17 Feb
2012
17 Feb
'12
4:16 p.m.
On Fri, Feb 17, 2012 at 01:34:27PM +0100, Jörn Nettingsmeier wrote:
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.
Any power supply problem would affect both outputs equally - the
'in phase' (hot) one isn't more 'positive' that the other...
The different voltages w.r.t ground are not necessarily a sign of
trouble. It all depends on what type of line output stage this is
supposed to be.
If it is of the type in which you essentially have two independent
unbalanced outputs driven in antiphase then the difference is a bit
too high to be explained by component tolerance for a device of this
quality class - you'd expect at most 2% tolerance resistors to be
used in such a circuit.
But it could very well be that those line outs are a bit more
sophisticated. It is possible to design them such that you get
a pseudo-floating output: if you short-circuit either hot or cold
to ground the voltage it would normally provide is just taken up by
the other one. In other words, the real output voltage (between hot
and cold) doesn't change. This amounts to increasing the common-mode
output impedance, the limiting case being a transformer isolated
output ('infinite' common mode output impedance).
With such a circuit it's quite normal to measure different voltages
on hot and cold, in particular if they are not loaded to ground.
Ciao,
BTW: mail delivery to your folkwang-hochschule address fails -
seems they don't know you anymore...
--
FA
Vor uns liegt ein weites Tal, die Sonne scheint - ein Glitzerstrahl.