gargletron distorting (in a bad way)

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Hi...

I built Tim E.'s Gargletron a while ago.

I had some distortion when playing anything other than a very gentle strum... I changed the input cap because it was a little murky sounding, and that brightened it up a bit, and I thought it reduced the distortion, but I still can't play a full on strum without it distorting. It is a very quacky, harsh clipping/ripping/noise sound, not at all nice sounding.

Any ideas on what could be modified about this circuit to reduce this? Try lower value input caps? Anything else? Any tips on how to troubleshoot where the distortion is actually occurring in the circuit is occurring? To my ears it is happening in the filter part, as it doesn't sound like the usual fuzz/transistor overdrive sound.

[CynicalMan]

The silicon diodes look a little suspicious... 

[Cliff Schecht]

You could increase the MPSA18's emitter resistance a bit, say 4.7k or so, to drop the input stages gain. Not quite sure what the diodes are there for but they'll most definitely rurn on and clip if your signal gets too big.

[fuzzo]

That schema looks intersting .

saturation can be caused by the high resistor values in the feedback loop of AOP ? AOP boosted by the MPSA transistor ?

[Top Top]

You could increase the MPSA18's emitter resistance a bit, say 4.7k or so, to drop the input stages gain. Not quite sure what the diodes are there for but they'll most definitely rurn on and clip if your signal gets too big.

I am a total noob when it comes to actual design of circuits, but I wonder if the diodes are meant to clip a little when it gets overdriven, to add some harmonic content to the filtered sound? I have no idea what I am talking about or if it would actually work that way.

I will say that the distortion is very bad, very different from what I associate with diode clipping. I don't think it was intended and it isn't present in the sound file that accompanies the circuit on his site.

Do you think it is worth de-soldering the diodes from the ground to see what happens? Would I need to run a wire straight to ground in their place?

One other thing to mention is that I don't notice this distortion at all when I have a distorted signal going in to the filter (ie from a fuzz box output). Only clean. When I put a distorted signal into it, it sounds good. Weird? Maybe the consistency of the distortion makes it sound fine.

Fuzzo: what is AOP?

[fuzzo]

Aop is Op-amp , I don't know if AOP is a correct word in english for IC. But , actually, I don't think the big value resistor in the feedback loops is the issue, Morley and other Wah based op-amp use that kind of values and they don't produce distortion by itself.

ne other thing to mention is that I don't notice this distortion at all when I have a distorted signal going in to the filter (ie from a fuzz box output). Only clean. When I put a distorted signal into it, it sounds good. Weird? Maybe the consistency of the distortion makes it sound fine.

Normal beaucse your signal is already satured you don't hear the saturation of this circuit.

[Top Top]

I'm going to give this another shot... Last night I experimented with different transistors, and found that a 2N2222, which I think is a lower gain than MPSA18, created less distortion, quieter in general... but it still does distort if I play more than a gentle strum.

So as for the two possibilities presented here - the diodes and the resistors in the filter feedback loop...

1) Could the diodes just be removed? Would I need to run that trace to ground in their place?

2) Any suggestions on other resistor values to try in the feedback loop? Will it affect the filter's cutoff/resonant frequency?

[fuzzo]

Maybe remove the transistor input gain stage, that can boost to saturation the AOPs (plus you've the diodes clipping) . Enter into them directely , maybe with a buffered signal before.

If you still have distortion, try putting a 1 or 2M in each feedback loop and see what good values gives a clean sound. If you've a low signal compared to the bypassed signal, add a final gain stage to increase the signal level.

they're just suppositions.