JLH headphone amp with FET's and servo - diyAudio
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Old 25th August 2008, 09:48 AM   #1
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Default JLH headphone amp with FET's and servo

I put a lot of time (and money) on my folded headphone amp project, but it didn't turn out as I wanted. The sound is just a bit too polite and unexciting. So I've been pi**ed of with amps for a while.

I thought I could use the board for something else, and since I've read all the raving about the JLH, I wanted to give it a try. With just some jumpers, it fit perfectly on the board, mirror imaged of course since I used N-channel input in the amp mentioned above. I choose JFET input because I wanted it DC-coupled, but since JFETs are so temperature sensitive there was over 100 mV difference in offset going from cold to warm. So I had to use a servo and input capacitors (WIMA MKS). I use an AD823 as servo, because that's what I had lying around. The virtual ground is a discrete rail splitter, and 3300 uF Sanyo WX capacitors.

Everything seems stable. The noise level is very low. The input FET runs at about 2 mA, the second stage at 4 mA and the output stage at 20 mA. It's housed in a Hammond 1455J1201, powered by 8 AAA's with a LM317T used as a trickle charger and a L78S10CV regulator when using wall power.

Now I understand the raving. It has a very warm and forward mid, fat and friendly bass and fairly clean treble. I think it has a slightly grainy sound, "the JFET sound". Maybe I'll try it with a 2SC2240 as input transistor later on, but now I'm just enjoying music. It sounds like my le Classe A headamp clone when I used JFETs as input stage, but warmer, with a golden glow, and more pronounced mid.

Schematic and images can bee seen at this link
http://www.esnips.com/web/KindofJLHwithFETandservo/

Most components are on sockets for tweakability. If you have suggestions for improvement, please let me know.
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Old 25th August 2008, 10:00 AM   #2
lineup is offline lineup  Sweden
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nelsonvandal
I am impressed
See my attached Schematic.

good & simple
but with high JFET amplifier quality
Great!

Lineup
... and easy to find transistors, too
Attached Images
File Type: png jlh fet w servo schematic_1.png (21.1 KB, 1273 views)
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Old 25th August 2008, 10:20 AM   #3
Nordic is offline Nordic  South Africa
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I suspect the grainyness is from the inherent low noise rejection...
Spend some effort on the PSU next.
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Old 25th August 2008, 11:00 AM   #4
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Quote:
Originally posted by Nordic
I suspect the grainyness is from the inherent low noise rejection...
Spend some effort on the PSU next.
It's battery powered. How can I improve the supply without losing too much voltage? I added those small resistors (2R7) to isolate the rails going to the first stages from the output transistors. I think it's called capacitance multiplier.

How would you describe the difference between bipolar and JFET transistors?

Or maybe it's the sound of the capacitors sinking the return current. I guess Sanyo WX isn't the best. Maybe some BlackGates?
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Old 25th August 2008, 11:21 AM   #5
Mooly is offline Mooly  United Kingdom
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Hi,
Quote:
Originally posted by nelsonvandal

How would you describe the difference between bipolar and JFET transistors?
You can't compare. They are fundamentally different devices each with it's own advantages and disadvantages. At it's most simplistic level you can say the Bjt is "current driven" and the FET "voltage driven" but there is much more to it than that. It's how they are used that matters and whether they are used correctly.
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Old 25th August 2008, 12:37 PM   #6
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Quote:
Originally posted by Mooly
Hi,


You can't compare. They are fundamentally different devices each with it's own advantages and disadvantages. At it's most simplistic level you can say the Bjt is "current driven" and the FET "voltage driven" but there is much more to it than that. It's how they are used that matters and whether they are used correctly.
I guess you're right. The folded cascode amp using the same SK170 as diff pair, isn't the slightest grainy. I wonder if the reason for JLH being so easy to like is beacuse of coloration or the lack of it.
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Old 25th August 2008, 12:43 PM   #7
lineup is offline lineup  Sweden
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I think,
after read so many sound reviews of JFET and Pass MOSFET based amps,
that when we let JFET / MOSFET be the important (input) stage
then we get another more pleasing sound.
At least to many ears.

Is probably one reason Pass amplifiers are so much enjoyed.
Another is of course TRUE CLASS A.
Which also give a very good sound.
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Old 25th August 2008, 04:10 PM   #8
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nelson,
Quote:
I put a lot of time (and money) on my folded headphone amp project, but it didn't turn out as I wanted.
I am personally not surprised, it was predictable.
Quote:
I think it has a slightly grainy sound, "the JFET sound"
The sound of heavily abused JFETs, I would say.
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Old 25th August 2008, 07:41 PM   #9
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Quote:
Originally posted by Lumba Ogir
nelson,

I am personally not surprised, it was predictable.
I haven't really given it up yet. The main problem is the ground channel, and I've just tore it out. It sounds very smooth and overly clean with resemblance of AD797, AD829 and ADA4841. There's no sign of any erroneously behaviour in this amp. It's just that its a bit soft, but it's nice to have it when my ears are sore from listening to more lively amps. Is this the "folded cascode sound"? So many posts here, but not a word on how things sound! Don't you trust your ears?

Lumba, what was predictable? What could I have done different, or is it the topology or layout that's bad?
Quote:
The sound of heavily abused JFETs, I would say.
I'm going to try a bipolar input, but I'm out of 2SC2240 at the moment. While I'm waiting for my order, why don't you just tell me how a JFET should be used.
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Old 26th August 2008, 07:21 AM   #10
Mooly is offline Mooly  United Kingdom
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The sound of a "Heavily abused FET" ---- Snap Crackle Pop I would have thought
And yes -- Trust your ears !!! -- and make sure the output impedance of the amp matches the 'phones -- experiment.
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