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audioslave  
#1 Posted : Saturday, January 4, 2014 12:51:08 AM(UTC)
audioslave

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I have a new dual mono COD DAC with the WM8804 SPDIF receiver and a Legato output stage. I have a Placid HD BP and Placid with a LCDPS for power.

I am having some terrible ground loop hum. I can hear music behind the hum. The hum occurs as soon as I connect the RCAs, even without the power cord to the DAC attached. The amp is a Denon 3802 reciever. The source is the Coax out from a Squeezbox Touch. Both the Denon and the SB Touch are 2 prong devices. It makes no difference if they are all plunged into the same outlet.

The LCDPS feeds the COD DACs. The Placid is feeding the WM8804. The Placid BP is feeding the legato. All of the PS are floating. I have tried connecting the grounds together and then two a star ground near the power entry module ( no reduction in HUM). The RCAs are isolated from the chassis (checked with DMM).

The humming would get worse when the SPDIF was connected. I implemented isolation transformer on the SPDIF input. That cleared up the HUM from the SPDIF but the RCAs still buzz like crazy.

Right now I have the PS floating. However I would like to ground them in the end at the starground near the power entrance. Any problem with this?

Any thoughts?

Mike
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DQ828  
#2 Posted : Saturday, January 4, 2014 10:11:15 AM(UTC)
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I'd make sure all of the boards are isolated from the chassis, then ground everything back to either 1 star ground or a star of stars. Say bring the power supplies to 1 star (which is isolated from the chassis) and the signal to another, then link the stars to the earth point which should be connected to the chassis.

That's roughly how mine are and they work fine.

David

PS: I had a noise that drove me crazy for ages when the DAC's weren't in cases & it turned out to be the proximity to the Squeezebox Touch, I sent MANY hours trying to find the problem when I finally move the SQT aside for another reason and the noise vanished, gave myself an upper cut for that one. Sometimes it's the last thing you expect.

Edited by user Saturday, January 4, 2014 10:16:18 AM(UTC)  | Reason: Not specified

avr300  
#3 Posted : Saturday, January 4, 2014 12:17:17 PM(UTC)
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Try disconnecting the reciever from the ariel (coax) connection.
Brian Donegan  
#4 Posted : Saturday, January 4, 2014 3:18:29 PM(UTC)
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Are the ground pins of your XLR connectors isolated from the chassis?

Also, you should try to avoid running power wires near the I2S wires (from receiver to COD). They will inject all sorts of noise into the power lines.

Edited by user Saturday, January 4, 2014 3:23:58 PM(UTC)  | Reason: Not specified

audioslave  
#5 Posted : Saturday, January 4, 2014 6:19:39 PM(UTC)
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The body of the XLR is grounded to the chassis. All the pins are isolated, unless I have the GND of the power supplies grounded to the chassis.

I will rework the I2S wires.


DQ828, I was referring to the SPDIF/RCA cable as a COAX. The receiver only has the SE outs from the DAC connected. Sorry for the confusion.

mike
audioslave  
#6 Posted : Thursday, January 9, 2014 6:59:09 AM(UTC)
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Still having hum issues. The AMP hums as soon as the RCAs are connected (even with no power cord attached). The hum gets worse when the power is applied with no connection to the SB Touch.

I cleaned up the I2S wiring and the DC power routing (see attached pics).

I noticed a few things:

I had some whiskers on the COD boards. One was bridging some pins on the DAC chip.

The voltage on the - side of the Placid BP is not stable. At times the voltage would drop from 12 to 10 VDC. I would adjust it back to 12 I have to wind up the trim pot all the way to get back to 12 volts.

If I measure Ohms from the RCA gnd to the chassis it is open loop. The RCA signal to chassis is 2 MOhm.

There is a -250mV from the RCA signal lead to the chassis.

The questions I have are:

Could the metal whiskers have caused permanent damage to components on the DAC board?

Any tips on where to start troubleshooting the power supply? I have checked the boards for cold solder joints, bridges...etc. Any key points I can measure?

I have another Placid in a BII DAC. I will swap it to see if the Hum is associated with the funky PS behavior.

Mike

Edited by user Thursday, January 9, 2014 7:02:45 AM(UTC)  | Reason: Not specified

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Brian Donegan  
#7 Posted : Thursday, January 9, 2014 6:51:56 PM(UTC)
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Do you get the same hum when the DAC is un-powered (but connected)?
audioslave  
#8 Posted : Thursday, January 9, 2014 8:16:15 PM(UTC)
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Yes. With no power to the DAC/Legato the receiver hums as soon as the RCA is connected. Even with the ground lifted.

Brian Donegan  
#9 Posted : Thursday, January 9, 2014 8:20:16 PM(UTC)
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Ok. Try disconnecting the RCA wires from the Legato, and see if the hum remains.

If not, add the Legato, but without any power or DAC connections. Then, working backwards, add the DACs, then the power supplies, until the hum returns.

This will allow you to determine the source.
audioslave  
#10 Posted : Thursday, January 9, 2014 9:55:18 PM(UTC)
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Ah....I see. That is exactly the kind of help I was looking for. I will narrow it down an let you know. I work in IT and control system automation, so i do a lot of troubleshooting. However it usually stops in when you reach the failed device. R&R and move on. I just needed a strategy to start with

Thanks,

Mike
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