The Smoking Mosfet Amp!

[Zero Cool]

I just aquired an amplifier that has blown up twice and been repaired twice. now its dead again and mine!!! Lucky me!

Looking at the schematics of this amp. there are several things i have noticed right away. first off, there is no output inductor. 2nd, there are no drain or source resistors so the output devices will need to be matched pretty carefully. 3rd, what an ugly BIAS arraingment!

The BUZ901P/906P output Mosfets are 200V 8a devices so i should be able to swap in some 240/9240 IRF's pretty easy. i have those on hand and there higher currrent capacity devices.

Suggestions on the rest? This will see pretty rough use so i want to make it as stable as i can.

Zc

[amplifierguru]

Hi Zero Cool,

You'll need about 6Vbe's in series with that bias pot to Tcomp the IRFP's. Simplest way is 6 x 1N400x against the heatsink or a suitable value thermistor say 47K (depending on coeff) across the pot.

Cheers,
Greg

[Workhorse]

HI ZC,

Its Either an OHM FL700 amplifier using Lateral Magnatec Mosfet or just a copy of it...a ditto....isn't it...

If you want to replace the mosfets with IRF types..then do as Greg has already said....

regards,
K a n w a r

[zinsula]

Zero Cool

Look here, seems basically the same except the output stage.
Power Amp Schematic

If you will use vertical MOSFET's like the IRF's, you need also source resistors, unless you can match them VERY tight.
Also, verticals have higher capacity, you may get in trouble driving 3 pairs with this circuit up to higher frequencies. Feedback will not be able to correct, and nasty distortion may rise in the upper audio band.
With the originally intended laterals (you can use Exicon, Renesas, Magnatec) everything is easier.

Tino

[Zero Cool]

This is actually a UK made Bass Guitar Power amp section! Supposedly a very good brand. but i have head many stories of these blowing up!

they have several amps that all use the same output topology. so i am looking to come up with a solid mod for these.

Thanks for the input so far. Keep it coming!!!


Zc

[Zero Cool]

Originally posted by amplifierguru
Hi Zero Cool,

You'll need about 6Vbe's in series with that bias pot to Tcomp the IRFP's. Simplest way is 6 x 1N400x against the heatsink or a suitable value thermistor say 47K (depending on coeff) across the pot.

Cheers,
Greg

So all siz 1N00x's should be in series, and that string in series with the BIAS pot?? Does it matter which side of the pot they are on??

I imagine i should mount each diode on each mfet or near it on the heatsink.

And what about source resistors? does it matter if i use .1 or .47 ohm resistors? i have both in stock.


Im hoping the fets really arent dead. most of the soundcraftsmen amps i have worked on that use the Hitachi mfets usually the drivers die and not the mfets. so maybe i will get lucky! HA, probably not hahahahah


Zc

[Ultima Thule]

Lateral FET's don't need temp comp/source resistors against thermal runaway which make them MUCH easier to work with.

Do you need new FET's then look below!

Cheers Michael

[Zero Cool]

This uses TO-247 devices so i cant use the J50/K135 set. i actually have some of those in stock...

[amplifierguru]

Hi Zero Cool,

Even the 6 diodes (in series and with the pot between the gate drive nodes) looks messy beside a single NTC thermistor. For a Bass Guitar Amp I'd go that way - just between the +/- gates on the heatsink.

It might cost a little more but easier to mount. Of course you will still need an extra 2-3V of Vbias so put in a green LED.

Cheers,
greg

[Zero Cool]

Where in the world would the LED go????

Here is a photo of the schematic with a NTC added. is this correct???