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Old 1st May 2002, 01:43 AM   #1
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Question Switching FETs for Audio?

Hi Guys,
I've been looking at a few data sheets, and it seems to me that it's possible to use switching FETs for amplifier output stages:
1: most FETs are designed for switching anyway
2: Switching FETs are much cheaper; I can get MTP3055 for around AUD 2 each, got some IRF740 free out of a junked power supply (so why did I pay AUD 18 each for a fancy Hitachi pair???)
3: If it's for a guitar amp, most Guitar hero wannabes are going using distortion anyway, so the amp doesn't need to be audiophool clean (an audiophool would use BJTs anyway)

Looking at the data sheets, it seems that the switching fets have higher cutoff voltages than the Hitachis, but surely this is just a matter of having the appropriate bias voltage? (I'm looking at the transfer charachteristics graph)
(I'm assuming also that the devices have acceptible Ciss & Rds)

Shoot me down in flames or enlighten me if you think I'm barking up the wrong kettle of fish. I'm going to try an output stage using the switching FETs first, then try it with the Hitachis (too chicken to risk $36).

Cheers, Pete McK
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Old 1st May 2002, 03:11 AM   #2
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1. switching devices often have a greater bandwidth and so if care is not taken in the design of such a circuit, then you are likely to have oscillations.

2. mosfets vs. bjt ..... dont count on it!

3. the hitachi fets are lateral fets and can happily be biased with a completely passive circuit however switching mosfets are the vertical type and usually like a like more complex bias circuit especially if bias is to be kept low where these devices have a +ve tempco.
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Old 1st May 2002, 09:46 AM   #3
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Try it out!

My guess is that most devices have not been tested for audio use. If you can get units for free, why not try them out and let the rest of us know?

If you can choose between devices, try the ones where the input capacitance varies least over the operating range, or select the ones where the input capacitance is the lowest.

Petter
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Old 1st May 2002, 11:48 PM   #4
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Default Switching Fets

Thanks for the comments Guys, gave me something to think about while I'm playing with these devices.

re: FET vs BJT; just stirring the possum, of course real audiophiles use toobes....

cheers, Pete McK
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Old 6th June 2002, 04:22 AM   #5
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Using switching fets for audio is my reason for being alive too.
See the thread
http://diyaudio.com/forums/showthread.php?threadid=3891

Not a whole lot of info yet but it's not just you alone thinking about this! Will post more to that thread as the experiments proceed. So far they are MOST encouraging

GP.
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