High pitched hiss with certain transistor+resistor pairs (bazz fuss)

[mark2]

I have a simple bazz fuss I work with often, and it's never given me problems with a variety of Q1 + R1 pairs.

Recently I had some built with an MMBTA13 + 10K R1 and it makes a high pitched squeal like this: https://soundcloud.com/mark-a-stratman/recording-1
Possibly related, the gain knob (B10K) scratches as it travels  and has a sudden jump in gain near the end.

Prior to this I had breadboarded with MPSA13 + 10k and it worked great.

To rule out the PCB itself I removed those components and replaced them with a MMBT3904 + 510k and the noise is gone.

Any ideas why this particular transistor is resulting in noise that many others didn't? (including MMBT6427LT1G which is also a darlington)
e.g. is it just higher gain and therefore more susceptible to power supply noise that the 3904 didn't amplify?

[mark2]

Coincidentally it's almost the same frequency as the noise coming from my truck's wheels... so bonus points if you recognize that too

[antonis]

Place a 1nF cap between Collector & Base (across D1)..
You'll experience significant gain lowering at high frequencies so you can lower cap value till squeal beginning threshold..

P.S.
You schematic calls for 510k for R1 (not 10k)

[mark2]

Place a 1nF cap between Collector & Base (across D1)..

Thanks. Why would that be the case for the MMBTA13 but now the 3904 or 6427?

Edit: And I just tried that and it cuts down the volume of the noise quite a bit (still there), but also muffles the sound of the effect more than I'd like.

P.S.
You schematic calls for 510k for R1 (not 10k)

Sorry for the confusion: That was for the 3904. I didn't draw up a separate schematic for this Q1+R1 pair since it's otherwise the same.  The MPSA13 (so presumably? the MBTA13) doesn't need nearly as high a value, and 10k seems to work.

[antonis]

MMBTA13 is a high gain Darlington pair BJT..

Alternatively to Miller cap (the one suggested above) you can try a 100k - 470k resistor from Base to GND..

[antonis]

Edit: And I just tried that and it cuts down the volume of the noise quite a bit (still there), but also muffles the sound of the effect more than I'd like.

See my edit above..

[Rob Strand]

The wiring inductance of gain pot wiring forms an oscillator with the buffer.

You need to add 100R to 1K in series with the base, then as a secondary measure add 47pF to 100pF to ground at the input side of the added resistor.

(The same problem that occurs with the SHO.)

Any ideas why this particular transistor is resulting in noise that many others didn't?

Not everyone has the gain pot.   Once wiring of the pot comes into the equation you will get all sorts of varied results.

[dennism]

You've already received good advice on the pedal, so I'll focus on your truck.   Your brake pad wear indicator is scraping against the rotor to let you know it's time for new pads.    And likely new rotors since most rotors these days are designed to be "disposable" and not worth machining back to flatness like they were back in the day.

[Rob Strand]

This thread is pretty much the same thing,

https://www.diystompboxes.com/smfforum/index.php?topic=122182.0

In that thread the small cap is at the base side of the added resistor.