My A-P-I clone and a feedback loop. Did I destroy it?

[Category 5]

So silly me, I hooked the output of the channel a to the input of the channel a. in closed ear headphones I wondered why no output from the mic. As I proceeded to crank the gain I heard a ringing sound. i removed my headphones and noticed my error.

It was the transformers ringing...audibly! I immediately pulled the cables off and proceeded to test the preamp... Whew, the mic worked and sound came through.

Tonight I had a stereo pair hooked up (matched mind you) and noticed the output of channel 2 seemed noticably brighter than 1. I swapped cables, channels, pans, mics...always channel 2 was brighter.

Now I am worried! Did I fry my op-amp? Did I fry my transformers? (is that possible?).

Are my 1973 2520s just "different" and I never noticed it before?




Also, on preamp one the voltage pots and dc offset pot are a little twitchy compared to the pots on module 2. Could this be because I had heat on them too long while soldering? Channel 1 was my first and I may have done this. Also, could a less accurate +/- ratio or DC offset setting cause the effects I hear? I'd rather replace the pots than the op-amp, or transformers.

If it's the Op-Amps, at least I can get A-P-eye to replace them, can't I? Anyone know what this costs?

Thanks!
Shane

 

[jrmintz]

Shane, were you working in phones? If so, did you turn the phones around? If you're working in speakers, did you check to see if you blew a tweeter? Did the brighter channel lose gain and bottom?

API charges $75.00 plus the old opamp. You have to return one to get a new one


Good luck

 

[tommytones]

I noticed something interesting (maybe minor) about the current 2520. I had a box open the other day and the opamps get quite hot as compared to any older version(s).

 

[Kev]

[quote author="tommytones"]I had a box open the other day and the opamps get quite hot as compared to any older version(s).[/quote]

same box ? ..... same rail voltage ?

I'm sure the 2520's have changed if only to accommodate component availability and supply issues over the years BUT as far as I know they only have the one version so to speak.

The other guys have slightly different models but I think all API2520's are trying to be the same thing.

 

[Category 5]

[quote author="jrmintz"]Shane, were you working in phones? If so, did you turn the phones around? If you're working in speakers, did you check to see if you blew a tweeter? Did the brighter channel lose gain and bottom?

API charges $75.00 plus the old opamp. You have to return one to get a new one


Good luck[/quote]

Yes, I switched the headphones around, switched the pans around, swapped mics, and swapped mic cables. The first channel always sounds less bright than the second. Maybe tomorrow I will swap the op-amps and check the voltage levels, etc. Could a difference in positive and negative voltage cause trouble (i.e. +17.4, and -17.3)? The pots on the first one were tweaky, and I had a hard time getting voltage to match.

I also couldn't get the DC offset to rest at 0 permanently. it would fluctuate up to 1 and back to 0 (but were talking millivolts!).

Both channels still sound fantastic. Were I not comparing with a matched pair of mics I would have thought it was the mics. Until I stated swapping things around.

Maybe the op-amps are fine, and I just have 2 from differen manufacturing batches. Being that they are from 1973 I wonder what the consistency was like.

Shane

P.S. Any way to compare the 2 channels' frequency response using a scope? How would I do it?