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Old 19th January 2007, 11:49 AM   #1
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Default Impressive class A headphone amp

http://www.hififorum.nu/forum/topic.asp?TOPIC_ID=57596

Martin from Sweden has measured with Audio Precision and the results of his class A mosfet headphone amp is very impressive. 0.00032% !!! and the noise is very low also.

http://www.hififorum.nu/forum/topic....TOPIC_ID=33313

I'll guess it's a bit hard to understand swedish but the measurements and the schematics may say something.
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Old 19th January 2007, 06:19 PM   #2
azira is offline azira  United States
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It's an impressive number. CCS driven Darlington VAS contributes to a very high Aol which probably really helps but also lowers the output impedance. I like the output stage a lot as well.

To my suprise, I recently found out that the IEC standard for headphone amps is for a 120-ohm output impedance. Most headphones (including my HD555s) are designed for that. While most traditional SS amp design attempts to create near 0 output impedances, I'm amazed that virtually no DIY headphone amp designs I have seen conform to that standard. IMO it could be as simple as a 120-ohm output resistor in series with the load or as complicated as attempting to control Rc/Re and hFe with Ic

What are anyone elses thoughts on this?
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Old 19th January 2007, 08:59 PM   #3
Nordic is offline Nordic  South Africa
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Naah, its pretty well known, I think most people aim for lower values first for less attenuation, once you killed any remaining hiss, Bob's your uncle...
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Old 20th January 2007, 02:19 PM   #4
AndrewT is offline AndrewT  Scotland
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Hi Azira,
it would be very simple to wire in a fixed 120R resistor after the output of any headphone amp. It would also make stability very easy to achieve.

However, I suspect that wiring in a switchable parallel resistor bank alongside the120r would show that better sound quality comes from quite low values of output impedance. Don't be surprised to find that 10r to 22r (and maybe a lot less) is probably near optimum.

Tell us after you have tried it.
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