LCR phono stages we know about

Audio Note UK also makes few models of phono preamps that employ LCR network.

Typical LCR network is sandwiched between two gain stages with the first stage being able to drive low input imepedance, typically 600 ohms like the Tango circuit, so a cathode follower is often employed and some purists might argue against it. But who cares... whatever works and whatever sounds good.

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Perhaps the 600 ohm "standard" stems from the pro side when making records in applying RIAA EQ, which was made using LCR network, inverse curve of course.

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One difficulty in using higher impedance LCR is that the inductance of the choke will be higher accordingly which is harder to wind. You gain some you lose some.

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But I really do not know how one can avoid having either an R or a C in the series signal path and still get to proper RIAA equalization.

Any tube gain stage has its inherent output impedance which can act as the invisible series resistor R in the EQ. That's been done in some designs by the late Allen Wright and others.

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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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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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