Velleman Voice Changer into "pedal" grounding issue?

[benrossm]

helllo, forgive me if im somehow in the wrong section of the forum but im basically a newbie to DIY and building electronics so i need some guidance.

ive been trying to find some help with this through the velleman forum with no luck but i think this might just been a simple electronics grounding issue so im hoping someone here may be able to help so here goes,

i built the MK171 Voice Changer kit (schematics below) and i swapped out the speaker with an audio out jack and the mic with an audio in. (im modding it so i can send from a mixer FX loop. it all works well and good when i use my iphone or a dynamic mic as the input signal and headphones as an out but once i plug it into my FX chain, (in this case just a a standard line mixer FX send/return) i get a nasty buzz (the effects still work but no signal is intelligible besides the horrible buzz. im assuming this is a grounding problem when its looped into a chain because if at least one device (input or output) is battery powered, it works fine but as soon as its two devices (line out of mixer and speakers for example)  it starts its screeching fit. . is there something im missing? thanks for the help!

*another thing ive done is replaced the battery power with DC wall wart input, the same thing happens with both power sources..

please help! 

http://bluestampengineering.com/wp-content/uploads/2015/06/VCSchematic3.png

[samhay]

Try adding an input capacitor. Looks like there will be DC on the input jack, which a cap in series with the input (i.e. between input jack and circuit input) will block.

[duck_arse]

and welcome to the forum, benrossm.

the numbers are a bit squinty, but R3 and R6 appear to be a bias supply for the original electret microphone. if you just lift R6 (4k7), the 100nf cap (I've given up with the numbers, too small) should serve as the samhay specified cap, and away you go, high voice/low voice.

[rutabaga bob]

Welcome aboard!

[benrossm]

and welcome to the forum, benrossm.

the numbers are a bit squinty, but R3 and R6 appear to be a bias supply for the original electret microphone. if you just lift R6 (4k7), the 100nf cap (I've given up with the numbers, too small) should serve as the samhay specified cap, and away you go, high voice/low voice.

thanks for the help everyone i really appreciate it. excuse my ignorance, but when you say lift R6 you mean bypass/jump it or just get rid of it?

btw here is a bigger look at the schematic ignore the notations..
http://www.captaincredible.com/abc/wp-content/uploads/2012/02/Bended-velleman1.png

[benrossm]

ok fellas i tried both suggestions to no avail... any other ideas?

[anotherjim]

Completely remove R6. R3 & C5 also not needed, but won't do any harm left.
You might remove R10 to lower the gain of the 386 amp. x20 ought to be enough?
R9 & C4 are not needed without a speaker & could make the 386 unstable.

[PRR]

I think you want to avoid the LM386 stage entirely. The changer core is unity gain. The changer chip allows a large gain in front so it will take a mike. You do not need gain of 20-200 on top of all that.

[wordstep]

like this. should build a virtual GND buffer before the LM386. then use a gain stage or use the LM386 (I did see commercial product use a LM386 as a line driver).

Another easy way is  to put a 1:1 audio transformer in some point.

just my 2 cents.

[benrossm]

oh man it works now. thanks for all the help everyone. dunno what did it but i went through and just got rid of most of the amp circuit and its perfect now!