Noise Issues in original (?) overdrive

[suryabeep]

Hi everyone,
I built a pcb for this original (it's just the gain stage from an ocd with a bax stack) overdrive that I built. For some reason there is a high-pitched whine when I turn the gain pot up over about 60%. There are filtering caps on both the main 9V supply and the 4.5V supply, the caps are placed right next to the 9V input wire and all 9V supply goes through those caps first before going to any other part of the circuit. However, the caps are not placed near the IC +/- pins. Could this be the reason for the gain pot behavior?

[GibsonGM]

Maybe. You could try a smaller value cap across the pins, like 100n or something, see what happens!   I'm assuming it only does that when used with a wall wart, and not with a battery?  And the wall wart is OK with your other pedals?

[amptramp]

You have back to back diodes going to Vbias which also feeds the input of the unused op amp and the bias for the input and output op amp stages.  One trick would be to feed the diodes to the output of the unused op amp so current fed into Vbias from the diodes does not contaminate the Vbias going to the input or output stage.

Also, a small capacitance of about 50 pF from the junction of R2 and R3 should prevent radio frequencies from getting in if that is part of your problem.

I concur with GibsonGM that a ceramic or film capacitor of 10 to 100 nF across the op amp power leads may get rid of a lot of this.

[Sophie]

Hi
I also agree that fitting ceramic filtering caps is the first move.
People often assume that because they have slapped a fairly big electrolytic across the power rails that any frequencies above a certain very low point must surely be filtered out but that is sometimes not the case.
No component is absolutely 100% pure especially capacitors, they also have a minute inductance.  It can happen that it is enough in high gain circuits to prevent very high frequencies from finding their way to ground.

If you look out for it, you will notice that where well designed circuits have a large electrolytic across the rails, there is very often a much smaller cap in parallel with it and that is why.   :-)

Lacking an oscilloscope, you might still be able to track it down by signal tracing, using just your guitar amplifier. 
To do this, make the contact to the earth of the effect from your jack lead earth-sleeve using a length of wire with a couple of crocodile clips on or similar.  Then you can CAREFULLY probe around the circuit with the tip of the jack-plug and see if the interference is present or louder in one place than another - although it might be that it's washing across the entire circuit anyway.

If you do go signal tracing, be extremely careful!  Don't short anything to anything else with your probe.  Keep the amp volume down on first pass too!
If you happen to put the probe on a +9v rail, the amp will just see that as a valid 9-volt transient and faithfully reproduce it as a huge clunk.

Two other things - screening.  I've had high-gain gadgets pick up noise just because I've taken the lid off - when you say you built this unit, is it in the box yet or just a circuit on the workbench?
Secondly...... sometimes trouble is caused by using op-amps that are just too damned good for the job!  A circuit that will behave perfectly with 741's in it will suddenly get just this sort of problem if you replace the rather pedestrian old 741's with a brace of super-chips that still have oodles of gain at high frequencies where a lesser chip will be rolling off.
The cure in that case is to flatten out the gain yourself above useful frequencies by putting some negative feedback across the op-amp with a tiny cap.

[antonis]

A 100R resistor in series with both Vcc & Vee supply would do no harm..

[GibsonGM]

If you make an audio probe as Sophie suggests, it is also a good idea to place a cap in series with the tip probe to block DC. 

Value is not that critical (.68u, 1u, 10u...etc.), this is not for audio quality testing (small cap = more 'tinny' sound), but make sure it is rated for the highest voltage you'll be probing...https://www.diystompboxes.com/pedals/debug.html