On Sun, 22 Feb 2015 19:35:14 -0000, "Woody"
wrote:


"Eiron" wrote in message
...
Hi experts,

How much DC offset would you consider acceptable in a power amp?
What can you do on a rainy Sunday afternoon? I was poking around
inside
an old Rotel RA-970BX amp. It works OK except that the DC offset is
about 100mv and 50mv on the two channels, which seems rather high to
me.
There is no adjustment, except for bias current, so any tweaking
would
probably have to be adjusting the current in one of the long-tailed
pairs
by swapping resistors.

--

Have you tried adjusting the bias to see if you makes any difference?

If you Google 'schematic RA970BX' you will get copious information
including the service manual which includes how to set the bias. I
suspect it will correct it.

You should ideally be aiming for 10mV or less offset.

That's a pretty fair ideal to aim for. However, in the case of a 60W
RMS per channel amplifier, even the 100mV offset isn't of any great
concern. It represents a mere 80mW dissipation in the bass driver coil
and an assymetric clipping level reduction by a tenth of a volt from
the +/- 31 voltage swing available to support driving a 60W RMS
sinewave into an 8 ohm speaker load.

Since this is an old model, the less than ideal DC offset might
simply be a symptom of elderly electrolytics suffering unusually high
leakage current due to lack of use. The DC offset might well improve
after a few hours use (or it just might get progressively worse).

I suggest the OP runs it for an hour or two and recheck the DC offset
for any signs of further drift over the first few hours and then
recheck over the next few days and weeks if nothing untoward develops.
--
J B Good