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Old 5th November 2006, 05:18 AM   #1
zox2003 is offline zox2003  United States
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Lightbulb IRFP... or MJ15003/15004_MORE POWER???

I have another question that would help some people:

On +/- 42V which combination would be able to give us more audio power IRFP240/IRFP9240 or MJ15003/MJ15004.So basically MOSFET vs. BIPOLAR?

Make coment about sound quality too,please?

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Old 5th November 2006, 07:04 AM   #2
quasi is offline quasi  Australia
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It depends on the configuration and other variables. There are simply too many to discuss in detail. For example with seperate rails feeding initial stages, a mosfet amp can be very efficient.

With traditional rails transistor amps are generally more efficient, but mosfet amps can be pushed harder (for similarly rated devices). It also depends on how many devices you use (how much they cost, will they fit? etc.)

Sorry but my answer depends.

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Old 5th November 2006, 09:45 AM   #3
AndrewT is online now AndrewT  Scotland
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Hi Zox,
a very simplistic way to assess output power capability is to calculate the total device dissipation and divide by 5, to gauge how much power is available for a load that does not exceed 45degree phase angle.

A pair of 15003/4 have 500W so can manage 100W @ 45degrees (driven all day at continuous high power when adequately cooled).

A pair of 240/9240 have 300W so can manage 60W @45degrees under the same severe conditions.

Note that many To3 packaged devices are rated @ 200degC whereas plastic packaged devices are often limited to 150degC.
This makes a big difference when running very hot.

When paralleling pairs, the allowable Vce can be increased and BJTs then become restricted by the limited SOA at elevated Vce. This must be designed for!!!!!
regards Andrew T.
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Old 5th November 2006, 08:17 PM   #4
zox2003 is offline zox2003  United States
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so,just running 1 pair of MJ15003/15004 would give me more power than IRFp240/9240.Right?
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Old 5th November 2006, 08:49 PM   #5
ilimzn is offline ilimzn  Croatia
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Originally posted by zox2003
so,just running 1 pair of MJ15003/15004 would give me more power than IRFp240/9240.Right?
You seem to be reading only that part of AndrewT's reply that you like?
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Old 5th November 2006, 09:01 PM   #6
zox2003 is offline zox2003  United States
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I am so glad I see you here again.You were very helpfull before.What is your opinion on this topic?
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Old 5th November 2006, 10:02 PM   #7
djk is offline djk
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"so,just running 1 pair of MJ15003/15004 would give me more power than IRFp240/9240.Right?"

If both are run with driver circuits using higher voltages than the outputs, then the power out will be essentially identical.

The calculations of AndrewT indicated that one pair of the IRFP240/IRFP9240 shold be restricted to driving 8 ohms, where the MJ15003/MJ15004 could drive 4 ohms (on 42V).

Hexfets require special attention to bias, and require matching for parallel use. Three pairs of paralleled MJ15003/MJ15004 can do up to about 300W into 4 ohms before running into SOA problems. It would require five pair of the IRFP240/IRFP9240 to have roughly the same safe area for the same 300W at four ohms. Many FET designs use less conservative amounts of safe area, but my experience has shown that those amps don't run as long before requiring a re-build either.

Sound quality?

I doubt many are going to try and build a class A 'X' amp from Pass Labs, so we will exclude that one. I haven't had a chance to hear the SKA from Greg Ball, so I will have to exclude that one too. FET amps have been commercially available now for about 30 years. With the exclusions noted above, I have not heard an FET amp that could best the best of the BJT amps. That is not to say that the best-of-the-best FET amps could not sound better than the less-than-the-best of the BJT designs.

I would like to see a modern version of William Chatter's servo FET amp from years back. With a grounded output stage (like a QSC), it would be easy to do. The FETs could be paralleled without matching, so large amps would be possible. The IRF hexfets are so cheap right now that it sparks my interest (even though the cost of the outputs is probably the smallest part of the cost of an amp).
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