A quick question about humming / buzzing

[markiemrboo]

I've just bolted all my gainclone stuff to the case, and I appear to be getting a very very quiet hum / buzz from the woofer. I find this very strange, as there was NO humming or buzzing when it was just sitting loose on the floor, and I have changed nothing!

When I short the input it's still dead silent, with no input connected (I know this tends to make oscillation but it was only for a second) the hum is a lower frequency than with the inputs connected.

Could this be the input wire picking up noise from power cable? The input wire right now is unshielded I don't think it's a grounding issue (in the sense of a ground loop), as there was no humming before bolting it down and it's still silent with the input shorted. Would I be right about this?

Thanks,

[richie00boy]

Sounds plausible. I caught myself out once with a humming amp, turned out to be the input lead picking up.

[AndrewT]

Hi,
four things for you to look at.

did you connect the safety earth to the audio ground?

does the input cable with the source connected produce more noise/hum/buzz than when only a shorting plug is inserted?

when you hear the hum of a lower frequency, where is it coming from?

before bolting into the case the amp was silent with the input connected to source. Now it is bolted to the case and the source connected input is causing humming/buzzing at the output. How can you say you have changed nothing? and how can you be sure it is not an earth loop problem?

[markiemrboo]

Originally posted by richie00boy
Sounds plausible. I caught myself out once with a humming amp, turned out to be the input lead picking up.

If it's possible, then this might just be my problem too. Thanks.

Originally posted by AndrewT
Hi,
four things for you to look at.

did you connect the safety earth to the audio ground?

Yes, via disconnecting network consisting of diodes and a 10R power resistor, exactly the same configuration as before.

does the input cable with the source connected produce more noise/hum/buzz than when only a shorting plug is inserted?

Yes. Though I am not shorting via the RCA socket, I am shorting it on the board connectors that the RCA socket plugs in to. I will try shorting at the RCA socket, this seems like it would be a good test to see if it's the wires picking up the noise! Why didn't I think of that before?

when you hear the hum of a lower frequency, where is it coming from?

The woofer still.

before bolting into the case the amp was silent with the input connected to source. Now it is bolted to the case and the source connected input is causing humming/buzzing at the output. How can you say you have changed nothing?

I can say I have changed nothing, because I have changed nothing EXCEPT it's now bolted on a piece of metal rather than loose on the floor! I don't understand how this change could cause a hum. I'm using the same power cables, same plug socket... even a very very very very similar layout bolted down to as it was loose on the floor! Have I misunderstood you here?

and how can you be sure it is not an earth loop problem?

I can't, and it could well be a ground loop problem. I'm not clever enough to know I am only assuming it's not, because it was perfectly fine before I bolted it down, and, as I said before, I have changed nothing except to have it bolted down. It just seems very odd.... don't you think?

[Nordic]

"Yes, via disconnecting network consisting of diodes and a 10R
power resistor, exactly the same configuration as before."

This network should go between starground , and mains earth

First thing to check, use a multimeter and make sure ground on input and output sockets are isolated from the case...

[AndrewT]

Hi,
I think you have a 2 channel amp.

You have added a metal plate.

You have connected the metal plate to the mains safety earth.

You have connected the safety earth to the disconnecting network.

You have connected the disconnecting network to the audio ground of channel one.

You have connected the disconnecting network to the audio ground of channel two.

Are any/some of these statements wrong? There has to be a change that is significant.


I've just thought of something completely new.
When wiring up a mains distribution board it is imperative that the incomers all come in through the same hole in the metal enclosure. Never feeding one of the poles in through one hole and another pole/neutral/earth in/out through another hole. The three core we use on the consumer side virtually guarantees compliance with the rule. But the double insulated single cores coming from the meter can be wired wrongly. In a power amp the input(flow) and output(return) to the speakers are almost always fed in and out through different holes. Could this be a problem for amps.
Could there be a message here?

I wonder if I should be starting a new thread?

[markiemrboo]

Originally posted by Nordic
"Yes, via disconnecting network consisting of diodes and a 10R
power resistor, exactly the same configuration as before."

This network should go between starground , and mains earth

First thing to check, use a multimeter and make sure ground on input and output sockets are isolated from the case...

That is where the network is, and the input and output are not attached to the case yet, so I they're pretty well insulated from it

Originally posted by AndrewT
Hi,
I think you have a 2 channel amp.

Yep!

You have added a metal plate.

Yep!

You have connected the metal plate to the mains safety earth.

Yep!

You have connected the safety earth to the disconnecting network.

Yep!

You have connected the disconnecting network to the audio ground of channel one.

Yep!

You have connected the disconnecting network to the audio ground of channel two.

Yep!

Are any/some of these statements wrong? There has to be a change that is significant.

I suppose something MUST have changed for it to start humming really, but honestly all I have done is bolt the boards to the bottom of the case. I would take pictures, but unfortunately my camera broke a couple of days ago.

I _will_ double check grounding of the amp boards after I eat though, just to make sure. It doesn't appear to be noise picked up by the input cables. Shorting it at the RCA socket was silent too.

I've just thought of something completely new.
When wiring up a mains distribution board it is imperative that the incomers all come in through the same hole in the metal enclosure. Never feeding one of the poles in through one hole and another pole/neutral/earth in/out through another hole. The three core we use on the consumer side virtually guarantees compliance with the rule. But the double insulated single cores coming from the meter can be wired wrongly. In a power amp the input(flow) and output(return) to the speakers are almost always fed in and out through different holes. Could this be a problem for amps.
Could there be a message here?

I wonder if I should be starting a new thread? [/B]

Way over my head Thanks once again.

[AndrewT]

Hi,
I am afraid I set a trap in those statements. Sorry.

You have two ground connections to the disconnecting network.

You have two ground connections through the RCAs to your source/s. If both RCAs are connected you have a loop. That could be enough to cause a very small voltage at your inputs causing the very low level hum/buzz.

Is the hum/buzz just as bad with only one RCA connected to source and the other RCA shorted?

Alternatively, the hum/buzz in coming from the source/interconnects.

Tell us which applies and we may be able to give the solution.

[markiemrboo]

Originally posted by AndrewT
Hi,
I am afraid I set a trap in those statements. Sorry.

You have two ground connections to the disconnecting network.

You have two ground connections through the RCAs to your source/s. If both RCAs are connected you have a loop. That could be enough to cause a very small voltage at your inputs causing the very low level hum/buzz.

Is the hum/buzz just as bad with only one RCA connected to source and the other RCA shorted?

Alternatively, the hum/buzz in coming from the source/interconnects.

Tell us which applies and we may be able to give the solution.

Dunno why you're sorry!

When you say I have two ground connections do you mean the two RCA sockets? Or the two amp boards going to the one place separately?

Quite frankly I am lost again (still having a stupidly hard time getting my head around the star ground thing).

Here's a crude diagram of how I have it grounded if it helps at all...

It's probably all wrong, but it's the best I could understand how to do it Try not to cringe too much!

[AndrewT]

Hi,
I have no right to cringe, your drawing is better than mine!.

I see a ground half ring going from disconnecting network to amp PCB to RCA barrel.

I see a second half ring doing the same to the other RCA barrel.

Now connect ONE source to ONE RCA. There is no ground loop so far.

Connect the second RCA to a second source (L & R in the same unit). I bet there is an common connection between the two RCAs in the source equipment. Now you will likely have a ground loop. Any electromagnetic field will cause a different voltage on the two halves of the loop. That voltage difference is what gets amplified and fed to the speakers.

Can you test the single interconnect and single shorting link to try and isolate the problem?