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Old 18th April 2014, 01:36 PM   #11
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Join Date: Oct 2009
Location: Hamburg
Dear Calvin and Jaclement,

Thanks for the suggestions.

Quote:
Originally Posted by Calvin View Post
the excellent ITH 02N250 in TO247
Oh yes it is available.

Sadly its a little expensive, more than the price of a triode!

So I have taken some inspiration form the circuits Calvin included and the good success Jaclement had with such a simple design.

I shall build a few, ideas and see how they all work. I have ordered a nice high voltage transformer and some high voltage capacitors.

I also ordered the "6SN7GT B" for a comparison as I think I will build a MOSFET and a valve based design and see which I like best.

I particularly like the idea of modding the opamp driven MOSFET with no output capacitor, as I have read (but don't know if it's true yet), that at such high voltages the capacitors "sing", maybe with a DC servo to set the voltage right.

Can I just use the "ITH 02N250" in place of the output MOSFET's in all of the designs Calvin posted or would I need to make many changes to bias?

Best regards

Owen
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Old 18th April 2014, 01:57 PM   #12
jcx is offline jcx  United States
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even film caps can become "piezoelectric" when they have high DC bias (polarization dependent electrostriction)

but this depends on the construction - the plates have to be able to move - used to be a problem in old oscilloscope ps with audio frequency chopper supplies and oval Mylar caps - the oval shape doesn't allow for even winding pressure, worse some were wound on removable round mandrels and then squashed to oval shape

high tension wound, or bonded layers will have much less problem

the really paranoid could stack+parallel a few of the biggest, highest V rated NP0/C0G ceramic caps - a solid, void free, nonpiezo material - at considerable cost


but the concern in electrostat amp output DC blocking is probably overstated - you also need high AC too and its usually the case that you are using 100x the expected ES headphone Cload so the output caps only see 1% of the audio AC

Last edited by jcx; 18th April 2014 at 02:09 PM.
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