dod 308 oscillation?

[deadastronaut]

hi guys, i built this 308 which works, passes sound etc, does its thing...

however it has a mad squeeling oscillation like a fast rising and falling lfo when powered up.  (think uk police siren its not as loud as the guitar signal, but its enough to piss you off.)

i have shielded the input, but its still doing it.... ive tested all other pedals on the same psu no issue....

so i know 100% its just this build....

only change from the standard schematic is 1MA gain pot instead of 500k .

the only other difference i see in my layout is the opamp is powered directly by 9v...
wheras the vb goes via a 100r.....hmmmmm........
i even added another cap across the +/- which sllightly tamed it....but not cure it.

its strange as it was quiet on the breadboard, with the same psu. (just usual expected squeel not lfo type osc when max gained..)

any thoughts. ?  cheers rob .

[deadastronaut]

update: out of curiosity i re breadboarded it,

removing the 100uf power filter cap almost removes all oscillation. not all...

wtf....

1  4.53
2  0.66
3  0.73
4  0
5  8.40
6  0
7  8.40
8  9.19

my vb is 4.04

edit: tested with battery, perfect...no problems.  so wtf.....

[anotherjim]

The unused opamp wiring is suspicious. Layout has output to +in instead of -in.
Also, that method  of tying off the unused amp is flawed since you have an op-amp that the input volts should not equal any supply voltage. It should be pins 7 & 6 tied together only and pin 5 to the Vref 22k divider.

[deadastronaut]

hi jim, cheers, yeah i thought the unused opamp looked moody on the schemo.......

i noticed i left out the 10uf vb cap to ground like a dumbass , which i have just remedied....

and voil'a all osc gone.....

[deadastronaut]

noticed all the 250, 308 schematics do the same with the unused opamp..........strange.

why not just incorporate it , hell even have the volume on it...

5 to vb, 6/7 loop...out...done....weird. 

[anotherjim]

The thing about not tying an op-amp input to a supply pin voltage is probably something you may get away with depending on the chip type. I don't think it is ever destructive but might increase current consumption a bit.
Most single supply or CMOS opamps, you can tie an input to the neg supply without issue. I suppose the designs could have been originally intended for a single supply chip that was only available in a dual.
In case someone wants to try other chips types, it's just safer to tie the pin to Vb.

Curious to know if this Dist+ variant has anything special soundwise?

[deadastronaut]

its just a classic overdrive really, been messing around on breadboard with various other clipping options.

in the feedback loop, etc   hard and soft. leds/diodes....pretty cool for a super simple od...

just did a test recording in cubase with drums, and bass guitar. , sounds ok....quite usable.