Crown PSA-2 outputs

[jsimmo03]

Hi all!

First post.. been reading diyaudio with great interest, but can't find much relating to the Amcron/Crown PSA-2. I read TomWaits' thread on his DC-300A with much interest. Well done Shawn!

The assortment of different outputs worries me. About half of the output transistors (originals Crown 4718) have been replaced with MJ15024s with various date codes. Crown say to replace them with 2SD555s, but aren't they faster than MJ15024? MJ21194 might be a possibility? 2SD555s are hard to find.

I'm also wondering about the pre drivers and op amps, but that's probably another post.

Schematics:
http://www.crownaudio.com/gen_htm/legacy/legacamp.htm

Regards,
Jake.

[djk]

Just use MJ15024 for outputs and 2SB600/D555 for drivers.

[jsimmo03]

Originally posted by djk
Just use MJ15024 for outputs and 2SB600/D555 for drivers.

Hey thanks for the reply!

I'm wondering what modifications, if any, would be required for stability when replacing all drivers and outputs with the MJ1502x. A service bulletin says to delete C401 (0.1uF) from the negative output modules when installing 2SD555s. The fT for that part is 7 MHz.

Also this might help (from the Theory of Operation):
"The topology of the NPN stage is different than any used in previous Crown products. Its principle advantage at this time is that the driver of this configuration must operate with minimum Vce which is less than that of the PNP stage, and this suggests the use of the PNP driver which retains its Ft with lower voltages than the NPN part. If PNP outputs were available to complement the rugged NPN devices, it would be possible to use the topology of the PNP stage for the NPN stage as well and obtain a similar advantage for the PNP driver by grounding its collector. The tendancy of three-stage Darlington amplifiers to oscillate at positive clip is often traceable to an Ft problem in the driver as its Vce becomes small."

Anyone out there done this sort of work to a PSA-2?

Cheers!

[jsimmo03]

Originally posted by djk
Just use MJ15024 for outputs and 2SB600/D555 for drivers.

Ah, sorry, just read your edit.

So if I use 2SB600/D555 for drivers, should I just follow the service bulletin's instructions a la removing C 401? That bulletin talks about installing 2SD555s for outputs, though.. not for drivers. Would it still be the same mod?

Thanks,
Jake.

[djk]

I'd try a 0.022µF, or just remove it.

[jsimmo03]

Hey thanks heaps for the tips.

Drivers: 2SD555/2SB600 seem hard to find.. Any alternatives? The same family as the outputs?

Outputs: What about MJ21194? I have the choice and the price difference is minor.

Looking forward to putting an order in with Newark tonight.