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Old 3rd September 2004, 01:07 PM   #1
argo is offline argo  Estonia
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Default Le Monstre MOSFET

Just toying with different pieces of circuits and here’s an utter heresy schematic I came up with - Hiraga’s Le Monstre front end with lateral MOSFET output. Sims OK but I am have no clue what the output impedance of this configuration could be?
Can someone please advice how to calculate for Z Out or guess the proximate range of it with this output configuration?



Regards

Argo
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Old 3rd September 2004, 01:26 PM   #2
MikeB is offline MikeB  Germany
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Hi !

As this circuit is that "simpel", why not just trying it ?
Numbers can't tell you really how it will sound...
Have you made AC-sweep ? The jfet-input might need
a small cap (~100pf) to ground. (Just connected to gates)

Mike
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Old 3rd September 2004, 01:42 PM   #3
argo is offline argo  Estonia
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Quote:
Originally posted by MikeB
Hi !

As this circuit is that "simpel", why not just trying it ?
Numbers can't tell you really how it will sound...
Have you made AC-sweep ? The jfet-input might need
a small cap (~100pf) to ground. (Just connected to gates)

Mike
Well, I have tried to many bad sounding MOSFET circuits lately. But seriously, even if the numbers can’t please ears they can calm down a restless spirit, as I am to not reach for soldering iron too hastily.
I mean if that circuit would have output impedance of 20 Ohms? Not good for driving even 16 Ohm speakers.


Argo
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Old 3rd September 2004, 01:51 PM   #4
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Argo: The output impedance is secured through the feedback loop.
After all the output impedance of the original bipolar collector's is also quite high, was it not for the feedback loop.

I think this design will work just fine, and probably you will get very good results from it. You can even use IRF540 / 9540 (much cheaper) with i would say - same result.

You don't need the 330 mOhms 'emitter' resistors when using a MOSFET. Just connect the sources directly to rails.

Le Monstre is one of the few truely innovative amplifier designs seen the last 20 years. Good idea to upgrade it with MOSFET's Argo!

Best regards

Lars Clausen
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Old 3rd September 2004, 02:21 PM   #5
MikeB is offline MikeB  Germany
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Hmm, in the case of irf540/9540 i would keep the 0.33ohms, or
you risk a thermal drift away.
With 15volt supply the to220 devices should be large enough.

Mike
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Old 3rd September 2004, 02:35 PM   #6
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MikeB: Before i started experimenting with MOSFET amps (in the real world) i would have been inclined to agree with you on this point.
However if you look on the datasheet (Vgs / Id curves) you can see the source already has an internal resistance of about 1 Ohm @ 0.5 A (which as i remember is the idle current of the Monstre)

So 1 Ohm or 1.33 Ohm ... das ist vohl egal?

If you need to parallel several devices, then of course it's another ballgame.

Best regards

Lars Clausen
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Old 3rd September 2004, 02:47 PM   #7
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Hallo Argo,

following your posted schematic it is doubtful if this amplifier will perform well. It is simple. But probably too simple. The main drawback is the high impedance of the driver stage (Q3, Q4). That feeds the large input (Cgs) capacitor of the MOSFETs. Even worse these capacitors vary with the signal amplitude. So you will have a significant pole with varying corner frequency. That is bad for stability. And it could cause ringing if the amplifier is stimulated with pulses. And music consist of pulses... So I would recommend looking for a more sophisticated schematic. Or you might enjoy the special sonic colour of instabel running power MOSFETs...

Arcolette
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Old 3rd September 2004, 03:02 PM   #8
MikeB is offline MikeB  Germany
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Ah, this would explain why my mosfet-amps (very early) had no thermal drift...
Ok, lets keep the resistors for measuring ?

Mike
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Old 3rd September 2004, 03:41 PM   #9
MikeB is offline MikeB  Germany
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hi arcolette !

The zenquito follows the same principle, and it's said that it sounds
wonderful !

Here's a list of different zenquitos:

http://perso.wanadoo.fr/jm.plantefeve/sche.html

At the bottom is a design similar to argo's, except that argo used cascodes.

In theory you are right, but it seems that these "design-flaws" sound great !

Mike
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Old 3rd September 2004, 05:51 PM   #10
argo is offline argo  Estonia
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Lars Clausen,

The source resistors in this particular circuit seem to help maintain the nice dominating even harmonic distortion pattern, produced by input cascades, through the output stage. Without the source resistors, the odd order harmonics tend to dominate as in usual push pull circuit. But this again is only in my simulations- in the real world this may be different though.

Arcolette,

I am aware of the shortcomings you mentioned, the thing what worried me more was an output impedance.

Mike,

Exactly, those resistors also became handy for bias current measurement


Argo
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