• Registered: 2020-09-25
  • Posts: 4

Topic: DC bias on ADI-2 DAC balanced outputs?

Hello
What level of dc bias current can I expect on the balanced XLR outputs?
I’m planning to connect the DAC to input transformers which are not happy with any dc current (100% mu-metal core).
I just realised that the outputs of the DAC are dc coupled, so I probably can’t assume zero bias current?
Thanks

2 Reply by KaiS (edited by KaiS 2020-09-26 19:29:16)

  • KaiS
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Re: DC bias on ADI-2 DAC balanced outputs?

I got:
DC Offset cold / warm (resolution 0.01mV):
CH1: 0.04mV / 0.03 mV
CH2: 0.35mV / 0.27mV
CH3: -0.03mV / -0.06mV
CH4: -0.24mV / -0.20mV

XLR Out L: -0.18mV
XLR Out R: -0.12mV

RCA Out L: -0.19 mV
RCA Out R: 0.07mV

Doesn't seem to change much with temperature.
The voltage is load independent.

Off course your individual specimen might show up different values.


Depending on the internal resistance of the transformer this might be OK.
What's your transformer's DC resistance and max. allowed DC current?


One thing to remember:
The ADI-2 is DC coupled all through, if you feed in a digital signal with a DC offset this will show up.

KaiS's Website

  • Registered: 2020-09-25
  • Posts: 4

Re: DC bias on ADI-2 DAC balanced outputs?

Thanks very much for taking time to make those measurements, I appreciate it.
This is the transformer:
https://www.sowter.co.uk/specs/9063.php
Sowter say no unbalanced current allowed, which is not too helpful but fair enough. Also they give total dc resistance referred to primary as 106 ohms, but no figure for the primary alone which I think is what we need to know?

With a bit of hunting I found this data for a different transformer, which gives a view data points for the effect of dc current:
https://www.sowter.co.uk/specs/9100.htm
This suggests that even 0.125 mA increases the LF distortion significantly.
The 9063s are on order, I won’t have them for a couple of months, so I can’t do any tests on them.

Were your measurements taken with no load across the outputs?

I wonder if I could use balancing resistors to null the dc offset?

By the way, thanks for pointing out that the digital signal can have dc offset, I had never considered this.....

4 Reply by MC

  • MC
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Re: DC bias on ADI-2 DAC balanced outputs?

0,35 mV with 160 Ohms are 2 µA (mikro Ampere). Simply forget it, there is zero problem unless you play back music with DC. And that can indeed happen. Fortunately the ADI does have a digital low cut (high pass) in the PEQ that will save you. Unless (again) you play DSD - then you are doomed as DSD typically has the most DC of all recordings.

Regards
Matthias Carstens
RME

MC's Website

  • Registered: 2020-09-25
  • Posts: 4

Re: DC bias on ADI-2 DAC balanced outputs?

Ok that is clear, I shall not worry about it any further.
Thank you MC (and KaiS) for such quick replies. Great support even on a weekend!

6 Reply by KaiS (edited by KaiS 2020-09-26 19:33:29)

  • KaiS
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Re: DC bias on ADI-2 DAC balanced outputs?

An oversight on my side:
I did measure the headphones outputs only.

I've added the Line Out XLR and RCA measurements to my posting above.
The new measurements did not show any different results.

But - the XLR output series resistance is 200 Ω, so transformers DC resistance doesn't matter in the equation.
DC current is limited to 1 µA (0.001 mA) with 0.20 mV DC offset into 200 Ω.

This is way less than a value that could cause any quality loss.
So don't worry.

KaiS's Website