[LAD] [ot] rme fireface: weird balanced output measurements

[Jörn Nettingsmeier]

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