Hi,
I just finished recapping two Adcom GFA-545 MKII amps. I replaced all electrolytics and set the bias. Both amps work great, however one of them has an annoying squeal during shutdown. The squeal occurs on both channels. One channel precedes the other by about half a second. It occurs about 5 seconds after flipping the switch off.
I can also make it squeal on powerup using a variac to slowly bring it up. It occurs when the AC input voltage is between 10 VAC and 50 VAC. The pitch of the squeal varies with the input voltage.
It appears to originate in the DC offset circuitry around the Adcom 3A IC. During power up it oscillates as viewed at pin 6 when the supply voltage on pin 7 is around 2 volts. As I bring up the input AC the supply voltage at pin 7 jumps to 10 volts and it immediately stops squealing.
Any thoughts on how to stabilize this op amp?
I just finished recapping two Adcom GFA-545 MKII amps. I replaced all electrolytics and set the bias. Both amps work great, however one of them has an annoying squeal during shutdown. The squeal occurs on both channels. One channel precedes the other by about half a second. It occurs about 5 seconds after flipping the switch off.
I can also make it squeal on powerup using a variac to slowly bring it up. It occurs when the AC input voltage is between 10 VAC and 50 VAC. The pitch of the squeal varies with the input voltage.
It appears to originate in the DC offset circuitry around the Adcom 3A IC. During power up it oscillates as viewed at pin 6 when the supply voltage on pin 7 is around 2 volts. As I bring up the input AC the supply voltage at pin 7 jumps to 10 volts and it immediately stops squealing.
Any thoughts on how to stabilize this op amp?