Pure Class A amplifier with FET - schematic wanted


2002-08-27 4:53 pm

This is my first post on diyaudio and I am looking for a Pure class A amplifier with Mosfet. I have searched in slovenia, but no luck.

This amp should be about 100-140W / 8Ohm

And it has to be with mosfets!

If any of you has an schematic or PCB, could it be posted here? Please?

My old 1R1 Mosfet design

For something challenging and somewhat complex, but with excellent sound quality look at the link below. It is has a fully class “A” output stage and is also a good foot warmer.

I designed this back in back in 1992 or so. My newer versions of this use a similar circuit arrangement. This project is perhaps a lot more complex than most DIY’er care for.


John Fassotte
Alaskan Audio
I will try to explain how MOSFETs work - this will give you an idea of why they are (inherently) not incredibly linear devices.

MOSFET is an acronym for Metal Oxide Semi-conductor Field Effect Transistor. (They are usually created from aluminiun and aluminiun oxide I believe)

A MOSFET relies on the field created at the gate to limit the amount of electrons that pass from source to drain - it is literally a field dependant resistor (note that it is a field created within the device). As field strength is proportional to the square of the distance from the source as the potential difference at the gate is changed in a linear fashion the field will not change with perfect linearity and therefore the device is not inherantly linear.

I hope this has cleared things up?

This is my understanding from a physics point of view, if anyone knows better why these devices are not linear I would be happy to listen to your idea.
The inputs are designed for balanced (idealy) signals, but can be used for unbalanced signal as well.

The input stage of the amp should be a differential stage (as this is the way to reject commond mode noise on the signal line) - in this case to use your RCA connectors simply connect the signal line to the + (i.e. no-inverting input) and tie the return to the ground and - input (inverting input).

I've not seen the schematic, but this should work.
...also while I'm thinking about it as this amp has a balanced input why not make a bablanced line driver to run it?

This will give much better noise rejection from unwanted signals on the cable and is quite simple - read this;


for more information. The line driver only requires power for the op-amps and you can steal this from the power supply in your source equipment (CD player etc). Also ESP is a great source of all kinds of hifi electronics information - I highly recommend it!

Hope this helps?
dshortt9 said:

dshortt9 the link you provided above has a very nice presentation of a good number of Aleph project diagrams.

I'm sure that all Aleph fans greatly appreciate seeing these diagrams in such nice format. The person who did this work deserves a pat on the back. Great job!

John Fassotte
Alaskan Audio