Sources of hum.

[JoeBob]

Well, I finally finished off the power suply for my amp and set it all up. But when I turned it on to my horror there was a humming sound, no trouble though, I didn't expect everything to work first time anyways.

The hum is there when volume is turned to minimum on the preamp, and as I turn up the volume the hum's volume doesn't change. When music's playing softly I can hear the hum, but when turned up it's inaudible. I've heard amps that hum when turned up loud, but that's not my problem here and I don't know where it comes from...

It hums on all channels of the amp, but some are worse or better then others. Any ideas of what could cause it would be apreciated, thanks...

Also, I'd like to add. The hum isn't just one tone, it comes out of the woofers as well as the mids, so I'm gathering it isn't a 60Hz ground loop.

[JoeBob]

I just noticed something strange, when I disconnect the inputs to my amp it becomes completely silent, no humming at all. But when I test the pre-amp alone it's perfectly clear.

So I turned my amp on, and plugged in the pre-amp, as soon as I plugged it in it started humming, so I thought maybe it was ground after all. But then I disconnected the pre-amp and it still buzzed. The thing is when I've got nothing connected to the inputs of my amp it's clean, but even where there's RCA cables connected (although they're connected to nothing), there's a hum.

How can just cables cause the hum? Is it because they're $2 RCA cables? Because I've never had this problem with them and other equipment. Or is it ground of some sort when the cables are attached.

It's rather puzzling for me...

Well, I just tried it with some thick expensive RCA cable, and it didn't sound any better, actually, it sounded slightly worse (but then aain, maybe it was my imagination).

[Sud]

It sound like (pun noise is being induced in the RCA cables from power cables.

Try moving the RCA cables around while they are connected to the amp and the pre-amp - perhaps away from the power leads.

[JoeBob]

I tried that, but nothing changes

[subwo1]

Try shorting the tip of the RCA cable to the shield to see if that changes it.

[rljones]

it might not be the cables (you could always try a different pair). my guess is a ground loop problem: possibly you have too many ground leads, or your signal ground leads are maybe connected too close to the power supply filter caps' ground point.

you would think all ground leads are the same (being zero potential and all), but they're not, especaially if a lot of current is flowing in them as it is adjacent to the filtering caps. i don't know about you, but i find laying out the ground is one of the more difficult aspects in circuit building.

[arnach]

What amp did you build?

Sounds like you have a ground-loop problem. My first guess would be that some part of the circuit is not properly connected to ground. Check the circuit grounds.

Also, when I built a SOZ, i had a major hum problem. Turned out it went away completely after I connected the ground of the RCAs attached to the heatsinks of the transistors, whereupon I then found the proper ground point and redid the wiring.

Also if you have a low-current circuit you can use the old finger conductor/capacitor technique.

-- Aaron

[PassFan]

I agree with Arnach. I built a Borbely preamp one time and as soon as I tried it it hummed. I went back and discovered a ground jumper from signal ground to chassis ground had been left out and as soon as I connected it no more hum. A 60hz power supply hum will also be present at the octaves. That is probably why you notice it in your speakers at different freq. . 60 hz, 120hz, 240hz,480hz,960hz, 1920hz are all harmonics of the base freq. 1st, 2nd, 3rd, 4th, 5th respectively. But, I think if you plug in an rca cable and just jumper signal ground to chassis ground it will probably go away. Good Luck.

P.S.- Try doing a search on star grounding. There's probably a lot of info here already.

[JoeBob]

Well, it's a five channel LM3886 amp, with the power-suply in a different box (so it's not that the ground is too close), 2 x 750VA, and 122000uF (so the finger trick won't do).

I tried shorting the signal ground to the chasis, but it didn't change anything. But my ground inside is connected to the chasis, should I disconnect that? Thanks for the responses...

[rljones]

I would again look at how you've routed your ground circuitry.

Another possibility is AC phasing. Try swithing the AC leads on the power supply chassis (you could have a 'bucking' type problem with interaction between you othe components).

Another related source of hum, is ground loop between various component chassis. Try lifting the ground lead off of one component power cord to see if this helps (leaving it this way is not best, but at least you can see if this is the source of the problem). A better technique is to try putting something like a 2 to 5 ohms resistor rated at 3 to 5W between your ground and chassis (ie, isolating the chassis). I'm suspicious of this as the source since the power supply is in a separate box and you may have a ground loop between the two chassis (and/or some other component).