DC Voltage on RCA Output

baxtr · 16th January 2009, 02:14 AM

I just bought a new Super Pro DAC707 and I noticed something odd. The RCA outputs have about 9V off DC current on them. This doesn't seem normal, shouldn't a output on have a few millivolts of DC? Can this high amount of DC current damage my equipment?

http://www.diykits.com.hk/dac_super_pro_dac707.htm

Thanks,
Dan

Geek · 16th January 2009, 03:41 AM

Hi,

Quote:

Originally posted by baxtr
I just bought a new Super Pro DAC707 and I noticed something odd. The RCA outputs have about 9V off DC current on them. This doesn't seem normal, shouldn't a output on have a few millivolts of DC?

Plenty of equipment has DC on the output. Output coupling caps being considered either 1) "not audiophile" or 2) too expensive of a luxury.

Quote:

Originally posted by baxtr
ICan this high amount of DC current damage my equipment?

Absolutely!

I personally consider a source switchbox that uses DC blocking caps essential.

Cheers!

baxtr · 16th January 2009, 05:26 AM

What is a normal amount of DC offset for most line outs? Also what would you consider to be unsafe for my preamp? Thanks.

Geek · 16th January 2009, 07:15 AM

"Normal" DC offset is zero.

If your preamp is AC coupled on the input (blocking cap), no worries. Not all of them are, like tube equipment is often DC in to the volume control.

Cheers!

sandyK · 16th January 2009, 07:40 AM

baxtr

It is highly unusual to have even 30mV DC out of a piece of equipment such as a DAC or CD/DVD player etc. Even most power amplifiers would not have any higher than 30mV DC offset.
This DAC either has a fault or is poorly designed .
If the potentiometer of the preamp/amplifier is before the preamp/amplifier's input coupling capacitor, this will lead to noise with movement of the volume control, even with 50mV DC, let alone 9V DC. I would check with the seller of the item, as it does appear to have a problem.

SandyK

oshifis · 16th January 2009, 11:35 AM

I can imagine the output DC coupling capacitor is floating because it has no resistor to GND. Measure the voltage with a 100k resistor between the RCA hot and sleeve. If that solves the problem, finish the manufacturing process...

baxtr · 16th January 2009, 09:20 PM

What type of damage could DC do to a preamp? I'm going to send the DAC back, does anyone else have a Super DAC 707 they could test?

Thanks
Dan

sandyK · 16th January 2009, 10:37 PM

baxtr
Try Oshifis suggestion first about testing with a 100K terminating resistor, before sending it back. I could imagine a small voltage via capacitor leakage if the output side of the capacitor did not have a high value (e.g.100K) resistor to earth, but not 9V due to the small loading effect (10M ?)of the measuring meter.
SandyK

16th January 2009, 02:14 AM	#1
baxtr diyAudio Member Join Date: Jan 2009	DC Voltage on RCA Output I just bought a new Super Pro DAC707 and I noticed something odd. The RCA outputs have about 9V off DC current on them. This doesn't seem normal, shouldn't a output on have a few millivolts of DC? Can this high amount of DC current damage my equipment? http://www.diykits.com.hk/dac_super_pro_dac707.htm Thanks, Dan

16th January 2009, 07:40 AM	#5
sandyK diyAudio Member Join Date: Apr 2007 Location: Sydney	DC Voltage on RCA Output baxtr It is highly unusual to have even 30mV DC out of a piece of equipment such as a DAC or CD/DVD player etc. Even most power amplifiers would not have any higher than 30mV DC offset. This DAC either has a fault or is poorly designed . If the potentiometer of the preamp/amplifier is before the preamp/amplifier's input coupling capacitor, this will lead to noise with movement of the volume control, even with 50mV DC, let alone 9V DC. I would check with the seller of the item, as it does appear to have a problem. SandyK

16th January 2009, 10:37 PM	#8
sandyK diyAudio Member Join Date: Apr 2007 Location: Sydney	DC Voltage on RCA Output baxtr Try Oshifis suggestion first about testing with a 100K terminating resistor, before sending it back. I could imagine a small voltage via capacitor leakage if the output side of the capacitor did not have a high value (e.g.100K) resistor to earth, but not 9V due to the small loading effect (10M ?)of the measuring meter. SandyK

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16th January 2009, 05:26 AM	#3
baxtr diyAudio Member Join Date: Jan 2009	What is a normal amount of DC offset for most line outs? Also what would you consider to be unsafe for my preamp? Thanks.

16th January 2009, 07:15 AM	#4
Geek diyAudio Member Join Date: Sep 2004	"Normal" DC offset is zero. If your preamp is AC coupled on the input (blocking cap), no worries. Not all of them are, like tube equipment is often DC in to the volume control. Cheers!

16th January 2009, 11:35 AM	#6
oshifis diyAudio Member Join Date: Mar 2004 Location: Budapest, Hungary	I can imagine the output DC coupling capacitor is floating because it has no resistor to GND. Measure the voltage with a 100k resistor between the RCA hot and sleeve. If that solves the problem, finish the manufacturing process...

16th January 2009, 09:20 PM	#7
baxtr diyAudio Member Join Date: Jan 2009	What type of damage could DC do to a preamp? I'm going to send the DAC back, does anyone else have a Super DAC 707 they could test? Thanks Dan

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