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current feedback electrostatic amp


kevin gilmore
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so sorenb really wanted this, so here it is

http://gilmore.chem.northwestern.edu/cfaelectrostat.pdf

the outputs are 300v darlington transistors  just need to find a suitable npn match for the stn9360, and this can be done all surface mount

tiny, fully complementary, fully dc coupled, decent input impedance (none of this 75 ohm nonsense) push pull output...

edit:  PBHV2160ZX looks great for the npn.

so you can add one more set of outputs and run up to 500v power supplies

balanced input only, and you need 4 of them

 

Edited by kevin gilmore
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It will need some heatsinking if we are to run it at reasonable current levels aka. mafia style.  That said, nothing too excessive. 


what are the typical ways of sinking smd parts? i saw kerry do it with the transportable dynalo and it seemed a bit unwieldy, hard to wrap my head around. are you just pressing the board against a surface?
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So we actually can do it as the schematic is drawn and make darlingtons out of 2 different transistors

So ksa1156 and ksc2752 for the output transistors, as the schematic is drawn they are 750mw each the driver transistors are 8mw each so definitely surface mount and the input transistors are 50mw each

I think there might have been a better match for the ksa1156, someone go find it.

also remember that Fairchild was bought by onsemi, so lets make sure the parts are going to be there for a while

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Just popped it into a Spice. I don't have models for the output devices, so used BC337/327s everywhere (they would blow up, but Spice doesn't care. :))

Bandwidth doesn't look too hot, even with the outputs unloaded. May have made a mistake, but the input stage looks like it's starved for current? Replacing the emitter degen Rs with 1k and bumping the ouput stage Rs accordingly (~3k) helps a lot.

Not sure how to attach the .asc, here are some screenshots.

eb6d9d6fac.png

0a368e0c15.png

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matching the output devices not a bad idea

board file posted, new software may not be compatible with some gerber viewers, I did it in rs274x although I can do the new gerber x2 version

changed a number of resistors, its less than 1db down at 20khz.  about 7.5 watts per board, 15 watts per channel, 30 watts total plus the power supply

joamat i'm expecting you should be able to have this done by the end of the day :D

 

 

Edited by kevin gilmore
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So, if I understand this, it looks like this design really likes to be driven balanced.  If you have an unbalanced input (e.g. RCA) the negative input is grounded and the stator gets no signal, unlike the differential input designs which become a long-tailed phase splitter if you ground the negative input, thus providing a balanced output.  Correct?

Edited by JimL
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no this is the krell/Marantz style of doing unbalanced to balanced via the low impedance - cfa input.

Marantz still does this in products today.  differential gain way less than .1db, look at the graph. schiit has trademarked this as "schiit pivot point"

my unbalanced/balanced/cast input board does the same thing.

if cavalli was not such an idiot, he could have done the same thing with his liquid cabon instead of a cheap front end opamp. By the way, liquid gold is the same fucked up mess.

Edited by kevin gilmore
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