Peavey Deca 528 Class D output question

[CaptDon]

I have a question about how Peavey did the Class D output on the 528. There are two Mosfets to pull the center point up to +75vdc. Those are IRFP9240 and are P-Channel. But they only use one Mosfet (IRFP240) N-Channel to pull the center point down to -75vdc. This seems very strange. It isn't one of those oddball bridges where a diode is in place of a Mosfet. It isn't an H-Bridge, only a half-bridge. Those Mosfets are Q409 and its mate Q411 and on the lower side is the lonely Q410. Sorry I can't upload the schematic but it is readily downloadable from several sources. Any ideas or opinions of why they did two for pull up and one for pull down? Thanks!!

[CaptDon]

I see one possible clue. The IRFP240 which is alone in its function has an RDS of .18 ohm at 10vgs. The IRFP9240 which has a parallel partner has an RDS of .5 ohm at 10vgs. Perhaps they felt it was best to have a lower total RDS and paralleled up the pair?? This unit can drive 140vpp or 70vp or 49.5vrms (with zero headroom) into a 4 ohm load. The amplifier is however rated at 250 watts RMS per channel into 4 ohms both channels sinewave driven. These were single rack space amps noted for low end transient available power and admired by bass players for thump without the weight.

[JohnG]

You are probably right about the on-resistance. Depending on how much feedback there is, having unequal resistance can give some substantial even harmonic distortion.

Using a p-channel for the high side makes the drive easier since the source reference is the positive bus. Some designers even used capacitive coupling to drive the high side.

The problem is that now you have a lot more capacitance for the high side switch, and this has its own problems, not the least of which is cost. It seemed like a reasonable idea in the 1980s, but designers have access to better high-side drivers for n-channel FETs, and you don't see so many p-channel power MOSFETs these days.

John