MOSFET Amplifier IRFP240/IRFP9240

[apexaudio]

Simple Full Complementary Simetry +/-80V. Voltage amplifier/driver biased with 30mA. This driver circuit stabilized BIAS in output stage. Exelent stability without oscilation, hum and noise, sound great.
RMS Power: 240W 8ohm, 350W 4ohm

[apexaudio]

Circuit diagram work good even in PA amplifiers. I made over 120 PCBs in last 5 years. Beginer with basic expirience in DIY can easy made this amp. Usefull not only for hobby.

[event horizon]

If you had used Lateral Mosfets in the output stage then i guess you could get away with such a simple biasing scheme. Lateral Mosfets have a negative temperature coefficient so they'll tend to reduce quiescent current as they heat up.

IRFP240 & it's complement have a positive temp coefficient (until they are drawing many Amps of current) so they'll conduct more current as they heat up. There is no allowance for this in your circuit diagram, you'd need something similar to what is used in a transistorised output stage.

I might be wrong, but i don't think so

[apexaudio]

Bias stabilized with Q7,Q8,Q9,Q10. Q9 and Q10 are on same heatsink with output mosfets. This circuit stabilized bias even in PA use at the summer heat. This is not experimental schematics, uses over 10 years in amplifiers.

[Rikard Nilsson]

If you want to reduce distorsion significantly, you could add another set of transistors identical to Q8, Q9 connected to R11 in the same way Q8 and Q9 are connected to R10. The collector of the "new" Q9 connected to ground. The emitter of "new" Q8 connected to the old Q8.

And the same for the negative side. This way, the VAS is configured as a differential amp and is very likely to produce severly improved linearity. I will try this in my own amp soon, but the sims show very promising results.

[event horizon]

Originally Posted by apexaudio

Bias stabilized with Q7,Q8,Q9,Q10. Q9 and Q10 are on same heatsink with output mosfets. This circuit stabilized bias even in PA use at the summer heat. This is not experimental schematics, uses over 10 years in amplifiers.

Ok, im not doubting you Q9 & 10 are effectively emitter followers though, if you mount them on the same heatsink as the output transistors surely they can't compensate for quiescent on the output stage?

Perhaps i'm having a stupid attack due to a few beers My apologies if this is so...

[apexaudio]

Q9 and Q10 are not emitter followers, R21 and R22 in colectors Q9 and Q10 are in the same time in emiter Q7 and Q8.

[Rikard Nilsson]

I too doubt there will be adequate temp stability, but at 30 mA and a large enough heatsink it might not matter. The bias can increase somewhat, but the temp will perhaps stay within reasonable levels so the amp doesn´t go into thermal runaway.

[nigelwright7557]

Originally Posted by event horizon

If you had used Lateral Mosfets in the output stage then i guess you could get away with such a simple biasing scheme. Lateral Mosfets have a negative temperature coefficient so they'll tend to reduce quiescent current as they heat up.

IRFP240 & it's complement have a positive temp coefficient (until they are drawing many Amps of current) so they'll conduct more current as they heat up. There is no allowance for this in your circuit diagram, you'd need something similar to what is used in a transistorised output stage.

I might be wrong, but i don't think so

You are right.
But, I found I could get away with a simple bias if I kept the bias current down to an absolute minimum.

I just barely bias the amp into none crossover distortion using a signal generator and a scope. Its suprising how little bias current you can get away with.

I think its peavey use a similar view of using a very low bias.

[apexaudio]

Q9 and Q10 run with 30mA, bias of the output tranzistor 200mA set by P1, and bias stay stabil. Maybie must corect after few minutes but that will be small corection.