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Old 30th October 2007, 09:22 PM   #1
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Default Low voltage all discrete amp, possible?

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.
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Old 31st October 2007, 03:26 AM   #2
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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!
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Old 31st October 2007, 05:13 AM   #3
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Mr Vandal, if power consumption is of major interest, have you checked any rail-rail opamp? Is it MP3 files you will be listening to?

EDIT: 200 mA in total How portable are you planning it should be / How long battery time do you want?
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Old 31st October 2007, 08:28 AM   #4
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My two cents worth.

For this application, use an op-amp.

Quick, easy, low Iq, almost zero offset (so no coupling caps), reliable and there are plenty out there that will run well off +-4V.
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Old 31st October 2007, 01:20 PM   #5
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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.
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Old 31st October 2007, 01:20 PM   #6
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Sorry, I didn't mean 12 AAA's. I meant 12 V = 10 cells.
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Old 31st October 2007, 04:19 PM   #7
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Seeing as you're in Sweden, why not get in touch with the folks over at Lundahl and see if they can fix you up with a 1:2 step-up transformer you can put between your input and output stages?

se
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Old 31st October 2007, 09:12 PM   #8
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Thank you for the link, but I think transformers are too expensive, and they're large. I have no experience using them, and I don't want to spend 200 USD on them unheard.
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Old 31st October 2007, 09:24 PM   #9
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Quote:
Originally posted by Steve Eddy
Seeing as you're in Sweden, why not get in touch with the folks over at Lundahl and see if they can fix you up with a 1:2 step-up transformer you can put between your input and output stages?

se

Oh, Steve you and your transformers
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Old 31st October 2007, 11:12 PM   #10
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Quote:
Originally posted by nelsonvandal
Thank you for the link, but I think transformers are too expensive, and they're large. I have no experience using them, and I don't want to spend 200 USD on them unheard.
You seem to have made some erroneous assumptions here. But as your mind seems already made up, I'll just leave it at that.

Good luck.

se
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