Seeking Grounding Help with the Lyman Amp (Improved Little Gem MkII)

[schrectacular]

After attempting to build the Little Gem MkII and running into problems, I came across https://www.diystompboxes.com/smfforum/index.php?topic=114995. I was able to follow the thread of the conversation and build a working "Lyman Amp" (but it was certainly made harder by the fact that Ben's pictures no longer exist). Here's my schematic and a pic of the breadboard:




There are a couple differences with my current setup - I put a 500k volume pot instead of 10k. I don't have a 1M5 resistor so I used two 3M in parallel and grounded all the preamp stuff together, then over to the main ground rail. I haven't added "F" in the schematic - when I try to put it between the preamp and ground rail I don't get sound. The ground rails are linked in the back.

I believe I have a grounding issue - whenever I add the gain caps I get a sort of buzzy oscillation. Anotherjim on the Lyman thread gives grounding instructions, but I'm too newb to interpret:

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) .

I don't understand this and I'm sure I'm not doing what he's saying, and I'm pretty sure it will fix the issue I'm having. I'm hoping perhaps he or some other friendly internet strangers may help point me in the right direction. I feel very close with this - I think once I get the gain working I'll put a switch to one 10u gain cap and a 10k pot to the other and then put it on a board and in a box. But before I do I'd like to remove the foolish bits.

Any advice appreciated!

[11-90-an]

The 1M5 value dousn't really matter much, you can use 1M5 or more...
I think (don't take my word for it) that the "F" portion should be from 9v to GND, but placed near the jfet....
Then after moving "F" you can tie the 1M5, 10k resistor, and 10k pot to GND...

Putting the source/emitter of any transistor through a cap (without any other way to ground) brings the volume very very veryyy low that you can only hear it when you crank your amp to max (given you were making a guitar pedal... )

[anotherjim]

"F" may be a hum breaker. The 10R goes to ground and the 0.1u cap goes across it in parallel -  you have them drawn in series which means the preamp doesn't have a ground. It may not be necessary so just connect "F" to 0v for now.

[schrectacular]

OK, I think I deciphered anotherjim's electronic cymraeg correctly - I gathered all the preamp grounds into a single rail, then ran that to my main ground rail via a 10ohm resistor and 100nf capacitor in parallel.

I can now plug capacitors across 1 and 8 of the IC and I don't get a squeal! I think I have success! Thank you!

Here's the current iteration:


Does anything else look "wrong" with this?

I also changed the volume pot to 500k and I feel like it works better, the wipe is pretty useful the whole way through and the amp is louder than with the 10k.

Experimenting around this morning I was able to couch in a simple pot/capacitor tone control in between C1 and P1, but these values don't do much for me:


I might just leave it alone for now and maybe put a tone control in version 2. But am I putting it in the right place?
preamp -> tone control -> volume/level -> amp

Thanks again for the guidance.

[antonis]

Can't figure out "these values don't do much for me"..

If you want to deal with a 500k pot, scale properly C1 & R1 values..
(or am I missing something totally different..??)

P.S.
As Jim correctly told you about 10R/100nF series/shunt combination, the former is a Zobel network (R5/C7) where the later (R3/C2) is a noise (hum) decoupling network..
(which, as also correctly said, might not be essential but it can't do any harm.. - 0.1% misbias for an FET buffer should be considered extraordinarily negligible..)

[duck_arse]

welcome to the forum, schrectacular.

on your circuit diagram, you show 1μF and 10μF caps on the "gain" pins. did you use differing values, or both the same?

[edited for mu's.]

[schrectacular]

Antonis - Even with a 100k pot there, the sweep doesn't actually do much tone modification - there is some, but not a lot. Since just putting something in didn't work immediately I think I'll shelve tone for next time, otherwise I'll never get anything in a box. It sounds great right now anyway.

Duck - Yes, I have different capacitor values on the two gain controls. Still iterating but the version I have now sounds nice and I think it's what I'll put on a board - it has side CCW lug of 10k pot, a 47u capacitor, and center lug of pot in series between 1 and 8 of one IC and a toggle switch and 10u capacitor on between 1 and 8 of the other IC. Sounds pretty nice - flip the switch for easy boost, or turn the knob for more control.

Edit: Oh, and thanks for the welcome, nice to be here! Thanks so much for the time looking at my beginners problems. There is just so much to learn...

[pinkjimiphoton]

i will call or text ben and ask him to check in on this thread

[Ben Lyman]

I just built another one of these for my niece last xmas. can anybody provide a free image hosting service to me? I have nothing but bad luck trying to post images on this forum. I'll add my pics, my perf layout is tight and super quiet

[pinkjimiphoton]

ask aron for your own folder here on the website. then you can upload to that, and link directly to your posts... i think that's how dino fixed all his old stuff when botosucket screwed us all over a couple years back.

[antonis]

botosucket

It took me more than 10 minutes to clarify that none of us is drunk, bro...

[GibsonGM]

Imgur is pretty decent and free.

[schrectacular]

I just built another one of these for my niece last xmas. can anybody provide a free image hosting service to me? I have nothing but bad luck trying to post images on this forum. I'll add my pics, my perf layout is tight and super quiet

Duuuude what I have now absolutely wails even though it's a bit noisy on the breadboard. I have a Jensen 6" speaker and it gets loud enough to make the wife come slam the door!

[PRR]

....I have nothing but bad luck trying to post images on this forum.

The "Add image" gives bad luck??

A man sat on his front step crying. Another man came along and said, "Cheer up. Things could be worse."
He took heed of those wise words and cheered up. Things got worse.

[Ben Lyman]

The "Add image" gives bad luck??

Well, frost my head and call me cake! Is this new? Why hasn't anybody mentioned this witchcraft before? I'll get right on it

[Ben Lyman]

Duuuude what I have now absolutely wails even though it's a bit noisy on the breadboard. I have a Jensen 6" speaker and it gets loud enough to make the wife come slam the door!

Great! when you box it up and shorten those ground connections, it will be even better. I run a 2x12" cab, loud enough to make my drummer ask me to turn down occasionally.

So, in that thread there were 2 different amps (among several experiments) sorry I'm not organized enough to for sure where everything is in my computer. Here's what I think was the final schematic for the single chip (TDA2222) amp with input booster, tone control and midrange toggle.




Here's the other, a modified Little Gem II. I don't know if I ever built this one besides on my breadboard.

[schrectacular]

Thanks so much for posting this, much appreciated. I still get some odd squeal sometimes with the gain but I think it's just picking up the AM radio. I live near a tower and whenever I touch a knob on something on my breadboard I can hear it pretty clearly.

When I put this on a board I'll share my final design, although I don't think it will differ much from the above.

[schrectacular]

Ok I finally got it all put together. It's not perfect but it works and I'm happy enough. This is the first amp I've ever fully built so there ya go I guess I'm a new man. It was also the first time I've used veroboard. Definitely had a mirror image mixup which luckily was resolved by bending all the legs on the IC chips all the way around.



I ended up with a 2.2uf capacitor on a toggle and a 22uf capacitor on a pot for gain. The distortion itself I think is rather pleasing, but my mistakes (outlined below) are literally amplified and it's just more fun playing on clean with a pedal.

I probably did something very silly like put my ground and power lines right next to each other or maybe my wires are too long or maybe having everything go through that hole is a problem... but it definitely picks up AM radio pretty well. It's ok clean but turning up the gain brings it out. I also still notice some high-pitched, squeaky-stuttery warble in the sustain, much moreso with the gain on. I probably also should have put some bass filtering on given the size of these speakers; I also get a little bit of buzz in the bass.

Here's a demo:

But overall, totally a success. It sounds pretty great at about 2/3 volume with my Barber Direct Drive pugged in (sorry, didn't demo that Oh and sorry for the ghastly musicianship). I glued up the hole in the case that I drilled, and putting the faceplate back on you get that Bose case payoff and it sound so much more full.

Thanks again for the help, couldn't have done it without what I've learned lurking.

Link to album:
https://imgur.com/a/nFEbWOG

[anotherjim]

Gotta love recycling.
An easy thing to try for the radio trouble is to fit a small value capacitor to ground on the signal input -  just solder across the input jack lugs. Cap value not too important, but not larger than 1nF or it will be cutting treble from the guitar all the time.
The other thing you can do is have the speaker wire pairs twisted together for as much of their length that you can manage - chip amps can pick up RF from those wires and the amp circuit feeds it back to its input and rectifies it making it an AM radio receiver.

[schrectacular]

Thanks Jim! I will make both those updates promptly and let you know!

However, in the meantime I have another quandary - I have an open-source sound computer called a "Zynthian". Part of what it can do is real time DSP (MOD effects stuff). When I plug that into this amp I get an awful pop-pop-pop sound like a capacitor filling and releasing:

The Zynthian people say this is likely due to a power source issue. And if I plug it into my little Blackstar Fly practice amp, sure enough it works no problem. Any idea what foolish mistake I made? Is it ok to just ground the guitar in directly to the 9v receptacle next to it?

Oh and I think the hip kids these days have a new term for this: "upcycling". Being a thrift store geek for years it's pretty interesting to see the generations of technology file through the store... like right now these ipod devices (anything with that earlier generation long receptacle) is basically trash. But there were SO many made. And now is about the time people are just junking them. Anyway, it's good for me because I get a relatively quality piece of equipment conveniently and cheaply enough that I really don't care if I junk it.