Discrete, Class A Lateral MosFET Headphone amp (not another follower buffer!) - Page 2 - diyAudio
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Old 23rd February 2012, 01:06 PM   #11
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Antoinel, I appreciate what you are saying, and to a certain extent I agree. Having said that, my particular phones require almost 10v pk-pk to achieve their maximum of 200mW, as they are, as I stated earlier, 64Ohm. Also the existence of phones up to 600Ohms needs to be considered for which considerably higher voltage drive is required.
The reason I have chosen to go with a gain of about 11 is that as sgrossklass suggested as gain is reduced other forms of distortion come in to play and new complications are required to combat these, as seen in the pretty complex compensation circuitry in opamps to enable their use at unity gain.
Without including these extra complications, and assuming we stick with a discrete design, the gain needs to be remain reasonably high.
When I build it, I'll include some opamp buffer based input circuitry to increase the input impedance without unnecessarily increasing noise or dc offset in the power amp input stage (although the opamps will of course add their own small levels of noise and distortion) and to allow for input attenuation.
Realistically, the 2v pk-pk signal from my source will attenuated to some factor more like 5 to about .4v pk-pk, then amplified by a factor of 11 by the power amp to produce a reasonable output of about 40mW into the phones.
I understand your objection in terms of gain structure, but I don't think there is much of a better way to do it whilst maintaining the ability to drive high impedance phones (read 600Ohm phones) fully and avoiding the problems associated with designing a low gain, low distortion discrete power amplifier.
I agree that in terms of power capacity the thing is way over engineered and like I said, Iq could be decreased, but for me that's kinda the point of DIY
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Old 23rd February 2012, 01:48 PM   #12
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It is not my place in this forum to object to anything a DIYer wishes to do. I apologize for projecting this attitude. I was simply pointing potential pitfalls which can lead to "yet another headphone amp" rather than one which is well differentiated from the existing pack.
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Old 23rd February 2012, 06:51 PM   #13
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Quote:
Originally Posted by mnturner View Post
you could definitely increase the vas current but given the low current needs of the mosfets I don't think it's necessary.
Actually my point was that compared to integrated solutions, currents already are relatively luxurious. (Without having looked at the schematic again, I bet your VAS alone draws about as much as many an integrated opamp.) Hence you get by with a lower degree of complexity just fine.

Obviously running the output stage (typically a major contributor to distortion) in class A also helps a lot.

Oh, and I do still get to about 15 kHz at elevated volume but that's pretty much it. Not quite 30 yet. There's a lot of factors playing in here, not the least being genetic disposition.
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Old 24th February 2012, 04:48 PM   #14
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Interesting, I had the almost exact same idea of building a discrete Lateral FET Class A headphone amplifier.

My design is attached.

It is already built and sounds really good.

Very detailed and controlled, with very, very low noise(read : Inaudible for all intents and purposes).

Hooked up both amp modules today for the first time, to some crappy gaming headphones from Sennheiser, but it sounds very promising.

Fully complementary floating JFET input stage biased at approx 3 mA per JFET, Darlington VAS'es, 2Sk1058/2SJ162 Lateral FET output stage biased at 180 mA per Lateral FET.

Gain is set at 2X.

The 100m output resistor is not included in my current test setup and is not really needed, unless you have to drive some seriously capacitive loads, then it would be wise to include it for stability reasons, but with normal headphone loads it is not needed.

DC coupled with a DC servo to keep the DC as clsoe to 0 as possible, testing shows the DC staying within a range of +/-1mV.
Attached Images
File Type: png LatFETHeadAMP.png (50.6 KB, 411 views)

Last edited by Neutrality; 24th February 2012 at 04:52 PM.
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Old 24th February 2012, 05:34 PM   #15
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It is a true fact: great minds think alike!
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Old 24th February 2012, 07:45 PM   #16
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I also made a No-thump board with a time delay before activating the output relay and a fast off function that detects when the AC for the powersupply drops output. No more thumps/pops/plops when turning on or off your headamp.

Also made a simple supply board using LM317/LM337 regulstors, nothing special or highend but should work well, with a ripple of around 1mV P-P with the full bias on both channels. It is not some ultra low noise design but it should work very well. Currently using a lab supply to power both modules and that is quiet as a mouse.
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File Type: png No-thump.png (43.5 KB, 381 views)
File Type: png Supply.png (50.4 KB, 360 views)

Last edited by Neutrality; 24th February 2012 at 07:48 PM.
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Old 25th February 2012, 05:56 AM   #17
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Nice, what made you go with fet input stage? Also, you could easily set this up for balanced input if you have a balanced source.
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Old 25th February 2012, 09:34 AM   #18
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Quote:
Originally Posted by mnturner View Post
Nice, what made you go with fet input stage?
I just really, really like they way the complementary floating JFET input stage works. It is a simple ann elegant way to get a good sounding and very well behaving input stage that is totally symmetric.

Normally one would use the low-noise 2SK170/2SJ74 JFETs for such an input stage, but since the 2SJ74 has been discontinued for several years it is getting harder to get and rather expensive, especially as you need many for matching of the input stage JFETs. The 2SK246/2SJ103 JFETs are a decent alternative that performs almost as good. A bonus is that they are very cheap and easily available.
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Old 25th February 2012, 01:19 PM   #19
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I have a question, why do you use a VBE multiplier for the biasing of the Lateral FET's, a fixed resistor will do just fine.

I used a pot to trim the bias on each module to around 180 mA bias and then I choose an resistor value as close to the value of the setting of the pot and mounted that into my modules, simple and effective. Ended up with 120 Ohm in the first module and then 97.6 Ohm in the second.

Bias is very stable, even after several hours of use it only swings +/- 5 mA from the 180 mA setting.
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Old 25th February 2012, 01:42 PM   #20
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Class A affairs in general seem to be quite uncritical when it comes to biasing.

IMO the main reason for using a constant voltage source would be removing the coupling of idle currents in all stages.

BTW, are you sure that symmetric drain resistors in the input stage give best performance? Normally I'd think that R4/5 < R15/18 would result in better symmetry.
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