LCR phono stages we know about

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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Hi Lewm,

Yes, the shootout was meant to get a feel on the basic characters of the units rather than serious comparison, which you really can’t do in a short time. If I understand correctly, you can order the complete phono from Thomas Mayer, with the option of supplying him with your own LCR module, or take the one he has. My friend’s unit used the Tango module.

I also agree that the SUT has great effect on the sonic, and it take considerable experimentation to match a cartridge to the SUT / phono. Since most of the other units in the shootout had built-in MC gain, we didn't prepare too many SUTs for the Thomas Mayer, which was unfortunate.

Regarding LCR, both the Thomas Mayer and a DIY SS unit that my friend built have a very polite, refine, relax and musical presentation, which make you just want to enjoy the music. It is in contrast to some of the modern phonos which give you the “in your face” resolution and dynamics. I am not sure whether this is a common character of LCR phonos, and I am very interested to audition the Allnic LCR models.

Hiho, The output impedance of the input voltage amplifier or gain stage has to be accounted for in the design of the RIAA. Therefore, as you may be suggesting, output impedance is a factor in determining the values of the reactive components needed to effect an RIAA equalization. But output impedance per se cannot substitute for Rs and Cs that make up the RIAA. Perhaps I don't get what you wish to say. Suffice to say that if you do anything to alter the output Z of the gain stage, you have then to change the values of Rs and maybe the Cs in the RIAA (or at least the value of the first R). This still does not suggest to me how the Emia RIAA topology is achieved. Maybe Dave Slagle will walk by this thread and stop to enlighten us.

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.