filtering goes it might be worth adding 1k in series with that inductor as a test.
However, I can see two problems:
The first is a layout problem. The ground track to the analog circuits is tapping off the noise input track. Ideally this ground track should ideally connect to the -ve terminal of your 100uF filter cap. But the input socket is already exposed to the noisy ground ...
The second issue is the power is switched using the input socket, as is normally done for effect pedals. The only problem is with switch-mode supplies is they pulse the input current and those pulses flow down the input socket ground and noise gets into the audio. You 220uF input cap helps but might not be enough.
I don't know if these are the cause in your case but they are very suspicious.
One way forward would be to remove the 2A fuse and wire the ground wire from the DC jack to the -ve terminal of the 220uF input cap. (This may also help the analog power tap-off issue but not 100%.)
If the ground through the input jack turns out to be the problem the correct solution is to use a MOSFET switch to switch the power and use the input jack to switch the MOSFET. That way the noise ground line does not pass through the input socket. The common way to wire this is a p-channel mosfet switching the *+ve* rail and the gate being switched by the input socket.
[EDIT: so the power socket ground gets wired to the -ve terminal of the 220uF input cap and the input socket ground wires to a 'quiet' ground point.]
There's a few schematics on the web about this. There was a thread where Marcus-Munky has this issue with an angry switch-mode on a bass with a lithium battery. I think there was also something on RG's site, perhaps on one of the schematics. I can't find the link or the thread at the moment.
Keep in mind you might find every one of the above points adds to the noise. When you remove the main cause the others could still be adding a little and it's not until you remove all the causes that the circuit is truly quiet as it can be.
Rob, filter inductor is 1uH. I will try to add some series resistance.
I have to 220uH caps. The first one is preceding SMPS, another one is between diode and inductor in SMPS. Shall I connect ground from DC jack directly to the later?
I really don't need input jack acting as a switch, since I don't use batteries here, so I can try to skip that if it will reduce noise.
I settled on 33k R1 / 1k R2 / 100p Ct for 555, which gives 412kHz SMPS. It seems to have less noise than before, at least to my ears, but I will measure it to check what's going on.
Tried all those ground points and noise seems to remain the same. >:(
Will try to add some resistance into filter.
QuoteShall I connect ground from DC jack directly to the later?
The correct connection for in DC input ground is to the -ve terminal on the 220uF input cap.
Quote
Tried all those ground points and noise seems to remain the same. >:(
Will try to add some resistance into filter.
It's a matter of working through the options.
FWIW, you could increase the value of the 2u2 output cap on the switch-mode.
I am OK with the noise level right now, it seems that increasing switching frequency fixed it. I have another problem now - preamp is distorting the signal, and it is pretty obvious on transients, I must turn the volume down to 1/2 to avoid that. And if I turn it to 3/4 or more, it is distorting almost always.
I tried to inject sine wave 100mV p-p into input, and this is what I get at Q4 base (it is clipped a little, but it is normal, due to nature of my signal generator) - so I can say it's not distorted there.
(https://i.postimg.cc/fJ1kzP7w/Q4b.png) (https://postimg.cc/fJ1kzP7w)
But when I look at Q5 base, I get this - like the bottom of the wave is flipped at some point.
(https://i.postimg.cc/YhLqsx5T/Q5b.png) (https://postimg.cc/YhLqsx5T)
Q6 base looks like this:
(https://i.postimg.cc/642wF9mC/Q6b.png) (https://postimg.cc/642wF9mC)
What can be the reason for this? Is it possible that simple treble/bass tone stack can distort the signal like this???
QuoteWhat can be the reason for this? Is it possible that simple treble/bass tone stack can distort the signal like this???
The inverted signal on Q6 comes from the fact Q5 inverts the signal. The cause is Q5.
As to what is the cause. I'd look at the biasing of Q5. Off-hand I don't know if there are any voltage lists for that circuit.
What might be happening is Q5 is overloading and that could cause cap on the base of Q5, and perhaps caps in the tone control, to charge-up. However, the fact weirdness occurs on the negate swing makes me think there's a bit more going on. Perhaps Q5 is overloaded on the positive input cycle into Q5, and that charges the input cap via the base-emitter diode. There will be a DC voltage on the input cap with the base side being negative. When the input to Q5 goes negative the DC voltage on the input cap cause the base to swing more negatively and breaks down the BE junction of Q5.
*BUT* the breakdown theory doesn't hold up because the voltages on your Q5 waveforms are too small. Are those CRO voltages correct? only 100mV? Are you using a x10 probe?
Check the DC voltages on Q5. Perhaps check and/or replace Q5.
No, probe is at x1. I have the voltage chart from the service manual, it doesn't show waveforms in different spots, but for 300Hz 100mV p-p input (My input is ±100mV. Is it OK, or I need to inject ±50mV?), Q5 base shall be at 400mV and Q4 base at 1.2V. Q6 base shall be at 800mV.
If I read this CRO well, I have 1.2V at Q4, 400mV at Q5 (but bottom is flipped) and 600mV at Q6. So Q5 is definitely a suspect, just have no idea what's going on...
I have also measured DC with shorted input, and this is what I get, but have in mind that my B+ is 23.7V, not 25:
Q... | Collector........ | Emitter.......... | Base............. |
1 | 25.0 (25.0) | 19.7 (19.5) | 18.8 (20.1) |
2 | 10.3 (11.0) | 0.95 (0.9) | 1.6 (1.5) |
3 | 24.9 (25.0) | 9.7 (10.3) | 10.3 (11.0) |
4 | 7.0 (7.0) | 3.1 (3.4) | 3.8 (4.0) |
5 | 12.6 (12.0) | 5.7 (6.3) | 6.2 (7.0) |
6 | 24.9 (25.0) | 11.9 (11.3) | 12.6 (12.0) |
Values in parenthesis are from the service manual. Please note that my B+ is 23.7V (not 25), so I scaled all my readings with the factor of 1.055.
Quote300Hz 100mV p-p input (My input is ±100mV. Is it OK, or I need to inject ±50mV?), Q5 base shall be at 400mV and Q4 base at 1.2V. Q6 base shall be at 800mV.
If I read this CRO well, I have 1.2V at Q4, 400mV at Q5 (but bottom is flipped) and 600mV at Q6. So Q5 is definitely a suspect, just have no idea what's going on...
The way I read it is you would need +/- 50mV to match the 100mV p-p (peak to peak) in the service manual.
It's best to try to match the service manual. Nonetheless it looks close (are Q4 and Q5 voltages flipped?). The tone control settings (or in inaccuracies in the pot tapers) can also cause slight difference unless the service manual says to set them to full or minimum.
QuoteI have also measured DC with shorted input, and this is what I get, but have in mind that my B+ is 23.7V, not 25:
The DC voltages are fairly close. Nothing stands out as bad (your Q1 base voltage is low due to meter loading). I checked a few voltages with hand calculations and the service manual voltages look like the transistors have a gain of about 400.
So maybe the problem is the gyrator circuit. That connects to Q5 so it could be affecting the waveforms. If the gyrator opamps are clipping it could do something like that. If you have a real inductor you could try that just see if it gets rid of the flipped waveform peak. You could also look at the DC voltages and waveforms on the gyrator opamps.
Are all the electrolytic caps loaded with the correct polarity?
I don't use electrolytics, I always use 1uF poly for audio path.
But I have almost the same distortion on another unit which has normal inductor. I will see it with CRO.
QuoteBut I have almost the same distortion on another unit which has normal inductor. I will see it with CRO.
A couple of things to try:
- change the Vari-mid frequency switch to see if it change the waveform shape. The idea here is perhaps the mid control resonance is making the waveform look weird when the transistor stage clips.
- It's probably worthwhile lifting the 2x6.8uF caps to isolate the transistor and mid control and see if the inverted tip is present with only the Q5 transistor stage.
Even when mid control completely disabled, there's the same distortion as before (and maybe even more). So it seems that one's not causing it.
Here is ltspice model of Acoustic 360 preamp, no idea what's going on. :icon_redface:
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QuoteEven when mid control completely disabled, there's the same distortion as before (and maybe even more). So it seems that one's not causing it.
That's good. So it can only come from that stage.
So maybe the next step is to drive the input of the tone control directly with a signal generator. That will isolate that transistor stage entirely.
QuoteHere is ltspice model of Acoustic 360 preamp, no idea what's going on.
Unfortunately I'm not running my engineering computer at the moment so I can do any spice sims. I would be nice to see the spice sim. If it shows up in the idealized environment of spice then it's a real effect.
Normally weird things like negative tip you are seeing are related to caps charging up. What I'm saying here is the flipped peak is actually a cap charging up in the positive direction. There's only the input cap and the tone control caps.
To charge the input cap it must come from:
- the transistor, which doesn't make sense under normal circumstances since the BE junction doesn't supply current. If the BE junction broke down it could supply current but the voltages are too low.
- from the CB feedback resistor. The CB resistor look too large to charge up a cap in that time and produce the peak.
So the only think that makes sense is a tone control cap control is charging up. That's where a spice sim would help. That effect would show-up reliably in spice.
Another thought is the flat-top on your signal generator going through the EQ of the tone control is promoting an odd-shaped waveform. You could try smoothing the output of your generator with an RC filter with a 300Hz cut-off. Alternatively just use your PC as a signal generator. That will give a clean sine output.
(BTW. you can often fixed distortion on sine-wave oscillators by tweaking the gain of the amplifier used in the oscillator. Lowering the gain lowers the hardness of the clipping and that cleans up the waveform.)
But Rob, all of my caps are non-polarized, metal film, except those with 6u8. I disconnected this varitone by simply removing the inductor, but they are oriented just as they shall be.
And this is the distortion of generated signal. It's too complicated for me to make a good CRO shot while playing a guitar, so I used generator. However, it's distorted regardless signal source.
And in spice it simply works normal - at least I think it does.
Quote from: Rob Strand on February 23, 2020, 06:28:04 PM
That's good. So it can only come from that stage.
Which one? The one before the tone stack or the one after it?
QuoteBut Rob, all of my caps are non-polarized, metal film, except those with 6u8. I disconnected this varitone by simply removing the inductor, but they are oriented just as they shall be.
Ideally you want to pull both the caps. If you just pull the inductor part there is still a feedback circuit from the collector to the emitter via the two caps and the pot. That could be doing *something* to the emitter voltage.
Pulling the inductor part does confirm the it's not cause by the gyrator or presence of the inductor.
When you get weird problems it's a good policy to strip everything back to the bare minimum so there's nothing else to blame - even if you think a lot of stuff can't possibly cause the issue. If you strip a circuit back to two resistors and it's not doing what you expect then you have a very good chance of seeing what the problem is.
QuoteAnd in spice it simply works normal - at least I think it does.
That's very odd then. Try different input levels. You need to match the tone control pot settings is spice.
You could just replace transistor Q5.
Unfortunately I don't have spice running otherwise I would have tried to break the circuit ages ago.
Quote
Which one? The one before the tone stack or the one after it?
Yes that one. The stage with Q5.