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Firstwatt M2 amplifier
http://diyaudioprojects.com/Forum/viewtopic.php?f=10&t=5491
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Author:  wdecho [ 22 Feb 2016, 07:44 ]
Post subject:  Firstwatt M2 amplifier

Nelson Pass discontinued the M2 amplifier and as he always does released the schematic for diyer's. There is a thread at,

http://www.diyaudio.com/forums/pass-lab ... matic.html

There is a group buy still going one for the boards if anyone cares to build this amazingly simple amplifier. I just completed mine last week and the sound is glorious with my horn speakers. The sound has a lot in common with better SET amplifiers but with more SS power.

Author:  Geek [ 22 Feb 2016, 18:05 ]
Post subject:  Re: Firstwatt M2 amplifier

Aren't the bias diodes backwards? :confused:

Author:  wdecho [ 23 Feb 2016, 08:00 ]
Post subject:  Re: Firstwatt M2 amplifier

Actually I think the optocoupler bias' the circuit. There was a discussion of the functions of the diodes on the thread that was very interesting. Amazingly simple great sounding amplifier with my horns. This is a worthy build for diyer's it you want to hear the sound of a Class A amplifier. You will need a pre though which raises the price. I like to use a tube pre with my setup but with the kind of music I listen to now I like a little more 2nd harmonic. Considering the cost of Class A amplifiers as a whole this is a very easy build with few parts. You have a autotransformer for voltage gain and a single stage push pull mosfet current gain output stage. The jfets at the input are just a buffer and contribute very little to the sound and can be omitted with the right pre.

I found the thread on the function of the diodes from Nelson Pass himself.


Every part justifies itself by making it work. The 385's allow proper
operation into low impedances. The diodes protect the output Gates and
give symmetric clipping.

Q6 and Q7 are the 385's and Q5 is the optocoupler.

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