I've been on a quest for the perfect portable headphone amp. I don't think it could be achieved using opamps since they tend to sound lifeless and colored. There are plenty of amps with 20 - 60 V supply that seem to be great, but I don't have the skill to transform them to lower voltage. Since I never play louder than 2 V, there's no need for a huge voltage swing. At the moment I'm using a closed loop class A buffer amp at 12 V supply, but I would like to have something like 6 dB gain since I'm using 300 Ohm phones and use a DAP with 1 V max output.
Is it possible to build a super quality all discrete amp for headphones. Preferably class A, DC-coupled, supply +/- 4 - 6 V, gain 6 dB. A current draw of about 50 - 100 mA/channel would be OK. Since size is an issue, it should be simple with as few components as possible.
Is this "Mission Impossible 4", or do you have any suggestions? Maybe something like the Dispre, or something with JFET input like Hiragas le Monstre but for headphones at a lower voltage.
Is it possible to build a super quality all discrete amp for headphones. Preferably class A, DC-coupled, supply +/- 4 - 6 V, gain 6 dB. A current draw of about 50 - 100 mA/channel would be OK. Since size is an issue, it should be simple with as few components as possible.
Is this "Mission Impossible 4", or do you have any suggestions? Maybe something like the Dispre, or something with JFET input like Hiragas le Monstre but for headphones at a lower voltage.
Nobody else is replying, so I will give a reply. It won't be very satisfying, because I don't want to give away all of my secrets. But yes, it is possible. And it will be easier if you use +/- 6 volts instead of +/- 4 volts.
The first thing to look at is the output stage and then work backwards. If you want a BJT output stage, then you are heading down the wrong road. I don't like to use feedback or output stages with gain. That leaves emitter followers. But you need a triple to avoid loading down the front end too much. But a triple will eat up 6 x 0.7 = 4.2 volts just for bias.
So the best output stage would be to use JFETs. They are depletion mode so you won't lose any voltage swing due to bias. A single pair of 2SK170/2SJ74 will run well with 25 ohm source resistors. The output impedance of this stage will be around 30 ohms. So this could be OK with high-impedance 'phones like you have. Or you could parallel more devices to get a lower output impedance. It's just that then you have to buy a lot of device and match them. Each pair will draw around 5 mA (using the BL grade, which is the best overall recommendation).
I would recommend JFETs for the input stage, as then you don't need coupling caps. The parts mentioned above will work well here also. The rest I will leave up to you. Have fun!
The first thing to look at is the output stage and then work backwards. If you want a BJT output stage, then you are heading down the wrong road. I don't like to use feedback or output stages with gain. That leaves emitter followers. But you need a triple to avoid loading down the front end too much. But a triple will eat up 6 x 0.7 = 4.2 volts just for bias.
So the best output stage would be to use JFETs. They are depletion mode so you won't lose any voltage swing due to bias. A single pair of 2SK170/2SJ74 will run well with 25 ohm source resistors. The output impedance of this stage will be around 30 ohms. So this could be OK with high-impedance 'phones like you have. Or you could parallel more devices to get a lower output impedance. It's just that then you have to buy a lot of device and match them. Each pair will draw around 5 mA (using the BL grade, which is the best overall recommendation).
I would recommend JFETs for the input stage, as then you don't need coupling caps. The parts mentioned above will work well here also. The rest I will leave up to you. Have fun!
How portable are you planning it should be / How long battery time do you want?
The 3 channel buffer amp I'm using now has a current draw of about 150 mA. I'm using 12 950 mAh AAA's. The playing time is quite sufficent for my needs. Even 200 mA current draw would be OK. This amp is housed in a Hammond J1201, but I realize that if I'm going to add a gain stage, I can't fit it into a box this small. I was thinking of something like a Hammond 1455K1201, 78x43x120 mm.
Opamps is not an option, I've tried so many of them already, always with a bitter taste. The best so far, I think, is ADA4899-1, and since this is very new, there's hope of "the perfect opamp" in a not too far future. I've not used it in front of buffers yet, maybe I'll give it a go with "diamond buffers". I'm also about to build an amp based on TPA6120 this week.
Yes, I listen to compressed material most of the time (OGG 256 kbit), but the quality is good enough to tell one amp from another.
OK, so you think I should make it an all JFET amp? I'm using 2SJ74/2SK170 as input buffer and BC549/559 plus MJE243/253 as output stage in the amp mentioned above, and I'm happy with the tonality. I've used BC550/560/327/337 and BD137/138 and MJE243/253 in diamond buffers at +/- 4.5 V supply with good results, not as good as with 18 V supply though.
I don't have the skills to design an all discrete amp with gain stage. I'd be happy if you could direct me to an amp as described above or show a good schematic.
Opamps is not an option, I've tried so many of them already, always with a bitter taste. The best so far, I think, is ADA4899-1, and since this is very new, there's hope of "the perfect opamp" in a not too far future. I've not used it in front of buffers yet, maybe I'll give it a go with "diamond buffers". I'm also about to build an amp based on TPA6120 this week.
Yes, I listen to compressed material most of the time (OGG 256 kbit), but the quality is good enough to tell one amp from another.
OK, so you think I should make it an all JFET amp? I'm using 2SJ74/2SK170 as input buffer and BC549/559 plus MJE243/253 as output stage in the amp mentioned above, and I'm happy with the tonality. I've used BC550/560/327/337 and BD137/138 and MJE243/253 in diamond buffers at +/- 4.5 V supply with good results, not as good as with 18 V supply though.
I don't have the skills to design an all discrete amp with gain stage. I'd be happy if you could direct me to an amp as described above or show a good schematic.
You are in a difficult position since you have made your mind up already. If you want low power consumption a good modern opamp may be a good idea since you can't design a discrete amp. You could take a peek at a current feedback amp. Such a amp can be made rather low power.nelsonvandal said:
OK, a CFB amp could be interesting to try. The only CFB I've tried so far is TPA6120, and it's quite good. Do you have a particular opamp or discrete design i mind? Your QRV-06 and 08 looks very good but too complex for me, I'm afraid, and still 30 V supply.
Yes my mind's made up, I want a "pure and golden sound". I don't really care if I have to use opamps, discretes, tubes or transformers, but I want my amp portable. I've tried quite a lot of opamps, but none of them are good enough. If opamps were flawless, people would't bother using anything else in their amps. I think it has to do with complexity. I've always prefered tonality and neutrality and not sophistication. I think an amp with fewer components in the signal path is what I'm after. I found this out by simply bypassing the gain stage (opamp) in some headphone amplifiers, just using the buffer (sometimes another opamp, sometimes discretes), with a huge improvement in sound quality.
If you know of any opamps with a better sound than AD825, AD829, AD744, AD8620, AD8058, LM6171, AD823, OPA2107, OPA2132, TPA6120, AD8599, LMH6642, LMH6654, LM4562, LME49720, AD8397, AD746, AD8045, ADA4899-1, OPA690, THS4221, I'm willing to give it a try. Finding a perfect opamp will make it a lot easier than messing with discretes.
If there are small, very good and reasonably priced transformers that could replace a gain stage, I'm willing to try.
Yes my mind's made up, I want a "pure and golden sound". I don't really care if I have to use opamps, discretes, tubes or transformers, but I want my amp portable. I've tried quite a lot of opamps, but none of them are good enough. If opamps were flawless, people would't bother using anything else in their amps. I think it has to do with complexity. I've always prefered tonality and neutrality and not sophistication. I think an amp with fewer components in the signal path is what I'm after. I found this out by simply bypassing the gain stage (opamp) in some headphone amplifiers, just using the buffer (sometimes another opamp, sometimes discretes), with a huge improvement in sound quality.
If you know of any opamps with a better sound than AD825, AD829, AD744, AD8620, AD8058, LM6171, AD823, OPA2107, OPA2132, TPA6120, AD8599, LMH6642, LMH6654, LM4562, LME49720, AD8397, AD746, AD8045, ADA4899-1, OPA690, THS4221, I'm willing to give it a try. Finding a perfect opamp will make it a lot easier than messing with discretes.
If there are small, very good and reasonably priced transformers that could replace a gain stage, I'm willing to try.
A very basic CFB amp consists of only a few transistors if you want to do it very simple. When I dived into the CFB subject I noticed that much is written but in DIY circles CFB is very unusual and I think this has to do with "never heard about it".
http://www.utdallas.edu/~hellums/docs/JournalPapers/SmithCFBamp.pdf
I have listened to a couple of the opamps you mention and then all sounds excellent so apperantly we don't share the same taste.
http://www.utdallas.edu/~hellums/docs/JournalPapers/SmithCFBamp.pdf
I have listened to a couple of the opamps you mention and then all sounds excellent so apperantly we don't share the same taste.
I would like the amp as small as possible, but the size of a Hammond 1455K1201 = 78x43x120 mm is acceptable. There must be room for batteries, either one or two 9 V's if the current draw is low, or up to 10 (or maybe 12) AAA's.
Isn't it possible to add a buffer after a transformer? There are unity gain buffers that sound good at lower voltage supplies. I'm open for suggestions. If there are transformers with unbalanced input to balanced output, there would be no need for gain at all, I think. Going balanced means 6 dB gain, doesn't it? Regarding the price, not more than 100 USD/two channels - that is if they're really good, better than opamps.
It's not that the opamps aren't good, It's just that I want less coloration.
Thank you for the information Peranders, I'm going to do some searching on CFB amps. I think you're right about "never heard about it", and I thought CFB amps were hard core engineering almost impossible to tame.
Isn't it possible to add a buffer after a transformer? There are unity gain buffers that sound good at lower voltage supplies. I'm open for suggestions. If there are transformers with unbalanced input to balanced output, there would be no need for gain at all, I think. Going balanced means 6 dB gain, doesn't it? Regarding the price, not more than 100 USD/two channels - that is if they're really good, better than opamps.
It's not that the opamps aren't good, It's just that I want less coloration.
Thank you for the information Peranders, I'm going to do some searching on CFB amps. I think you're right about "never heard about it", and I thought CFB amps were hard core engineering almost impossible to tame.
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