carlmart said:
R22 is discussed here, and further down in the thread:
http://www.diyaudio.com/forums/showthread.php?postid=1310083#post1310083
D7 and D8 here:
http://www.diyaudio.com/forums/showthread.php?postid=1323187#post1323187
It is actually a class A/B design with a good 70-80 mA idle current in each output.
No I've not built it and have no plans to at this point, it is just a reference design for comparision to other amps. I was helping out in the other thread and I have a version with even lower distortion.
Pete B.
Conrad Hoffman said:
I see ... it's always interesting to experiment with new ideas.
I would try a no-global-feedback design with a more powerful amp for speakers where you might see some clipping since I believe that this is a main advantage of this type of design. Headphone amps seem good for experimenting with class-A since they're lower power obviously. Just my opinion.
Pete B.
I would just use a good quality IC buffer buffer. For most applications, the output from the pre-amp (I assume typical value of 1V) will be sufficient - no additional gain required.
One point that's important in my view is to make sure you drive th e headphone amp with a low output impeadance and avoid the series attenuator resistor.
I think this a basic reason why dedicated h/phone amps sound much better than the headphone socket output on a typical integrated amp. There was a test in Hi-Fi News (I think) a month or two ago and in the result s you can see the very big frequency response deviarions on typical headphones from ideal due to the use of a series resistor. In the article, they talk about a typical value of 120 Ohms when driving 32 Ohms headhpone transducer - that will certainly kill the sound.
One point that's important in my view is to make sure you drive th e headphone amp with a low output impeadance and avoid the series attenuator resistor.
I think this a basic reason why dedicated h/phone amps sound much better than the headphone socket output on a typical integrated amp. There was a test in Hi-Fi News (I think) a month or two ago and in the result s you can see the very big frequency response deviarions on typical headphones from ideal due to the use of a series resistor. In the article, they talk about a typical value of 120 Ohms when driving 32 Ohms headhpone transducer - that will certainly kill the sound.
Yes, there's little need for more gain, so a buffer is all the circuit has to be. I do drive headphones with resistor attenuators from a large power amp with great success, but never just a series resistor. I usually use 25 ohms with 1 ohm to ground, mostly because I have them lying around. Lower would be even better, but that still gives an output impedance of under an ohm. With my 32 ohm phones, there shouldn't be significant response deviations. The attenuator also lets the amplifier operate at a reasonable level, pushing any low level crossover, noise and distortion, down by the same ratio.
Bonsai said:
I am not so sure about that. The series resistor will protect the headphones too.
And as long as it's the same value as the headphone impedance, there shouldn't be any problem, major or minor.
This resistor should be good quality though, and probably higher wattage, like 1 watt or two.
Bonsai said:
I am not reading Europe or US audio magazines anymore. They are more expensive than books over here.
But if the question is using a 132ohm series resistor for a 32ohm transducer, then that would certainly be bad.
The resistor must be as close as possible to the transducer's impedance.
Impedance matching, if that's what's being talked about, has no relevance to the situation. IMO, the term never should have made the crossing from the RF world, to the audio world. The headphones are designed to be driven from a constant voltage source, and they certainly have impedance variations over the audio band. In this case, a constant voltage source means something with a low output impedance. Thus, if you drive through a series resistor that's a significant percentage of, or even exceeds the 'phones Z, expect response variations. BUT, if you drive through a resistive divider, assuming the amp has a low output impedance, the source impedance seen by the phones will be the parallel combination of the two resistors of the pad. In my case of 25 ohms and 1 ohm, the 'phones are driven from a 0.962 ohm source. Not zero, but close enough to avoid problems. In addition, the amplifier is lightly loaded and operated at a higher voltage level. That means that distortion products are both minimized and attenuated. I still wanna build a special purpose amp 
I'd have to search on head-fi for exact references but iirc Sennheiser's engineers as well as people from Grado have said that their headphones were designed and tested with amps with (close to) 0 output impedance in mind. Older headphones (especially beyerdynamic and akg) were most respectful of the IEC 61938 norm, specifying a 120Ohms output resistor.
A sennheiser hd580 for example certainly doesn't sound the same with 0, 120 or 300R output impedance. And the best is without contest 0.
A sennheiser hd580 for example certainly doesn't sound the same with 0, 120 or 300R output impedance. And the best is without contest 0.
I wasn't aware of the 1200 ohm spec- no idea how many 'phones use it today. IMO, even if they want a 1200 ohm driving impedance, a resistive pad will still give a lot of benefit. I'd use the above described pad, maybe with a couple ohms in the lower leg if more signal is needed, then come off the tap with the 1200 ohm resistor. This will get the amp operating at a decent level, suppressing noise and distortion. Maybe I should put these in a box, pot it to hide what's inside, and sell it to the high end crowd for big bucks?
You know that of course ? It's from Joe Grado
Btw, it's 120 Ohms, not 1200 ohms
An externally hosted image should be here but it was not working when we last tested it.
Btw, it's 120 Ohms, not 1200 ohms
A very simple balanced headphone buffer
I listen mostly with 300ohm Sennheiser HD650 headphones and my source is a Zhaolu D2.0A with a Zapfilter mkII installed. The Zapfilter delivers enough voltage swing balanced but doesn't drive my headphones directly very well so I made a very simple buffer using a single bjt (2SC4793) biased at 100mA using a Variable +/- 13V PSU. I can you tell you it sounds quite awesome as it is. Imho it already sounds better than any other headphone amp I've heard to date, commercial or diy.
So does anyone have any comments on this? Would there be any advantage going with a Push-pull configuration instead, like a diamond buffer or something else?
I listen mostly with 300ohm Sennheiser HD650 headphones and my source is a Zhaolu D2.0A with a Zapfilter mkII installed. The Zapfilter delivers enough voltage swing balanced but doesn't drive my headphones directly very well so I made a very simple buffer using a single bjt (2SC4793) biased at 100mA using a Variable +/- 13V PSU. I can you tell you it sounds quite awesome as it is. Imho it already sounds better than any other headphone amp I've heard to date, commercial or diy.
So does anyone have any comments on this? Would there be any advantage going with a Push-pull configuration instead, like a diamond buffer or something else?
I played around with headphone amps in the past. Here are my observations:-
Drive from as low a source impedance as possible - Your <1 Ohm is a good way to go Conrad
No caps - DC coupling only. On a good pair of phones, you can hear the difference straight away
Using an op-amp based cct followed by some transistors (with appropriate f/back around the whole thing) gives much better results than you get out of the headphone jack of a typical integrated amp.
If you can afford it, try to spend >$200 on your phones. Once you get above $400, you can blow just about any speaker system away in terms of sound quality.
Drive from as low a source impedance as possible - Your <1 Ohm is a good way to go Conrad
No caps - DC coupling only. On a good pair of phones, you can hear the difference straight away
Using an op-amp based cct followed by some transistors (with appropriate f/back around the whole thing) gives much better results than you get out of the headphone jack of a typical integrated amp.
If you can afford it, try to spend >$200 on your phones. Once you get above $400, you can blow just about any speaker system away in terms of sound quality.
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