linuxaudio.org

hi everyone!
yesterday, i visited my friends' new studio, to help them shake out some

bugs in the patch and fix a monitor problem. there i came across a

really weird issue with the line outputs on two rme fireface 800s:
we put a test tone out in logic (yeah, they run a mac shop), and i went

to measure the outputs. in addition to really high fluctuations from one

output to the next (with identical digital input signals), i measured

huge differences in voltage on the hot and cold side, such as
hot to ground: 2.00 V

cold to ground: 1.82 V
despite the fluctuations across several channels, this trend was pretty

constant. so i figured, maybe this box has a problem with its negative

voltage rail. they had another ff 800 in there, which we measured for

comparison. same issue.
i figured, maybe apple wrote an oscillator with a dc offset, so we

applied a known-good test tone .wav file. same result.
is my thinking flawed, or are rme really delivering such crappy output

stages on such a pricey box?
my multimeter is not a calibrated one, but it's in the 100+€ range, and

i used AC true rms measurement mode.
i understand my meter has a very high input impedance, and that's what

line level connections should have, right? operating more or less

open-loop, without significant currents flowing. or should i use a shunt

resistor and measure across that? if so, what value is recommended?
thanks for any insights, best,
jörn
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