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Old 18th April 2005, 08:34 PM   #1
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Default headphoneamp with demands..

hi Guys!
-can you help me here?
i am looking for a schematic for a headphoneamplifier that correspond to my demands:

*Discrete design, no ic:s.. (mosfet or bjt:s)
*Single end (no output capacitor)
*Runs on 9V single supply (maximum 9V! or in worst case dual 9V supply)
*Class A
*Low noise
*As simple as possible, no more components than necessary.
*preferrably zero-feedback

-any circuit ot there than can match my slightly hard demands?


ps. i would be happy if this circuit can act as a good preamp as well.. 8-) its intended to drive 32ohm cans..
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Old 18th April 2005, 09:24 PM   #2
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I can't answer your question but did you try Headwize? They have lots of projects listed.

http://www.headwize.com/projects/
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Old 18th April 2005, 09:40 PM   #3
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How can you want simple but also not want an IC.

ICs work great. I use the LM4881 in every headphone amp I build. It's small, has a lot of power, and only requires a small output cap. (the surface mount one I use is much less than 1/2" wide and tall). Plus it runs on less than 9VDC, it runs from 3-5VDC.
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Old 18th April 2005, 10:34 PM   #4
PRR is offline PRR  United States
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*Discrete design, no ic:s.. (mosfet or bjt:s)
*Single end (no output capacitor)
*Runs on 9V single supply (maximum 9V! or in worst case dual 9V supply)
*Class A
*Low noise
*As simple as possible, no more components than necessary.
*preferrably zero-feedback


Your "demands" are very specific. They do not say "sounds good", "long battery life", or "kind to headphones". In fact there isn't a gain or power specification. Since these goals are not mentioned, I'll ignore them.

This plan has no output cap, eats 9V single-supply, is Class A, will have low noise (referred to input), couldn't be much simpler, and insignificant feedback (negative or positive).

Click the image to open in full size.

It will also melt headphones and drain batteries. If not, then it will run around 25% THD at full output.

> my slightly hard demands?

Some of your "demands" are contradictory. OK, I can do "single supply" AND "no output cap", if you give me a choke or transformer. No output cap (or choke) and no phone-frying means control of the DC, and DC-coupled amps tend to be less-simple. Or "simple" if you use an IC, which after all is the same parts as you buy discrete. And BJTs can't be tamed without some feedback.

Maybe instead of the buzz-word feature-list that you want to appear on the advertising, you should say how you will use it (in-home, walk-about), what headphones (makes a difference), and solicit suggestions from people who are "happy with their amps"?
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Old 18th April 2005, 11:27 PM   #5
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the Szekeres Class-A Mosfet Follower fit all the requirements if +/- 9 volts is the power supply but NOT nine volt batteries !
you will need both polarities to cancel out the DC on the output or isolate with a transformer as PRR said.

Even better better would be a higher voltage.Nine volts in itself limits the design

Class-A into 32 ohms and little tiny batteries unless paralleled in a large quantity is not a mix that is happening unless the output current is severely restricted and in that case the 32 ohm headphones are out.

you will need to re-evaluate your "tight" demands if you want a workable solution not opamp based
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Old 19th April 2005, 08:13 AM   #6
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thanks or all yer replies!
i was in a hurry when i wrote the "demands", i correct myself and gives you green light using dual supply, but still only 9Volts per side..

Transformer on the output is not an alternative for me..

This amp is for indoor use and will not be run on batteries but on a giant powersupply, although almost as silent as if it was batteries..

MosFet would be preferred as you said, bjt's acting stupid with no feedback..

The Szekeres amp would be fine if it wasn't for the output coupling capacitor, are there any variants of this with dualsupply?
( http://www.headwize.com/projects/szeke1_prj.htm )
by the way, it has somewhat low gain, about 6dB?

ps. of course it has to sound good!! 8-)
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Old 19th April 2005, 07:59 PM   #7
guzzler is offline guzzler  Scotland
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try the project addendum

g

Quote:
Originally posted by Mr. Triatic
thanks or all yer replies!
i was in a hurry when i wrote the "demands", i correct myself and gives you green light using dual supply, but still only 9Volts per side..

Transformer on the output is not an alternative for me..

This amp is for indoor use and will not be run on batteries but on a giant powersupply, although almost as silent as if it was batteries..

MosFet would be preferred as you said, bjt's acting stupid with no feedback..

The Szekeres amp would be fine if it wasn't for the output coupling capacitor, are there any variants of this with dualsupply?
( http://www.headwize.com/projects/szeke1_prj.htm )
by the way, it has somewhat low gain, about 6dB?

ps. of course it has to sound good!! 8-)
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Old 19th April 2005, 08:02 PM   #8
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hehe, it seems that i cant read... ;-)
thanks!
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Old 19th April 2005, 08:42 PM   #9
joensd is offline joensd  Germany
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Quote:
by the way, it has somewhat low gain, about 6dB?
The circuits without any ICs in the addendum are just plain followers so will have no voltage gain. (read the article ) With a CDP you should get enough volume though.
Quite healthy in some way...
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Old 20th April 2005, 06:12 AM   #10
PRR is offline PRR  United States
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http://www.borbelyaudio.com/eb602210.asp would work, I think, on +/-9V, though I don't know why this +/-9V is carved in stone. I think it would work without any parts-change, though the output power in hi-Z loads would be very much less.

It does use feedback. You'd have to be a monk to eliminate ALL forms of feedback from your system. (Vacuum triodes have built-in feedback, which is why they can be tolerated without any explicit feedback connection.)

I have a lovely simple little DC-coupled +/-9V design, but it uses a chip and a BJT. Several people like the way it sounds, it should give good numbers, and I think I leveraged the two bits of sand against each other so they give no audible offense. Oh, and it does not need super-clean power (it does need a lot of power). But you are not interested in an IC design with NFB.

> would be fine if it wasn't for the output coupling capacitor

All "simple" designs have coupling capacitors. The standard "OCL" loudspeaker-amp passes huge chunks of audio through its power supply capacitors. The Cmoy has no cap at the amp output, but the rail-splitter is a coupling cap (and common to both channels). The un-simple alternative is to have additional amplfiers, such as Cmoy with "active splitter", which of course add additional points of distortion.

Oh, and the basic Szekes runs huge feedback. Cathode(Source)-Followers are all feedback. 100%. The open-loop gain would be around 35dB, the closed-loop gain is 0dB, so there is ~35dB NFB.

Hmmmm.... a truly Class-A bridged amp could work without any output coupling caps, explicit or hidden in the power supply. The sum of the two sides of each channel would have no audio current in the power rails. And it could run on single-ended power (just +9V) if you don't mind the headphone jack being "live" (+4.5VDC). That would however, for stereophones, require a 4-wire headphone, instead of the common 3-wire jack. And don't ask me for details: the last true-Class-A headphone amp I built used eight opamps, four big rude BJTs, and sinful input caps, all eating +/-20V. (And it isn't Class-A for loads less than 55 ohms.)

Massively parallel plate-loaded 6c33 at +/-9V? You might burn the house down (or burn your cans up) before you got under 1% THD, even with the large (hidden) NFB of a low-Mu vacuum triode, but Ah! The Glow!
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