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DIY Stompboxes => Building your own stompbox => Topic started by: Ben Lyman on September 07, 2016, 03:20:46 PM

Title: Little Gem MKII not working on the BB
Post by: Ben Lyman on September 07, 2016, 03:20:46 PM
Sounds bad, gated, splatty, LM386's getting painfully hot to the touch. Ive stripped it out, double checked, triple checked, shortened up, neatened up, rebuilt 5 times... same results.
Got 2 LM386N and 2 LM386L from Tayda and they all worked fine for my Smokey with several different speaker combos.

Can one of you please take a close look at this schematic and tell me what could be missing/mis-labeled? Does it look perfect as it is? I think people have made this amp before
Scemo:  http://www.runoffgroove.com/littlegemMK2.png (http://www.runoffgroove.com/littlegemMK2.png)
Thanks!
Title: Re: Little Gem MKII not working on the BB
Post by: boppy100 on September 07, 2016, 08:49:41 PM
Speaker polarity??
Title: Re: Little Gem MKII not working on the BB
Post by: Ben Lyman on September 07, 2016, 08:59:45 PM
Looking at the schematic linked, I hooked up the LM386 pictured above to the "plus" side of the speaker and the lower LM386 to the "minus" side labeled speaker. tried a 4" 8ohm, 2x 4" 8ohm in series and parallel. Also tried a 4" 30ohm and a 12" 8ohm, then 2x 12" 8ohm in series.
Title: Re: Little Gem MKII not working on the BB
Post by: PRR on September 07, 2016, 11:44:39 PM
Speaker polarity does not matter.

The schematic is fine.

What did you *really* build (repeatedly)?

Great heat does suggest reversed *power* polarity (how would I know that?).
Title: Re: Little Gem MKII not working on the BB
Post by: Ben Lyman on September 07, 2016, 11:56:53 PM
Speaker polarity does not matter.

The schematic is fine.

What did you *really* build (repeatedly)?
I don't know Paul, I'm mystified. I moved on to the Ruby, seems the same but simpler, only 1 chip and works fine as I BB it, as does the Smokey. Ruby has a 100nF pin 7 to ground and a 220uF from out pin (5) to speaker+ ...remove it and chip melts down.

Please tell me where the Little Gem MKII differs in that it does not need a cap there or why there is nothing on pin 7
Thanks
Title: Re: Little Gem MKII not working on the BB
Post by: PRR on September 08, 2016, 12:19:33 AM
> I moved on

Movin too fast for me.

Is this one of those nebulous projects that has no Volts in it? LINK (http://www.diystompboxes.com/wiki/index.php?title=Debugging)

The cap on pin 7 (bypass!) defines gain and suppresses power supply crap. It really should be used.

Pin 5, Output, sits at DC voltage of half supply. If you run this through 8 Ohms to ground, way too much useless DC current flows. "All" single-rail grounded-load audio amps need an output cap. The bridge-tie (two outputs) variant has both outputs at 4.5V DC, so no DC current flows in speaker or amplifiers. (Why I was askin about Volts.)
Title: Re: Little Gem MKII not working on the BB
Post by: Ben Lyman on September 08, 2016, 12:51:08 AM
Sorry, "moved on" was poor choice of words. I just mean that I'm trying simpler things for a minute to make sure I understand this chip.

Right now the Ruby is kicking serious butt from my BB into a 12" 8ohm Mesa cab, loud and clean or louder and raunchy.

So I am almost prepared to tackle the LG MKII again. No caps needed on the out pins of both chips, got that.

So... pins 7 and 7... I should have some caps there on each chip going to ground?
Sorry if I misunderstood that part but I will give it a go with caps if that's what you mean.
thanks
Title: Re: Little Gem MKII not working on the BB
Post by: PRR on September 08, 2016, 12:54:12 AM
First figure out why the dual-amp plan gets HOT.

Voltages are clues.
Title: Re: Little Gem MKII not working on the BB
Post by: Ben Lyman on September 08, 2016, 01:02:08 AM
ok, will do, thanks
I had to work fast because the chips were heating up, I think it is only number 2 chip that gets hot.
Readings were moving around a lot and I just had to grab a number sometimes and move on.

Lets call the upper pictured IC #1 and the Lower pictured #2

Q1
D 2.0
S -4.0
G -4.5

 IC1
1 4.2
2 5.0
3 5.0
4 5.1
5 2.0
6 4.1
7 -3.6
8 -7.0

IC2
1 5.1
2 5.7
3 5.9
4 5.9
5 .06
6 4.5
7 -1.0
8 3.2
Title: Re: Little Gem MKII not working on the BB
Post by: balkanizeyou on September 08, 2016, 06:14:13 AM
the voltages look way off - are you powering it with a dual power supply? You should be getting supply voltage (9V) at pins 6 of both ICs, pins number 4 should be at ground - in your case pin 4 is at HIGHER potential than pin 6, which is weird to say the least.
Check the power supply connections, also a picture of the build would be a great help
Title: Re: Little Gem MKII not working on the BB
Post by: bluebunny on September 08, 2016, 08:01:37 AM
+1

Bad (or no) ground connection?  There's nothing much that looks like 0V, like IC1.2, IC1.4, IC2.3, IC3.4.  No obvious power at pin 6 either.  And given that IC1.3 is connected directly to IC2.2, they should have exactly the same voltage.  Plus what Paul said about the output pins.

If this is all on a breadboard, I'd be inclined to take a deep breath and start over.
Title: Re: Little Gem MKII not working on the BB
Post by: deadastronaut on September 08, 2016, 08:55:35 AM
i was bored...here ya go, 8)
(https://dl.dropboxusercontent.com/u/7464107/WAVES%20FOR%20PELLE/GEMII.jpg)
Title: Re: Little Gem MKII not working on the BB
Post by: Ben Lyman on September 08, 2016, 09:39:18 AM
Thanks guys, ya I know it's all way off, numbers were changing with the temperature, I have taken several breaths and start-overs already. I'm using a 1-spot, it measures 9.4v

I just used these chips to BB a Ruby on the same BB and everything worked at that time.

Thanks DA, I will lay it out one more time exactly as you show.

Thanks
Title: Re: Little Gem MKII not working on the BB
Post by: deadastronaut on September 08, 2016, 09:50:10 AM
no probs, thats how the schemo says it is..

good luck. 8)
Title: Re: Little Gem MKII not working on the BB
Post by: Ben Lyman on September 08, 2016, 12:49:02 PM
Same results. Seems like IC2 is always the culprit, I switched speaker wires around and IC1 got hot. I thought maybe my 4ohm cab and/or speaker cable was loading it down or something? I don't know, I tried the shortest speaker cable I have, about 3 feet, and same results.

I thought it might be cool to try a panoramic shot of the BB:
(http://a63.tinypic.com/3502deb.jpg)
Title: Re: Little Gem MKII not working on the BB
Post by: Kipper4 on September 08, 2016, 12:56:10 PM
How come you dont have jumpers from the top power rails to the bottom ones mate?
Title: Re: Little Gem MKII not working on the BB
Post by: deadastronaut on September 08, 2016, 01:07:01 PM
+1 and make sure you have continuity across those rails

some breadboards are split in the middle...
Title: Re: Little Gem MKII not working on the BB
Post by: Kipper4 on September 08, 2016, 01:35:28 PM
Not just the jumpers in the middle Rob.
My BBs dont have continuity from the top power rails to the lower (+ and -)
Title: Re: Little Gem MKII not working on the BB
Post by: Ben Lyman on September 08, 2016, 01:54:03 PM
good eyes, that was a test  ;)
actually, this bb has continuity horizontally across and the reason I didn't put a vertical jumper was because I only used the top for plus and the bottom for minus... but, umm... oops, I just noticed the 100uF needs some attention. I'll fix that but rest assured it will not change anything.
Thanks again guys

Updated pic with 100uF corrected. Same problem
(http://a67.tinypic.com/10z4u12.jpg)

(http://a64.tinypic.com/14ugpxy.jpg)

(http://a66.tinypic.com/w7mqfk.jpg)

(http://a67.tinypic.com/alnkvm.jpg)
Title: Re: Little Gem MKII not working on the BB
Post by: Kipper4 on September 08, 2016, 03:32:48 PM
I burnt my thumb print off earlier tonight by badly connecting LM78l05 regulators. Dammed those Underside view datasheets and my stupid old eyes.

At least I could commit a burglary without Thumb print evidence now.

Sorry I cant be more help with the debug.
Have you tried it with some different speakers.
Are some of the designs resistor rated higher than our std 1/4" watt?

I hope you get it sorted mate.
Title: Re: Little Gem MKII not working on the BB
Post by: Ben Lyman on September 08, 2016, 06:08:53 PM
Good questions Rich, as far as I know this should work with 1/4watt resistors.
I have tried it with a single 12" 8ohm, a single 4" 8ohm, 2x12" 8ohm series & parallel, 2x4" 8ohm series & parallel, all with the exact same results.

These chips can still do a Ruby or Smokey just fine but not the LG MKII, perhaps as they try to do this "different kinda task" they simply lose their minds.

How about a possibly bad Q1 doing tricky stuff to the chips?

Def IC2 that gets red hot, I can't even read the print on it anymore because it's been heated up so many times, but still drives a Ruby/Smokey circuit with the Mesa 12" cab.

Here's todays readings:

Q1
D 9.3
S 3.4
G .03

IC1
1 0.21
2 0.01
3 0.29
4 0.01
5 5.2
6 8.5
7 4.5
8 1.0

IC2
1 1.0
2 0.41
3 0.01
4 0.44
5 2.5
6 9.3
7 4.8
8 2.0
Title: Re: Little Gem MKII not working on the BB
Post by: jonnyeye on September 08, 2016, 09:04:21 PM
Sounds like you may have ultrasonic oscillation; instead of the 10 ohm plus 0.1uf damping/Zobel network across the two outputs (pin 5), try a 10 ohm plus 0.1uf from each output (pin 5) to ground, using the shortest wiring you can.

Also, the LM386 is only really spec'd for 4 ohm operation and thus the minimum load it can drive when bridged like this is 8 ohms; even then the output stage is current starved. You'll get most power out with a 16 ohm load (your 2x8 ohm speaker cabinets with the speakers in series will work, once the amp does).
Title: Re: Little Gem MKII not working on the BB
Post by: Ben Lyman on September 08, 2016, 11:52:48 PM
Thanks jonnyeye! I'll do that for sure ASAP.

I thought LM386 took an 8ohm speaker?
Anyway, the ROG LG MKII said use 4ohms or more for this circuit.
Oh well, I'll see if I can get it going. Anybody else wanna BB this? I'd really like to know if I'm working on something that only exists on paper  :P

Edit: no go on the 2x 10r plus 2x .1 pins 5 to ground. bummer

I pull IC2 out and IC1 starts working like a champ (more like a Ruby). replace IC2 (with new chip) and volume splutters, chip heats up.
Title: Re: Little Gem MKII not working on the BB
Post by: slacker on September 09, 2016, 03:34:53 AM



I pull IC2 out and IC1 starts working

Something's wrong here, if you just pull IC2 IC1 on its own can't make any sound because only one side of the speaker will be connected to anything. Sounds like you're connecting the speaker wrong, one side goes to IC1 pin 5 the other side goes to IC2 pin5 and no where else. You're not connecting one side of the speaker to ground are you?
Title: Re: Little Gem MKII not working on the BB
Post by: Ben Lyman on September 09, 2016, 04:09:39 AM
No, the speakers are hooked up just as you say, pins 5 and 5 only. Not sure why it worked without IC2. Is it possible for the speaker to work if (-) goes to (+) through the 10r and .05uF? Because that was still there. I think something is wrong with my 1/4" in/out jacks w/DPDT ground wires or my breadboard when it comes to this circuit, anything else works fine though so I dunno.

btw, I just went back to the Ruby to test these chips and they are both still working fine. They are LM386 N1 which sounds like maybe lowest of the low for heat tolerance. Maybe they heat up and pass all kinds of signals through them, then cool down and work again for the Ruby/Smokey.
Title: Re: Little Gem MKII not working on the BB
Post by: Phoenix on September 09, 2016, 04:38:56 AM
Is it possible for the speaker to work if (-) goes to (+) through the 10r and .05uF?

No, if you pull IC2, only one end of the 10R and 50nF are connected to anything (except the speaker in parallel of course), there is no return loop.

Do you have your input and output jacks mounted to a metal plate or something so that the sleeves of both jacks are shorted together?
The output jack MUST be isolated from ground for this bridge mode operation to work.

Also, if you leave IC2 and pull IC1, does IC2 still get hot, or does it work in the same way as the reverse?
Title: Re: Little Gem MKII not working on the BB
Post by: Ben Lyman on September 09, 2016, 09:49:56 AM
I'm not using the output jack so I don't think it should matter but maybe I'm missing something here. I will bb it again later and go from guitar straight to bb. Now I wonder if unwanted connections are being made within my bb but I've made and remade this thing so many times in different positions and on 5 different breadboards, doesn't seem likely I'd get the same issue everytime by coincidence.
Title: Re: Little Gem MKII not working on the BB
Post by: anotherjim on September 09, 2016, 10:38:38 AM
Ben, are those voltages with or without speaker connected? Without speaker, you should get typical 386 voltages, but if something is amiss in that case, particularly output pin 5 volts different, the low resistance of the speaker is trying to pull the outputs together against thier wishes. Unless both outputs of a bridge amp are proven to have the same DC (within a few 10's of millivolts), don't even try to connect the speaker. Do that with a proper meaty power amp, and you fry the voice coil and other choice bits.

In case oscillation is a problem, try 100nF ceramic disc from pin 6 of each amp to ground. I think always fit one with IC's, even if it makes no apparent difference!

Try 2 Zobel networks. One for each output to ground rather than 1 across the speaker. I've just searched "bridge connected IC amp schematic" and every one uses 2 networks.
Title: Re: Little Gem MKII not working on the BB
Post by: duck_arse on September 09, 2016, 10:55:45 AM
how come both pin 6's are connected to the same supply, but show different voltages? maybe power off and test continuity, to prove your links are linking like you're thinking.

also both pin 4's. should be 0V, or whatever your meter reads when you short the probes together (my chitbox wanders quite a bit with its zero reading).
Title: Re: Little Gem MKII not working on the BB
Post by: Phoenix on September 09, 2016, 10:58:13 AM
I'm not using the output jack so I don't think it should matter but maybe I'm missing something here.

So how are you connecting to your speaker? Crocodile clips or something?

I'm quite convinced that you're shorting the output of IC2 (pin 5) to ground - what's labelled as the "negative" terminal - entirely meaningless and should have been omitted from the ROG schematic, as well as the "Pay attention to the speaker polarity" which doesn't matter at all, and they missed out the most important detail which is that the "negative" speaker terminal must be isolated from ground.
This would explain why IC2 is heating up when on the breadboard, and why the circuit works (at half power) when it is removed.

Please follow my above advice and remove IC1 from the breadboard, I'm certain that IC2 will still heat up, and will produce no sound.
Title: Re: Little Gem MKII not working on the BB
Post by: anotherjim on September 09, 2016, 11:53:17 AM
I found this...
(http://www.seekic.com/uploadfile/ic-circuit/s201142631055622.jpg)
Note cross connection of the unused inputs  - not grounded. Balance control between pin 7's? Hmmm....
Title: Re: Little Gem MKII not working on the BB
Post by: Phoenix on September 09, 2016, 12:02:25 PM
I found this...
(http://www.seekic.com/uploadfile/ic-circuit/s201142631055622.jpg)
Note cross connection of the unused inputs  - not grounded. Balance control between pin 7's? Hmmm....
Pin 7 is the Vbias output, normally used to attach a bypass cap for better PSRR, but connected in that manner it can also be used to help balance out DC offset between the two chips at the expense of a small amount of headroom.
It's not relevant to the problem that Ben is having though.
Title: Re: Little Gem MKII not working on the BB
Post by: Ben Lyman on September 09, 2016, 12:55:14 PM
Thanks everyone, I'm trying as fast as I can to catch up to all your suggestions.
I had it working a minute ago.
I eliminated all extra jacks and connections, went straight from guitar cord to BB, straight from pins 5&5 to speaker (yes, gator clips everywhere) but even doing this was no fix for the problem.
I am 99.99% certain I have everything exactly correct to the ROG schematic.

Then...
As I lifted IC2 up and almost out, I heard a noise. So by resting it gently on top of it's BB holes and adjusting IC1 in the same manner, I was able to get a very loud and "real" sounding guitar amp.
Then both chips heated up so I shut it all down again.
It must be my BB, I am very careful, slow, patient, etc. I usually don't have any BB connection problems.

Do you think, when I push the IC down into the holes, the fat part of the IC pins could be spreading the BB terminals and making them all short together?

It almost seems like that is happening but I have made tons of IC projects and never had that happen before.

So, it was sounding great (for what it is) and seems promising, except that the chips still heated up way too hot.
Title: Re: Little Gem MKII not working on the BB
Post by: anotherjim on September 09, 2016, 03:47:35 PM
All pins are different V. If adjacent rows were shorting, the pins in them would be shorting & have same voltage.
Pin5's are very different which is cooking the chips.
IC1 = 5.2V. IC2 = 2.5V (unless you read the DMM backward there? ;)  2.7v across speaker. DC R of speaker approx 3R?
Then current = 900mA. The chips no like, even if speaker R=4 it won't like. Maybe 386 outputs have different self limit sensitivity to source or sink current fault - One shuts down & stays cool, the other gets hot.

Even when volts are ok, I think a 4R speaker is too much load. Bridge operation is a wonderful trick - it doubles the voltage swing across the speaker. That doubles the current. Power is proportional to Volts x Amps  (it is AC and load is inductive, so the formula isn't exact, but it's near enough). You then have about 4x the power for a given supply voltage compared to a normal single amplifier. A car audio amp rated 22 to 28W RMS per channel is very likely to be a bridge amp. 6 to 7W RMS rated ones are single.



Title: Re: Little Gem MKII not working on the BB
Post by: Ben Lyman on September 09, 2016, 07:08:37 PM
OK, chips are cool, volts are stable, got some accurate readings although I don't really know what they mean.
Problem is the volume is super low, clean, useless.
Pull IC1 pin3 and volume cranks, overdrives, sounds great, then IC1 overheats  :(

One Spot 9.4v

Q1
D 9.37
S 3.36
G 0

IC1
1 1.19
2 0
3 0
4 0
5 4.95
6 9.23
7 4.72
8 1.14

IC2
1 1.20
2 .01
3 0
4 0
5 4.52
6 9.43
7 4.76
8 1.28

I call BS on this circuit  >:(
Your challenge, should you choose to accept it:
Prove me wrong using a breadboard  ;D

Thanks for all your help everybody, it really sounded great in that brief moment but I want to build something I know will work so probably going to start a noisy cricket or ruby instead.
That's not to say I'm giving up on this one, if you guys have anymore ideas I will keep working on it because it's definitely louder than a single LM386
Title: Re: Little Gem MKII not working on the BB
Post by: Ben Lyman on September 09, 2016, 09:24:00 PM
Update: got it working perfectly (almost) by trying a bunch of 386's, maybe they needed to match up better or something, maybe I fried one or two previously, I dunno. Strange that they still work on their own for a ruby. 

Components and leads are solid, I have not changed a thing from the original schematic, sounds great driving a 12" 8ohm, loud enough to jam out on, probably even gig with a mic on it, depending what kinda music you play.

Both chips get too hot, very hot. I have to stand by my claim for now, the challenge stands... any takers?  ;)
Title: Re: Little Gem MKII not working on the BB
Post by: Ben Lyman on September 10, 2016, 01:16:34 AM
Nobody wants to try this? Too bad, I just had a breakthrough moment and discovered the missing component to the ROG schematic!
This thing cranks a 12" speaker, I like it best at lowest gain, it's super clean and loud like a PA or something.
If you wanna know the trick, PM me and I will send you my paypal address  :icon_mrgreen:

Just kidding, I was just running through all your suggestions one at a time when I decided to put a volume pot on it the way the data sheet shows in the "common uses" schematics.
A 10k pot solved all the problems, lost the teeny tiniest wee bit of output volume but well worth the trade off. Lug CCW to ground, lugCW to .22uF, wiper to input pins, I guess a voltage divider would work too.

I plan on fussing with it some more tomorrow, I don't want to build it until I'm sure it will remain stable.
I still wish someone else would tackle it and see what happens
Title: Re: Little Gem MKII not working on the BB
Post by: anotherjim on September 10, 2016, 08:40:22 AM
Good going. I was going to suggest the inputs could be interacting when just tied together like ROG scheme shows, but needed to read up the data sheet. I thought separate input caps might do it, because the data says they don't have to be ground referenced by the input. It looks as though common ground reference can work. I have a worry though that in some cases, the offsets between the 2 chips could still be bad.
Title: Re: Little Gem MKII not working on the BB
Post by: duck_arse on September 10, 2016, 10:39:07 AM
so, if you did fit a pair of 220nF from the fet instead of the single connection, what DC voltages do/would you have at the two inputs?

and if a pot cures it, maybe a series resistor to each input from the single cap would fixxe?
Title: Re: Little Gem MKII not working on the BB
Post by: Ben Lyman on September 10, 2016, 11:39:50 AM
I like those ideas, thanks guys.
I seem to have another problem, I noticed my forearm burning as it rested on my guitar strings, feels like more than 9v and it goes up and down as I turn the volume knob.

My guitar cord goes straight from guitar to the bread board, tip to Q1gate, sleeve to the ground rail
Title: Re: Little Gem MKII not working on the BB
Post by: Kipper4 on September 10, 2016, 12:08:14 PM
That don't sound right.
If you smell a whiff of burnt flesh it defo aint healthy.
I guess the input cap is not ok. Does it have an input cap?
I go look at the scheme.

Hmm NO input cap....
Title: Re: Little Gem MKII not working on the BB
Post by: Ben Lyman on September 10, 2016, 12:31:51 PM
Ya Rich, no input cap shown so I put a 100nF there but to no avail.
I still want to work on this because it intrigues me, it's very loud and sounds good enough for a solid state practice amp.

BUT... does anybody have a better project that will give me one or more watts in a mini amp?
I have a TDA(somethingorother) chip that is a possibility, I'll go look for it. What simple amps can be made with that, or any other simple amps at all? thanks
Title: Re: Little Gem MKII not working on the BB
Post by: balkanizeyou on September 10, 2016, 01:14:37 PM
if you want to keep it simple, just get yourself a laptop power supply and use it to power a straight-from-the-datasheet circuit for the TDA chip (you should be able to get a few watts into 4 ohms out of the TDA2040 in this configuration for example). Add your favourite preamp in front of it and you have a really loud and cheap amp. Just make sure to use appropriate power rails filtering, because a lot of laptop supplies are not filtered too well.
Title: Re: Little Gem MKII not working on the BB
Post by: Ben Lyman on September 10, 2016, 02:55:14 PM
Update: my guitar ground was janky, got the shocker sorted. I think I should be able to ditch the gator clip direct guitar to board thing anyway and go back to my regular set up with the jacks.

Still working great, chips get a little warm when I play loud but not hot to the touch.

 :icon_question: Does anyone think there is a problem if I only use one cap across pin 1&8 and leave the other chip clean?  :icon_question:
ROG says put a switch on each 10uF to lift them and go clean. I like lifting one cap for a gritty OD. Is that bad? One chip at gain of 20 and the other chip at gain of 200?

Thanks balkanize you, sounds like a fun project. For a preamp in front of that amp, can I just make a one transistor distortion like an Electra and install it permanently in the enclosure with the amp?
Title: Re: Little Gem MKII not working on the BB
Post by: anotherjim on September 10, 2016, 03:14:50 PM
Personally, I'd use one of the many amp sim pedal designs out there to front end a chip amp. What amp would you want it to sound like?
Think of the power supply first. If using something you already have, then you can select an amp chip to suit. 12v DC is good if you can get it, because there are all the car audio chip amps to choose from. A single ended 12v chip amp into 4ohm would be close to a 5W tube amp volume wise. PSU current rating 2A minimum.
Run the amp sim circuit off a 9volt reg & it should do its tube amp-like clipping before the power amp itself, which isn't very nice.


Title: Re: Little Gem MKII not working on the BB
Post by: Ben Lyman on September 10, 2016, 06:15:50 PM
okay, thanks Jim, I'll keep that in mind for my future amp experiment.
For now, what about my idea of lifting the 10uF from only one of the LM386 chips in this bridged amp?
It sounds really cool to be able to get 2 different OD sounds plus the clean sound as a 3rd option by pulling both 10uF caps but I don't know if it is bad for something.

Something like this, a volume knob and two switches for Hi boost, Low boost, both boost, or no boost.

(http://i68.tinypic.com/33xfxnr.png)
Title: Re: Little Gem MKII not working on the BB
Post by: duck_arse on September 11, 2016, 10:45:31 AM
I lashed this to the BB today, but I have "GL386" chips. dunno how difference they are, but the work in the standard circuit. EXCEPT, I hadda pull both 10uF caps, otherwise it was just 'broken', loud cracks and blurts and no good guitar noise coming though. running dead cold, they worked with single 220nF cap same as a pair of caps.

and to answer one of my questions, with separate caps, one chip input read 13mV, the other 11mV (and my meter was reading 5mV as 0V). outputs were 4V35 and 4V16 into 4R computer speaker.
Title: Re: Little Gem MKII not working on the BB
Post by: anotherjim on September 11, 2016, 03:52:44 PM
Ben, I don't think there's a problem if one amp is "hotter" than the other. If one of them was silent, it's o/p is just sitting at V/2 and just behaving like an active virtual ground.
Title: Re: Little Gem MKII not working on the BB
Post by: Ben Lyman on September 11, 2016, 05:45:26 PM
hadda pull both 10uF caps, otherwise it was just 'broken', loud cracks and blurts and no good guitar noise coming though.
Thank you so much for testing it Duck! I really appreciate that, sounds like you got the same result as me once I ironed out the wrinkles in my breadboarding. No good guitar sounds with the caps in, only crackles and splutters. Did you try putting a 10k volume pot between the 220n and inputs? or maybe just a 10k resister right there going to ground would work.

At any rate, it didn't perform for you as shown in the schemo, so I stand by my claim: BS!
Who's next? anyone? Bueller?

Ben, I don't think there's a problem if one amp is "hotter" than the other. If one of them was silent, it's o/p is just sitting at V/2 and just behaving like an active virtual ground.

Cool Jim, thanks. I don't think it's exactly silent but I don't know the theory behind it. Anyway, I've been playing through it with a 12" 8ohm and it's still working fine. Super clean and loud with both caps pulled, add a 1uF to one chip and get nice bright sparkly OD, replace it with a 100uF and get big fat OD, then put 100uF and 1uF at the same time for big crunchy distortion.

What about if I build this thing with a power toggle switch and LED indicator? Should be okay right?
Also, it doesn't work if I run the +9v through a 1n5817 plus 47r before the circuit?!?! So, should I put a different kind of polarity protection on it?

Title: Re: Little Gem MKII not working on the BB
Post by: PRR on September 11, 2016, 10:47:05 PM
> it doesn't work if I run the +9v through a 1n5817 plus 47r

A fully loaded 8 Ohm amplifier looks like about 48 Ohm to the power source.

Bridge-loaded makes that close to 12 Ohms.

A 47+12 divider puts about 1.8 Volts on the LM386s. Even they can't eat so little.
Title: Re: Little Gem MKII not working on the BB
Post by: Ben Lyman on September 11, 2016, 11:02:20 PM
... Even they can't eat so little...
Thanks but do you mean "this won't work" or "it should work fine" unless I boogered something?

also, is there a better way to "polarity protect" and filter power. I'm not planning on building this with a battery snap, it's only going to run off a One-Spot 9v adapter
Thanks again!
Title: Re: Little Gem MKII not working on the BB
Post by: anotherjim on September 12, 2016, 06:14:04 AM
A relatively high resistance in the supply is the reason you aren't cooking the 386's. Usually power amps have little or no series resistance, because it absorbs power you want to go in the speaker. If there is a resistor, it's low R with high power rating.
Title: Re: Little Gem MKII not working on the BB
Post by: Ben Lyman on September 12, 2016, 10:45:59 AM
Ok, thanks.
So I'll not go with any resistor for power filtering, just the 100uF placed close to the IC's

Have you ever heard of placing the ground wire next to the ICpin4 and doing a "star ground" from there to all the other parts?
Title: Re: Little Gem MKII not working on the BB
Post by: anotherjim on September 12, 2016, 03:50:58 PM
Correct. As direct as possible to chip-amp power pins and filter cap close to them. You "star" connect other parts (pre-amp especially) from the amp supply points via a filter R & C network as you do in a pedal. That way, any increase in supply ripple when the amp is working hard has little effect on the pre-amp. If ripple does reach the pre-amp, you end up amplifying the ripple (op-amps are not as fussy about this as discrete pre-amps) .
Title: Re: Little Gem MKII not working on the BB
Post by: Ben Lyman on September 12, 2016, 04:05:35 PM
Cool, thanks again Jim. Just tried it on the BB and this amp is working great, even after I pulled my 10K volume pot... what was that I said about BS?
ummm... I'll take a big helping of crow with a side order of my foot, thank you! I guess I owe ROG an apology, sorry ROG.

It still sounds strange when I turn my guitar volume down and/or pick softly. It's like this fuzzy/static distortion that goes away when I turn up/play louder.
I'm hoping it's just some weird breadboard side effect that will go away with a proper layout.

I think for now I am going to go back to the simplest possible plan as a test run on the build, no volume no switches, just plug'n play.

There's one more issue that is baffling me... how can I install a 1/4" speaker jack so I can use this with my 2x12" cab?
Do they make plastic jacks with isolated rings or sleeves or whatever it's called?
or is there another way besides having a hole with two wires dangling from it?
Title: Re: Little Gem MKII not working on the BB
Post by: Kipper4 on September 12, 2016, 04:12:54 PM
"There's one more issue that is baffling me... how can I install a 1/4" speaker jack so I can use this with my 2x12" cab?
Do they make plastic jacks with isolated rings or sleeves or whatever it's called?
or is there another way besides having a hole with two wires dangling from it?"

Sure they do Ben

(http://www.mojotone.com/Mojo-3-Conductor-PC-Mount-1-4-Phone-Jack-3-Pin-Plastic-Nut-image.jpg)


I did the same with my noisy cricket and used the switched kind so when the cab cable is plugged in the amps speaker is bypassed.

Bosh
Title: Re: Little Gem MKII not working on the BB
Post by: Ben Lyman on September 12, 2016, 04:17:38 PM
Thanks Rich. Dammitalltohell!!! I just ordered a bunch of jacks from BLMS and didn't know I would be needing one of those  :P
Title: Re: Little Gem MKII not working on the BB
Post by: PRR on September 12, 2016, 04:36:54 PM
> Dammitalltohell

So get an all metal chassis-grounding jack, solder wires, but DON'T mount it in chassis. Wrap with tape or mount in a plastic pill-bottle. Let it dangle outside the case.

By the time you get tired of the "ugly", you'll probably be placing another order somewhere.
Title: Re: Little Gem MKII not working on the BB
Post by: Kipper4 on September 12, 2016, 04:43:28 PM
> Dammitalltohell

So get an all metal chassis-grounding jack, solder wires, but DON'T mount it in chassis. Wrap with tape or mount in a plastic pill-bottle. Let it dangle outside the case.

By the time you get tired of the "ugly", you'll probably be placing another order somewhere.

I like them onions :)
Title: Re: Little Gem MKII not working on the BB
Post by: Ben Lyman on September 12, 2016, 05:01:28 PM
> Dammitalltohell

So get an all metal chassis-grounding jack, solder wires, but DON'T mount it in chassis. Wrap with tape or mount in a plastic pill-bottle. Let it dangle outside the case.

By the time you get tired of the "ugly", you'll probably be placing another order somewhere.
If this idea came from anyone else, I'd be shaking my head and groaning. Here's a funny true story:

Every time PRR has given advice in one of my threads, I have either taken the advice and put it to good use...
or, not taken the advice and tried another way, then a few days later, sometimes a few months later, I have ended up, one way or another, doing it just as PRR advised me in the first place!

So, for now, I am trying to think of another way to do this but I know, in the end, I will be busting out a freakin' pill bottle and dangling it from the back of my amp!

So go ahead and shoot this idea down: I have a big plastic box to put the amp in...  What? Why not? RF? Electro-static from the neighbor's ferris wheel? What? Something else?
Title: Re: Little Gem MKII not working on the BB
Post by: Kipper4 on September 12, 2016, 06:13:42 PM
Rename it the Tupperware?

Ferris wheel. Geez.
Mexican radio receiver with a pill bottle Ariel ?
Pictures please.:)
Title: Re: Little Gem MKII not working on the BB
Post by: PRR on September 12, 2016, 11:08:30 PM
> Electro-static from the neighbor's ferris wheel?

I just heard a story, that a traveling ferris wheel, foldable for travel, folded-up while people were on it, triggered by smart-phone radiation or somesuch.

What have you got to lose? A leg? A life? A plastic container?
Title: Re: Little Gem MKII not working on the BB
Post by: duck_arse on September 13, 2016, 11:11:14 AM
mount the metal jack on some etched PCB. mount the copper-less pcb to the metal chassis (4 bolts/rivets), with a hole bigger than the jack so it doesn't contact. whoosh-kah (as kipper tries to say).

or just do what Paul tells you. twice now.
Title: Re: Little Gem MKII not working on the BB
Post by: Ben Lyman on September 13, 2016, 12:01:04 PM
thanks guys, for now I am going to try the plastic box. It already has two holes in the lid slightly larger than a jack but I used some oversized washers to make them stick.
I have no idea what these boxes came from but they appear to be some kind of waterproof electronic device enclosure with two buttons on top
(http://a66.tinypic.com/2qasmyh.jpg)

(http://a65.tinypic.com/2znsg81.jpg)

(http://a67.tinypic.com/2mn28n8.jpg)
Title: Re: Little Gem MKII not working on the BB
Post by: Ben Lyman on September 13, 2016, 01:34:00 PM
I kinda rushed through the assembly so it's not much to look at but it works exactly as the ROG schematic shows, no volume knob, I have the DPDT gain boost switch on both 10uF caps and I added an extra switch to add one 10uF cap for a lighter gain boost.
(http://a65.tinypic.com/huhxev.jpg)
Title: Re: Little Gem MKII not working on the BB
Post by: Ben Lyman on September 14, 2016, 10:04:33 AM
what's the best way to add a volume pot without sacrificing the max output?
Data sheets show a 10k b/t buffer and IC's but there's a loss of maximum volume, I'm guessing because there is always 10k going to ground no matter what.
Title: Re: Little Gem MKII not working on the BB
Post by: Phoenix on September 14, 2016, 10:08:38 AM
Replace the 10k jfet source resistor with the 10k log pot, and connect the 220nF cap to the wiper of the pot instead of the resistor/source junction.
You're correct about why it drops the volume, a 10k load is too much on the jfet source follower.

EDIT:
Could also use a larger value pot like 1MEG after the cap (in the same position you had the 10k pot presumably), as that will not load down the jfet so much, and also has the benefit of not having a DC voltage across it, so you won't have a scratchy volume control.
Title: Re: Little Gem MKII not working on the BB
Post by: Ben Lyman on September 14, 2016, 10:42:33 AM
Thanks Pheonix, sounds like a plan. I had thought about trying a 1M pot but I thought maybe the sweep would be weird. I'll give it a try anyway, then maybe a reverse log and see how it reacts.
I also like your idea of replacing Q1s r with a 10k pot. cool thanks.

What about building a preamp onto the front end with gain, volume, and tone? Can that just be added right onto the buffer input?
Title: Re: Little Gem MKII not working on the BB
Post by: Phoenix on September 14, 2016, 11:12:34 AM
Keep in mind that replacing the 10k source resistor with a volume pot may result in a scratchy volume control due to DC being present on the pot, as I noted in my edit to the previous post. It may or may not be objectionable, try it and see.

The input impedance of the LM386 is typically 50k for both the inverting and non-inverting inputs, you're driving two in parallel, so the load on the pot would be ~25k. That's definitely low enough to mess with the taper of a 1MEG pot. If you used something like a 100k linear pot, that will not further load down the source follower to any significant degree, but matches up nicely with that ~25k load as a tapering resistance to give you an approximately logarithmic response.
This would not effect a 10k pot placed in the jfet source, as the following load (~25k) would not load it enough to taper it, so you would be best to use a log, not linear pot in that position.

If you made a preamp with tone controls and clipping you could omit the buffer, as the preamp itself would be a buffer - it's just there to prevent loading your guitar too much. Other circuits such as the Smokey amp load the guitar on purpose as part of their tone, that is why they don't include the input buffer.
If you put in a passive tone stack at the end of you preamp you may need a buffer in between it and the power amp though, as 25k is a pretty low load impedance - depends what and how you try to implement.
You wouldn't want much gain/output level from a preamp, because the LM386's already have a gain of 20, or 200 with the boost caps engaged. You've already got the 10uF boost capacitors on a switch though, so that will make experimentation easier.

Another option for gain control would be to add a pot (10k would be a good choice) wired as a variable resistor in series with the boost caps. You could do this for both LM386's with a dual gang pot, or you could do it on just one of the LM386's for asymmetrical distortion.

Hope this helps.
Title: Re: Little Gem MKII not working on the BB
Post by: Ben Lyman on September 14, 2016, 12:00:22 PM
Hope this helps.
Tremendously! thanks
Title: Re: Little Gem MKII not working on the BB
Post by: Ben Lyman on September 15, 2016, 01:57:41 AM
I'm learning from experimentation as well as the info from you all.

TS-9 in front, sharing the same daisy chain 9v adapter works with the amp in "un-boosted" mode. Boost switch ruins it.
TS-9 on it's own battery into the amp works in all modes.

So I guess I would have a problem building a preamp into the amp because a 9v 1-Spot won't provide juice for all of it?

I could build another LG MKII without boost switches and build in a preamp because it sounds great with the TS9 on all the time thru the clean setting, gives me 3 controls, gain, level, and tone. It is also way louder when using the TS-9 as the source of tone and volume into the clean LG MKII.

TS9, simple as it is, still seems like too much for me to want to add to the LG MKII though.

The ROG Tonemender looks like a possibility too, but it's just a clean booster with tone controls and I'm not sure how that would work out with the boost switches either:
Tonemender schematic http://www.runoffgroove.com/tonemender.png
Tonemender html http://www.runoffgroove.com/tonemender.html



Title: Re: Little Gem MKII not working on the BB
Post by: Phoenix on September 15, 2016, 02:50:54 AM
TS-9 in front, sharing the same daisy chain 9v adapter works with the amp in "un-boosted" mode. Boost switch ruins it.
TS-9 on it's own battery into the amp works in all modes.
A genuine 1 Spot has more than enough current available to run both, can you describe what you mean by "ruins" it? No output at all? Unpleasant distortion? Choppy/gated sound? Oscillation/squeal?

So I guess I would have a problem building a preamp into the amp because a 9v 1-Spot won't provide juice for all of it?
That's jumping the gun a bit, first we need to figure out what's causing the incompatibility when running both the LtlGemII and TS-9 off the same power supply.
Title: Re: Little Gem MKII not working on the BB
Post by: Ben Lyman on September 15, 2016, 09:38:09 AM
Cool, thanks Pheonix. Unpleasant, gated, choppy is pretty accurate. I'll try to make a demo vid later showing how the boost switches affect the combo.

Edit:
it seems to be sounding fine, go figure.
I must've been doing something wrong late last night, maybe a bad cord or something, I dunno.

The other effect I really like in front of the amp is a sort of Electra type OD that I made a while back
Title: Re: Little Gem MKII not working on the BB
Post by: Ben Lyman on September 16, 2016, 02:13:57 AM
Still working after an hour of loud use today with the TS9 cranking it up. Definitely loud enough to use with my band through my 2x12" cab.

I got out my TDA2822 and checked the data sheet. It is a lot easier to do the bridge thing with a single IC like this, worked first try on the BB, no noises or anything bad.
There is a schematic on the data sheet for bridged mono output. I added a 2n5088 buffer with a 22nF input cap and took out the 4.7uF, replaced the 10k pin 7 to ground with a 10k pot.

Here's a link to the data sheet, the schematic is down at page 4/11:
http://www.st.com/content/ccc/resource/technical/document/datasheet/9e/18/f7/cd/2e/b8/43/62/CD00000134.pdf/files/CD00000134.pdf/jcr:content/translations/en.CD00000134.pdf

Its very loud and clean,it looks to me like theres no way to adjust gain like on the LM386? But I think because of the simplicity, its a good candidate for a preamp.

Where would you adjust tone if you wanted a little more treble out of it? Its kind of bass heavy
Title: Re: Little Gem MKII not working on the BB
Post by: anotherjim on September 16, 2016, 03:59:44 AM
I'd use a coupling cap into the chip. It has 100k input impedance, so in the 4.7nF to  22nF range to cut bass.
Try a big muff tone control. It will be lossy, but can be set to give some mid-scoop similar to a traditional tone stack.
Title: Re: Little Gem MKII not working on the BB
Post by: Ben Lyman on September 16, 2016, 11:39:13 AM
Thanks Jim. I think this is what I have now. For R3, is it possible to use a 100K pot because I think it will increase the volume even more?
And then, put the BMP tone stack right after that going to Pin7?

(http://i63.tinypic.com/a9rayq.png)
Title: Re: Little Gem MKII not working on the BB
Post by: anotherjim on September 16, 2016, 03:50:00 PM
Hi Ben,
You do have a base bias on Q1?
I don't think 10k should be loading it that much, but sure 100k could work (there's 47k pot in the radio application in the data).
Bass limit cap would be between emitter and vol pot. If pot=100k, it would be about 1/10th of the range I suggested above - but it's working into 50k (the pot and chip input impedance in parallel).
To run a tone stack, the front end should have some gain - copy an LPB1 for the front end - that should do it.
(http://fuzzcentral.ssguitar.com/schematics/lpbschem.gif)
Or if you have some J-fet, use a Fetzer stage. Then you can claim tube sound ;)
If true polarity bothers you and the pre-amp is inverting, you only have to swap the wires for the speaker jack to put it right.
Title: Re: Little Gem MKII not working on the BB
Post by: Ben Lyman on September 16, 2016, 04:59:28 PM
Awesome! Thanks again Jim. Yes forgot the bias, I'm using a couple 220K like this, corrected schematic:
(http://i63.tinypic.com/504a2u.png)

I tried it with an A100k vol pot, so far it's good. I also put a 22nF cap at the input to the buffer base, and another 22nF at the emitter to volume pot... is that weird? It just seems to cut some of the lows that way, it's still not perfect but getting close.
Title: Re: Little Gem MKII not working on the BB
Post by: Ben Lyman on September 17, 2016, 03:09:15 AM
It sounds great with the BMP tone stack, LPB-1 as a booster is working. It didn't take kindly to a TS9 or RAT until I put the buffer back on. I really like it now, very loud, tone is bright or dark as needed and sounds great with pedals. I went back to the 10k volume pot just because. What about all those caps? Can C11 4.7uF go? Anything else that should go? Or something missing?
Here's the updated schematic:
(http://i65.tinypic.com/fnsdoi.png)
Title: Re: Little Gem MKII not working on the BB
Post by: anotherjim on September 17, 2016, 05:09:42 AM
Both sides of C11 are ground referenced, so you should be able to lose it, although making it smaller is another opportunity to cut bass overall.

10k volume is loading down the tone control. 100k will give a lot more back. With those tone values, you have high/low shelving control, but no mid scoop. That's fine if it suits your sound.

You shouldn't have to put a buffer in front. It has similar impedance to the LPB, and having no gain, shouldn't be doing anything for you. If anywhere, the buffer can go after the tone. Then you can have 10k volume without loading the tone stack so much.

I would try putting power filtering RC on the pre-amp supply. You don't want supply ripple fed into the preamp, especially not now it has gain. This may also be key to why you're getting interaction with some other pedals. The C doesn't have to be very large - 10uF? and the R can be larger 470R?.
Title: Re: Little Gem MKII not working on the BB
Post by: duck_arse on September 17, 2016, 11:08:45 AM
If true polarity bothers you and the pre-amp is inverting, you only have to swap the wires for the speaker jack to put it right.

now jim, you know that's not fair.
Title: Re: Little Gem MKII not working on the BB
Post by: anotherjim on September 17, 2016, 12:46:30 PM
Stephen, I claim special exemption for bridge amps. Since both ends of the speaker are live outputs. Honestly, I'd never suggest that with single end amps, even though you'd get away with it. Ground on tip is never right!
Title: Re: Little Gem MKII not working on the BB
Post by: Ben Lyman on September 17, 2016, 01:00:33 PM
You guys are talking over my head but I'll figure it out someday.
Anyway, for now I like the idea losing R11, tossing the buffer, changing vol pot back to B100k, so it's just ->booster->tone->amp.

About power filter for the booster, would that be something like this? Imagine this schematic without the buffer on it:
(http://i66.tinypic.com/m9t5w9.png)
Title: Re: Little Gem MKII not working on the BB
Post by: Ben Lyman on September 17, 2016, 03:01:33 PM
This might be a winner. I noticed the need for the buffer seems to somehow be related to the B10K volume pot. When I changed that back to 100K, no buffer needed. Does that make any sense?
I also changed the values of the tone stack to get as close as I could to the BMP and it sounds really good now. The mid scoop in the middle seems to be trying to get close in sound to a blackface Fender maybe.

Let's say I wanted a mid-boost toggle switch, is that even possible?

Still not sure if I understand the power filter, here's what is on the BB now:

(http://i63.tinypic.com/xprpzr.png)

I like how simple it has become, thanks for all the help Jim
Title: Re: Little Gem MKII not working on the BB
Post by: anotherjim on September 17, 2016, 03:54:59 PM
Getting there.
That's the power filtering exactly.
This is now the classic basic guitar amp layout. Preamp - tone stack - power amp.

Off the top of my head, you might use a switch to modify the tone by adding a bypass from C10 to the wiper of the tone pot. Switch in some resistance & reduce the mid scoop. The tone pot will still work to an extent, changed depending on the bypass resistor value. Put a cap in series with that resistance, or a cap only, and the bypass can add a treble boost. I'd call it a boost switch. A centre off switch could do 2 different bypasses.

True polarity - when the signal out of the guitar swings positive, the speaker should push forward. Most amps keep this standard. Use a Rangemaster for example and the speaker is reverse - because the RM is inverting. Nobody minds.
The pre-amp here is inverting, so it isn't true polarity.

Title: Re: Little Gem MKII not working on the BB
Post by: Ben Lyman on September 17, 2016, 04:20:31 PM
...when the signal out of the guitar swings positive, the speaker should push forward....
The pre-amp here is inverting, so it isn't true polarity.

Ahh, this I understand. Very cool info, thanks.

Here's a band pass that I tried on the BB, big mid/volume boost. Am I over thinking this? Seems like there should be an easier way.
EDIT: just changed R2 & C1 in the band pass to 10K and 10uF
(http://i68.tinypic.com/2a4psnk.png)

It is getting there. Sounds good. One weird thing that might just be BB layout and maybe no big deal; but if I share the One-Spot with a pedal (bypassed or not) AND turn the BMP tone below 12noon, mass oscillations ensue. Not gonna worry about it until I have soldered it all up and discover it continues... then I'll just not share my One-Spot anymore.

Now all I need to test out with it is how it handles a power switch, LED indicator and reverse polarity protection, none of which currently reside on the BB. Any thoughts? Try a a 1n5817 in series with +9v? or just a 1n4001 from +9v to ground? Something else?
Title: Re: Little Gem MKII not working on the BB
Post by: anotherjim on September 18, 2016, 09:22:48 AM
The IN5817 in series should be ok. I don't think you'd lose much with a series 1N400x type either. Me, I don't like shunt protection diodes unless I can put them after an RC filter, so the resistor in that limits reverse fault current. You can't have significant resistance in the chip-amp supply, so series protection diode makes more sense.

Are you sure you have 10k & 10u in that filter? That passes all audio. It should just slam off all your signal. Do you mean 10nF?

You're finding out that amps should have their own power supply.
RC filters can only do so much. They won't completely iron out very low frequency supply ripple, such as sag when you whack a big chord. Turning the tone to the bass side showed that up.

When the supply voltage regulator some distance from the load, any resistance in the round trip to the load and back (wire, plug contacts), can prevent the regulator from doing its job properly. It's partially blind  - it doesn't know what voltage the load is actually getting - it only knows what it's sending out from its end. Additionally, devices sharing the same supply must suffer any voltage sags caused by any other device that's current hungry.

Best thing is to have the power supply in the amp. Get the shortest route possible between it and the load.
The small chip amplifiers don't require huge power, but it is more, and more erratic in its nature; than most fx pedals impose. For these amps, a normal power brick supply should be fine, so long as it only powers the amp.

Title: Re: Little Gem MKII not working on the BB
Post by: Ben Lyman on September 18, 2016, 11:29:29 AM
Thanks Jim, I'll have a closer look at that band pass today when I try a series diode for protection.

I was looking at the BMP Analysis:
http://www.electrosmash.com/images/tech/bmp/bmp-tone-control.png
And I was trying to pass everything below the (top side of the) mid scoop, around 1.6KHz,
then shave off everything below the scoop, from about 725Hz and down.

I made all that up as I went along  :P
How can I pass only the lost mids through the tone stack?

Can I buy an internal power supply from an electronics supplier or is that something you guys usually make yourself?
Title: Re: Little Gem MKII not working on the BB
Post by: anotherjim on September 19, 2016, 06:45:44 AM
You've got that right. The BMP is HP & LP in parallel. The area where they overlap causes the scoop. It can be set without overlap so no scoop. HP & LP in series does the opposite - take out above the mid with the LP then then below the mid with the HP.

You might like some tools to find the values. A single pole filter is easy to do by guess and listen. A complex filter has too many variables - you need calculation to find the right sort of values, then confirm with listening.

First, there's Duncan tone stack calculator. That's a free program that needs installing. It has a BMP tone emulation (you can try different values and see on a graph what they do. Most other amp tone stacks are in there too.

Then bookmark this...
http://sim.okawa-denshi.jp/en/Fkeisan.htm
In the filter tools there are simple RC LP & HP. You type in values and get the corner frequency (and a lot of other techno gumbo). It understands 10u, 1M, 47k, 100p etc shorthand, but you must use decimal points for fractional values, so for 2.2n, it doesn't like 2n2 -  it will swear at you in Japanese if you try.

Keep R values >10k <100k if you can. You should rarely have to use a C over 100nF in tone networks.
Title: Re: Little Gem MKII not working on the BB
Post by: Ben Lyman on September 19, 2016, 06:41:18 PM
Thanks again Jim, I like the sound of the values in it right now so I'm going to go ahead and build it like this. I can always make another one if I'm not happy with it; the components, wires and perf are only about a buck anyway, haha.

I bookmarked the link, it looks like it could be very useful, thanks.
There is a site called "learning about electronics" and I used their calculator to come up with those values; they have a bunch of calculators too.

I made a layout, components are going on the perf and should be soldered up by tonight for a trial run before I "temporarily" install it in a recycled 1590BB that's already been many different pedals before this. Ultimately, the layout should easily fit in a 1590B with power switch, LED light, Mid switch, Vol and tone knobs all on top; guitar in, DC, and speaker jacks all on the sides.

Edit:
Jim, you're right about the 10k and 10u. I don't know what I was doing that made that somehow work on the BB, maybe I had a dodgy ground connection or something. Anyway, the perf is all soldered and I had to cut those two out of the mid boost section and now it's all working.

I'm not sure what it makes my mid boost switch now, just a 1n and 220k passing the tone network but it sounds good. Next time I'll try a 2n2 and 100k in the mid boost. I need to study up on the band pass thing a little more because I really just wanted to accentuate that one mid zone, which is what I thought I was doing  :P
Title: Re: Little Gem MKII not working on the BB
Post by: anotherjim on September 20, 2016, 09:49:51 AM
I think your bypass is treble boost -  if it starts low enough, it will reduce the mid notch too.

You may like this if you can run to another pot...
http://www.muzique.com/lab/tone3.htm
Title: Re: Little Gem MKII not working on the BB
Post by: Ben Lyman on September 20, 2016, 11:21:45 AM
Thanks Jim, ya, treble boost or treble bleed I guess. It seems to bump the mids well enough to make my amp a sort of tweed/blackface... or blackface/Marshall... or something  ;D

Here's a new schematic even though I have assembled it already. Notice the 1N4001 shunt... sorry folks  :( the series diode made the amp distort too much, not in a bad way, just too much.
(http://i68.tinypic.com/hun6z4.png)

And here's a vid of it in my 10x repurposed 1590BB.
Strat -> TDA2822M -> 2x12" Eminence Patriots.
Then w/Rat at 1/2 gain.
Title: Re: Little Gem MKII not working on the BB
Post by: anotherjim on September 20, 2016, 04:17:01 PM
Very cool Ben. You should be proud of that. It sounds a lot bigger than a Champ.

Title: Re: Little Gem MKII not working on the BB
Post by: Ben Lyman on September 20, 2016, 06:12:37 PM
Thanks for all your help Jim, much appreciated.
I think I might have figured out what went wrong with my attempted band pass filter, I'll be experimenting some more with it when I can.

I made a new layout that is narrow enough for a 1590A but I'm not sure about squeezing in all the pots, switches and jacks.

I almost forgot to make a vid of the Little Gem MKII, here it is, same set up with strat, 2x12" and then with the Rat. I think the LG MKII could benefit from the booster/tone combo, it's just a little quieter than our TDA2822 creation
Title: Re: Little Gem MKII not working on the BB
Post by: duck_arse on September 21, 2016, 10:59:37 AM
it's always fun listening to your demo's ben, even when they are only running at "half-rat power".

and the "I" setting makes me wonder - if the idea of a bridged amp is to feed the same signal only bigger to the speaker, what does the speaker think/do when it gets clean on one terminal and half-filth on the other? sum? divide? average? something else?
Title: Re: Little Gem MKII not working on the BB
Post by: Ben Lyman on September 21, 2016, 12:13:34 PM
and the "I" setting makes me wonder - if the idea of a bridged amp is to feed the same signal only bigger to the speaker, what does the speaker think/do when it gets clean on one terminal and half-filth on the other? sum? divide? average? something else?
Good question Duck. I did ask about it earlier on in this thread before committing to it. I think the answer was that there shouldn't be any problems and so far it seems fine.
I don't know what it's doing, maybe a subtle tubescreamerish thing with a little bit of clean signal, barely audible and mixed with the gritty signal?
Title: Re: Little Gem MKII not working on the BB
Post by: anotherjim on September 22, 2016, 04:03:28 AM
The speaker only moves the cone in response to the difference in volts across its voice-coil terminals. As long as there's a difference, it moves.
Having 2 different signals across the coil gives an average mix. It might well sound like TS distortion when one is distorted and the other clean. It's a good trick really. The added harmonics from the distorted amp won't get the near x4 power advantage of a bridge amp, since it's only single ended.

Then I think you may be worrying about the "clean" amp's health - it's output is being dragged about by its evil twin with only the speaker impedance in the way. It has the same power handling, it has the same output impedance. It can handle it.
Really, an amp is a variable power supply. When you use an op-amp to buffer a voltage divider for a stronger Vref, it's the same kind of thing. In fact you can use a chip amp as a high power Vref buffer. One amp can be silent (so long as it's output is sitting close the working amps Vref), or doing it's own thing.
Title: Re: Little Gem MKII not working on the BB
Post by: Elijah-Baley on May 06, 2018, 05:23:04 AM
It seems kind of I missed this thread. Nice works, Ben Lyman! ;)

I'd like to ask you something. I think you changed something in the last version of your Little Gem amp. Did you post the final schematic?
And... if you had the layout to show, will be great! ;D

Thank you.