linuxaudio.org

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

quoted text
> 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.
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