Kettle pot zendrive clone

[Harkonnen]

Hi all,

I am in the process of building my first enclosed pedal, a Zendrive clone as per http://revolutiondeux.blogspot.com.es/2008/05/zendrive-project-at-fsborg.html

I have breadboarded the thing and it works faultlessly. The problem came when I implemented the circuit on a veroboard and mounted it on a metallic pedal. Everything works as expected, except with gain/voice/tone controls to max, when  a high pitched kettle pot whistle like sound (I'm guessing feedback) comes in at approximately 4 Khz. The sound does not occur with no input connected, and is not present on the breadboard version. Using a signal generator and an oscilloscope I have tested that the outputs of both circuits are the same, except for the high pitched whistle which kicks in with the pedal version. It would be very helpful if any of you could provide any leads as to what may be going wrong.

Some information as per aron's post on debugging:

- Although the schematic is the same as presented in the link at the beginning of the post, the veroboard layout is custom. If it helps I can post it as well.
- The circuit is negative ground. I have used the pedal case to make ground connections throughout the circuit, and sometimes potentiometer backs when it was convenient wire-wise.
- The behavior is the same whether on battery or on DC adapter.
- The high pitched noise only occurs with a guitar plugged. In fact, it does not happen with the signal generator. With the breadboard version, it only happens if I touch the jack tips, and only occasionally.
- There is another forum post relating a similar issue, but in this case the high pitched sound only occurred with no input connected. The pedal worked fine with the guitar plugged in.
- I have initially not measured voltages across the circuit, as it seems to be working fine except for the whistle. I will do it if you think it could help in any way, though.
- Edit: if it helps in any way, I can record a youtube video that illustrates the issue. Maybe hearing the whistle would be helpful.


Many thanks for reading, and thanks in advance for your help!

[Harkonnen]

Well, auto-update. It is a high-frequency feedback issue that, for some reason, doesn't manifest on the breadboard as severely as on the enclosed version.

I forgot to mention, I *did* modify one "small" detail in the original circuit, as I am using an NE5532 instead of an AD712. I guess maybe they are more prone to parasitic oscillations? A temporary solution has been to change RB in the original schematic from 1K to 2.2K, to reduce the maximum gain of the first amplification stage. The results are satisfying, but the problem still manifests using pickups without a treble filtering circuit (e.g. the bridge resistance in a Standard Stratocaster).

In any case, if you have any suggestion that could improve the issue I will be grateful to hear it.

Thanks!

[midwayfair]

Does turning down your guitar volume a tiny bit help?

[Harkonnen]

Yes, turning down the guitar volume makes it stop. Also turning down the tone control for the selected pickup.

When turning down gain, voice or tone on the pedal the issue also disappears.

Turning down the volume of the pedal or the amp does nothing.