Problem solving is fun, especially when PA out is involved. However much you know your kit it will still manage to throw a few red herrings your way. After months of puzzling why my bass makes a funny buzzing sound I seem to have got somewhere.
I noticed yesterday it stops if I unplug the compressor - one would assume it's the compressor then (this was red herring one). Except it also stops it I touch anything metal and grounded, including the strings and flight case, and nothing else buzzes with a compressor plugged in.
The buzz was happening between 2k and 6k, far to wide to notch out. And the lack of a massive spike at 50hZ suggested it wasn't getting signal dumped into it from the mains.
So there I am thinking my compressor has a grounding issue when I adjusted the tone knob, and the buzzing stopped. A tone knob has a capacitor going to ground - so it must do some decoupling or something right. This was red herring two.
Thoughts about capacitive loading of opamp inputs happened - maybe an edge case sent the opamps in the compressor into oscillation?
So I tried ground lifting my DI, which attenuated it by about 3dB. Given that the compressor and guitar were now galvanically isolated it couldn't be a complex interplay between the tone cap and the opamps in the compressor, especially given that there's a "high-quality" xenyx preamp in the middle.
So it made sense that the bass was the problem. I turned the pickups down and the buzz went away. Given that the ground was still connected ground issues now seemed unlikely. Waving the bass around changed the sound of the buzz - must surely be an antenna now.
Puzzled where the signal was transmitted from, I put the guitar on it's hanger, looked up at my PAR16s and remembered how noisy the cheap Ikea dimmer is if it's next to my head when I sleep.
I turned the dimmer off and the buzz disappeared!
It may be red herring number three, but my bedroom lighting setup involves a dimmer dimming a 10m extension lead travelling round the perimeter of my room.
This was probably the transmitter I was looking for!
Red herring 2: debunked
It makes sense that turning the pickups down stops the buzz, as it was being induced in either the strings, and then transmitted to the pickups, or the pickups themselves directly. But why did the tone pot affect it?
Tone pots shunt some of the high frequencies to ground by using a capacitor - effectively a very simple low pass filter. The pot is actually wired as a variable resistor, which acts to prevent all the hf escaping - as it's value is lowered, more current can flow through it, meaning more hf is removed.
Read that again: "hf is removed", which would include any buzz at 4k.
Red herring 1: debunked
The final question is how was the compressor invovled. After much puzzling, I realised how obvious it was, and how stupid I'd been.
I built my insert cables as a balanced jack to XLR and XLR to 2 off mono jack. This makes them much more versatile and means they can be extended with a Mic cable when the rack's just a bit too far from the mixer.
So to disconnect the compressor's signal I unplugged the XLR. Unfortunately what this did was cut the signal path through the mixer - when a plug's in the insert jack the signal leaves through the tip (usually), goes off the wherever the cable goes. In my case this was now air! The signal then comes back from somewhere (air again here) and goes to the rest of the channel strip. In short, all I'd done was unplug the buzz from the speakers, and then tried to work out why I couldn't hear it.
Trouble shooting is easy - a methodical approach makes it fairly easy to find the problem. Until you think too much and fall into the traps the equipment sets for you that is.
Protip: check the bleeding obvious before you break out the multimeter/oscilloscope!
And to end, Captain Jean-Luc Pickard after working out the real reason he couldn't hear the CD through FOH:
We've all been there captain...