IRF 640 for AlephP ?

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Hi folks,

Some time ago I bought a couple of IRF 640 (manuf. by IRF)
for a preamp in AlephP / Balzen Style.
I bought them because they were available here instead of the IRF610.
After reading the articles a bit more carefully I am hesitating because they do not seem the right choice for a Pass-style preamp.

What can I do with this MOSFETs ?

They are quite similar to the IRFP240 (a bit less "strong" and smaller, i.e. TO220 instead of TO247).
Is a (somewhat downscaled) Aleph Amp possible with that transistors?
Or a Zen? Maybe a SoZ is even better, because the resistors will take most of the heat.
I think it is probably possible to mount them with a bar to get proper contact to the heatsinks.
Any experiences?

On this very day I also bought a couple of IRF240 (in TO3 case) not manufactured by IRF.
This was maybe stupid, but so what, this was what I could get hold of.
Are these Transistors useful for a job in high-class ClassA amplifier?
Again, can anybody of you Pass-experts (or Mr. Pass himself) offer some ideas ?

best whishes & thanks
What to do

If you bought a couple of 640's, you did not waste too much money ... as they probably are about 50-100cents a piece depending on quantity etc. They could probably be used in a Zen or whatever if you mounted them directly onto a piece of copper with some electrically conductive thermal grease and isolated this assembly from the heatsink. This would make Rth junction to case the limiting factor. Compare this number against the 240.

Or you could make a low power Zen (headphone amp???) -- fewer devices, less capacitance.

Or you could use them as current sources for an X-100(0) knockoff

Or you could use them as "relays"

Or you could add an extra driver stage ...

Or you could throw them away ...

The problem with 640's is that they have more input capacitance than you want for an input stage. They would probably be fine in an application not requiring too much thermal efficiency. I also suspect that they would probably provide quite good bass response.

Now for the 240's -- they would be great in an Aleph, an X knockoff or whatever. Take your pick! If you don't have complementary devices (9240), you need to make an n-channel output stage such as Zen, Aleph, or other concoction.

TO3 case is good -- the best as far as thermals go. What I don't like about them is that they are made of ferric material (at least it looks like that) and cost + the fact that leads have to go through heatsink. Still nobody else seems to be complaining about this so it is probably a hangup I have.


[Edited by Petter on 07-18-2001 at 10:35 PM]
Thanks Petter for your answer.

I think your hint with the copper piece / heatsink assembly is quite clever.

BTW, american transistors are sadly more expensive here (in Austria), so it was more like three bucks for an IRF640 than 50 cents...

For the Non-IRFmade IRF240:
Nelson Pass always stresses the fact that the transistors he uses for his amps are made by IRF. Is that really so important? (Maybe all other MOSFETS are more for controlling electro motors or whatever.)
I think the fact of different brands of the same transistor type is much less discussed than say kinds of capacitors and resistors, although they are the actual amplifiing devices and therefore probably of greater influence on the sound.
I was (hobbywise) "raised" with german types of transistors (like the BCxxx or BDxxx types), always thinking their american or japanese counterparts or equivalents were better in certain aspects, but my experiences in the last years often showed up that this is only very rarely the fact.
Any experiences ?
Regarding semiconductor devices of the same type from different manufacturers, although the discussion of interest is on MOSFETS, I would like to mention one experience I had with Power Transistors.

I assembled a 175watt/275watt (into 8 and 4 ohms respectively) amplifier that was published in Elektor sometime ago, which used NE5534 for the input stage, MJE15030/15031 drivers and MJ15003/15004 for the output stage. I replaced the IC with a type TLE2141 (also tried OPA604) and had Motorolla output devices. The amplifier sounded quite good in all respects. For some reason I had to remove the output devices and replaced with the same type but from a different manufacturer, viz., MOSPEC. All other components on the board were retained except for the replaced output devices, and lo and behold, the bass was almost gone and the amplifier was sounding like some cheap STK series kind of thing. Since the PCB was the same and the only change that I had made, made all the difference, I have simply packed away that particular amplifier till date. Some day I plan to replace these with Motorolla devices.
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