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Author Topic: 55W Fet Poweramp  (Read 2094 times)


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55W Fet Poweramp
« on: June 18, 2011, 09:53:18 AM »
Hi guys, I'm asking the advice for the  power amp that I designed.
It's a almost all FET discrete amp(except the bipolar current source and mirror, which only improves the linearity of the amp, and doesn't affect the sound).
I'm going to use the kind of Marshall MF350 negative feedback which is also used in MG series.

I know how to design voltage feedback, but I'm not familiar with current feedback.Can someone explain me how this feedback works, so that I can design PCB and share whole project with you guys.



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Re: 55W Fet Poweramp
« Reply #1 on: June 18, 2011, 04:36:34 PM »
Great! One question: Is this any better than chip amps in terms of clipping?


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Re: 55W Fet Poweramp
« Reply #2 on: June 18, 2011, 11:38:36 PM »
Hi mensur,
              Still searching for some magic tricks?  8|
              Maybe Re Read Teemu's book?

As to chasing holy grail tone? NO this won't resemble a Valve power stage at full tilt. :'( :'(

IMHO, *I* FB does little as I did quite a few experiments with it and for all the trouble
it brings little real world improvement, On the other hand some people rave about it?? go figure.

I very much doubt that a SS powerstage using the common designs which are all very similar
in basic concept will ever be able to reproduce the saggy power effect of a real Valve Amp.

The closest you will get is to use very simple designs and just add some power rail droop.
a couple of extra Caps and small series R of say 50 Ohms in the supply rails
will reap better sonic rewards.

If you desire more in depth explanation there read this;


The soft Clipping is explained halfway down that page and that (unlike I FB) *does actually work quite well*.
IMO, it comes closer to reproducing the effect of a Valve power stage running flat out
and will impart far more realism than messing with feedback tricks.

Another cheap trick is to insert a series light bulb into the mains side of the power supply circuit, (same as the light bulb load limiter except you just keep it plugged in)
That I have done also and I'm still kicking myself for swapping that little Amp as I forgot to draw the schematic :grr

All of this still won't give you the extra sustain of a Vpwr stage :(

If you want that to happen try a Voltage driven Output transformer.
In which case build **KMG's stunning Fet Amp** which is much closer to the real deal. :))))

a small KMG can be made using very cheap *Line transformers* as output transformers.

Oh I have no idea of FB values but my guess is the I FB on you circuit should be something like this; see pic.
« Last Edit: June 19, 2011, 12:31:17 AM by phatt »