LCR phono stages we know about

Not all RC RIAA phono pre's place a large value resistor in series with the signal. I have a fully differential phono pre designed by Kevin Carter of K&K Audio that does not. The large value resistors (and caps for that matter) are all shunt connected, not series. In its first incarnation this was not the case. This will get a little technical here but bear with me. The input stage consists of a pair of vacuum tube/JFET (now MOSFET) cascodes. In the original design the tubes had plate loads and the plates were direct coupled though 33K resistors to the grids of the driver tube (the series connection). With a later redesign the plate load resistors were replaced with Constant Current Sources (cascoded MOSFETs). This required changing the 33K resistors from a series connection to a shunt connection (from the plate to ground) leaving just a 10 Ohm grid stopper in the signal path.

What does all this mean for the sonics? A way more dynamic sound, macro and micro. A much better sense of touch. It brings closer the sense of real music being plaid by real musicians. I heard this change with everything else in my system the same. So I can say without reservation that removing that large series resistor makes for a big difference.

BTW, Kevin's latest generation of his single ended parallel feed phono pre the Maxed Out uses this same arrangement. You don't have to opt for a LCR design to reap these benefits.

" is that it places only an inductor in series with the system, which has very low DC resistance compared to the resistor MANDATED in RC type RIAA "

My point in posting was to highlight the fact that not all RC RIAA implementations require that high value series resistor and that yes, removing it does yield improved dynamics, palpability, touch etc. I have heard the difference in two different phono pre's: a single ended parallel feed design and a differential series feed design. In each case switching from series to shunt resistance resulted in big improvements in the above mentioned areas.

So you are right. Providing a low resistance path through the phono stage is beneficial in my experience. I just wanted to point out that LCR is not the only game in town when it comes to doing that. You seem to have implied that with the above quote.

If you load the input stage cascode with an active current source you can effectively achieve an infinite output impedance. This allows you switch the RIAA series resistor from a series connection to a shunt connection. No issue with RIAA response changing as the tube ages. The excess current from the plate load CCS is shunted across this resistor to ground and sets the plate voltage. Neat trick. To maintain the set plate voltage the cascode pair needs to be biased with a cathode/source (tube or tube/SS hybrid cascode) CCS.

Yes you can have a LTP differential stage with a current source tail and current source plate loads. I have one. The trick is the shunt resistor. It bleeds excess current from the plate load CCS to ground and establishes the plate voltage at the same time. This does present a problem if you are using an all vacuum tube cascode in that imbalance currents between the tubes will change as the tubes age and this will lead to differences in the two plate voltages of the pair. If you are direct connected to the second stage as in my phono stage, this is bad and will eat into the dynamic voltage "headroom" of the preamp. However if a SS/tube hybrid cascode is used, the bias is mostly determined by the SS devices used (matching is a must) which generally doesn't change over time. If the plate load current sources are mounted to the same heat sink so they track thermally the problem is solved. It's a shame that the Sonus Veritas phono pre which used this topology never got any traction in the marketplace. One TT manufacturer who will remain nameless compared in to the Ypsilon and he preferred the Sonas.

With the single ended version the cathode CCS is bypassed with a capacitor.

You need a cathode bypass cap for the same reason you need one in an ordinary single ended triode circuit. The tube for the sake of analysis can be viewed as an AC signal generator whose output is Eg*u were Eg is the signal voltage presented to the grid times u, the gain factor of the tube. The AC signal loop with a cathode bypass cap consists of two resistors, Rp and Rl in series where Rp is the dynamic plate resistance of the tube and Rl is the load. This acts as a voltage divider on the output of our theoretical signal generator. The greater the load resistance in relation to the plate resistance, the greater the percentage of the signal is dropped on the load. With a CCS load the gain of this triode stage thus approaches u.

Now remove the bypass cap. The signal still has to return from ground to the cathode. It no longer has nice near zero impedance cap. It must pass through the cathode resistor. The AC voltage drop is -i*Rk where i is the AC signal current and Rk is the cathode resistor. The output of our tube (signal generator) which was Eg*u is now (Eg-iRk)*u. If Rk is very large this gain stage will have very little to no output.

Lewm, the bypass cap is only necessary for the single ended version. The differential LTP version of course does not need a bypass for the cathode CCS as the signal passes from cathode to cathode directly. I have a complete schematic for the beta version of the K&K Audio Sonus Veritas Venice which used this circuit for its input stage (BTW the Venice is fully differential). I only have a "paper" version of this schematic and don't have a scanner unfortunately. Maybe if you talk to Kevin about this he will be willing to email you the schematic.

The schematic tends to look pretty busy due to all the current sources and shunt regulators. One plus sonically is the only caps affecting the signal are the two RIAA caps that are shunt connected across the two balanced signal paths. No caps in series with the signal path. No bypass caps either of course and the PS is well isolated from the signal (CCS "tail" impedance between cathodes and ground, separate CCS and shunt reg. between B+ and PS for each gain stage). Also, the only series resistance in the signal path are the 10 Ohm grid stoppers.

One way of thinking about this input stage arrangement is to consider it a parallel feed design without a parafeed cap (we aren't trying to block DC in the shunt resistors). The shunt connected RIAA resistors are now the tubes'e plate load with bias supplied by the plate CCSs. One issue that Kevin had when he went to this arrangement is that the tubes' capacitance now enters more into the RIAA calculation. This required some rejiggering of the RIAA capacitor to obtain an accurate response.