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Old 17th October 2008, 10:52 AM   #1
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Unhappy mos/bjt buffer in class A for headphone amp

Hi,
i have 300ohm headphones at my home ... i would like to make a headphone amplifier using tube at the input and havig a fet or bjt buffer at the output....

i was looking at different schematics from Broskie circuits, SDS Labs headphone amp, Borbelly headphone amp concept, Raymond headphone amp concept that is using 5670 tube and a simple FET buffer ( http://raymondaudio.nl/index.html ), some Szekeres amps and mod's, Sijosae Hybrid v2.0 and Nano, Millett Maxx and Starving Student....some others not mentioned also....

so... right now i am a bit confused.... which buffer to use.... i need relatively simple buffer that would be able to drive my 300ohm headphones with quality and drive them easy....

can someone tell which buffer is really good.....
i am listening jazz, blues, ambient... so it should perform good with acoustic and electronic music.... demanding music and simple music......
please help...!!
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Old 17th October 2008, 07:26 PM   #2
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sad to see that there are no answers..... tough question i guess...oh well....
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Old 18th October 2008, 09:04 AM   #3
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300 Ohms headphones need not so much current.
In fact they take almost 10 times less current than 32 Ohm.

For example 3 Volt peak output is only +- 10 mA.
(this corresponds to a measured output RMS Voltage of ~ 2.0 Vrms)

One Class A push-pull transistor buffer, biased at 10 mA idle,
can give out close to 20 mA peak output ( 2 x 10 mA ).

To reduce distortion and have some margin,
I would suggest a push-pull stage like 25 mA (=max 50 mA peak out)
Theoretical max Class A output voltage:
50 mA x 300 Ohm = 15 Volt peak! ... this is 10.6 Vrms
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Old 18th October 2008, 09:29 AM   #4
lineup is offline lineup  Sweden
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Click the image to open in full size.
http://www.diamondstar.de/

This is a push-pull Diamond Buffer that is used for Headphones.
As high gain input transistors is used,
you can set the input impedance quite high.
This way buffer wont load your tube output much anything.

For 300 Ohm you may only need one output pair MJE243/MJE253.
And you can use BD139/BD140, as well.

And also use some other TO-92 transistors for input Q6, Q7.

To adjust, set the bias Output current you want, you trim with P1
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Old 18th October 2008, 03:31 PM   #5
jcx is offline jcx  United States
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I'd want 200+mWrms clean power
+/-12 V, 40 mA would be my minimum target for typical 96-98 dB/mW sensitivity 300 Ohm headphones to avoid clipping on 120 dB dynamic peaks in well recorded music

the main online source for diy headphone amp designs is www.headwize.com
check the library

look at
http://headwize.com/articles/hearing_art.htm
for a hint at how loud dynamic peaks may be in real music
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Old 19th October 2008, 06:24 PM   #6
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thanks guys....

i have been on Headwize.... a lot of examples there - and the main problem is that i do not know what to choose ...

i like the Szekeres buffer with LM317 connected as ccs.... seems to me that it is easy to make .. ... but - again i am not sure about the result....

any opinions about that buffer....????


other thing - i have a "feeling" that i need a lot more bias current than 10 or 20mA ... it's just that my head's did not work very good on amps that had low bias.... so i would like to try with have biased class A - something like 100mA or 150mA..... any opinions on that one???
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Old 20th October 2008, 05:11 PM   #7
jcx is offline jcx  United States
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if you want to just drive 300 Headphones then the bias current values may look small next to designs intended for 32 Ohm headphones

more bias/signal ratio does reduce distortion - I would probably consider 2X bias to peak current demand to be the edge of dimenishing returns - unlike multi-way dynamic loudspeakers single driver headphones don't have deep impedance dips
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Old 20th October 2008, 06:34 PM   #8
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o.k. - thanks for the answer..... it is just that i "feel" that I need more current....because i had some head amps that did not performed well with currents arround 10-20mA....it was not good enough... i never tried a lot more current - so maybe i need more - i am not sure any more....

i would like to try mosfet amps with a lot more bias current than needed.... maybe this will solve the problem - i really do not know any more.....confused a lot ....
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