Preamp trouble? Ideas?

[lost_in_ABQ]

Howdy! I've been using a Hafler P3000 amp with a Spectro Acoustics P-101 preamp, with pretty satisfactory results. However, a couple of weeks ago, I cranked it a bit louder than normal (about halfway) and PFFFT! The left channel died and smoke rose ominously from the Hafler. I pulled it apart and found the problem: one of the FETs burned up. No biggie. I ordered new ones and thought I'd sub in another amp I had, a Sonance 260. I plugged the preamp into it and fired it up. Sounded good until I put the tiniest little volume on it. The 260 started shutting down on peaks. I could hear a relay clicking, like it had detected a short. I couldn't get it loud enough to be heard properly no matter what I did. I viewed the preamp with suspicion, so I got my scope out and looked at the output. No DC, no wild voltage swings, nothing out of the ordinary. In fact, very, very quiet. But the 260 definitely doesn't like what I'm feeding it, regardless of source. I even connected a ground wire between the preamp and the amp, just to eliminate ground loop. No difference. Am I right to suspect the old preamp? Thanks for any ideas!

 

[dspear99ca]

Sounds like coincidence. The amp will almost certainly have a cap on the input to eliminate DC. What speakers are hooked up? If it's a very low impedance load you may be just pulling too much current for EITHER amp to handle comfortably. One overheated and one went into protection... just a guess.

 

[lost_in_ABQ]

Sounds like coincidence. The amp will almost certainly have a cap on the input to eliminate DC. What speakers are hooked up? If it's a very low impedance load you may be just pulling too much current for EITHER amp to handle comfortably. One overheated and one went into protection... just a guess.

Wel... makes sense, but these speakers have been connected to the Hafler for a while now without issue. They're 8 ohm loads that always worked well. But I will change them out to test your theory. Thanks.

 

[lost_in_ABQ]

Okay, well, I swapped out my mains with the speakers from my garage system. Exact same problem. At very low levels, all is good. Bump up to mild listening levels (only a couple of watts) and the input protection kicks in. I'm going to have to pull the preamp apart and see if I can make sense of this.

 

[slimecity]

Where did you get the replacement FET?

Are you satisfied it is a genuine part?

 

[lost_in_ABQ]

I haven't repaired my main amp yet. This is an amp I had sitting on a shelf. It's a known good amp. I still think this points to the preamp. Dont know what else it could be.

 

[lost_in_ABQ]

I haven't repaired my main amp yet. This is an amp I had sitting on a shelf. It's a known good amp. I still think this points to the preamp. Dont know what else it could be.

I said it was a known good amp. I think I just proved it isn't. I plugged the tuner directly into the amp after turning down the input levels. As I turned them up, it began cutting out again. I now have the amp open, looking for something obvious