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Old 20th June 2006, 07:29 AM   #1
stlblue is offline stlblue  United States
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Default T-Amp = Headphone amp

Greetings:

Thinking of buying some headphones and everything I've read says a good headphone amp is a "must." Could a Charlize or other T-amp work well in this application?

Many thanks,

StlBlue
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Old 20th June 2006, 08:03 AM   #2
Pano is offline Pano  United States
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Does anyone ever search this forum before posting?
Sigh.....

FYI. T-amp + headphones = bad idea.
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Old 20th June 2006, 10:28 PM   #3
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Quote:
Originally posted by panomaniac
Does anyone ever search this forum before posting?
Sigh.....
Oh I'm hearing ya...

but man oh man, I've searched this forum for hours on end before finding an answer to a question. Yeah sometimes the answer is right under our noses but sometimes it isn't.
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Old 21st June 2006, 03:56 AM   #4
stlblue is offline stlblue  United States
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Yes I did look, hence my question. Thanks for taking the time to answer.
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Old 21st June 2006, 04:52 AM   #5
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Default Headphone question?

One would think that the headphone driving problem begs for a class A solution due to the low power requirements. It is true that a lot of successful low power class A designs have been done but there is some magic with class D that canít be denied. There are some chips available that possibly could work but most are made for 5 volts or less and this gives no headroom for the higher impedance phones. Also most are open loop types, not good! Does anyone have any ideas? How about an all discreet UcD type design? Anyone feel like doing a PCB?
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Old 21st June 2006, 05:19 AM   #6
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It's late ~ maybe this is dumb (or maybe not ), but... What about using a transformer to step up the voltage from a T-Amp to drive headphones? I know some kinds of tweeters need transformers for this purpose.
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Old 21st June 2006, 06:18 AM   #7
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Default Transformers?

This could be done but at what cost? Quality transformers are expensive and would be bulky. This would indeed solve several problems like the fact that headphones have a common ground and are single ended in the way they connect up. If you could find something surplus that would offer 1:1 windings with a 1 watt or so power handling it would be worth a try. The transformers would also offer additional filtering of the switching frequency. Good luck on your search.
Roger
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Old 21st June 2006, 08:36 AM   #8
Pano is offline Pano  United States
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Quote:
Originally posted by stlblue
Yes I did look, hence my question.
Yeah, I know. Things can be hard to find on this forum, not sure why.
Sure wish we could get a sticky post at the top to direct people to the Wiki/FAQ.

Class-D is a bad idea for headphones for several reasons.

1) More power than you really need. Headphones require very little power compared to normal speakers, so many of the problems of power amps are not much of a concern for headphones. As stated above, Class-A amps would seem a good choice here.

2) Tons of noise. Class-D amps are noisy, by headphone standards. If you plug headphones directly into a class-D amp, even a small one, you'll hear what I mean! This noise is not a problem with normal speakers as they are so much less sensitive than phones.

3) The output filter. Switching amps use an low-pass output filter the is usually designed to work with a 4~8 ohm load. Headphones are often 32 ohms or more, so the filter response will be wrong.

4) H-Bridge design. The small Tripath chips used in the Sonic, Charlize, Fenice 20, AMP6, AMP3, etc. are bridged. Thus the left and right grounds can not be tied together. You would have to rewire your headphones to dual mono.

For the same money, size and trouble, a good Class-A amp, solid state or tube, even opamp, should give you better sound.

Hope that helps.
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Old 21st June 2006, 05:40 PM   #9
BWRX is offline BWRX  United States
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Default Re: Headphone question?

Quote:
Originally posted by sx881663
but there is some magic with class D that canít be denied.

How about an all discreet UcD type design? Anyone feel like doing a PCB?
I haven't heard any good class a amps to compare to but class d definitely has magic compared to class ab!

If someone can come up with a schematic I'd be willing to do the PCB layout.

Quote:
Originally posted by panomaniac
1) More power than you really need.
2) Tons of noise.
3) The output filter.
4) H-Bridge design.
1) We could design the amp to be low power.
2) We could use an LCLC output filter or some other measures to combat noise.
3) The output filter could be designed specifically for a higher impedance load.
4) H-Bridge isn't a problem if we could implement a single ended UCD design.

Michael has some good points, but if a low powered discrete UCD type design is built specifically for headphones it could work very well! Any takers? At this level of power all of the components could easily be surface mount and switching frequency could probably be increased above 1MHz for even easier filtering.

These are all just ideas of course
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Old 21st June 2006, 07:24 PM   #10
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Default The headphone question

All valid points;
1) The lower power issue is one to think about. I personally wouldnít want the amp to have voltage headroom issues with either the input or output side. High impedance phones can require quite a bit of drive voltage so I would say +/-12v should be considered a minimum.
2) The output filter is potentially a real problem, as stated they are designed for a specific load. I guess a fixed load could be added to narrow down the range of loads it could see but I would think at the expense of added noise. With a double filter configuration I donít see how a UcD type feedback arrangement could be worked out. Once again a transformer could be the answer as an additional filter and to break the ground paths.
2a) This lower power amp could be run at 1mHz or even more and this should simplify the filter and noise issues. A UcD type design with all low voltage surface mount parts would be real nice. Looking at the Ztec offerings for output devices, their N and P type devices in the same package could make the drive single ended and a lot simpler. No level shifting or bootstrapping required. A chip type comparator could be used. Remember we only need about 1 watt max. More could be nice to drive small speakers. Maybe 3 watts max?
3) What impedance do you design for? Phones vary from around 32 ohm to over 600 ohm. With a 32ohm fixed load added the range would be from 16 to 30 something and this is doable as we are dealing with 4-8 ohm range right now.
4) Yes, this is why the UcD was brought up in the first place.

This is quite a project with the end result a real question. The results could be worth it or it could end up a waste of time. Sounds like a worthy DIY project to me!
If there is enough interest a new thread should be started (UcD type headphone amp?) with stated specific design goals.
Roger
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