Sources of hum.

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.
 
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).
 
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

Member
2001-04-07 10:03 pm
USA
Hey

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
 
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.
 
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...
 
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).
 
Well, I'll try the inverting the AC leads, but as for the ground of the psu, it's got two diodes, a 100nF cap and a 10R resistor between the ground and the chasis ground. I tried disconnecting the RCA ground from my pre-amp but then it starts "hellicopetering". When you said "Try lifting the ground lead off of one component power cord", you meant disconnecting the ground from the wall? Wouldn't that not be safe?
 
It's not safe as general practice, as it doesn't protect you against an AC wire touching the chassis and electrocuting somebody. But as an experiment to find the source of the hum it should be okay.
The ground loop between two pieces of equipment is the audio ground between the components, and the AC power line ground from both components back to the circuit breaker panel.
JoeBob, have you got the AC power line ground connected directly to chassis ground, and then chassis ground connected via the diodes/cap/resistor to the power supply ground? This should be enough to break the AC ground loop.
But from what you said earlier the hum is still there if the input cables are plugged in but not going anywhere. This is confusing.
 
Joe Bob:
Your rca jacks that you used for your chassis, do they have the insulating wafers to insulate the ground lug from the chassis or does the ground lug come into contact with the chassis. This being a multi channel amp if the lug comes into contact with the chassis and then goes from signal ground to chassis ground elswhere as well this would create a lot of potential loops. Have you looked for dc on your output? Is it a 60 hz ac hum or could it be dc offset on the outputs? It does seem weird that it will do it with a bare cable plugged in, and it's not easy when it's not right in front of us. Don't give up though eventually you'll get it and we'll all gain a bit.:confused:
 

Jamie F

Member
2001-12-15 12:33 am
UK
JoeBob

I did warn you! If you remember, I told you I was having similar problems last year with 10 LM3886's.

I haven't finally fixed the problem - my project has lost a little momentum - but I have spent quite a lot of time (and money) on this.

Like you, I get buzzing. It is worse on some outputs. I have star grounding, signal ground is generally isolated from the chassis, only connecting to safety earth at the power amp. Scoping the outputs, I have seen up to 50mV pk-pk of intermittent rubbish, including power supply frequency spikes (100/120Hz). Can't say if this is what I am hearing. Unfortunately this sort of problem isn't easily measurable.

I was convinced the problems were due to the number of interconnects. I tried a single signal return to the preamp, but this made matters worse (no common mode cancellation). More significantly, I have implemented ground cancelled interconnects between my pre- and power amps. This has still not solved the problem.

I have verified that the problem isn't due to power supply ripple. I tried a supply with worse ripple and didn't get the problem.

The thing that has made the biggest difference is removing the transformer from my pre-amp chassis. I have now removed the transformer from my power amp chassis too, but haven't tested since.

Interested to know how you get on.

See Doug Self's site for the best description on noise sources and ground loops.

Jamie

P.S. I also meant to say - the one thing that is troubling me about my amp layout is that many amps have the star point at the signal input connectors - I do not. My signal connections now go direct to each amp module, then I have a thick ground connection to the star point. I don't see why this should be a problem, but it is a little different, so I am suspicious.
 
Ya, I de remember you warning me, that's why I made it easy to take apart and reassemble (which I have a few times)... I replaced all the 22 gauge waire carying both the ground and the signal in my preamp with solid 14 gauge wire, and I also did the same with my power amp (obviously the outputs were large wire to begin with), and it helped, still there, but less so.

I've got star ground, but it's not on the inputs, but a specific place on my board. And my transformers aren't in with my power-amp, nor in my preamp (it uses a plug pack). Strange we get a simmilar problem with rather different configurations...

Since I rewired it the buzzing's gone down greatly, even with an RCA cable connected (before that made it intolerable), but now when I connect my preamp it gets louder. Ugh, I never thought grounding would be such a pain...