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Old Yesterday, 08:04 PM   #1
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Join Date: Jun 2007
Default Single rails - buzzing issues

I've recently built an STA508 amp and notice that when the amp is powered on (24v single rail supply) there is a buzzing noise.

If I connect a source (preamp, dac) the buzzing goes away.

Would this be caused by not having the -ve side of the 24v supply grounded to the chassis? Or possibly not having the RCA jacks grounded in some way?

I also notice that when I put my finger on the power button or on the front of the chassis the buzzing gets worse. But when the preamp is connected it is silent as a grave.
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Old Yesterday, 08:37 PM   #2
rayma is offline rayma  United States
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Location: Upper midwest
Quote:
Originally Posted by lordearl View Post
Would this be caused by not having the -ve side of the 24v supply grounded to the chassis? Or possibly not having the RCA jacks grounded in some way?
The ground shell of the RCA jacks and the return side of the 24V supply both connect to the circuit common.
Then you should connect a single wire from the circuit common to the chassis so that it can act as a shield.
This chassis point should be the same as the one where the utility safety ground is connected.
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Old Yesterday, 08:44 PM   #3
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Quote:
Originally Posted by rayma View Post
The ground shell of the RCA jacks and the return side of the 24V supply both connect to the circuit common.
Then you should connect a single wire from the circuit common to the chassis so that it can act as a shield.
This chassis point should be the same as the one where the utility safety ground is connected.
Ok thanks, although there is not a utilty safety ground point as I am simply using a 24v dc laptop power supply with a 2.1mm jack. So in this case, I should connect a bolt to the chassis and wire the -ve side of the power supply to the ground of the input rca jacks?

What about the ground of the speaker outputs?
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Old Yesterday, 08:51 PM   #4
rayma is offline rayma  United States
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Join Date: Apr 2011
Location: Upper midwest
Quote:
Originally Posted by lordearl View Post
Ok thanks, although there is not a utilty safety ground point as I am simply using a 24v dc laptop power supply with a 2.1mm jack. So in this case, I should connect a bolt to the chassis and wire the -ve side of the power supply to the ground of the input rca jacks?

What about the ground of the speaker outputs?
There are a number of circuit returns that are connected to a "common" which is then connected by a single wire to chassis ground.
These may include RCA jack shell, speaker negative, power supply negative (in your case), amplifier common, etc.

If you post the schematic, I think it will be easier to discuss.
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Old Yesterday, 09:01 PM   #5
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Ok thanks.
Unfortunately I don't have the schematic as the manufacturer pre-fabricates the boards as modules (and won't give me the schematic....go figure!).

So I simply installed them into a chassis with a dc power socket, one set of rca inputs and the speaker posts as the output.

In terms of troubleshooting, is the first step to connect the -ve of the power supply to the chassis and see if that stops the buzzing on idle?

(All of this is of course assuming that an amplifier shout NOT having any buzzing when there is no source connected and it is powered on?)
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Old Yesterday, 09:08 PM   #6
rayma is offline rayma  United States
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Join Date: Apr 2011
Location: Upper midwest
Quote:
Originally Posted by lordearl View Post
Ok thanks.
Unfortunately I don't have the schematic as the manufacturer pre-fabricates the boards as modules (and won't give me the schematic....go figure!).

So I simply installed them into a chassis with a dc power socket, one set of rca inputs and the speaker posts as the output.

In terms of troubleshooting, is the first step to connect the -ve of the power supply to the chassis and see if that stops the buzzing on idle?

(All of this is of course assuming that an amplifier shout NOT having any buzzing when there is no source connected and it is powered on?)
The board should have a negative speaker return point somewhere. It may be near the negative power supply point.
Doing the wrong thing could cause a meltdown here.
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Old Today, 12:15 PM   #7
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Thanks for the help here.

Attached is a picture of the board in the chassis - as you can see it is a very simple set-up.

Ok I had a look at the board and there doesn't seem to be a speaker ground return point at all?!
Attached Images
File Type: jpg IMG-20141022-01080.jpg (730.3 KB, 2 views)
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