Some to consider are from allnic, ypsilon and zanden...
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LCR phono stages implement the RIAA equalization using passive components (inductors (L), Capacitors, and Resistors) between the active gain stages, in much the same manner as a loudspeaker crossover, instead of doing the equalization within an active stage. Some preamps use both techniques, one for the bass boost, and another for the treble attenuation.
Mordante et al, I apologize for not defining the acronym. I thought most analog aficionados would know about LCR RIAA circuits. But here goes a definition:
In analog history, nearly every commercial phono stage has been based on using RC filters to achieve RIAA equalization. An RC filter is one that uses resistors and capacitors to achieve each of the three 6db per octave "shelves" in the frequency response that constitute an RIAA correction. Most such RC-based RIAA networks require the use of a fairly high value resistor in series with the phono signal coming out of the first gain stage. And some require the use of at least one capacitor in series with the signal, as well. The advantage of an LCR network, used to achieve the exact same RIAA correction, 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 networks. Capacitors and resistors are used as well, but all of these are in parallel with the signal, connected to ground. Theoretically that does less "damage" to the signal, where "damage" can mean different things to different audiophiles. Proponents of LCR type RIAA correction circuits believe that they sound better, pure and simple. However, implementing such a network presents a lot of problems to the designer. Thus = expensive.
Yes, JFrech, I knew that Allnic (in the H3000 and H1500) make LCR type phono stages, and I knew about Zanden. Did not know about Ypsilon being LCR type. All of these are VERY expensive. (Well, the Allnic H1500 is not too far off the charts.) Have you heard any of these 3? In fact, I would love to hear from anyone who went from a very fine RC type phono stage to a top of the line LCR type. I would also like to hear from anyone who has actually listened to any LCR type phono and who has an opinion about the advantages based on real world experience.
That's not quite correct or at least possibly confusing as written, Sqlsavior.
LCR vs RC has nothing to do with active vs passive RIAA. Either technique can be implemented in a passive RIAA filter. It's just a matter of how the filter is created, with inductors in series with the signal vs with Rs and Cs in series/parallel.
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.
John, I am very familiar with your association to K&K. I agree they make very fine products. However, what has any of that got to do with the price of eggs or the topic of this thread? I guess I can be faulted for not having catalogued every possible way of designing an RC RIAA network. For example, you did not mention that it's also possible to use a capacitor as the first element in an RC RIAA, tapped in series off the plate of the input voltage amplifier, instead of a resistor. Nor did I wish to turn this thread into a debate about the relative merits of each type of RIAA filter. I just want to know more about LCR type phono stages that are already out there as commercial products, and how they sound in comparison to other good phono stages.
"I would also like to hear from anyone who has actually listened to any LCR type phono"
I have listened to the Ypsilon (driven by a Lyra, on a Yorke parallel arm, S Yorke 10 TT). Mind you, I just heard it: I never had it at home.
Well, it is very detailed and gives the feeling of not adding or doing anything to the sound. Strange because it has a tube somewhere in the circuit, caps & inductors -- bu that's the impression it conveys. In other words, as if it's not in the system at all.
Sonic result: The details are most noteworthy: there is detail even within what may sound like a dynamic crescendo with another phono (say, the big Pass or the small FM acoustics). Typically, you will listen at high volume, the music is magical. I don;t know if the Y's dynamics are equal to other top phonos (or worse or better) -- I couldn't really tell.
Unfortunately, I'm nowhere near being able to afford $26k for aphono -- or anything else for that matter!
Lewm, I've heard the H3000v in my system...and have heard the H5000 in friends systems. The H5000 is likely one of the the very best out there. The H3000v I heard definitely presented things is space with much more palpability. Just eerily real soundstage and how the instruments are portrayed...I can only guess that this is from the LCR network...
I agree with Jfrech's analogy. The Allnic does have a palpability and involving presentation that is addicting. I have owned the Allnic H3000 for about 8 months and still learning it and still trying to figure out the best load as there are 16 options! However, with my Jubilee MC there are realistically only about 10 options but they do each present differently.
" 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.
I the van den Hul Grail SB a LCR phono pre amp. This pre amp is one of the best I have ever heard. Maybe even better then my previous "reference" the Pass XP-15 and XP-25.
But I have not heard them side by side.
Has anyone here heard the van den Hul Grail and Grail SB?
See for info:
Jazdom, Hi. Can you say more about the Kahn phono stage. I presume the name refers to our mutual friend in Silver Spring. But maybe not.
John, I own a Silvaweld phono stage, designed and produced by the same Mr. Park who later went on to produce the Allnic products. He evidently had a huge change of heart after Silvaweld went under. Silvaweld phono stages are all RC types, use a capacitor as the first element in the filter, thus avoiding high value resistors at that point in the signal path (or any resistor, for that matter). However, using a capacitor has it's own theoretical issues, since capacitors are also imperfect devices; one might say they are more imperfect than resistors in fact. So the Silvaweld circuit is a far cry from the LCR used in the Allnic H3000 (or H1500). The Silvaweld also uses a JFET input to add gain for MC cartridges, followed by an all-tube RIAA correction network that by itself develops sufficient gain for any MM. In contrast, Allnic phonos use built-in SUTs for this purpose. I use my SWH550 only for MM cartridges, so I bypassed the JFET permanently, and then I rather drastically modified the tube circuit downstream, with some help and advice from a more knowledgeable DIYer. (Kept the capacitor-led RIAA as is.) The result is shockingly good. I have a neighbor up the street who just got hold of a LCR MM made by Dave Slagle and his business partner. I do not know the commercial name. I heard that the other day and am VERY impressed. I am familiar with my neighbor's system and had heard it previously with two other very good phono stages, Coincident and Dolshi. By comparison, the Slagle unit sounded very open, airy, effortless. He's made some other changes in his system along the way, so one cannot attribute all of the improvement to the new phono stage, and I would not say that the new phono "kills" the previous two, but.... it's very beguiling.
So, here's a list of some of the LCR phonos mentioned above:
Allnic H3000, H5000 (not a real-world possibility for me), H1500
"Kahn" Experience Music
Zanden (did not previously know that was an LCR type)
I know there are others in between the extremes of cost ($32K for the H5000 vs $250 for the Lounge [maybe the Ypsilon is even more costly than the H5000]), but my mind has gone blank. The Dave Slagle design costs ca $5000, plus the cost of any added SUT for MC. There's another one available via eBay from the orient that has received favorable mention. I am really curious about the H3000 and am on the look-out for a used one.
For the really far out theoreticians, I have read about LR-based RIAA filters, which use no capacitors at all. Apparently that's very tricky to execute because the values of the inductors must be very precise in order to achieve good RIAA compliance.
Addendum. After composing that long treatise, I did some surfing to find out more about the Slagle LCR phono stage owned by my neighbor. It turns out that the phono stage is called "Emia" and is marketed by Dave and Jeffrey Jackson, in collaboration.
Further, the "Kahn" Experience phono stage is also a product of this same collaboration, only it's more expensive than the Emia.
So these two would be important players in the LCR game, albeit hardly mass-market.
Dave, I looked it up. Not much info regarding the innards. Seems to be solid state. Does not seem to have adjustment for load R and C. However, it does have a mono switch. (I don't know how you can build a phono stage with high end pretensions and not include a mono switch.) As you know, I'm partial to tubes, but I'd like to hear this one. Do you have direct knowledge?
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.
Yes, that's one problem with designing a tube-based LCR; the needed 600-ohm impedance on the input side is hard to achieve with tubes. There are many interesting threads on that subject. One such claimed that you only need 600 ohms on one side of the LCR, not on both input and output side; I am not sure about that. Some use a transformer to achieve the needed impedances. But another approach is to build an LCR that can be driven by a higher impedance, 10K ohms being another more recent standard. A very smart DIY guy I know in Australia has built a high-gain LCR RIAA using ~20K ohm LCR, built from parts supplied by Dave Slagle. I have the schematic. I actually have no idea how or why 600 ohms was ever adopted as a standard or why it is anathema (to some) to deviate from that value. It could merely be that the original LCR modules, made by Silk and/or Tango, in Asia, were set at 600 ohms, and everyone jumped on that bandwagon when those were the only LCR modules available. I welcome input from others who may know more about this.
Good point. That's where Dave Slagle jumped in, to design and build chokes that will work well in "high-impedance" LCR designs. Dave can create very precise modules.
One of our readers has done a little digging into the Emia phono stage. Apparently it is NOT an LCR type. Rather, it is a novel CR type wherein the high value resistor(s) in the signal path have been eliminated. In fact, my correspondent says he was told that there are NO R's or C's in the series signal path between the input stage and the output stage; All Rs and Cs are shunted to ground. (I am only the messenger here.) Thus the plate of the input tube is direct-coupled to the grid of the output tube, with all the filter components shunting to ground. I can only say it sounds great, no matter what the topology. 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. On the other hand, the two designers are smarter than I.
Nearly a year back, we did a phono shootout, including a Thomas Mayer LCR phono, please see some of the write up in the following thread (mainly in the 2nd and 3rd page from the most recent)
Thekong, Thanks for mentioning the Thomas Mayer LCR phono stages. I had been meaning to do so as well. He seems to sell an LCR phono in kit form, using Dave Slagle LCR modules (aka "Intactaudio"). Very tempting. As to this shoot-out, it is not very helpful, although it seems the participants were having great fun. For one thing, several different SUTs were used with the Mayer LCR; I think the SUT will a major effect on sonics, if not THE major effect. I am somewhat familiar with the Scott Frankland phono stage mentioned. IMO, it is good but not a contender among today's best options. But you've added Thomas Mayer to our list. I wonder what he charges for that kit version?
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.
Yes, the shootout was meant to get a feel on the basic characters of the units rather than serious comparison, which you really cant 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 friends 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.
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.
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.
Jazdoc, Now that we know the Emia is not an LCR type, do you know anything more about the topology of the RIAA that you could reveal without violating any confidentiality agreement with the two designers? I am still wondering how they effect direct coupling and whether the accuracy of the RIAA equalization would be affected as the gain stage tube ages, and its output Z drifts accordingly. I think Hiho said it first; the virtue of the high value resistor at the output of the gain stage in typical RC RIAA networks is that it ameliorates the negative effect of tube aging. No matter; the Emia sounds excellent in the home of my neighbor, and he is thrilled with it.
I thought more about the Allen Wright RTP. As Hiho mentioned, the RIAA equalization depends upon the constancy of the input impedance of that 50K attenuator seen in the schematic. But at the same time, the signal must pass through a 2.2uF capacitor between the plate of the gain stage and the input of the attenuator. Those are at least minor drawbacks. My MP1 has a second gain stage downstream from the dual-differential cascode input stage. THAT second gain stage then drives the attenuator, thus isolating RIAA equalization from the attenuator. However, there's no free lunch; you still need a largish coupling capacitor between the output of the second gain stage and the attenuator. I use a smaller value capacitor (0.68uF) and an attenuator with a higher input Z, 100K ohms, to achieve roughly the same low frequency cut-off.
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.
There is also the Wavac LCR-X2 phonostage which uses LCR type RIAA equalization. Apparently, Wavac's founder is considered the originator of the LCR concept as detailed in some internet pages (don't flame me if you think otherwise, I am just passing along info). I have the Wavac and also the DIYHiFi Supply's Cole LCR phonostage which was my first phonostage. Have always found them to be musical and palpable but not sure if it's because of the LCR equalization or other system design contributions.
John, The input stages of the Allen Wright RTP (shown in the schemtic posted by Hiho) and of the Atma-sphere MP1 are both dual-differential cascodes , i.e., balanced topologies. Thus in both cases, a CCS is used optimally at the junction of the cathodes of the two "bottom" tubes in the cascodes and ground. (Take a look at the RTP schematic; I think AW used an LM317, which is definitely not my choice, but it's a CCS nevertheless.) Thus there is no place for more CCSs on the plate side. However, your point is relevant to a single-ended topology. But in a single-ended topology, the output Z would be the parallel sum of the impedances of the CCS and of the cascode stage below. The impedance of the CCS is ideally infinite (but is always finite in reality, albeit very high). However, would not the impedance of the cascode below be a very finite number, thus dominant in the calculation of output Z? And also subject to change as the tube(s) in the cascode age? Please correct me if I'm wrong; you probably know more about this stuff than I.
Now I re-read your post, you seem to be talking about a dual-differential cascode with CCSs between cathode and ground AND between plate and B+. AW discusses that possibility in his TPCB. Have you ever tried that?
Ddrive, That's very cool that you have the Wavac. As I recall, it's "big bucks". I forgot all about the Cole; I remember reading about its pro's and cons on Romy the Cat's blog; that was an early contender in the LCR game. I'd love to hear either. That's the only way I am going to gain a feel for these beasts.
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.
"With the single ended version the cathode CCS is bypassed with a capacitor." This is a sidebar to the topic, really, but are you imagining a single-ended phono input with CCSs between both the cathode and ground AND between plate and B+? Using a bypass capacitor around the lower CCS does not fit in with my understanding of the need for a bypass capacitor, the value of which needs to be inversely proportional to that of the more typical cathode resistor, in order to pass audio frequencies of interest. In this case, the CCS impedance is so high that a bypass cap would seem to be superfluous. Never seen that done.
I am guilty of going off topic in a thread that I myself started up.
I found a nice thread on Wigwam, among some English guys, one of whom built a LCR phono for another member, using inductors built by Dave Slagle. The builder explains that for an LCR, it either has to be driven by a source with an output impedance equal to its input impedance (e.g., 600R), in which case it can be terminated by an impedance that is ideally 10X higher, or the other way around. In other words, when I wrote that there needs to be 600R on either side of 600R LCR network, I was wrong. But 600R impedance is still very difficult to attain with tubes, without resorting to transformers, etc. This guy used 7K inductors made by Slagle.
Well, my Cole is available for sale. It's currently a 230v unit but I think it can be converted (need to rewire the input transformer). Will let it go for $500 excluding shipping and paypal charges.
I bought my Wavac used. That's the only way I can afford it but it has been bringing great joy to my vinyl system ever since it arrived.
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.
I designed and built a handful of inductor-based phono stages about fifteen years ago, both tube and solid-state. I used pre-made LCR networks from Tango and S&B (both now out of production), and a couple of LCR and LR networks of my own design with inductors custom-wound by Sowter. And while I certainly have no inherent prejudice against inductive components in the signal path, I ultimately felt that it wasn't the best way to go . . . but I'll share a few of the conclusions (at least that I can remember).
As others have pointed out, designing the network to a lower impedance reduces the inductor values and makes them easier to wind, but it at the same time pushes the inductors' ultrasonic self-resonance frequency higher, and further from the audioband. The Tango and S&B units I had were both 600 ohms, and one of my Sowter-based networks was significantly higher (about 2K IIRC), and the latter's transient response was poorer in an immediately obvious way, even though the RIAA response through the audioband was at least as good.
The other impedance idiosyncrasy (not surprisingly) about all the networks was that distortion was markedly lower when driven by a zero-impedance source, and terminated by its characteristic impedance, rather with "equal" impedances at each end. The noise figure is obviously lower as well. It also seems that the better the winding techniques and core materials are, the more sensitive the inductor is to any amount of DC current leakage. So I'll admit that I'm scratching my head a bit as to why LCR equalization and (especially low-feedback) tube electronics seem to be frequently associated, as the "impedance comfort zones" of LCRs and tubes don't much overlap, and some traditional tube biasing techniques that put DC through the inductor are definitely a no-no with a precision LCR EQ network.
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.As I read it, the operating conditions you describe are identical to a conventional transconductance amplifier with a plate-load resistor to B+. To the audio signal, the load resistor goes to ground either way, it's just that in the traditional topology the current is capacitively coupled to ground through the B+ supply capacitor. The source impedance is still the tube's output impedance in parallel with the load resistor, and since this resistor loads the signal, its value is still the limiting factor for the open-loop gain. Just because there's a constant-current-source yanking the DC parameters into submission doesn't change any of this; rather, is simply adds one more uncorrelated noise source to the equation. Maybe I'm not seeing something fundamental about your circuit description, but (no offense) I don't get how this is any kind of improvement over the basic tube circuits of seventy years ago.
But this does highlight how many designers make broad topology decisions in conjunction with their approach to RIAA EQ . . . so in evaluating various designs, it's hard to know what are the characteristics of the LCR approach in general, and what's simply the sonic signature of the circuit design as a whole. Incidentally, my favorite of the tube designs from the aforementioned exercise was five tube stages from three tubes per channel: WE417 followed by 12AT7 (both plate-loaded) and a 12AX7 cathode-follower, with global NFB around these three for a very low output impedance. I then used the Tango iron loaded at 600 ohms, and then the other sections of the 12AT7 and 12AX7 (also plate-loaded and cathode-follower, respectively, and with global NFB around the two) as additional gain and buffer. Sounded great especially with an MM cartridge.
My favorite solid-state approach was two 990 opamps, the first one giving active 318uS and 3180uS EQ and driving a LR network (IIRC 200 ohms or so) for 75uS EQ, then the second for additional gain and an output buffer. This was better for MC cartridges, especially when I added a Jensen 346-AX input transformer for a bit more gain, and so could reduce the electronic gain of the first stage.
At some point I removed the LR network from the SS preamp and substituted an RC . . . and found I liked it better. I recall the RIAA tolerances for the LCR-based units to be better than +/- 1/4dB or so, the RC/LR design was significantly tighter as there are fewer interactions, that is the resistor values can be easily twiddled to match the inductors and caps. The S&B units seemed to have a bit of an ultrasonic peak that I tamed down with a small-value Zobel network - the same approach wasn't completely successful with the higher-impedance LCR network.
John, I think there is a misunderstanding between us that comes from trying to describe a circuit in words rather than by schematic. Can you reference a schematic that illustrates your point? For my part, I can only say that I've taken a particular interest in balanced differential input stages, and I have never ever seen a schematic wherein the CCS ideally used between ground and the cathodes of the two tube sections that constitute the balanced input is bypassed by a capacitor. Plus, my understanding of theory tells me that it's not necessary, would even be detrimental. However, I stand ready to be educated; if you can point to a schematic it would help.
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.