Pass A75 nearly finished!

Greetings all

I have nearly finished the Pass A75. This weekend I was listening to it for a few hours in the lab and it sounds good so far.

Right now I 'm looking at the output stage bias, and it seems to be all over the place. I'm using IRF630N/IRF6215 TO-220 output transistors (not the best possible choice, but I had not found this site when I had ordered the parts, and the specs. seem to match up well.) I matched them per Nelson Pass's instructions at 15v with a 55 Ohm resistor to within .05V (I ordered 100 of each). But still my output biased is all over the place, measuring the voltage across the 1 Ohm source resistor, I see any where from 50mV to 300mV and that from these matched devices. This has caused some heat sinks to run very hot and others to run rather cool. Mark Finnis wrote something about matching output transistors, I thought he maybe matched them at the operating current and voltage?

Any suggestions/experiences on getting the output devices to becaome a little more uniform. I smoked two output devices this weekend as they seemed to run out of control!

BTW I have all 48 devices spread over 12 medium sized surplus heatsinks, using a Kapton pad without grease to isolate them, with 4 fans running at low speed.

Mr. Pass and Jam,

Thanks for the input. Last night I unhooked the fans and the levels evened out significantly. I guess I was not getting even cooling of the heatsinks.

Tonight I will ground the input and adjust over the evening hours.

Jam-> when you mentioned matching the output devices at the bias. I did bias the MOSFET to ~.19A, but this was at 15v not at 42V which is my current rail loaded down. Therefore they got quite warm bu no where near the temps that I am seeing inside the amp.

Thanks again for all your help.