Bass-ackward amp selection...

[Racket Scientist]

Just wondering if there are any Pass DIY designs that can make use of a dual mono power supply with +/- 75 volts? I have a huge PA amp that died leaving behind a carcass and a big power supply. I plan to gut it and put something DIY in there. Unfortunately my skill level is modest, so "tweaking" an existing design is not a good option. Any suggestions?

[AndrewT]

+-75Vdc should be good for 200W into 8ohms.
Two channels from your power supply will require a big, well designed PSU.
If you use 4ohm loading the power demand is doubled. Can your PSU supply that doubled demand?

For big power amps, you will find that MJ21193/4 are excellent in the output stage. 1pair could supply 16ohm, 2pair for 8ohm and 4pair for 4ohm would be a good starting guess.

[Racket Scientist]

Thanks. Any specific Pass designs I should be looking at? The original amp put out about 450 watts into 4 ohm.

[AndrewT]

Pass are ClassA designs.
They are generally not suitable for enormous ClassA power into low impedance loads.

His extremely expensive get up to around the target power you have in mind but they cannot be powered by a skimpy PSU from a PA amplifier.

You need to re-assess your requirements and then see if the PSU suits your new target.

[Racket Scientist]

I'm aware of that. I'm not looking for enormous power, I just have a robust power supply available to use. I'm interested in a fabulous sounding class A amp with 50 to 80 watts into 8 ohms. I'm not trying to rebuild the same amp.

[Racket Scientist]

I wouldn't call a dual transformer PSU weighing in at about 40 lbs "skimpy" BTW.

[AndrewT]

Originally Posted by Racket Scientist

Any specific Pass designs I should be looking at? The original amp put out about 450 watts into 4 ohm.

in terms of what you stated here, any PA PSU will be skimpy.

Was that specification 450+450W into 4ohms?
If so then that would indicate a transformer VA of between 900VA and 2kVA.
It should also have between 20mF and 100mF on each supply rail and these should be rated for 100V duty.

[Racket Scientist]

Sorry, I was slightly off. Its 400 + 400 watts into 4 ohms. I'll plan to look later to see what capacitance it uses. So no Pass designs have rails of around +/- 75 volts?

[Nelson Pass]

If the primary voltage taps on the transformer can be set
for 240 volts, then you can extend the range of possibilities
by operating that at 120V.

[Racket Scientist]

Thanks for the response Mr. Pass. I'll have to look it over and see if I can figure that out.