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Old 21st July 2009, 05:17 PM   #1
percy is offline percy  United States
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Default High linearty low-power BJTs and Class A headphone amp

Hi,
I am researching for building a classic single ended Class A headphone amplifier. First of all are there any good designs around that I could start looking at ?

Secondly, if I design this from scratch I would like to use BJTs with high linearity (in hopes of using as minimal negative feedback as possible). Since this is for light headphone duty I don't expect current requirements to be in any access of ~20ma per channel. Any suggestions here ? Do you know of any good ones ?

Over to you for your thoughts....
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Old 21st July 2009, 05:31 PM   #2
jcx is offline jcx  United States
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headphones differ greatly in impedance and sensitivity: 16-600 Ohms and 88-119 dB/mW for dynamic headphones

so 20mA may not be enough - look at Gilmore's "A Pure Class A Dynamic Headphone Amplifier" on Headwize

http://headwize.com/projects/index.htm

quickest way to high measured linearity over the audio range is to use op amps - composite amps with separate high current output stages inside the feedback loop either discrete or using recent DSL driver CFA op amps can have unmeasureably low audio frequency distortion - well below the 120-130 dB spot noise floor of the better audio ADC cards

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Old 21st July 2009, 06:14 PM   #3
percy is offline percy  United States
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I forgot to mention - I want to really keep it 'simple'. Simple as in just one active device. Just one BJT per channel. I already have a good opamp amplifier(Corda Headsix) so want to try something different now.

am not as much concerned about 'measured' linearity as much as intrinsic/natural linearity of the device itself, before feedback.

yes I realize the wide range of headphone power requirements, but I don't intend to build a 'marketable' product to cater to that range. This is custom. If it works for the two headphones I have(AKG 501, Koss KSC75) that should be enough. And after some tests & trials I have concluded that I wouldn't need much over a 1V swing and 30ma draw.
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Old 21st July 2009, 07:02 PM   #4
jcx is offline jcx  United States
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simple and very linear is just using iPod's output for 1V, 30mA

"High linearity" isn't going to happen with a single device

if you expect a different sound from a "simple" circuit it will most likely be because the amplification is less accurate/linear


the best linearity from a single device would come from heavy bias so you use a smaller fraction of its nonlinear response curve

then you could argue over whether degeneration helps or if more low order distortion is less noticeable than the feedback introduced higher order low level distortion components

which also depends on the music you listen to - its commonly pointed out that "audiophile" demos often use "simple" music such that the IMD products with the higher levels of low order distortion doesn't muddy the sound as it would with a complex symphonic work
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Old 21st July 2009, 07:10 PM   #5
paulb is offline paulb  Canada
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If you really insist on one BJT per channel, your options are very limited. Just use a transistor applications textbook. I suspect you'll be unhappy with its performance.

To see what the simplest practical circuit would be using BJTs, take a look at this article to see why the author went from one BJT to three:
http://sound.westhost.com/project36.htm
This can be built up as either a speaker or headphone amp.

Also have a look at this:
http://www.tcaas.btinternet.co.uk/jlhphones.htm

If you don't insist on BJTs, and don't need gain, look at this:
http://headwize.com/projects/showfil...szeke1_prj.htm

And finally, this may also be a useful article:
http://www.passdiy.com/pdf/zenamp.pdf
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Old 21st July 2009, 11:47 PM   #6
CBS240 is offline CBS240  United States
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Quote:
Originally posted by jcx
"High linearity" isn't going to happen with a single device
especially from silicon.

If you must use a single device, I suggest you use a triode. It might have better linearity.
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Old 22nd July 2009, 01:41 AM   #7
lineup is offline lineup  Sweden
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Quote:
Originally posted by jcx

the best linearity from a single device would come from heavy bias so you use a smaller fraction of its nonlinear response curve

then you could argue over whether degeneration helps or if more low order distortion is less noticeable than the feedback introduced higher order low level distortion components
You can have a look at my:
Complementary Class A Preamp using BC368-369
Complementary Class A Preamp using BC368-369


Features:
- fully complementary with JFET input stage
- suitable for headphones because of
- high current output >100mA
- uses paralleled small signals output
- has very low open loop distortion (0.15% at OLGain=512 and 1 Vrms into 100 Ohm)
- and high open loop bandwidth by reducing gain in vas

Here is the schema from my open loop testings:
http://www.diyaudio.com/forums/attac...amp=1215949714

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Old 22nd July 2009, 02:04 AM   #8
iko is offline iko  Canada
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Quote:
Originally posted by CBS240

especially from silicon.

If you must use a single device, I suggest you use a triode. It might have better linearity.
It's not a bad idea.

Perhaps even a dht triode. I'm myself in the final stages of a new headphone design using one DHT triode directly coupled to a p-channel mosfet and output transformer (I'm not very fond of capacitors). I got some very nice simulated results; it remains to be seen if it also sounds good.
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Old 22nd July 2009, 05:33 AM   #9
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Default Re: High linearty low-power BJTs and Class A headphone amp

Quote:
Originally posted by percy

Secondly, if I design this from scratch I would like to use BJTs with high linearity (in hopes of using as minimal negative feedback as possible).
You hope does not match what would you like to get, since small amount of feedback does more of harm than it's absence, or a big it's amount. I advice you to read last pages of Bob Cordell's topic about negative feedback to understand what I mean.
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Old 22nd July 2009, 09:03 AM   #10
h_a is offline h_a  Europe
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There are a few nice and simple headphone ideas out there, like Grey's simple mosfet follower

MOSFET follower headphone amplifier

In this thread you will also find a very nice jfet follower headphone amp (DAO). Unfortunately the power jfets are hard to get.

Have fun, Hannes
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