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HEXFET headphone amp !?
HEXFET headphone amp !?
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Old 4th December 2017, 02:43 PM   #1
xXBrunoXx is offline xXBrunoXx
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Talking HEXFET headphone amp !?

Hello, can I design a portable headphone amp runing on 5- 12v with these hexfet's ? http://skory.gylcomp.hu/alkatresz/irf7319.pdf
Maybe an op amp driving them ?
They are out of an DVD 5.1 , 2pairs for one output , with some weird Ic's before them 3state buffer sop20 ( fairchild P27AB MM74HC )
I only want to use the HEXFET'S
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Old 4th December 2017, 02:50 PM   #2
Mooly is offline Mooly  United Kingdom
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HEXFET headphone amp !?
Very unsuitable for low voltage design work I would say (have a look at the transfer characteristics in the data sheet ?).
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Old 4th December 2017, 03:10 PM   #3
xXBrunoXx is offline xXBrunoXx
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on the original schematic ( DVD ) they work on 15v
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Old 4th December 2017, 03:25 PM   #4
Mooly is offline Mooly  United Kingdom
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HEXFET headphone amp !?
There is a massive difference between 15v and your stated requirement of 5-12 volts. A 5 volt rail would not allow any drive circuitry to generate sufficient G-S drive voltage.
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Old 4th December 2017, 05:24 PM   #5
xXBrunoXx is offline xXBrunoXx
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Gate Threshold Voltage would be the " on " ? like Vbe 0.6 for BJT's to make them " on " ? ( it suggests 1v Vgs ) or How much voltage I need across G-S to " open " the mosfet?
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Old 4th December 2017, 05:43 PM   #6
Mooly is offline Mooly  United Kingdom
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HEXFET headphone amp !?
Gate/Source threshold is the point at which the FET begins to come into conduction. The data sheet quotes 250uA at that point, which won't get you very far driving headphones

Nothing happens below that voltage point, and to turn it fully on requires as much as 5 volts, and you can go even higher than that to squeeze the last drop of 'conductivity' out of it.

They just wouldn't be my first choice given you mention going as low as a 5 volt supply as part of the design brief.

Don't let that stop you from experimenting though
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Old 4th December 2017, 06:20 PM   #7
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I haven't use mosfets just once in a design , there was around 4v g-s it had some distortion. I dont really know how to bias them. In a push pull config , Headphone Amplifier Can i replace Q1 Q2 with these fet's , how do I calculate R5 /6 , vbe multiplier instead of D1 D2 maybe ..
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Old 4th December 2017, 07:23 PM   #8
Mooly is offline Mooly  United Kingdom
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HEXFET headphone amp !?
It would not be straightforward. The supply voltage (-/+15) is fine but the whole bias network is totally inappropriate for FET's. D1 and D2 would have to be replaced by something generating a higher voltage... you might get lucky and a combination of series diodes or diode and zener or even LED got you the desired current, or you might have to design an adjustable multiplier to give fine control over the bias setting. At least the 10 ohm resistors should keep any thermal effects in reasonable check.
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Old 4th December 2017, 09:48 PM   #9
xrk971 is offline xrk971  United States
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HEXFET headphone amp !?
Use a DC-DC step up converter and run them at circa 18v to 24v and they should work well. You could use any number of SE Class A designs available here or even push pull designs like an F5 topology. One intersting thing to try is to have the N-channel set up as a CCS and the P-channel as the output stage in a SE Class A. It would be very compact.

Heat removal might be an issue with such a tiny package - maybe metal core SMT PCB's?

I have used these - they work really well and noise free if passed through a simple CRC filter:
DC DC boost converter Constant Current Mobile Power supply 10A 250W LED Driver-in Integrated Circuits from Electronic Components & Supplies on Aliexpress.com | Alibaba Group
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Old 4th December 2017, 10:03 PM   #10
johnnyx is offline johnnyx  United Kingdom
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You could use logic-level FETs. Maybe the VN2106 and VP2106 as a complimentary pair. They could be paralleled up to drive headphones.
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