Eiron wrote:


How much DC offset would you consider acceptable in a power amp?


** An offset of +/- 100mV is Ok for regular 8 ohm speakers - but not for an ESL57 of the like which has a resistance of 0.3ohms.


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.

** That amp is a dog's breakfast.

It uses complementary differential pairs at the input so ought to be almost input current and hence DC offset free - if they merely matched the Hfes and got the resistor values right.

The input has 47kohms to ground while the overall feedback resistor is 12kohms.

Good luck fixing it.


.... Phil





--
Eiron.