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Hybrid Cascode Phono State


dsavitsk
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A friend asked me to come up with a phono stage that is of reasonable quality but not too over the top. Here's my first go. Basically, it is a hybrid cascode gain stage followed by a passive RIAA filter and a cathode follower. The idea is that the single gain stage should not only provide enough gain, but it should be reasonably quiet. The Zout is also governed by the plate resistor which keeps the filter stable even as tubes age. In a sense, it is much like Pete's pentode phono, other than that the cascode should be a bit quieter than a pentode. That's a lot of should's, so it remains to be seen whether it actually works. The only really tricky part I anticipate is selecting the JFET for having the right current with the right gain resistor. A negative supply such as Vacuum State uses with a similar (though differential) set up would alleviate this issue, but I am trying to keep it as simple as possible and I do not want to add another PS, or a virtual ground on the bias supply if I don't have to.

cascode_riaa.png

For the power supply, in order to keep things as simple as possible, I wanted to use an off the shelf power transformer. Antek has a bunch of toroids with a HT winding and a couple of 6.3V windings. So, for the heater, I just used schottky diodes and a LDO reg. Their windings are generally spec'd at 2A or something, so there should be a bit of voltage headroom. For the bias which needs ~10-12V, I used a voltage doubler follower by a LDO linear reg, followed by the shunt reg shown in the schematic. B+ is a CLC or CRC followed by a simple shunt reg (CCS and a mosfet biased by some zeners).

cascode_riaa_brd.png

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I'm sure I'm missing something , but why use tubes at all in this circuit? Besides setting up the RIAA network, looks like all the ECC88 does is act as the upper half of the cascode and an output follower. If nothing else, a completely solid state circuit eliminates the need for a HV supply. On the other hand, if tubes are desirable, a three stage split RIAA design using triodes can easily be made dead quiet and can be implemented with no more components than shown here.

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All reasonable points. The person who asked me to design something requested tubes. And, I'll take the Allen Wright position that tubes sound better, even as followers. I'm not sure I believe this, but I'll say it anyway.

But, as for comparing against triodes, the issue there is that as the tubes age, or as a different tube is used, rp changes which influences the riaa acuracy. You can use a large output resistor, but unless it is really large, tube changes can still matter. With a cascode, or pentode for that matter, since the Zout is almost completely governed by the load, the filter is much more tightly controlled even as tubes die.

And I've built an all triode phono stage. Seemed like something different might be interesting, and my ear prefers pentodes to triodes these days smile.png

The irony is that the person also needs a line stage, and for that I basically copied SY's Heretical line stage, but I did so with all sand parts.

Edited by dsavitsk
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You have a point about about rp variation in triodes. This is a very real problem when using the high gm types such as a 417a or 7788, but not so much when using 6SL7/6SN7 types. After several attempts with the high gm types, I reverted to a 6SL7/6N7/6AH4 line up. I like a high/medium/low gain arrangement. Using a low mu/low rp tube in the third stage for gain instead of a follower gives you a buit in line stage with reasonably low Z out. Full function preamp in three tubes. Been using this for over 10 years with the original tubes. Now I'm curious to see what, if any, changes have occurred over the years.

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