Strange Electric Mistress noise...

[blueduck577]

So lately, I've noticed that my modded reissue DEM has been making some strange noises lately.  They kind of sound like odd chirping noises.  At first, I thought it was clock signal, but after I added a delay time knob (15k pot in series with clock trim), I can set it at an extreme setting to hear the real clock signal, and the other noises are still present.  The only other mods I've done are changing a few capacitors (C6, C20, remove C3) to give it better high-frequency response.

The strange thing is, I found that when I have my Boss SD-1 on in front of the mistress, the noise disappears.  But when I bypass the SD-1, or just plug the guitar directly into the DEM, the noise returns.  Maybe an impedance issue?  I have no idea whatsoever.

I made a youtube video demonstrating the problem, it's much easier to watch the video than for me to explain it:
http://www.youtube.com/watch?v=6REUmJWxi7w

Also, the schematic:

[PRR]

> remove C3

Why??

Stock, response goes to 10KHz. That's more than you want on guitar.

C3 is probably there to suppress odd chirps when the input is not firmly terminated.

Put C3 back. If you really can't stand the insignificant loss, go 100pFd or 68pFd, but don't leave it out.

[blueduck577]

> remove C3

Why??

I found pictures online of the guts of a reissue DEM that had one of the popular series of mods done to it, so I just went along.

Strangely enough, I unplugged my telecaster and plugged in my strat and the chirping seems to have vanished... strange.

[Dave W]

Required reading:
http://www.geofex.com/circuits/what_are_all_those_parts_for.htm

[12Bass]

Sounds like aliasing noise, perhaps induced by oscillation from an input which is not terminated properly.

FWIW, I made some low pass modifications to my SAD1024A A/DA flanger clone and it makes similar noises with high feedback levels.  Aliasing can also be heard if it receives a 10 - 15 kHz sine wave signal, but I can't hear anything bad with normal music.  IMO, the increased openness in the highs is worth it.