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Old 26th December 2013, 09:24 AM   #1
EUVL is offline EUVL  Europe
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Default Pass STAT Headphone Amplifier Concept

This is our interpretation of Nelson's original concept first published in 2010 :
Semisouth JFET's: What are they good for?

Such a simply but nice concept, that it seems a waste not to make some use of it.
As an exception, we have included the Spice file so that you can play with different parameters.


Happy New Year,
Patrick

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Last edited by EUVL; 26th December 2013 at 09:34 AM.
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Old 26th December 2013, 02:36 PM   #2
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Patrick I would look at the voltage divider on the mosfets, make that lower impedance. High impedance make it susceptible to deviations on transients.

Jan
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Old 26th December 2013, 02:54 PM   #3
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How low, factor of 5 ?
That will also increase dissipation and cap size (not that it is an issue).


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Old 26th December 2013, 03:19 PM   #4
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Just changed as you suggested (5M->1M, 500k->100k, 10->47, etc.) and checked frequency response in Spice.

No apparent benefit. Frequency response remains identical.
So the limits were set somewhere else (feedback network fro upper limit and mu-follower bootstrap cap for lower limit).
The FET has really very low capacitance.

I think no change for now.


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Old 26th December 2013, 04:01 PM   #5
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This is a Very interesting Thread !!!
I have been contemplating such a design for quite sometime.
I have finally gotten some HV fet's to work with.
I have some IRFBG30's and an assortment of various ones of 900v, 950v and 1200v Fets from ST as well.

I do have a 900v powersupply from an old Heathkit HW-101 that I may use, But I am thinking about a simple switching supply as well due to safety and the cost of filter cap reasons when I start venturing above 900v.

I will be watching for further development of this project!!!

Cheers !!

jer
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Old 26th December 2013, 04:41 PM   #6
EUVL is offline EUVL  Europe
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IRFBG30 seems to have quite a bit higher capacitances. Transconductance also somewhat lower.
I think both bandwidth and distortion would suffer.

But of course you can try it out first in Spice and compare.


Patrick
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Old 26th December 2013, 05:18 PM   #7
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In my Previous Sims in Circuitmaker2000 it seemed to work pretty good but then I was also using and Opamp to drive them.

I haven't explored the same circuit it LTspice yet.

It is some times hard to find FET's with high voltage ratings with low capacitances.

Here is the list of ST devices I have,

STW4N150 1500V Nfet,

http://www.st.com/web/en/resource/te...CD00050744.pdf

STF6N95K 950V Nfet,

http://www.st.com/web/en/resource/te...CD00261184.pdf

STP21N90K5 900v Nfet,

http://www.st.com/web/en/resource/te...CD00255284.pdf

And some HV NPN BJT's BUL128D-B,

http://www.st.com/st-web-ui/static/a...CD00001675.pdf

The 950V Nfet has the least amount of Capacitance value of the lot at only 450pf input and 30pf output values.
The 1500v Nfet is not too bad but it is up there, But acceptable at 1300pf in and 120pf out.
It is much less than many others I have found in this voltage range.

Here is the datasheet to the IRFBG30's I have,

http://www.vishay.com/docs/91124/91124.pdf

I have posted the schematics of my previous sim's earlier in another thread and one working model using IRF510's/740's/840's and such at 200v.
But again, I was using an opamp to drive the stacked FET and I was using a Power resistor for the top CS too.
It worked very good I just haven't had the time to get back into it since then (2008).

jer

Last edited by geraldfryjr; 26th December 2013 at 05:34 PM.
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Old 26th December 2013, 06:12 PM   #8
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I'm quite interested. I'm running tubes now, but I've always wondered what could be done on the solid state side.
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Old 27th December 2013, 04:34 AM   #9
EUVL is offline EUVL  Europe
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http://www.fairchildsemi.com/ds/FQ/FQP3N80C.pdf

Crss 5.5p
Coss 54p
Ciss 543p

Yfs 3S at 1.5A

Not easy to beat.


Patrick
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Old 27th December 2013, 11:58 AM   #10
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PCB first draft.

Since the active devices are so low cost, one can afford to spend on the passives.
Resistors are e.g. Caddock MK132 & Susumu 0805 thin film.
MK132 is rated at 400V 0.75W, compared to most axials like RN55 which are ~200V 0.25W.
And they take less footprint as a bonus.




Patrick

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