High Pitched Squeal from 2 MXR Micro Amps

[Lotetem]

Hi All,

I'd like to ask someone smarter than me to verify if I'm right or wrong.

I built 2 MXR microamps, they sound great but when both are on max gain and feeding directly one to the other I get a really high pitched squeal. It changes slightly in pitch when I switch pickups.

If the guitar is unplugged there is only normal 50Hz (EU) hum, there isn't any if I turn down the tone knob about halfway. No hum or squeal on bypass. The power source is a 9V battery.

My guess: The guitar (pickups?) acts like a radio antenna and that noise is always there but inaudible, but due to the huge gain it gets amplified to audible levels. When I turn the tone knob It filters it out, and I can not change that without changing the input cap on the Micro AMP such a way that it would significantly change the tone. So the solution is just not running both of them on 11.

Did I miss something? Is my thinking correct?

[antonis]

Hi & Welcome..

Each MXR micro amp has a max gain of 22 (27dB) so both of them in series have an overall gain of 484 (54dB), without applying any form of negative feedback (either local or overall)..

If you are happy enough with the amount of overall gain, try to raise the value of both feedback caps..
(the ones between IC output and inverting input..)

For a first try, double their value (100pF or bigger) and see if squealing is reduced or even vanished..

[iainpunk]

what happens it you lower the amp's volume? does it persist?
it might indeed be the caps Antonis mentions, but it might also be microphonics of pickups, or the pickups are antennae like you suspect.
3 problems with 3 somewhat different solutions.

cheers

[Lotetem]

Hi & Welcome..

Each MXR micro amp has a max gain of 22 (27dB) so both of them in series have an overall gain of 484 (54dB), without applying any form of negative feedback (either local or overall)..

If you are happy enough with the amount of overall gain, try to raise the value of both feedback caps..
(the ones between IC output and inverting input..)

For a first try, double their value (100pF or bigger) and see if squealing is reduced or even vanished..

Hi Antonis,

Just tried it, it did not work, there is no change in pitch or volume.

[antonis]

ΟΚ..

Are squealing effects boxed..??
(grounded metal enclosure..)

[Lotetem]

ΟΚ..

Are squealing effects boxed..??
(grounded metal enclosure..)

Yes, they are. I checked the grounding as well.

what happens it you lower the amp's volume? does it persist?
it might indeed be the caps Antonis mentions, but it might also be microphonics of pickups, or the pickups are antennae like you suspect.
3 problems with 3 somewhat different solutions.

cheers

Hi Iainpunk,

If I lower the volume on the amp it still persists, if I lower on the guitar it vanishes at around halfway.

Guys, I'm super thankful so far, I also made a video, I probably listened to the squeal far too much... I am kinda enjoying it. But I still want it to go if possible.

[iainpunk]

i was thinking about it while riding my bike, and it came to me that i had a similar problem with a Mosfet distortion i once build, which i found out that the bypass switch and offboard wiring had enough capacity to couple signal back into the input. this capacitive coupling and the impedance you change with your guitar knobs and pickup choice create a high pass filter for the positive feed back of the signal.
this makes sense, because the impedance of your guitar lowers when you turn down the tone knob, or the volume knob beyond half way.
i remedied this problem with a buffer BEFORE the bypass, and gave up the true bypass. looking back on it, i think shielding the wiring as much as possible would be a great place to start, if doing that doesn't work as desired, i'd try the buffer option.

cheers

[Lotetem]

i was thinking about it while riding my bike, and it came to me that i had a similar problem with a Mosfet distortion i once build, which i found out that the bypass switch and offboard wiring had enough capacity to couple signal back into the input. this capacitive coupling and the impedance you change with your guitar knobs and pickup choice create a high pass filter for the positive feed back of the signal.
this makes sense, because the impedance of your guitar lowers when you turn down the tone knob, or the volume knob beyond half way.
i remedied this problem with a buffer BEFORE the bypass, and gave up the true bypass. looking back on it, i think shielding the wiring as much as possible would be a great place to start, if doing that doesn't work as desired, i'd try the buffer option.

cheers

U basically described my messy build, thank you so much. I have a lot of wires, not just a lot, but beefy as well, it was definitely a bad design choice. It did not produce this on a breadboard so I was surprised. But now I see why and how adding the wires changed the circuit.

Kudos

[iainpunk]

when it comes to wires, shorter is better.
you could use grounded aluminium foil to de-couple the wires, just wrap the wires, starting with input path and then output path until it doesn't whine anymore.
keep it in place with electrical tape

cheers

[anotherjim]

If you have 3pdt footswitches, use the middle switch lugs for the LED switch and the outer ones for the bypass. This can separate the signal wires enough to stop positive feedback via the capacitance between adjacent switch contacts.