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Old 7th May 2006, 10:03 PM   #1
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Join Date: Apr 2004
Location: Monroe Township, NJ
Question Hybrid Cascode ?

Allen Wright has had success using a low noise JFET in the bottom of a cascode in his phono section designs. Does anyone here have experience the other way around with a high gm triode on the bottom and a FET on top?

I ask because the supply of 12BY7s is dwindling. That implies new small signal circuitry will have to be developed for H/K Cit. 2s.

If the sections of a 12AT7/ECC81 are paralleled, the gm is approx. = to that of the 12BY7. So, that's a possibility for the bottom of a cascode. Now to find a suitable top. Please keep the need to keep small signal heater draw < 1800 mA. in mind.

Of course, resuming 12BY7 production would solve the problem. New Sensor are you "listening"?
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Old 8th May 2006, 07:30 AM   #2
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Eli,
You can use just about any MOSFET on the top which means you don't need to worry about elevated heaters supplies, extra current draw etc.

I assume that you are not using a huge rail or huge current in which case you just might get away with my favourite low power, high voltage "E-line" (TO-92 ish) Mosfet the ZETEX (and others) ZVN0545A.

Gain in a cascode is approx equal the load on the top device x gm of the lower device - this tells you the top device is not super critical.

Check the likely power dissipation to see if ZVN0545A is really suitable.

Cheers,
Ian
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Old 9th May 2006, 11:13 PM   #3
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Ian,

Thanks for the reply.

The B+ rail in the H/K Cit. 2 is about 485 V. I'm thinking of regulating it down to run the cascode.

A SWEET spot for the 12AT7/ECC81 is 210 V. on the plate and Ia = 3 mA. A ZVN0545A rates to be OK passing 6 mA.

I wonder how such a hybrid cascode will sound. Maybe follow in Stu Hegeman's footsteps and use some drain to grid local NFB.

DC couple a cathode follower to the cascode. The 2nd section of the twin triode would be the regulator pass device. 6FQ7s and 12BH7s seem to be candidates for this "slot".
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Old 9th May 2006, 11:35 PM   #4
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Eli,
The regulating is a good idea - cascode has high effective rp and so rail noise rejection is not good - you either need a very good clean stable supply or to use cascodes each side of a diff amp and rely on the diff amps CMRR.

If you are going to run a DC coupled cathode follower then you can just about effectively shunt regulate the supply by matching the cascode current and the cathode follower currents.

OR take the "full bore" approach and have the spare triode half of the cathode follower dual triode wired as a SHUNT regulator. With even more added silicon (another MOSFET as a unity gain inverter) you can actively drive it off the audio signal in antiphase. This is starting to go "over the top" BUT it allows experimenting with frequency shaping and scaling of the antiphase signal in the shunt reg triode. I am aware of a commercial design which does this and they scale the antiphase shunt reg current to approx. 2/3 of the active stage current and start rolling off its frequency response very early - like about 500Hz.

Good luck with it.
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Ian
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