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Old 7th February 2002, 05:31 PM   #11
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Location: Limoges, France
Hi,

I've tried a quick simulation (see attached image). It gives good results but no sound .

Well, regarding the gate voltage for the cascode mosfets, a rough approximation gives :

Starting from the input mosfet : gate @ 0V
Allow 10-15V more on its drain. Add 4.5V for the Vgs of the second mosfet. You get about 20V at the gate. Be sure that Ibias x Rload voltage drop let sufficient room for the second mosfet to behave correctly (10-20V more positive than the gate). That's only a quick way to go, but seems to work... And maybe I'm completely wrong

Hope this helps
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Old 7th February 2002, 07:05 PM   #12
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Ok, a couple of quick observations:

Steve, I suggest you use pots of R8 and R9, and you
probably want to lower the value as Q1 and 2 probably
don't need so much voltage across them to perform
well and it cuts into your output swing. I assume
that R16 and 17 are simulation loads, and would otherwise
be removed.

Francois, I think you'll end up wanting a few hundred
ohms output resistance to isolate you a bit from the
outside world. Also, V3 and V4 can easily be the same
source.
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Old 7th February 2002, 08:44 PM   #13
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Nelson,

Thanks for your kind observations. Of course, it's far from a practical circuit, it was only for quick simulation purposes, just to test the feasability (?) of some ideas. It's a long way from computer screen to music listening

Steve,

I don't think that noise is really an issue here. My BLS is dead quiet, provided that power supply is correctly filtered.
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Old 7th February 2002, 09:15 PM   #14
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In this case it's quite a short path. I believe you can
use it as is.
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Old 7th February 2002, 09:20 PM   #15
SteveG is offline SteveG  United States
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Nelson,
Thanks for the advice. I will try a smaller value of pot (50K?) to dial in the gate voltage. Is there a better way of biasing the cascode fets into conduction?
I'm still not exactly clear on how to determine the operating point of the mosfets... I believe that in this case it is controlled by the current source, but I'm not sure how to determine the optimal value of load resistor and current. Does the gate voltage not matter, as long as it is biased above 4-5 volts?
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