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santonel 4th April 2012 06:30 PM

High pitched squeal from DIY guitar amp
 
Hey everyone, I'm still pretty much a n00b when it comes to electronics so forgive me if this is an easy problem with a well known solution.

As a first project I've designed and built a little battery powered practice guitar amp for myself and it seems to be oscillating. Sort of a thin, high pitched, tearing screech that varies in frequency with the sound from the guitar. The frequency also varies when I touch the controls. However, if touch the input jack or even touch the PCB the noise just disappears and it sounds great.

I'll post a schematic later when I have some more free time but basically it consists of a couple JFET (all J201s) gain stages (a simple common source followed by a "mu-amp" stage), followed by a Big Muff tone control into an LM386 output stage. It's non-inverting so I'm guessing that's conducive to oscillation as the output is in phase with the input. What's the standard for guitar amps anyway? Should they be inverting or non-inverting?

It's not in any sort of chassis yet, I've just been bench testing it. I built it on a generic protoboard PCB and I've heard that the capacitance between the traces can cause problems. I don't know much about layout but I put as much space between the gain stages and the LM386 as I could.

I just got it all put together late last night so I haven't had much time to fiddle around with it yet. I thought I'd ask here though in the hopes someone could save me the trouble.

Thanks in advance!

firechief 4th April 2012 06:42 PM

Post pictures and a schematic. Be thinking about some kind of metal box to put it in. Also it might be that you can reduce the gain just a bit and solve the problem.

Nigel Goodwin 4th April 2012 08:27 PM

Quote:

Originally Posted by santonel (Post 2972829)
What's the standard for guitar amps anyway? Should they be inverting or non-inverting?

It makes no difference at all.

santonel 6th April 2012 02:55 AM

1 Attachment(s)
Schematic:

epicyclic 6th April 2012 09:34 AM

Change R13 to 150R i believe you have to much positive feedback around LM386

santonel 19th April 2012 04:37 PM

I fixed it! Turns out I had the input jack wired backwards! D'oh! :o

Nemuikuma 20th April 2012 01:43 AM

Quote:

Originally Posted by santonel (Post 2992054)
I fixed it! Turns out I had the input jack wired backwards! D'oh! :o

it happens to me time to time, usually it is such a trivial error, nothing serious.

The interesting part when you stare at the PCB ad the schematics for hours and you don't seem to find the problem, then you lay back for a few minutes, and you realize what have you done wrong.

The morale of the story is that it is really important to take brakes, even if you are really obsessed with your project.


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