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Old 24th February 2004, 08:48 AM   #1
JanDH is offline JanDH  Netherlands
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Default BAsic questions Aleph-X

Hi,

Some basic questions:

- Resistors in the BOM are 1/2 watt unless otherwise specified?

- What are for audio purpose the most important specs from a power FET.

- Do you have to double the input capacitance when applying a second driver and compare this with applying a FEt with more power. So applying two IRFP240 or one IRFP1405?

- Where can I find this CLC-filter schematic.

Thanks,

Jan
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Old 24th February 2004, 12:44 PM   #2
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Quote:
IRFP1405
Don't use that one. It is an excellent switch, but a bad amplifier. Also, it is TO-220 and is definitely not suited for high power losses.
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Old 24th February 2004, 12:50 PM   #3
JanDH is offline JanDH  Netherlands
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Thank you Holger,

What is the typical difference compared to the suggested fet types? According to IRF-engineers their components are not suited for audio at all.

Regards,

Jan
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Old 24th February 2004, 12:53 PM   #4
wijgers is offline wijgers  Netherlands
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Default IRFP140

And What About the IRFP140.

Need I adjust some resistor values in the schematic?

Nils
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Old 24th February 2004, 01:47 PM   #5
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Concerning Power FETs in general:
Every manufacturer gives us unbelieveable power ratings and current specs, especially for the modern low-voltage types (have a look at IRF2804 for example). This is alle very pretty, but absolutely useless in RL. Those specs are extrapolated values for chip temparatures of 25C or so. You won't be able to keep the die on that level, no matter what you try.

Also, those current ratings of +150 amperes are BS, because the case pins just can't stand it. At least IR has a very small note in their datasheets covering this issue. As far as I remember, they allow max. 50 amperes on a TO-220 pin.
So, all those modern high power types don't really help us because we can't use them at their theoretical limits. When chosing a suitable FET, I would care for the following points:

- low capacitance figures. This usually applies to the higer voltage types (100-200V).
- a case that is capable of transferring some power to the heat sink. This is TO-247/TO-3P these days, as the classical TO-3 is more or less dead.
- As you won't use the FET at more then 50 Watt power loss and 10 Amperes of current, the related specs are not that important.

Of course there are some more points: linearity, distortion figures, and so on. I would stick to the types that seem to do a reliable job: IRFP240 (wich should do better then IRFP140), IRFP044 for example.
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Old 24th February 2004, 01:57 PM   #6
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--Half-watt resistors will be just fine. You can even use 1/4W if you want. Signal levels are really quite small until you get to the output. Be sure to use larger resistors (I used 3W) for the output current sensing array.
--The important numbers for a MOSFET are the voltage, current, and watts of heat dissipation that it will take. It's not a bad idea to stay below half of the published specifications. If you want to take it a level deeper, you can start looking at the Gate capacitance and such. In general, less is better, as it takes more current to drive a higher capacitance. The gate capacitances will add together for devices in parallel.
--A CLC filter is so simple that you don't neeed a schematic: Cap from rail to ground. Inductor in series, heading for the next cap. Cap to ground. Then off to the circuit. Some books refer to it as a PI filter because when you look at it on paper it's shaped like the Greek letter.
--TO-220s work fine for outputs. I used IRF644s for my Aleph 2s. Yes, the TO-220 has less heat dissipation capability than some of the other packages, but as long as you as you watch your wattage, it'll do the job.
--The IRFP140 won't require any changes.

Grey
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Old 24th February 2004, 06:22 PM   #7
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Quote:
Originally posted by JanDH
According to IRF-engineers their components are not suited for audio at all.
Gee, I guess I'll have to give back all the money I've made over
the past 13 years.
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Old 24th February 2004, 08:10 PM   #8
grataku is offline grataku  United States
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Never mind the IRF engineers, they got plenty of money.
If you feel like giving money away for no particular reason give _ME_ some!
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Old 24th February 2004, 09:42 PM   #9
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Well, engineers also said that 4558 was a good audio opamp.
Actually i think a coupple of parameters worth checking before choosing fet: Drain-Source resistance during on-state, especially if You plan to use the amp with higher Q speakers.
Most important, amount of blue smoke contained in the device.
This is easily checked by, for instance, creating a short in a driver transistor and thereby letting the entire supply reserve rush through one of the output fets. The amount of blue smoke it then lets out is directly proportional to how good it could have sounded.
Seriously : The new TO 247 and other plastic siblings of TO3 are good but need to be bolted very tight to the heatsink.
Just one tiny m3 screw wont do, use washer or metalplate to distribute fastening torque evenly across the transistor body and always use heat transfer compound even if You use silicon isolators.
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Old 24th February 2004, 11:54 PM   #10
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Quote:
a coupple of parameters worth checking before choosing fet: Drain-Source resistance during on-state
I hope this belongs to the joke part of your post...
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