hifi engine

Quick question for you electronics gurus!

I have 2 perfectly healthy Adcom GFA 565's. They both perform exactly as they should sonically. The question I have is related to DC offset. When I first turn them on from being cold the DC offset readings are as follows:

After 1 minute... left-40 Mv. Right- 2 Mv
After 15 minutes... left- 15 Mv. Right- .5 Mv
After 1/2 hour..... left- 9Mv. Right- 0 Mv
After 1 hour...... left- 1.1 Mv. Right- .1 Mv

I've made these same checks several times and the numbers repeat themselves fairly close each time. Id like to know if there is any sensible reason why one amp stabilizes the DC offset right away and the other takes an hour. I do realize that these are all acceptable numbers. Once they reach their lowest DC numbers, they both stay low and very stable as long as they are left on. Thanks for any input on this. Thanks



analogdino's picture

Re: Quick question for you electronics gurus!

I suggest that there is a bias component in the LH channel that is changing with operating temperature, likely a resistor. As you say, 40 mV down to 15 mV in 15 minutes is not a problem, but it is irritating to us purists! Caveat: you might make it worse by shotgunning each resistor! If you have an accurate DMM, you might try reading a series of voltage drops across identical L and R channel bias resistors and look to see if anything is changing with operating temperature.
There is also the chance that an electrolytic capacitor leakage is changing over this period. but my guess is that this is far less likely than a resistor value changing.
Good luck.
Roger