Kitty Hawk M3

[lowell]

This amp is having an intermittent feedback, very low in frequency. It's definitely something to do with the reverb as it goes away when the reverb control is down. The circuit is opamp driven. I've checked voltages on the opamp and it has these voltages.

Pins
1:12v
2:12v
3:12v
4:0v
5:12v
6:12v
7:12v
8:24v

The 12v on most pins seems normal except for pins 1 and 2. It seems to me that these should both be 0v. ??

I currently cannot get the thing to feedback with the amp out of the cabinet so I'm thinkiing it's an acoustic coupling issue. However I reinserted the amp into the cab and now it still won't happen. Please let me know about the opamp voltages. Thanks!

[Enzo]

No, they should not be zero. Unless driven to some other voltage, op amps should have outputs sitting at the mid point between the supply rails. When it is +15 and -15, that mid point is zero. But this corcuit operates the op amp between +24 and ground. Mid point is 12v. See how both halves of the IC have a pair of 100k resistors across the 24v rail to set a 12v bias?

Your reverb pan was feeding back acoustically. The same fixes apply to any amp, and we have discussed this before for some other models.

You may have cured it by pulling the pan out.

[lowell]

Thanks Enzo! I'll peruse some other threads about this.