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HEXFET headphone amp !?
HEXFET headphone amp !?
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Old 5th December 2017, 02:21 AM   #11
PRR is offline PRR  United States
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Yes, they need 4V drive... for 20 Amps flow!! We don't need to hit them that hard. Headphones don't need even 300mA, 0.3A.

The 1V @ 250uA is not directly useful, no. But this and the 4V @ 20A define a curve that we want to find mid-points on.

The total range 250uA to 20A crosses at least two MOSFET modes, and we can't know the exact curve of the line. But it will be roughly straight, with a bend, plotted on the log/lin paper the spec-sheet uses. I've attached my guess. I think the headphone current range is 1.5V-2V. This is very approximate, and the bias MUST be tuned to the actual devices.

This is not a "bad" voltage for a 12V headphone amp. However the next question is to fill in the rest of the plan. Which is really a request to give free design services so that some very specific "free" MOSFETs can be used. Which is considerable thinking time, and more time to write/draw it up.
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Old 5th December 2017, 12:47 PM   #12
xXBrunoXx is offline xXBrunoXx
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That step-up converter would be great and cheap , but not avaiable where I live. Im from Romania.
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Old 5th December 2017, 12:50 PM   #13
xXBrunoXx is offline xXBrunoXx
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I've tried to make myself an dc-dc step up From 5v by 555 and irfz44n but failed.
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Old 5th December 2017, 12:55 PM   #14
xXBrunoXx is offline xXBrunoXx
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If I can't really use these hexfet's , easiest way would be to use some op amp + BD139-140 by rod elliot's headphone amp schematic on + - 2.5v , 0.6v for the transistors , would I get at least 1v - 1.5v output on 5v supply ?

Since my phone/ mp3 player doesnt have the "omph" for my headphones ( 32ohm ) I want a small portable headphone amp ( 5v would be great as I have one external battery 5v 2amp output ( 6amp capacity )
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Old 5th December 2017, 01:42 PM   #15
xXBrunoXx is offline xXBrunoXx
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Weird thing , I've tired some schematics in LTSpice to " play " with mosfets, see their Vgs , how much to open them, etc. Every mosfet even at 4 - 6v Vgs it draws a few 5-10 - 30 amps from the rails, this is weird .
Even this one only with one 1n4148 across gates I get 11v Vgs and 88A from rails. Even with 4-0-4 V RAILS I get 3amps .. Something wrong with my LTSpice maybe...?
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Old 5th December 2017, 01:47 PM   #16
Mooly is offline Mooly  United Kingdom
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HEXFET headphone amp !?
Your 5 volt requirement is the killer for all these.

Why not have a look at something like the TDA2822 chip which operates down to a couple of volts. Fig 1 in the data sheet is all that is needed. Add a volume control and possibly a low value series feed resistor to the headphones and your done.

http://www.st.com/content/ccc/resour...CD00000134.pdf
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Old 5th December 2017, 01:50 PM   #17
Mooly is offline Mooly  United Kingdom
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HEXFET headphone amp !?
If you post your .asc file I'll take a quick look.
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Old 5th December 2017, 01:54 PM   #18
xXBrunoXx is offline xXBrunoXx
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Here, Thank you .
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Old 5th December 2017, 01:59 PM   #19
Mooly is offline Mooly  United Kingdom
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HEXFET headphone amp !?
M2 is your problem Have a close look.
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Old 5th December 2017, 02:05 PM   #20
Mooly is offline Mooly  United Kingdom
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HEXFET headphone amp !?
Its upside down.
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