Poor RF rejection

[6267]

I'm still working on the "big" amp, but I've noticed that it has pretty poor RF rejection (not that the local radio station is playing by the book--I can get most of the commercial equipment to pick it up too at high gain ). If I plug it into the computer to test it with mp3s while I mess with knobs, I can faintly hear talk radio (it's AM, I forget the callsign & frequency). I can also hear the computer monitor...moving a window causes very distinct low amplitude noise. Other than that, the amp sounds pretty good. I have to note that this is a 2x12 chassis, with the power amp on the "floor" and the controls on the roof. Moving wires has no appreciable effect on the RF performance. It's dead quiet without cables or computer plugged in, and also quiet "terminated" by a guitar. However, it'll do it with just the cable sometimes too...this being a "stereo" amp, one side is grid leak and one side Ck biased. Ck biased side has a grid-stopper. Grid leak resistor in the grid leak side is carbon film. Is this a computer sucks problem, an amp sucks problem, or a radio station sucks problem? Anyone have any experience with this?

[stokes]

Are the wires that connect the controls at the top of the amp to the amp at the bottom shielded?If not,using good quality shielded wire should help.

[6267]

Yes, shielded wires are used, with grounding pretty well thought out (as good as the chassis I scrounged will let me...seems ok with just a guitar). I think so far I am going to blame the computer for the noise, because I can get it on the other amps when they're plugged in. It's just weird, because it's not so present on headphones. I can't believe a computer that was probably intended to work on AV is so damn noisy! I suppose it could be oxidized contacts in the audio jacks acting like detector diodes or something. I guess it's time to buy a scope. I think the reason the noise is louder on my tube gear is that all my tube gear is guitar stuff, with input Z of like 500k-5M, and the solid state stuff is mostly PA with probably a 250k max input z.

[Enzo]

AMp chassis have an open side. Is your open side covered with a metal shield grounded to the chassis? That is what the awful aluminum foil inside Fender amp cabs is all about, or the metal plate stapled to the ceiling in some other amp.

[Steve Conner]

You may well be forming a ground loop that picks up RF and computer noise, when you plug it into your computer.

I've been into home recording with computers for years, and they are horrid noisy beasts. They have tens of amps of noisy DC flowing across the motherboard and internal wiring, and the IR drop due to this appears between the AC inlet ground and your soundcard's ground. The CPU's current draw modulates according to how hard it's working, and that's why you can hear noises when you move windows around.

The noise isn't RF, it's actually audio frequency to start with, so no amount of ferrites and RF stoppers will help. The only thing I ever found that stopped this ground loop problem was a pro soundcard with balanced I/O, or better still digital I/O to an external converter rack.