I just spent the whole day – daytime day, I quit at dinner – rewiring my studio to better isolate the AC power lines from everything else, because it’s pretty clear at this point that our AC power is just scary with RFI, and the new high-gain preamp is AC all the way to its motherboard, unlike everything else, which is DC power once you get past the adaptors.
Result? All this did NOT A FUCKING THING for the high-gain preamp. In fact, it’s worse tonight. Enough that were I trying to use it, it’d be difficult and I’d have to be stupidly careful with cable placement, or something, and hope I get lucky.
At that point I thought I’d achieved nothing at all, and had in fact fucked it up somehow – which didn’t make any sense but that’s never stopped me before – until then I poked at my primary input board for semi-unrelated reasons, and HOLY HOPPING CHRIST ON A POGO STICK the difference.
SO MUCH QUIETER so much quieter
Even at max gain, zero input shows up as zero on my main input board. It’s not actually zero – I can hear it – but it’s too far down to light the metres, which puts it at somewhere around -75db.
So at least I didn’t fuck things up. That is a thing which is good to know. I actually achieved my design goal for the rewiring, to a surprising – nay, astonishing – degree of success!
And it did absolutely fuck all for the problem.
I’m still sure AC Power is a problem though, because if I plug the preamp straight into the wall, the RF pick up is REALLY LOUD; through a computer power strip (which filters, some) it’s less; through the battery-backup and power strip, less still. I suppose it’s possible there’s another reason for that, but I don’t know what it would be.
eta: I took apart the preamp, convinced myself of a few problems, maybe improved it, maybe the RFI just calmed down a bit on its own. Who knows? Here’s a recording of a suboptimal cable position after the resoldering. That’s boosted all to hell, of course; at normal recording levels, it’s inaudible, but I hate that it can be found. It’s also still cable-position-dependent. I tested literally every cable I have; this was the most RF-resistant one. If I have it on the right side of the mic stand, it sounds like this. On the left: no audible signal.
eta2: I opened up the case again and twisted the internal side of the AC power lead (9v AC) as tightly as I thought I could get away with. The test afterwards contained far lower RFI than previous tests, but that’s not necessarily meaningful – it’s a different time of day. (All my previous daytime tests have been mid-afternoon.)
That said, for the first time, there was no sign at all of KUOW 94.9, which may be a first. And I didn’t hear BBC World Service in there either. I was picking up KIRO, dimly – but I don’t know whether I was picking up the AM or FM station. Most of the time they have separate programming, but they simulcast during Seahawks handegg, from pregame through post, and that’s what was on. And I had to dig at it to get that much, which is actually good – if I can’t stop it entirely, I can at least make it something you really have to work to find.
So, yeah. Ongoing. I think that twisting the internal AC power lead bumped it down another level. But I’m not sure. It gives me some hope that the RFI chokes I’ve ordered might help.