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knutn 6th January 2012 05:17 PM

NOStalgia - the all-FET JLH/PLH Class A amplifier
1 Attachment(s)
What to do when you have two large heat sinks (0.3 K/W) and a large toroid (2x18 V, 500 VA)? The answer came to me when reading Mr. Pass' article about the PLH amplifier, see Mr. Pass started with the JLH amplifier from 1969, the schematic of this amplifier is shown, at, and made his own MOSFET two stage amplifier (omitting the input transistor). So I decided to try an all-FET three stage amplifier.

knutn 6th January 2012 05:19 PM

NOStalgia - the all-FET JLH/PLH Class A amplifier
1 Attachment(s)
The schematic of the PLH amplifier is shown. Please observe especially the potentiometer P2, which varies the mode of operation between Single ended and Push Pull. The PLH amplifier is, as mentioned, two stage and it is inverting, and only MOSFETs are used.

knutn 6th January 2012 05:22 PM

1 Attachment(s)
The schematic of NOStalgia is shown. I am using a plus and minus 24 V power supply, so I don't need an output capacitor. Since the input transistor is a JFET, I don't need an input capacitor either. The use of a self-biasing JFET makes it possible to avoid a current generator or a resistor (which need to be tied to a "very clean" voltage). The two last stages are very similar to the PLH amplifier with the exception of different N-channel Power MOSFETs. I am using a switch instead of a potentiometer between Single ended and Push Pull.

vizion 6th January 2012 05:36 PM

How does it sound ?

knutn 6th January 2012 05:51 PM


Originally Posted by vizion (
How does it sound ?

I really don't know. I am finishing the testing these days. My hope is that it should sound much alike the JLH or the PLH amplifier.

knutn 6th January 2012 05:57 PM

1 Attachment(s)
The layout files are attached. The size of the PCB is about 52x99 mm.

knutn 6th January 2012 06:10 PM

I have tested the amplifier (with a rather poor) signal generator at 1 khz and 16 W output into 8 ohms using a spectrum analyzer. In the Push pull mode the second and third harmonic was down 67 dB. However this is close to the signal generator distortion, so I would not be too sure about this result. In Single ended mode the second harmonic was down 49 dB, the third down with 58 dB, the fourth down with 70 dB and the fifth down with 79 dB. Mr. Pass observed a similar difference between the Single end and Push pull mode, and was not preferring the sound from the "pure" Push pull mode. I presume the NOStalgia will have a sound similar to the PLH amplifier in this respect?

Dzius 10th January 2012 06:40 PM

So how does it sound right now ? Have you got any other Nelson amp to compare ?

knutn 11th January 2012 06:53 PM

Please be patient. I have left the measurement setup, but the assembly is not finished yet. However, with matched JFETs (IDSS about 20 mA), I have found that the difference between the two channels was very small. And I did not match the power MOSFETs.

JRKO 23rd January 2012 01:21 PM

Hey Knutn - how is the development going? I could be interested in this one :D

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