I have a new to me pair of Threshold SA2 Optical Bias amplifiers. They sound fantastic in their original condition unrestored condition. I will eventually have them restored but my budget may not allow for now. I have noticed 1 of the 2 mono amps is running slightly warmer than the other, about 5 degrees on top cover and about 7-8 degrees at the center of the heat sinks (one amp at 100 degree and other at 107 degrees on center of heat sink at idle 20 minutes). Is this something an adjustment to the bias would resolve? Does anyone have the procedure for doing so? Is the small difference even worth addressing at all? Thank you. Charles
Bear in mind that you could make things MUCH worse with a slip of the probes.
This is a job only for an experienced technician with all the proper test equipment.
Best to leave this until the restoration happens.
This is a job only for an experienced technician with all the proper test equipment.
Best to leave this until the restoration happens.
Hi Zen Mod. Thank you for the pdf. It appears page 1 is missing. Would you happen to have page 1 of the pdf? Thank you!
The bias adjustment full doc it somewhere on this site, Check on the new front end thread I have seen it there.
The procedure posted above is for the E-line products. To adjust the bias on the SA2, the rear panel must be removed to access the bias controls.
Of course a DVM must be used to monitor the emitter emitter voltage drop. Somewhere between 100 and 125 mv is typical. Use clip leads and the pots are sensitive so be gentle. Once an adjustment is made, put the rear panel and the top back on. Let the amp run for 1-2 hours and measure the voltage and the temperature from the rear heat sink near the output terminal posts. The optimal temp. is 50C. 45-48 is OK especially for older units that are not Optical Bias equipped.
Of course a DVM must be used to monitor the emitter emitter voltage drop. Somewhere between 100 and 125 mv is typical. Use clip leads and the pots are sensitive so be gentle. Once an adjustment is made, put the rear panel and the top back on. Let the amp run for 1-2 hours and measure the voltage and the temperature from the rear heat sink near the output terminal posts. The optimal temp. is 50C. 45-48 is OK especially for older units that are not Optical Bias equipped.
Thank you for all the info. I ended up adjusting both amps to 150MV cold start. Both amps are now dead even for heat - 46-47C in 72 degree room ambient temp. The offset on one amp is 16MV and the other 38MV. I know these could use a refresh but does that offset seem acceptable?
