DC Voltage on RCA Output

Hi,

baxtr said:
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.


baxtr said:
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

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
 
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
 
10 or 20 millivolts but nah send it back with that 9 volt action!



My understanding is that the unbalanced RCA audio format was invented primarily to a) cut down on costs - less wire! b) enable DC only / positively biased AC signals to travel between "components" of a "component system", which they got away with initially mostly because everything back then used transformer balanced 3 wire connections - which reset the ground to create the AC again (eg signal goes from 0 to 2 volts fully positive alternating current before... now it goes -1 to +1 into the negative volts also). If the components of the component system were all from the same manufacture (RCA?) then it can work OK-ish.


This enabled them to discard the inverted side of the signal (it's picked up from ground instead) saving money: lets say your source had DC bias in it, instead of putting an output transformer to bring it to rest at 0 upon silence, they can whack it straight out the "line-out" out, and the next bit of gear mite not use a bias now, no transformer either, as the source "operates directly" upon the power amps input decoupler, via the volume knob only limit.



Here is when having a top grade desk with transformer balanced input everywhere can help I reckon.



Anyhow use of transformers got reduced lately. Digital DAC equipment should be able to directly create AC signals by splitting half the buckets to each side (15 bits per side at 16 bit and 23 for 24 bit one bit is for the - minus sign). So I went hunting schematics for DACs. I failed. I could not find any good circuit schematics for how a 24 bit DAC creates the sound (a lot of tiny capacitors are charged or not charged and then a clock empties them via a transistor!).




I improved my computers sound by attaching a stereo tube-DI to the mini-jack headphone out on my Ryzon 7 and my god it does sound much less USB noise with the DI-box connected, earth-linked.
 
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
Depends on the preamp, but if its discrete transistors you could fry an input device, or degrade its noise performance permanently. A well designed preamp will have some series resistance on the input to help protect itself (but not so much its noisy).


The expensive damage is to the speaker cone as it flies across the room as you plug in the cable to the preamp!!
 

ICG

Disabled Account
Joined 2007
The expensive damage is to the speaker cone as it flies across the room as you plug in the cable to the preamp!!

Well, the gain (volume) on the preamp determines if it's just a 'pop' or a voice coil going flying. And it's mainly a problem of DC coupled amps, for DC servo amps or amps with an input capacitor it's just a minor problem for a second once you switch to that input but afterwards not anymore. The easy hack is to introduce a input or output capacitor but fixing the problem is probably much more beneficial though.