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58dB Hybrid Phono Stage

July 6, 2016

Several years, I built a quick and dirty phono stage and it had promise but I set it to one side until I came to sell my AR turntable that is fitted with a Sumiko Blue Point Special Cartridge. I decided to sell the two together. The BPS is a high output moving coil design however, I felt that higher gain than the 44dB or so that my typical design (two stage with passive RIAA) provides, was desirable. This phono stage actually started out in 44dB form. Now, the BPS has a mixed reputation and this was a challenge to me. I had realised that it probably does not tolerate capacitance well being a high impedance moving coil and thought that perhaps a cascode input phono stage might help. And so I modified the design to incorporate a fet / triode cascode in the front end, using a 6GM8 battery type tube for the triode. I have used the 6GM8 in a couple of other projects and it sounds quite good altogether. The new design worked, all of a sudden the BPS went from having a congested and rather lifeless sound to being immensely open, articulate and above all, musical. I had worked the design out further in a re-build of a commercial integrated amp (link) and continued to be pleased. And so come the time to sell the AR, I cleaned this one up and the new owner reports experiencing much listening pleasure with the combination. It does hail from my tube voltage regulation days, now I would use current regulators. Here is the schematic:

58dB Hybrid Phono Stage Schematic

I haven’t bothered to show the raw B+ gubbins, anyone who has the ability to use the schematic offered can sort that out easily. Note the +6.3V is from a LT1085 regulator that also feeds the heaters. It must be superbly clean. Oh, the stain at the top right is from  chocolate ice cream (every detail matters).

And here is the thing:

58dB Hybrid Phono Stage 1

It is still on its wooden baseboard, I added proper terminals, a copper ground plane under the phono stages and a power switch.

58dB Hybrid Phono Stage 2

Here’s the AR, with Audiocraft unipivot arm and Merrill platter, the phono stage and my 2A3 line stage are behind:

AR, Merrill & Audiocraft 1

AR, Merrill & Audiocraft 2

It’s a damn fine sounding set up and caused me to do some maintenance on my Benz – Sota – planar triode set up which was not comparing well! It turned out that the arm ground was flaky, phew! It is a good idea to treat audio connectors with Deoxit once in a while. Even if there is not audible noise, lousy connections sound just that, lousy.

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