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Old 8th May 2013, 11:12 PM   #11
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The J110 (Q3-12) is rated for only 25V VDS, so you can hardly expect it to hold off 300 V. It's only good for ~150 mW realistic power dissipation, so you can hardly expect it to stand up to (300V X 0.1A)/10. So, Q13 is the top half of a cascode, biased by current source Q17 and resistor R37. It pins the drain voltage of Q3-Q12 at ~15V (actual VDS ~10V). This keeps all those little fets from blowing up due to overvoltage or overpower. Q13 is rated for 800V VDS, and is a solid TO-247 device that can handle 30W dissipation with an adequate heat sink. You want a high voltage rating for Q13, as its drain can swing considerably above the +320V rail when the amp is driven hard, expecially if the load is accidentaly removed. I also have a fair number of the 800V fets around (a cheap Electronic Goldmine buy), so I have no qualms about using them

The board is laid out so I could alternatively use a single Lovotech jfet in place of all those J110s, but I'm saving that mischief untill later (much later).

This project came out of the initial discussion in the "Beast of a Thousand JFETs" thread, where I mentioned that I had a little project going with a few J110s in parallel cascoded by a HV mosfet. That particular board was a bit wimpy (only 3 fets), so I decided instead to gin up something that could command a little more respect in the living room. If done right, this amp should be good for somewhere in the vicinity of 10-15W/channel.
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Old 8th May 2013, 11:22 PM   #12
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Ok, good! I thought it was a cascode...

So is the point of the HV to use the output transformer? Or just because?

Regardless, it's very interesting...
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Old 8th May 2013, 11:50 PM   #13
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I use the output transformer and HV as an effective way of putting 100ma to work. It also causes anguish to the easily offended.... This amp is in good company, as I have two other solid state designs that use high voltage and output XFMRS. Check out the "Sic Puppy" and "Das Ist Aber Schade" threads for examples.

A more extreme implementation of this project could be ginned up using a lot more jfets and a lightbulb/resistor/current source load, a more solid-state type supply voltage, and capacitive coupling to the load, but my hair has enough grey in it as it is without trying to match up yet more jfets (look what it did to NP). Besides, JFETs in TO-92 package are getting to be a very scarce commodity, and I dislike tying up too much of my precious stock just to prove some bizarre point.

Imagine this amp as a solid-state version of a 2-stage partial feedback SE amp, with pentodes for both input and output duty. Having said that, power pentodes with 50-100 mS of transconductance are rather thin on the ground. A beam triode like the 6HV5A comes close, but those tubes require scary plate voltages to make them wake up.
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Old 9th May 2013, 03:04 AM   #14
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Old 9th May 2013, 05:07 AM   #15
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Distinguished silver - right- I've heard that one before. I guess grey/silver locks are a badge of honor for those who have spent years attempting to unscrew the inscrutable...Dealing with customer types that want three or six or so impossible things before breakfast (for no money) will also bleach out the locks, if you haven't ripped them out already in frustration.
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Old 14th May 2013, 04:29 AM   #16
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I'll be slogging on with the basic RC bias version of the amp for the initial implementation, as it "just works", and I don't have to do any extra tweaking to get a working first-pass amp. Those who decry expedience can scoop me by building and debugging an LED-biased version of the amp. Any volunteers? I thought not.
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Old 14th May 2013, 09:50 AM   #17
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Old 14th May 2013, 09:20 PM   #18
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That's me when this project is done.
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Old 14th May 2013, 09:26 PM   #19
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Can I get one?
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Old 15th May 2013, 12:12 AM   #20
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Can you get what?
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