2 A12's 1 not making strange noises

[daveseviltwin]

I built 2 A12's. The first one works fine. I did all the tests and everything worked great. I installed them put them in the chassis and hit record. The first time everything was good as far as the way it sounded except for the one in question didn't have gain staging at the first 4 clicks but then it "opened up" and sounded fine. Within 10 minutes it stopped passing clear signal and started making a sound that is very similar to overloading something with digital distortion. I haven't broken these open yet but I was hoping you could give me a good place to start. Thanks

[tpryan]

Always start with the DC voltages at the op-amp. Is one voltage regulator noticeably hotter than the other?

[daveseviltwin]

Yes, I'll finally get a chance to look at it again today but when I did the initial tests everything checked out except phantom but I had the jumpers wrong. Like I said, it worked great for 10 minutes or so. I'll update you on what it reads as soon as I get it back out of the chassis. Thanks Tim

[daveseviltwin]

So I tested exactly +/- 16 on the U1 tests. I switched opamps and that did not help. I resoldered and checked my work and didn't see any solder bridges. I did the tests on the molex and that checked out. I'm sure it had to be something I did but I can't quite put my finger on it. All the diodes are facing the correct way and the resistors are all placed correctly as well as the caps. Maybe the key to solving the problem is the fact that the first four clicks on my gain knob have no output and then its distorted on the fifth click. Maybe an unnoticeable solder bridge on my gain knob?

[tpryan]

What's the DC voltage at U1, pin 6? You can also measure it at the "OUT" terminal of L1. Are you sure that C3 is installed correctly and that the solder joints are good? Re-flow the solder joints at R23, R24, and all of the resistors close to the rotary switch.

[daveseviltwin]

When I started the pre up, the measurement between pins 4 and 6 of U1 started at .30 volts dc with the meter set to 20v. After five minutes it increase to .55v/dc. After 10 minutes, it was at .70v/dc.

[daveseviltwin]

Too much fluctuation of resistance at L1 to measure and c3 also doesnt show a constant resistance.

[daveseviltwin]

Well, I think maybe one of the jumpers may have been a hair loose. It appears to be working now. Sounds awesome. Hopefully I don't have any more problems with it. The other worked great from the start. I love these preamps.

I ordered 2 of the t15's. Can't wait to do another build. I hope to get the money up soon for at least 2 n72's and the j99's. Great design! Thanks for all your help. I finally figured out where you were going with it when you asked me to measure at U1. Thanks!