Why does my OD circuit become noisy when I set the volume control down?

[Transistor-Transistor]

So I'm designing an overdrive pedal. It's basically a mix of a Tube Screamer and a Big Muff Pi. It sounds very nice, except that when I turn down the volume pot my circuit gets very very noisy. Is there a way to fix this? It's just a simple 100k Log pot acting like a voltage divider. I've tried putting it before the output buffer and after the output buffer and have gotten the same result. Thanks in advance!

[antonis]

Is there a way to fix this?

Improve S/N ratio..

P.S.
More seriously, plz post a schematic..

[ElectricDruid]

Antonis is right. Without a schematic, we're just guessing, sorry.

Post a schematic of what you think you've built and some photos of what you've got so we can comapre and we might be able to help more.

[Transistor-Transistor]

Here's the schematic! I'm still working on it. Diodes are 1N34. Gain(a) and (b) is a 100k dual pot.

[ElectricDruid]

C3/100uF should be from R5 to Ground to decouple the Vref, not from the top to the bottom, where it does no good except provide a (probably tiny) amount of power supply filtering.

R1 should be larger. There's no reason to use such a small value for the connection to the Vref supply.

There are other problems with the DC bias through the rest of the circuit, I think, but that schematic is a bit tangled and I can't figure it out right now.

HTH,
Tom

[GibsonGM]

Need cap before the tone stack? DC on the volume and tone pots? And of course then would need biasing for the output buffer.   

Is it in an enclosure (that is grounded), or on the breadboard??

[ElectricDruid]

Need cap before the tone stack? DC on the volume and tone pots? And of course then would need biasing for the output buffer.   

Is it in an enclosure (that is grounded), or on the breadboard??

Yeah, exactly, all that sort of stuff.

[Transistor-Transistor]

Is it in an enclosure (that is grounded), or on the breadboard??

It's on the breadboard, but I know its not breadboard noise because the noise is gone when at full volume. I guess it really isn't biased correctly. I should probably work on that. Thank you guys so much for seeing what I missed!

[antonis]

Delete T1 and bias IC1A via a 100k - 220k resistor..
(in case noise persists, place a low value cap across bias resistor..)

[Rob Strand]

Here's the schematic! I'm still working on it. Diodes are 1N34. Gain(a) and (b) is a 100k dual po

There's no bias on the base of T2. You need to add an input cap base bias resistor to VB.  Copy off the T1 circuit.
(GibsonGM also mentioned it.)

To avoid scratchy pot issues you need a cap after IC1B or before the 100k volume pot.  You don't want any DC getting to the volume pot, or R11 for that matter.

A short-cut solution to both of those issues would be to connect the ground side of the 100k volume pot and ground side of  R11 to VB instead of ground.   That solution doesn't need any extra caps or resistors.

[PRR]

> There's no bias on the base of T2.

There is bias, because that tone control leaks DC. A "designed" approach would have a blocking cap or a more holistic scheme.

[Rob Strand]

There is bias, because that tone control leaks DC. A "designed" approach would have a blocking cap or a more holistic scheme.

When the volume is dialed down the bias will cut-off.  (I agree, not *none* entirely as there is some bias when the volume control is dialed up.)

[PRR]

It still does not match the report- more "noise" when Vol down. It may not be wired the way it is reported.

[antonis]

I'd say about C1/R1 input HPF corner frequency (about 3.4kHz..!!) but I realized that R effective resistance is 10k + (68k//68k) due to VB capacitor absence..
(dunno if 770hZ should be considered OK for input HPF..)

[Rob Strand]

It still does not match the report- more "noise" when Vol down. It may not be wired the way it is reported

It's not clear what the nature of the noise is.  I thought he might be experiencing the pot scratching.  He also said he moved the pot after the buffer but if he put the volume pot on the emitter (instead of after the 3u3) there's still DC on the pot.

If he did move the pot to after the 3u3 cap and it's getting noise with the volume pot turned down, that would be odd because it's essentially shorting the input of the amp.   It could mean bad wiring on the sockets or ground but then you would expect the noise to be there for all settings of the volume control.

It doesn't quite add up.

Perhaps the OP can clarify?

[Transistor-Transistor]

So by noise I mean that when I turn down the volume pot, I get a lot of buzz. It sounds like the same buzz of a very high gain pedal except that when the volume is all the way up there is no buzz. I'm starting to think its a grounding issue as well because if I were to pull the wire from the output so that nothing goes to the amp, I get the exact same noise as when the volume is all the way down.  Changing the other knobs does not affect the character of the noise

[GibsonGM]

Could you describe the ruckus?  (Breakfast Club reference)   Is it a 60Hz/120Hz buzz?   Is there anything nearby such as a desk lamp, computer that would be causing interference?   Can you record the buzz for us?

There ARE circumstances where lowering a pot a bit can change a resonant frequency inside a circuit, and then things like oscillation or RF interference can become audible.   Check those grounds first.

Some ppl shield the thing temporarily with a sheet of tin foil that is grounded.

[Rob Strand]

So by noise I mean that when I turn down the volume pot, I get a lot of buzz. It sounds like the same buzz of a very high gain pedal except that when the volume is all the way up there is no buzz. I'm starting to think its a grounding issue as well because if I were to pull the wire from the output so that nothing goes to the amp, I get the exact same noise as when the volume is all the way down.  Changing the other knobs does not affect the character of the noise

For your original schematic with the volume before the transistor, if you dial down the volume far enough the transistor cuts-off and there's only the 10k there to shunt the signal.   It's possible to get noise like that but 10k should be low enough to shunt most noise away.   In your second experiment where you moved the volume control to the output the transistor should be on and no such effect occurs.   Also when the volume control is set to zero the volume pot should short the amp input to ground and there should be no buzz.  So it's looking like the buzz problem is elsewhere and ground wiring is high on the list.

(Buzz problem aside variable base-bias via the volume pot probably should be fixed using one of the methods mentioned earlier.)

[Transistor-Transistor]

So I did the biasing and I must have done something very wrong because now my circuit sounds awful. Its squealing and phasing. I took out the volume pot in this image. Could be a dying battery.

[Transistor-Transistor]

The battery is not dying. I'm going to deconstruct the circuit and rebuild it from the beginning and see if I have the problem again.