NOStalgia - the all-FET JLH/PLH Class A amplifier

[revintage]

Heja Knut,

Since the input transistor is a JFET, I don't need an input capacitor either.

How do you expect to get 0V at the output with the FET gate tied to ground? As long as there is a current-flow through the feedback resistor R14 and a source/gate-voltage in the FET this will be impossible. But I might have missed some hidden twist?

Other FET variations is to be found in the gigantic JLH-thread. JLH is a nice topology anyway!

[knutn]

Thank you for your interest. Hopefully I will be finishing the assembly in about two weeks. So I am looking forward to the first listening test after that....As always: enough time is a problem.

[JRKO]

Originally Posted by knutn

Thank you for your interest......As always: enough time is a problem.

JLH/PLH has been floating in my mind for some time......

Unusually I have too much time on my hands......and not enough brains

[knutn]

Originally Posted by revintage

Heja Knut,



How do you expect to get 0V at the output with the FET gate tied to ground? As long as there is a current-flow through the feedback resistor R14 and a source/gate-voltage in the FET this will be impossible. But I might have missed some hidden twist?

Other FET variations is to be found in the gigantic JLH-thread. JLH is a nice topology anyway!

The voltage at the source of the JFET is about minus 1.8 V when the output is at 0 V. This is enough to have a suitable current in excess of 1 mA in the P channel JFET. it works just fine.

And I agree with you about the topology.

[revintage]

I smell something burning.........

[knutn]

Data sheet for the JFET: http://www.fairchildsemi.com/ds/MM/MMBFJ177.pdf

[Dzius]

That's very interesting amp indeed. Some DIY-ers are concern about resistors and capacitors. More advance care about transistors. But there is more in what comes out of speakers - sound characeristic i.e. harmonic distortion content. With PLH one can change from Push-Pull signal amplifing method to Single Ended, or choose and many stages between the two. (Like 2/3 AC current ratio suggested by Nelson). From this point of view it's very interesting to see in what extent sound change accordingly. That's why I think every DIY should built PLH but probably only some understand what it's about. I would like to build a one. Knut your cirquit is particulary interesting since there are no capacitors at the output, so one thing less to bother. Looking forward your final results. Thank you for PCB drawings. Pawel

[gusto142]

Hej Knut,

Can you inform us about the current status of this project?

Regards,
John

[LBHajdu]

Those that find this amp interesting, may also find the Amp Camp Amp #1 interesting:
Amp Camp Amp - ACA (The designer has a decent reputation)

It has some similarities:

• JFet input, though the Amp Camp Amp #1 uses the LSK170 (2SK170) which has 4 to 8 times less noise compared to J175 used in NOStalgia. LSK170 0.9-1.9 (nV/Sqr(Hz)) @ 1kHz @ 2mA(ID) vs. J175 8.0 (nV/Sqr(Hz)) @ 1kHz @ -5mA(ID). It is a little apples and oranges comparison based on the data sheet with less current (-0.2mA) the J175 noise goes up to 15. Then again now that 2SJ74 has been discontinued with no good replacement, like the LSK170 that replaces 2SK170, good p-type jfets are hard to come by. I guess if you try it and don’t hear any hiss (popcorn noise they call it in jfets, I think) it will not matter much.
  
  
• Same output devices (2x ) IRFP240.
  
  
• Depending on where you tap the output you can move between pure SE and having the top current source contribute some gain, in the Amp Camp Amp #1. And the auto bias adjust in the Amp Camp is also very nice.

And there is going to be a kit for it including parts (not just PCB) here at DiyAudio

Originally Posted by Dzius

Knut your cirquit is particulary interesting since there are no capacitors at the output, so one thing less to bother.

I would have thought quite the opposite, far more to bother with, like DC offset adjustment and drift for example, and making sure voice coils don’t get fried. I can’t help thinking people make a bigger deal about having the cap. before the speaker as opposed to after it in the power supply, though I have not done subjective testing on this. Psychologically I guess it does provide less to bother with because the power supply schematic is typically drawn on a different page, so you don’t see the rail caps, just a pretty ground symbol , even though it’s still in series with the speaker, out of sight out of mind. In some cases when the cap is made too small I guess that can have an effect on base. I change my own mind on this topic all the time and can argue both sides of the fence.

[Dzius]

I've got First Watt F3 with nice battery of 15 ELNA silmic II capacitors per channel and that's very impressive amplifier. It's a pity I don't have time to finish PLH project. PCB's are waiting for their turn.