Super simple mixer self oscillating?

Anybody else had this issue? My mixer works fine but when nothing is plugged in it starts to self oscillate, you can change the pitch by turning the knobs, it stops at certain points, then comes back, weird. I followed the stripboard layout perfectly, the only thing i have added is an attenuator on the output (100k lin with unused lug grounded) I cant think the attenuator is causing the issue. I’ve also checked all my grounds/connections for continuity, even re-flowed the entire board. The only thing I can think of is a bad 072, but i dont have any spares to hand at the moment. I’ve built circuits way more complicated than this, I cant believe i cant work this out, lol. Any ideas much appreciated.

Have you terminated your unused opamp.


Good suggestion but this circuit uses both op amps, I got excited for a second XD Screen Shot 2021-01-14 at 22.29.03

What is the oscillating frequency? Is it a humming (50/60Hz)?
And what did you plug on the output? Any long cable?


The frequency varies with the pot position, it goes from lfo clicks all the way to inaudible! I’m pretty sure its not a mains hum, my supply is well isolated and i’ve never had any 50hz hum before. I used a long cable to test it first time and then mounted it in my case and patched it up with 50cm patch leads, same problem. I’m stumped.

ah, the mains hum frequencies. :slight_smile:

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I think i might pull an 072 from a known good module and see if thats the problem, hopefully it wont pop it!

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Indeed that’s weird! Can we have some pictures?
And if you toggle all your mute switches to the 2-3 position, do you have the same issue? Maybe try to ground the muted inputs by flipping the switch: pin 1 to input jack, pin 2 to pot, and pin 3 to ground.
Another thing: are you sure that the non-inverting inputs of the opamps are on ground, and that you have a symmetrical power supply (+/-12V)?

Hi, I didnt use the mute switches, kept it super simple and went by Sams stripboard layout. The input jack goes straight to the CCW pot lug, it comes out the wiper to the mixer input stage. The CW lug is grounded. Power supply is good.

have you connect your panel to the GND ?

I’m a complete donut, I started poking round with the continuity tester again, I didnt ground the board to the panel or any of the jacks/pots…! I can’t believe I missed that. The first time I did a continuity test I must have had an input or output connected to my test rig, which gave a false reading. Rookie mistake!

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The thing about rookie mistakes is you get better at them as the years go by :laughing:


glad it works !

(morrrrreee carrrac …ter)