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Zero Cool 3rd December 2005 12:29 AM

Best Design Using 9240/240's??
I just scored 2 brand new amps that use IRF 9240/240 mosfets with +/-75VDC Rails and extra secondaries a bit higher for the driverboards.

These are Renkus Heinz Commercial amps, P2400's. and i want to use the chassis, sinks, trafo's etc to rebuild into some good sounding home amps.

Each channel uses 4 pairs of 9240/240's, so i would potentially have 8 pairs per chassis to work with. I was thinking of building something ALA Pass Style. but i want to keep the high power output. these amps were originally rated for 250@8 per side and 350@4.

I am looking to keep these at 200W or higher per block.

Obviously these were made for commercial use, and as such not designed for high fidelity. and i am quite aware of the differences. but i believe the PUS and output section to be made from some decent quality parts. with the addition of new driver boards. these should sound no different that if i hade built everything from scratch myself.

Is there a Pass Style design that would lend itself well to such an application? Something like the X-250 maybe?

The schematics are freely available on there website:


BobEllis 3rd December 2005 03:34 AM

At those rail voltages you are in X1000+ territory, unless you run the primary at the 220V setting and reduce them to 37V. That ought to get you into the X200 range.

One option is an older Pass-Thagard design - the A75 run in AB. I bumped the rails up on my version, but only to 60V on the output. You'll need to update the input differential, since the dissipation will be too high for the IRFD type devices. I bent the pins on IRF610s and 9610s to fit the AudioXpress boards. My front end rails are 65V.

You may need heatsinks on the input devices unless you reduce the current. You should also adjust the zener current setting resistors to keep the dissipation in the zeners and resistors reasonable, although you'll probably want 1/2 watt resistors there.

Run the numbers and check dissipations, you'll want decent heat sinks on all front end devices. use multi turn pots for all three.

At low bias you'll want some sort of thermal compensation. I moved the Vgs multiplier to on top of one output device, and it seems to work.

Depending on how much bias you use, four pairs of devices would probably be OK, so you could build four channels. Is there room more pairs on the heat sinks? converting to a monoblock with 8 pairs ought to allow you to bias fairly heavily, if your heat sinks are up to it.

Others may have better ideas.

amplifierguru 4th December 2005 04:46 AM

Hi Zero Cool,

Isn't that a co-incidence, my 300W 'modulated bias' cascode 300W amp uses the same complement for 300W into 8 ohms from +/- 75V rails! It wouldn't need the boosted supplies as it is already R2R with the common source output stage. See -

However I wouldn't use this high a supply for 4 ohm and suggest something more like +/- 60-65V if 4 ohm use is envisaged.


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