HELP: Distortion that oscilates with high gain

[Dutchie]

Hey Guys,

I'm progressing on building my distortion with amz/bigmuff tonestack,
it all works but i got 1 major problem , when i max the gain it oscilates like crazy.....how can i prevent it from doing that? it produces high and low pitch oscilations. when the guitar volume is turned down to a certain point  it stops. and with some tonestack settings it gets less or more.

at lower gain settings it is fine, i think that when the opamp starts clipping it triggers it.

anybody know how to solve this??

Thanks

[Dutchie]

Whooohooo i figured it out the highfreq oscilation, seems like a cap of 100p parallel tot the feedback resitor does the trick.

to bad when you look things over the head, but eh thats life hehe

hmmm but the low end rumble is still there ...kind of a ground loop sound,
can you create a groundloop within a pedal? this also kicks in when the gain is at max

anyone?

[Dutchie]

There must be some guru that can help....i'm really stuck here

[markusw]

Do you use a battery or a power supply? in the latter case you might add a low pass filter to the power plugs (e.g. 50 Ohm + 100 µF).

[Dutchie]

Nope i use just a plain 9v battery
thanks anyways.

[Paul Marossy]

I think in your case, you might be trying to set the gain too high.

[Dutchie]

hmm, dont think thats a problem, the gain stage resistors are the same as of a mxr distortion+

[RDV]

Add a 10k resistor between your divider network and pin 3 and see what happens, Paul's right.

RDV

[Dutchie]

how is that 10k resistor gonna lower the gain?

i tried it, didnt work  

the hum is there even at minimum gain, when i touch the aluminium box it produces a click.

when i put it on bypass with a dpdt and dial in maximum gain i hear oscilations ( high )comming though the amp low on volume ,  i dont get that, it is true bypass

it seems all very strange to me? what am i missing here, the sound of the dist. is otherwise very nice.

AAARRRGGGHHHH  :shock:

[Dutchie]

i'm making progress hehe

the oscilation when in bypass i solves by putting a C of 10uF from Vref to GND on the 1M devider network.

still is a riddle to me why it could be heard when in bypass.

the "groundloop" kind of hum is still there and its loud

[Dutchie]

uhm not 10uF...dulls out the sound way too much....33n seems better

[free electron]

the hum is there even at minimum gain, when i touch the aluminium box it produces a click.

is the box grounded?
Regards
Peter

[Peter Snowberg]

the hum is there even at minimum gain, when i touch the aluminium box it produces a click.

is the box grounded?
Regards
Peter

I'm wondering if the whole amp is grounded? The hum and clicking upon touching is the symptom you get when you lack a ground all together.

Best of luck!
-Peter

[RDV]

If you change R1 & R2 to 100k & add another 100k between them and P3 should work well to make the bias right. The hum may be a ground problem though.

Regards

RDV

[puretube]

bypass osc. will go away if you use the "grounded input" bypass switching to be found somewhere here:
http://www.geofex.com/

[Dutchie]

Hmmm how can an amp not be grounded at all?

if there is nocommon ground there is no refference, hence no potenial difference, so no signal to the amp.

and there is sound comming out of the speaker.

i use a fender princeton 112plus for testing, works fine with the guitar straight in.

i'll check with other commercial pedals to see if they have the same problem, but i cant remember ever having this prob.

Thanks for all the help guys

[Dutchie]

Yep....seems like the amp is the problem  

put in a big muff and there was the hum again...

Thanks again for the info...helped big time!!