I'm experimenting yet again with the following audio mixing circuit:

enter image description here

On the scope with signal gen as source, everything looks great. When hooked up to the input of a mixer with a laptop as the source, I get the following audio noise (recorded with no audio playing):


The ticking and staticky blips/bloops are coming from the circuit, the slight background hum is not. Is this noise caused by opamp oscillations?? Or what is causing it?

What I've tried: I get the same results plugged into the mixer's unbalanced input or when adding an extra network to balance the output and using the mixer's balanced input. I've tried removing the virtual ground and using a dual +-6V supply, the noise is still there. It seems to be coming from the NE5532 opamps, adding more buffers increases the noise. Initially the diff amps were biased by the virtual ground, and the NE5532's were biased using a voltage divider. Scrapping the voltage divider and connecting everything to virtual ground reduced the noise by about half, but it's still present. The entire setup is on a breadboard, but I've tried gluing an NE5532 on it's back and wiring it that way and the noise is still present.

Update: After trying different scopes and getting some external feedback I was able to see what I'm dealing with at the output that is causing the noise. Here are the scope results:

30 Hz Spikes

Zoomed In:

27-28MHz Waves

The spikes are pretty much dead on 30Hz (with 60Hz power grid) and zooming in they're a relatively constant frequency of around 27-28MHz, but sometimes measured around 21-22MHz. Any idea what might be the cause? Some type of antenna pick-up?

I've tried unplugging or cutting power to everything in the vicinity and beyond, changing outlets, adding/removing filtering surge protectors, the only thing that seemed to affect it was when the light switch, which switches the adjacent outlet on/off was switched on, the 30Hz peaks increased significantly, whether anything was plugged into that outlet or not. But nothing eliminated the spikes.

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    \$\begingroup\$ Most likely your ground is not robust. Since you are using a breadboard, you probably have your ground as a single sequence (called a daisy-chain) of of wires connecting your ground points together. You should use thicker wires and connect your grounds into a grid. The ideal is a ground plane, but this is usually only possible with a printed circuit board. \$\endgroup\$ – WhatRoughBeast Nov 6 '17 at 2:28
  • \$\begingroup\$ @Beast:Interestingly I had the exact same problem with a similar circuit about which I've posted a few times that used transformers as opposed to the balanced line receiver ICs.After over a week of trying every suggestion I received and more, I started randomly placing capacitors around the circuit, and a strategically placed 1nF capacitor from signal to ground completely eliminated all traces of noise. That was with a similar if not messier breadboard layout than I have currently. I initially thought it was the resulting LP filter that fixed it, but placing an LP filter as shown doesn't help. \$\endgroup\$ – User7251 Nov 6 '17 at 15:54
  • \$\begingroup\$ Does the fact that that solution worked give you any information as to what (ground related, etc.) may have been causing it? \$\endgroup\$ – User7251 Nov 6 '17 at 15:58
  • \$\begingroup\$ Yeah. You show an admirable policy of 1 decoupling cap per IC. However, your simple ground symbol does not correspond to reality, as there are all sorts of parasitic resistances and inductances in your breadboard. In the example you cite, you got lucky and found a spot near a critical IC, where the capacitor actually managed to have enough effect to counteract the effects of your setup. That is, the cap actually worked the way you thought it would. My belief is that this is not happening in your current setup. \$\endgroup\$ – WhatRoughBeast Nov 6 '17 at 20:14
  • \$\begingroup\$ @Beast: While I think you're right that this somehow involves grounding, I wish I were as convinced as you are that this is a ground robustness issue. I would really like to have more of an understanding of specifically where and why it's happening. I went so far as to deadbug some of the components on the last system, completely removing the breadboard and was still struggling with the noise, I really feel like there is some opamp quirk that is playing a role here as well. If I completely pull power from the system, the noise continues until cap C1 or C17 discharges completely, which can ... \$\endgroup\$ – User7251 Nov 7 '17 at 7:34

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