LCR phono stages we know about

Here's one example from a Wright circuit. As you can see, there's no series resistors, the output impedance is the invisible resistor. The downside of such circuit is that tube aging can affect plate resistance and therefore R's value and RIAA accuracy. Wright's circuit uses a lot of regulations to perhaps make the circuit as stable as possible and not affect EQ accuracy. I believe such circuit might need tweaking the RIAA network when changing tubes. There might be other "tricks" in the circuit that I might have missed.

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I am very familiar with that circuit. I revised my own Atma-sphere MP1 phono input (already a dual-differential design, like the RTP, but with an all-tube cascode input stage, unlike the RTP) to some degree to mimic the RTP shown here. However, I did nothing to the standard MP1 RIAA network (except see below), so I had not noticed, as you correctly point out, that the RTP has it's RIAA in shunt topology. (I don't like the term "invisible resistor"; it's the output impedance of that hybrid cascode to which you refer, and that's a factor in any RIAA network. The cascode will have a high-ish output Z in fact. But I take your point. Thanks.) Note however that the RTP is rather unusual in that there is no second gain stage after the RIAA (It's not needed because that hybrid dual differential cascode at the input develops tremendous gain by itself); nor is there a cathode follower at the output of the phono section so as to reduce output impedance to drive the attenuator and the linestage section.

When I installed a similar hybrid cascode gain stage at the phono input of my MP1, all I had to do to maintain correct RIAA was to change the value of the first (series) resistor in the MP1 RIAA network, to correct for the change in the output impedance of the gain stage. I listened to it before changing that R value; there was virtually no bass response. When I re-adjusted the series resistor value to compensate for the altered output Z of the gain stage, the bass came back. (Just to make my point that the output impedance is a factor in any RIAA network.) Now you've got me thinking I could possibly adopt the RTP RIAA network to my own phono section. Cool.

Lewm: "Note however that the RTP is rather unusual in that there is no second gain stage after the RIAA (It's not needed because that hybrid dual differential cascode at the input develops tremendous gain by itself); nor is there a cathode follower at the output of the phono section so as to reduce output impedance to drive the attenuator and the linestage section."

Yes, that phono stage has to work in tandem with the line stage, ie, approximately 50k input impedance (and whatever capacitance in the internal cables), whereas many phono stages with cathode follower are to isolate the RIAA from outside influence and can work with external line stages or loads.

Sidenote: the Manley Steelhead is a variation of RTP circuit (SE not balance) with a White follower so it can drive anything.

Lewm: "When I installed a similar hybrid cascode gain stage at the phono input of my MP1, all I had to do to maintain correct RIAA was to change the value of the first (series) resistor in the MP1 RIAA network, to correct for the change in the output impedance of the gain stage."

I agree that having a series resistor makes tweaking accuracy easier. Since you only have to adjust one resistor, taking the output impedance of previous stage into account, the rest will fall into place. I guess what the purists object to is the high value of the series resistor but the higher the value the less affected by the first stage's output impedance and less fussy to deal with. It comes down to how obsessive you are about such things.

Anyway, looks like you're having fun with your mods.

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As I age, I tend to think, "less tinkering, more listening". I've got around 2200 LPs that need listening to. That's why I am much more likely to purchase an LCR phono stage than to build one of my own, although I have a friend who would walk me through that process.

Lewm --

Sorry I missed your post from last week. The 'Kahn' in Kahn phono stage refers to the metalworker who implemented the design of the box -- very steampunk!

As to the performance...spectacular. I have experimented with tube rolling and am very pleased with where things ended up. These pieces (phono and line stages) are very special, bespoke pieces. And yes, Dave Slagle's handiwork is integral to the implementation. The EMIA product offers a large measure of the Kahn for much less money.

Best part: both Jeffrey and Dave are first class music geeks who are extremely fun to work with.