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Old 6th October 2011, 01:23 AM   #1
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Default my Class-A MOSFET headphone amp

Just thought I'd open a thread about my project I showed @ BAF. I was in the Tesla room. Phones I used were the Denon AH-D2000.

The circuit is rather straight forward except for the diff pair. I'm trying something different.

Input transformers are Jensen JE-11P-1 not shown in the schem. The amp can drive speakers just fine, too. Idle current right now is at a beefy 1.3A
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Old 6th October 2011, 03:25 AM   #2
Mikett is offline Mikett  Canada
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Hmmmm.. I got 4 pairs of those output mosfets and in need of a headphone amp.
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Old 6th October 2011, 08:35 AM   #3
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Jump in, the water is delish. I'll post a more accurate schem soon
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Old 6th October 2011, 04:19 PM   #4
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Currently thinking of making a 4-channel version at higher rails in prep for a Denon DN-A7100 surround preamp to do the rear channels.
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Old 6th October 2011, 08:45 PM   #5
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At 1.5Vrms 1kHz output into 8 ohms (real speaker), 0.0095% THD+N. Rockin' Residual shown as mostly noise, too. I forget the residual of my sine generator, but it's something around 0.0035%

10kHz Square wave performance not so good, though. A bit of ring, but I'm working on it in spice to see how to arrest the 1MHz ring without losing slew.
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Old 6th October 2011, 11:25 PM   #6
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I will be waiting to audition the next beta

--Rob
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Old 6th October 2011, 11:34 PM   #7
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1.3 A bias. (!) If I read the schematic correctly, |V+|+|V-|= 30 V so that's ~40W idle.

Those Denon cans are 106dB/mW and 25 ohm. 50 mA bias would be more than sufficient to maintain class A operation under real world listening conditions.

What is you rationale for driving them with such a high power amplifier?

I'm curious as I went through much the same thing recently, and came up with a much, much lower figure.
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Old 7th October 2011, 12:21 AM   #8
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Spot on rjm... 40W. The fins are big enough. The TO3s cook to 41C.

1) mainly for fun to see how far it goes. I like seeing how the Hitachis back off 10% once they warmup. Gotta love that negative tempco
2) it was meant mainly for headphone, but will also be used with some small speakers, too.
3) I reversed it down to 350Ma idle per channel calculating for 2W output, rather than thinking max SPL. See the chart (hope it is readable) ->
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Old 7th October 2011, 12:29 AM   #9
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Thinking of increasing the transistor count by 4 for my next one.. I'm still thinking about it... This solves the ring.
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Old 7th October 2011, 12:32 AM   #10
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Quote:
Originally Posted by hamoohead View Post
I will be waiting to audition the next beta
You will be the first to see it. I really can't do much about avoiding you if I wanted to.
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