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buzzforb 27th August 2011 12:13 PM

RO85 Zen
 
I was wanting to try and build a R085 version of Zen. I have a pretty decent grasp of what is happening with the standard version, but have a question about using the depletion mode device. I want to use a standard ccs(non aleph) in place of bulbs or resistors because i can only get a 50V rail out of my current trafo selection and need the extra efficiency. I am having trouble figuring out what determines the drain potential of the the signal fet when using both a ccs and a self biased Jfet. Does the ccs look like an infinite impedance and is therefore ignored, with the speaker taking the place of the Rl.

Nelson Pass 27th August 2011 05:03 PM

That's a good question. The Zen amp does have feedback, and you can use
that to determine Drain voltage, but it gets a little complicated when you have
a depletion mode device, as the Gate voltage has to be negative with respect
to Source. You could use lots and lots of degeneration (large Source resistor
to ground), but that cuts into your open loop gain.

If you have a small low current negative bias voltage available, you can use
it to bias the Fet through the feedback loop and this way you can run the
Fet without degeneration.

:cool:

buzzforb 27th August 2011 05:16 PM

Without dual rails, that is going to require a second trafo and PSU, correct. How much is enough and how do i come to that figure? Would a bootstrap cs perform better than DeLite setup? How would it compare to version in previous question, which i believe would be the illegitimate cousin of the F2. Sorry about all the questions:)

Nelson Pass 27th August 2011 05:55 PM

1 Attachment(s)
You can rig a "negative supply generator" off of your existing transformer.

Here's one.

:cool:

buzzforb 27th August 2011 06:21 PM

Thanks. I don't have all the parts, so i will build the DeLite today and do this one next. Do C2 and R1 pull the signal down to lower diodes?


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