Here's a recap of my first foray into the bias adjustments on my stack of Adcom amps. Today I just started on the pair of small GFA 535 II amps I use to power the 4 Klipsch overhead ATMOS speakers. The main thing that suprised me a bit was the one amp measured about 5 uV bias in both channels with the other amp measured 10+ uV in both channels? That doesnt look to have been a result of age drifting, more likely the tech's were aiming for a different target at the factory for some reason? The factory spec on the 535 II bias is 7 uV and the DC offset is self adjusting. Does so very well BTW, I couldn't measure "zero" offset at all 4 of the channels speaker terminals. Also since both amps had identical readings in the L & R channels it doesn't look as if any drift at all was a play in either amp after 25 some years of use.
Adcom 535 II #AP15041-142 Japan
Cold 10 mV both channels.
Hot dropped to around 8mV both channels
Adcom 535 II #AP25331-342 Taiwan
Cold 5 mV both channels
Hot dropped to around 3 mV in both channels
Factory recommended bias is 7 mV for the GFA-535 II.
My adjustment procedure was to first measure the bias with the amp cold (after 5-10 minutes idling). Then drive the amp with a 1khz signal to roughly 5 watts (6V) across the dummy load for about 10 minutes which made the heat sinks quite hot. Bias would drop about 1.5 uV from idle to very hot so I set the bias to about 8.2uV at cool idle which should give a nominal 7uV at a normal running temperatures?
So far so good guys, I didn't blow up the first two amps.
Two down a three to go.