distortion noise

[BrepoBrepo]

Hi everyone

This site has been a great help so far, so thanks for that. But now I've got a problem that I can't really find a solution to.

I'm building a distortion pedal, I've built kits in the past, but this one is the first that I'm doing myself on perf board. It's a variation of a DOD250 circuit, with more bass, higher gain and a tone knob. Now, when I plug this into an amp, it makes A LOT of noise, like a very aggressive sounding hum. The signal come through, the sound itself is good and all the controls work fine.



This is the schematic, I don't think there's anything wrong with it, because it worked just fine on breadboard, without noise.

Here are photos and videos of the project, with the sound. https://photos.app.goo.gl/NETtZwbHqrrEQdB37

As you can see on the videos the noise increases with gain and when I touch the gain knob, but not other parts of the circuit. It is present with our witout guitar plugged in.

I have replaced the IC and the gain pot, and I've checked all the voltages and the continuity of all the ground connections.

Anyone have any idea what could cause this?

Thanks

Tijs

[Mark Hammer]

Make sure that everything which should be grounded IS connected to ground.

Do yourself a favour and make the Volume pot 100k instead of 10k.  You will hear a big increase in maximum volume.  Also, put a 22-33pf cap in the feedback loop of the op-amp to help remove some of the hiss.

[amz-fx]

That is not hum, it is oscillation. Try putting 10pF from pin 2 to pin 6 to see if that tames it.

The feedback loop has a LOT of resistance in it. You would be better off if you scaled it down.

regards, Jack

[BrepoBrepo]

Thanks guys, I'll try the capacitor and reducing the feedback resistance.

As far as the volume pot, there's plenty of volume right now, but I'll give it a go.

Let's see how it goes!

[iainpunk]

That is not hum, it is oscillation. Try putting 10pF from pin 2 to pin 6 to see if that tames it.

The feedback loop has a LOT of resistance in it. You would be better off if you scaled it down.

regards, Jack

i echo this and i'd like to add: put a 100nF ceramic or film cap on pin 4 and 7 of the opamp, this can also tame some oscillations in my experience and its also generally the proper way to do it.

cheers, Iain

[antonis]

By double echoing, I should call it "high pitch motorboating"..   

Additionally to the above well said, fit a 470μF cap right after R1 (and before V+)..

P.S.
Bipolar op-amps, like 741, don't like 1M5 feedback resistors..
edit: Jack already mentioned it..

[11-90-an]

Welcome, @BrepoBrepo, naw, 5 replies (1 is OP's) in this thread, and nobody else said welcome? 

And to triple echo, Antonis suggestion was to tack a 470uF from V+ to ground, just to make it clear..

Also, Iaian's 100nF cap, place it as close to pin 4 and 7 as possible for it to work best...

HTH

[antonis]

And to triple echo, Antonis suggestion was to tack a 470uF from V+ to ground, just to make it clear..

Clear now..?? 

[BrepoBrepo]

Thanks for all the very helpful and clear suggestions, and for the warm welcome.

I'm pretty new to this and I obviously have a lot to learn. I think I'll start here:
https://www.electronics-tutorials.ws/opamp/opamp_1.html

Tijs

[iainpunk]

i just realized i forgot this:
welcome to the forum!

tijs - sounds Nederlands/Belgisch

cheers, Iain