Driver stages for my SE Class A amp

Ok, I have built a SE Class A MOSFET output stage. IRFP140N's as output devices. Runs from +12 , -12v 50Ah batteries. CCS similar to the NP CCS's used in the Zen's and Aleph's.

Thinking about changing the output to a CFP.

Could I use a simple High Voltage gain stage on the input?


I would like to stay with the battery supply if possible.

Otherwise, I will just leave them as a gain of 1 with a CFP output.

Sound quality? amazing... bass is great, midrange and vocals are warm... better than my 6B4G PP amp.

No sound of SS artifacts...



p.s. how do I insert an image???

Nelson Pass

The one and only
Paid Member
2001-03-29 12:38 am
With Mosfets, you hardly need a Sziklai for a follower.

OK, the answer is yes. Since you are only using +/-
12 volts, just about anything will drive it. How about a
BoSoZ, also known as the Balanced Line Stage? It'll
swing that, and is happy seeing the capacitance of the
Mosfet, and if you want a little more power, then you
can run the output stage balanced.

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Nelson Pass said:
With Mosfets, you hardly need a Sziklai for a follower.

I'll second that. With a hexfet type mosfet all the yucky stuff seems to happen in the range of 0 to 0.5 amps, 1 amp if you're fussy. Well actually it depends on the size of the fet you are using, more like once the current is above a certain percentage then things really start to straighten out. With my own unfinished class A source follower amp it runs 3.5 amps and looks really quite nice on the scope with no nfb. It has a 30v rail and so it will pull +28v into an 8 ohm resistive load with no obvious visible or audible distortion (I'm sure you could measure it easily though). So don't get too stressed about making a CFP; it may not make a great deal of difference since there is not that much lacking in the first place.

Have a look at the blurb on my amp here it's the best advice I can give without repeating myself. I hope you find something useful here to incorporate into your amp.

Drivers... maybe not..


I really need to look at the output waveform on a scope again.

I am using a 6H30 tube pre at the moment to drive it... sounds good to me.


Do you have any recommendations on improving the CCS used in the Zen amps? I want to stick with a fixed 3.2A sink... not have a variable one as in the Alephs.

CCS sink...

Oh.. I think you misunderstood me.

At the moment I have a sink essentially like the original Zen amps. With a MPSA42? sensing current across a 0.2 ohm power resistor. This drives the gate of the IRFP140N.

The collector of the BJT is connected via a 2.2K / 100uf / 2.2K combo to my +12v. The 100uf is connected to -12v.

I have a small value cap to swamp out any capacitance issues at the gate of the IRFP140N...

Is there any way to make this sink any better?

Would increasing the current through the MPSA42 be better?
I have ~4mA through the BJT right now.. 10-20mA?