Le Monstre MOSFET

argo · 3rd September 2004, 01:07 PM

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

MikeB · 3rd September 2004, 01:26 PM

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

argo · 3rd September 2004, 01:42 PM

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

Lars Clausen · 3rd September 2004, 01:51 PM

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

MikeB · 3rd September 2004, 02:21 PM

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

Lars Clausen · 3rd September 2004, 02:35 PM

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

Arcolette · 3rd September 2004, 02:47 PM

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

MikeB · 3rd September 2004, 03:02 PM

Ah, this would explain why my mosfet-amps (very early) had no thermal drift...
Ok, lets keep the resistors for measuring ?

Mike

MikeB · 3rd September 2004, 03:41 PM

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

argo · 3rd September 2004, 05:51 PM

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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3rd September 2004, 01:26 PM	#2
MikeB diyAudio Member Join Date: Apr 2004 Location: Gütersloh	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

3rd September 2004, 01:51 PM	#4
Lars Clausen diyAudio Member Join Date: Oct 2003 Location: The Lab	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

3rd September 2004, 02:21 PM	#5
MikeB diyAudio Member Join Date: Apr 2004 Location: Gütersloh	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

3rd September 2004, 02:35 PM	#6
Lars Clausen diyAudio Member Join Date: Oct 2003 Location: The Lab	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

3rd September 2004, 02:47 PM	#7
Arcolette diyAudio Member Join Date: Jun 2004 Location: Germany	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

3rd September 2004, 03:02 PM	#8
MikeB diyAudio Member Join Date: Apr 2004 Location: Gütersloh	Ah, this would explain why my mosfet-amps (very early) had no thermal drift... Ok, lets keep the resistors for measuring ? Mike

3rd September 2004, 03:41 PM	#9
MikeB diyAudio Member Join Date: Apr 2004 Location: Gütersloh	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

3rd September 2004, 05:51 PM	#10
argo diyAudio Member Join Date: Feb 2001 Location: Estonia	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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