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Grounded Grid
Cascode Phono Preamp
Mr. Rozenblit points out that it is amazing how many people still want a good phono stage. His design uses the Common Cathode amplifier with a Cascode stage riding on top of the second triode for extra gain, which then cascades into an SRPP stage. Feedback is taken from the grid of the SRPP's bottom tube so that the SRPP is outside of the feedback loop. The output is inverting, which is not a liability; this non-problem is easily rectified by inverting the leads on the phono cartridge. In fact, a very real advantage to having an inverting preamp is that it is less likely to break into oscillation due to the output signal inductively coupling to the input of the amplifier when the input jacks are too close to the output jacks for instance, as any re-circulation of signal will decrease the output, not increase it.
(Actually, this was the only circuit in Mr. Rozenblit's book to make my eyebrows climb up my forehead. Just how noisy is this circuit? Phono stages, unlike line stages, require the lowest noise operation possible and this circuit looks noisy to me. Maybe a better design choice for a low noise tube phono preamp would be a Grounded Grid Cascode with a P-channel FET driving its cathode instead of the
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Common Cathode arrangement or even maybe forgoing the Grounded Grid feature altogether and just using a Cascode for the first stage with the feedback fed into the top triode's grid. Of course, since I have not heard this preamp, maybe it is quiet enough as is.)
Conclusion
I heartily recommend this book. It is not perfect. It could use a rewrite (just like everything I write). It could use a few more illustrations. It could be longer. Still, I cannot think of a better book to recommend to those hundreds of tube fans who have asked me for advise and help on how to become more knowledgeable in the ways of tube circuits. I thank Mr. Rozenblit for making my recommendation easy.
// JRB
Price: $29.95 USD
Available from:
Transcendent Sound, Inc.
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