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Old 11th March 2007, 01:10 AM   #1
zlast is offline zlast  United States
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Default Hummm...

I have a three channel Leach amp. I am getting hum at the speakers and its driving me mad! Here are the particulars:

channels A,B,C

If I have RCAs connected to the input of just channels B,C - QUIET

If I have RCA connected to the input if just channel A - QUIET

Any combination of RCAs connected to A-B, A-C or A-B-C - HUM

I can tone through both ground points on channel "A" to chassis, from the RCA ground to chassis...everything seems to be as it should. Is this a faulty component on the board for channel "A"?
Channel "A" plays fine. Just have to get rid of the hum
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Old 11th March 2007, 08:31 AM   #2
AndrewT is offline AndrewT  Scotland
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Hi,
how have you interconnected the three signal grounds and the three power grounds from the Leach PCBs?

How have you interconnected the three 0v commons from the PSUs to the audio grounds?

How have you connected the three RCA grounds to the audio ground/s?
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Old 11th March 2007, 08:41 AM   #3
Salas is offline Salas  Greece
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Try lifting A from ground with a 10 Ohm resistor if other methods fail.
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Old 11th March 2007, 04:17 PM   #4
zlast is offline zlast  United States
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Default Hummm

(A) how have you interconnected the three signal grounds and the three power grounds from the Leach PCBs?

(B) How have you interconnected the three 0v commons from the PSUs to the audio grounds?

(C) How have you connected the three RCA grounds to the audio ground/s?


(A) Both signal and power ground from each board connect to a central point (with three power supply 0V) which is connected to a Bridge rectifier, resistor (5 ohm was out of 10) capacitor as outlined here http://sound.westhost.com/earthing.htm. THe output side of the bridge rectifier, resistor, capacitor is connected to chassis with AC ground.

B) See A

C) RCA outer connecter goes to the SIGNAL ground input as specified on PCB : http://users.ece.gatech.edu/~mleach/...cs/Compnt2.gif

I dissassembled the entire amp last night...and found a ground loop (crimp on terminal ring on one of the capacitors) I corrected this issue and with the AC\Bridge rectifier ground lifted there was no contuity from any of the boards to chassis. "I have found the problem thought I"...I put everything back together, and the HUM is still present in channel "A".

salas

Try lifting A from ground with a 10 Ohm resistor if other methods fail.

All three boards are isolated from AC ground by bridge rectifier/resistor/capacitor as outlined above, the strange thing is its only channel "A" that exhibits the HUM, and the same channel is fine if no other RCAs are connected ( so it would seem to a ground issue rather than noise picked up).
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Old 11th March 2007, 04:29 PM   #5
Salas is offline Salas  Greece
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There must be a ground loop forming between A and B&C returns, maybe through A connected equipment. Differentiate A (either with other value resistor or no resistor) to see what happens so maybe we get some clue.
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Old 11th March 2007, 06:26 PM   #6
AndrewT is offline AndrewT  Scotland
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Hi,
check the PCB central ground next to the signal ground from the RCA input.
It should be 10r or so from the central ground connection over by the output side, on all three channels.

I am surprised that you get no hum when both B & C channels are connected by interconnects from your source. It appears there is no external hum loop, I don't understand that.
What have you done to avoid the loop?
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Old 11th March 2007, 08:54 PM   #7
zlast is offline zlast  United States
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[ANDREW]
check the PCB central ground next to the signal ground from the RCA input.
It should be 10r or so from the central ground connection over by the output side, on all three channels.


This would have to be done with the signal ground and central ground lifted from the star grounding point correct ? (basically measuring R50)

I am surprised that you get no hum when both B & C channels are connected by interconnects from your source. It appears there is no external hum loop, I don't understand that.
What have you done to avoid the loop?

Initially I thought it was a ground loop between components. However after some testing, with only preamp connected to the amplifier, I found that channel "A" creates the HUM...B anc C are fine....channel "A" creates hum with either left of right channel input (as long as either "B" or "C" is also connected ) But "A" alone is hum free...

With both central grounds lifted I get the proper 82 ohms for resistor 51 measuring between the central grounds on each board.

Measuring from the RCA ground to each central ground on all three boards I get .1 ohm (central ground) and 82 ohms (signal ground).
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