Kirkus, Thanks for your patience in preparing a detailed response to my questions. Your take is very interesting and thoughtful. Meantime, I am wondering whether we should all be beating the bushes for vintage phono stages to use with these vintage MMs. But that would partially negate the premise that we can get more out of them now than ever before with modern well designed preamplification. Here are low-priced products that jump out at me as being possibly well suited to MMs: the new K&K, the Aqvox (SE for MMs, balanced for MCs), the Herron dedicated to MM and now out of production, any of the Hagerman products (I have high respect for his design capabilities), the Allnic products (they add on a SUT for MCs, so maybe they would work well for MMs, especially the H1200), the Whest2.0, possibly older Audio Research phono stages (maybe a tweaked SP3 would be great). Raul's 3160 may be at the top of the heap if price is no object. Any other ideas, guys? |
Hi Kirkus, because the side-effect of the twisting is an increase in cable capacitance, which is exactly what we DON'T want for our MM cartridge. This is fascinating for me right at this moment due to the fact that I have just mounted a Garrott P77 MM cartridge to my Continuum Copperhead arm which has balanced input terminations into the Halcro DM10 preamp. The tonearm wiring is fine gold (or copper) uninsulated Litz type wiring (I think), which is very tightly wound together to reject the noise (hum) to which you refer. This seemed to work for LOMC cartridges such as the DV1s and Universe (which only produced a slight hum when the volume was turned up considerably), but fails miserably with the MM which produces an incessant hum at any volume. The question has been asked of Mark Doehmann if he has listened to any MM cartridges through the Cobra and Copperhead arms as none of the dealers believe anyone has actually mounted a modest MM in such exotic and expensive arms? We are still awaiting the answers but your statements are intriguing indeed. What do you venture, would be a solution to this problem? Regards Halcro |
As in other audio areas here there are good, regular and bad designs. Damn straight. Couldn't have said it better myself. |
For the reasons cited, it is much more difficult to obtain a true balanced signal from an MM cartridge as compared to an MC one. Feeding a signal with an imbalance of noise on one phase vs the other to the balanced gain stage will result in the amplification of that noise, i.e., it will not be rejected because it is not identically present on both phases. Lewm, this is a fair approximation of most of what I was saying. Maybe add to that . . . if the "balanced" input stage doesn't do a bangup job of eliminating magnetically-induced hum in the real world, then does it really need to be "balanced"? This is of course a question that each circuit designer will have to answer for him/herself. Right now, I'm personally leaning toward "no" . . . but it wouldn't suprise me if I changed my mind in the future. Why is the noise to which you refer not similarly amplified by an SE topology? (It's that "3 db" boost of the noise that I don't quite get.) Noise is noise(?) In a good SE phono stage for MM, is the "ground" isolated from chassis or earth ground? That would seem to be a good idea. Well, in this case "noise is not noise". The noise to be avoided in poor grounding is a result of the amplification of ground currents flowing across ground connections of finitely-low resistance (that is, everything except superconductors), and should be completely eliminated in a good design. This is what I am referring to in response to Axel's comment below. But the 3dB minimum noise increase from an actively-realized "balanced" input stage is a different noise source altogether - here I am referring to the (mainly) thermal noise from the input semiconductors/tubes and their associated passive components. In a differential "balanced" input, there are double the number of devices, each producing their own uncorrelated noise, acting in series. When combined, they will produce double the noise voltage, which is 6dB higher. That can usually be offset by the fact that each side of the differential input stage is now looking at half the source impedance, which can reduce the noise by 3dB . . . so that's where the net 3dB noise increase comes from. This is a big reason why the vast majority of low-noise preamplifiers for ANY kind of low-impedance (hence low-noise-voltage) transducer is usually unbalanced in architecture. But IMO there can be plenty of logic to the idea of swallowing 3dB higher thermal noise (especially if the circuit is still very quiet), for improved hum rejection from a balanced input stage. The inevitable and unavoidable 'ground contamination' influences of capacitors etc. that makes the other argument for differential/balanced vs. unbalanced. Hi again Axel :) - There should be no such "ground comtamination", regardless, in a good circuit design and layout. This is "simply" a matter of the designer carefully analyzing the ground current flow in each part of the circuit, and understanding and considering the subtleties of such things as careful local bypassing, power-supply impedances, and ground-trace routing. But judging by many of the commercial products I see, this seems to be a particular challange, and differential-balanced circuits can sometimes be more forgiving of these sorts of faults. Why? It is that the differential circuit also cancels even-order harmonics in the process of common mode rejection, so you wind up with a bias toward odd-order harmonics, and that is not so 'natural' to our ear. I'm of the opinion that while low-distortion is only one of several required characteristics for a good-sounding circuit . . . I feel that in a high-quality phono preamplifier, ALL harmonic and IM distortion should be completely and totally buried in the noise floor, which in itself should be very low. Yes, low-order and even-order products are less disconcerting to the ear . . . but who wants any of it at all? |
Dear Axel: +++++ " some renown reviewers / testers having found, that even in a balanced design (not pseudo) the single-ended still sounded more 'natural'. " +++++
the main subject here IMHO is the kind of topology in the design, there are very good balanced designs and there are too not so good balanced designs. This not so good balanced designs performs a " step " down the unbalanced good design performance but IMHO not because the unbalanced is better.
As in other audio areas here there are good, regular and bad designs.
Regards and enjoy the music,
Raul. |
Kirkus, I would like to try to re-state your argument in short form, to see if I understand it:
For the reasons cited, it is much more difficult to obtain a true balanced signal from an MM cartridge as compared to an MC one. Feeding a signal with an imbalance of noise on one phase vs the other to the balanced gain stage will result in the amplification of that noise, i.e., it will not be rejected because it is not identically present on both phases.
Is that more or less correct? Why is the noise to which you refer not similarly amplified by an SE topology? (It's that "3 db" boost of the noise that I don't quite get.) Noise is noise(?) In a good SE phono stage for MM, is the "ground" isolated from chassis or earth ground? That would seem to be a good idea. |
Hi, Axel, it was this comment that I probably misunderstood: "I have the idea that Raul will be the person to explain to you in his words why he has opted for two **dedicated** phono-pre option in his 3160 and NOT a one-serves-all (just with a jumper to switch from 40 to 60dB)." I understood the portion within the parentheses to mean that the 3160 does not use switches except for a jumper to set gain. My mistake.
Tom |
Hi Kirkus
VERY insightful, thank you. I'll have to chew over all of this with my 'consultant engineer':-)
But now one question: Common mode rejection, yes well... everybody points this out on the differential circuits, and nobody is running 20m or more ICs (other then in the professional world).
But it was the OTHER item:
The inevitable and unavoidable 'ground contamination' influences of capacitors etc. that makes the other argument for differential/balanced vs. unbalanced.
It seems this (at least to my current take) is to be weighted against the ~ 'imbalance(s)' in a balanced design.
A lot could be said about some renown reviewers / testers having found, that even in a balanced design (not pseudo) the single-ended still sounded more 'natural'.
Why? It is that the differential circuit also cancels even-order harmonics in the process of common mode rejection, so you wind up with a bias toward odd-order harmonics, and that is not so 'natural' to our ear.
Be nice to have your take on this side of it.
Thanks,
Axel |
Hi Lewm . . . so for the subject of balanced inputs for phono preamps. First, its important to understand exactly what type of noise a balanced input is capable of rejecting. The overwhelming majority of cartridges and tonearm/turntable cartridge wiring treat the cartridge winding as a balanced source -- this alone is wholly sufficient to avoid hum pickup from ground-currents flowing between the turntable and preamp, provided no mistakes are made. Suceptibility to RF interference is determined by the design of the input stage itself. That leaves magnetic hum pickup as the main type of noise that we're designing the balanced input to reject.
The effectiveness of the hum-rejection in a balanced-input system is directly related to how closely matched the positive and negative conductors are impedance-matched to ground - that is, if the + and - conductors on the input have different impedances, the magnetic field will cause different amounts of interference in each one, which in turn will manifest itself as signal voltage. This is why (IMO all decent) balanced-signal cables use twisted-pair or star-quad configurations - the tight twisting keeps the impedance very much the same between the conductors, for good hum rejection.
But in a tonearm, the wiring is very rarely twisted-pair . . . they're usually just stuck side-by-side through the tonearm tube. And this isn't necessarily a bad thing, because the side-effect of the twisting is an increase in cable capacitance, which is exactly what we DON'T want for our MM cartridge. And if you want to minimise capacitance in the leads between the tonearm and the preamplifier, then you're also probably looking at simple coaxial cable types instead of shielded twisted-pair. Then there's the matter that having carefully-balanced output impedances is probably only very rarely considered in the design of the phono cartridge . . . because the vast majority of phono preamps over the years have unbalanced inputs.
So in any case when we're designing a phono preamplifier, we CANNOT assume that the impedances coming from the + and - leads of the cartridge will be well-balanced, impedance-wise. So if we want magnetic hum rejection, we need to build a balanced input that is fairly insensitive to these imbalances. And the way to do this is to keep the differential-mode (cartridge loading impedance) as low as possible, and keep the common-mode impedance as high as possible, as it's the ratio of these two that determine the effect of the source imbalance.
So for a traditional, say 5-ohm LOMC cartridge driving a 50-ohm transformer input, this is pretty easy to obtain, because the transformer will have a common-mode impedance in the tens of megohms, say 50 Meg. The ratio between the common-mode and differential-mode input impedances is thus 1,000,000,000 . . . and since the cartridge source impedance is so low, the maximum impedance imbalance will be a fraction of an ohm anyway. So hum rejection can be reasonably effective, regardless of the type of wiring used.
If we're to do this with an all-active input, we would still have the same 50-ohm loading resistor, but to effectively manage i.e. input-bias currents and offset, the input impedance of each side to ground would be probably at the highest maybe 470K, making the common-mode input impedance 235K, and our impedance ratio 4700. Not as good as the transformer, but still worth it.
But when we go to an MM cartridge, the source impedance is usually something like 1.5K and rises with frequency, so the impedance imbalance of the cable is then more likely to be a handful of ohms, and also rising with frequency. Per our earlier discussions, the differential-mode input impedance needs to be about 100K. And assuming JFET inputs, the very highest you can probably get away with for common-mode (without having the offset go through the roof, or cap-coupling) is 2 Meg resistors . . . making the common-mode impedance 1 Meg, and our impedance ratio is at 10.
So the endgame: the higher output impedance of an MM cartridge will make the impedance mismatches in the wiring more apparant, and at the same time makes the necessary design criteria in the phono preamp more suceptable to these imbalances. And the most effective way to reduce the imbalances in the wiring (twisted-pair construction) raises the capacitance, which is exactly what we DON'T want for our MM cartridge.
So then there's implementation - for MM cartridges, we can't use transformers, which leaves us with active realizations, which have a couple of major disadvantages over unbalanced inputs. The first is noise . . . you usually end up with twice as many uncorrelated noise sources, and can only make up for it by the fact that each side will see half of the impedance, giving a minimum 3dB noise penalty. The second is that many input stage designs don't work as well in the presence of significant common-mode voltage (which if we're trying to reject it, means it exists), and with high common-mode impedances, some sort of protection diodes, series resistors, etc. will probably be necessary to keep the input stage from getting fried when a ground wire gets disconnected and suddently there's 30V of common-mode voltage.
While I won't pretend that my conclusions on the matter are definitive, all of the above makes me think that for an MM cartridge, active balanced inputs are unlikely to deliver enough hum rejection to be worth the complications. |
Dear Axel: Why in a MC commercial world any phono stage or any Phonolinepreamp manufacturer have to take care about the MM alternative? when the MM alternative almost does not " exist " by commercial business.
So in the last years almost everyone commercial manufacturer build phono stages for the MC market even the ones that are for MM because it does not have enough gain for MCs but that they recommended using with external SUTs.
Only a few of us ( like Kirkus. ) that understand and like the MM virtues start to care on the subject and that through a research find that the MM needs are way different that the needs for the MCs.
That's why our units has separate and dedicated MC and MM phono stages that cope totally the MC specific needs and the MM specific needs. These different topologies make a quality differences on the sound reproduction?, ABSOLUTELY YES.
Lew sum up correctly: " One topology does not ideally fit all. "
IMHO the MM alternative is so good an friendly that even if the phono stages are not designed on its specific needs the sound reproduction quality on these cartridges is just glorius.
Now, on the convenience to switch from one stage to other or to choose load impedance through switch/jmpers or the like Lew say " I'm purist " an even that my Atmasphere has those jumpers I solder the resistors directly. A switch/jumper or the like is always a compromise even if that compromise is at minimum even if you can't hear it. When we are talking of MC/MM we are talking of very delicate signal at very low output/gain where these low signal are easy to contaminate even if you " bread ". That's why I prefer ( between other things. ) integrated units ( Phonolinepreamplifiers. ) instead phono stage that has to be connected to a line stage through additional cable/connectors.
Btw Lew our uit is differential/balnaced too in the MM stage.
regards and enjoy the music,
Raul. |
Tom,
>>> ... confused a bit by your comment that the 3160 has a jumper to change from 40 dB to 60 dB of gain.<<<
That is not what I said, neither implied, since the 3160 has dedicated (like two seperate units) MM & MC stage(s) inside the line-pre-amp.
Jumpers are just somewhere in the hierarchy of 'switches' and perhaps a bit preferable (but not for comfort). They are normally very closely placed to where things need to get 'switched', Raul will NOT have any of those either, I'm sure.
A solder joint, or no joint at all, is still THE BEST (as long as it does not create a diode :-)
A. |
Hi, Lewm, nicely stated explanation. And after reading Raul's responses again I see what was implied, that convenience may override preamp performance. I can certainly understand the convenience factor.
But Axel, I'm confused a bit by your comment that the 3160 has a jumper to change from 40 dB to 60 dB of gain. Are jumpers considered to be acceptable, that is, they do not degrade signal path, at least in the amplification portion of the circuit? Or did I interpret your comment incorrectly?
Tom |
In my view, one of the hard to resist features of the 3160 (Raul's product) is the completely separate MM and MC phono stages, not to mention the fact that both are balanced circuits (or at least I know that the MC one is balanced).
Raul might respond to Tom's question about why "high-end" phono preamps have switchable load resistances by saying that there is a demand for it among end-users, for the sake of convenience, not optimal sound quality. Most end-users of such products want remotely switchable everything, so they don't have to get off the couch. Raul wants optimum performance as his end-all and be-all. What I meant to infer when I said that most of us would not know whether switches degrade the sound is that most of us have not done the experiment properly in order to make a judgment. Most of us have one phono stage which either does or does not have switches in the signal path. And no one (other than Raul) has come forward to say that removing switches from phono inputs has made an audible improvement in sound. Or that adding same caused a deterioration. For my part, I own an Atma-sphere MP1 in which I have made considerable mods to the phono stage. The MP1, like Raul's preamp, has no switches in the phono input, but one can change load resistance by installing resistors between two screw terminals on the outside rear of the chassis. Purist that I am, I have never used those screw terminals for loading or for anything; I solder my load resistors right at the grids of the input tube (or the gates of an MAT02 NPN transistor, in the case of my modified unit). Oops, I am off-topic.
One thing that comes out of this so far, thanks to Kirkus and Quiddity, is the concept that there truly IS a difference, primarily at the input voltage amplifier stage, between an ideal MM phono and an ideal MC phono. One topology does not ideally fit all. I'd still like to hear more from Kirkus on balanced mode with an MM cartridge. Anyway, I've decided that I do want a separate dedicated MM phono stage, which is kind of where I started but now I have more info to go on. |
Hi Tom,
you say:
>>> Just seemed like a similar question in that having phono stages designed specifically for the type of cartridge engine makes sense. <<<
I have the idea that Raul will be the person to explain to you in his words why he has opted for two **dedicated** phono-pre option in his 3160 and NOT a one-serves-all (just with a jumper to switch from 40 to 60dB).
Having read all my and Kirkus' comments should have made the point, but let's see if Raul can put it into a 'nut-shell' for us.
Greetings,
Axel |
Oops. Meant to include a thank you to Axel and Lewm for answering with understandable information. And Axel, I didn't intend to sidetrack the thread with my question about dedicated MC phono preamps. Just seemed like a similar question in that having phono stages designed specifically for the type of cartridge engine makes sense.
What I'm not sure about is whether you'd be able to hear the difference (as Lewm suggests) between a dedicated minimalist design vs a preamp with variable gain/loading options. I often read that it does make a difference, that having switches or plugs or jumpers degrades the signal, but as Lewm also suggests, if the problem is that noticeable, why do even high end manufacturers provide gain/loading flexibility in their phono preamps?
Tom |
This thread has been informative and I'm better understanding some of the nuances of phono stage design, both MM and MC architecture.
And I'm beginning to see the value in having a dedicated MC phono preamplifier if LOMC cartridges are preferred. Why bother with 40 dB gain and 47K loading for MM cartridges. The design would, at most, have binding posts to change loading resistors. Being able to change the amplification (gain) would be handy but not necessary if the output voltages of the preferred cartridges were within a fairly close range.
Is such a phono preamplifier available?
Tom |
Raul, Just another one of life's compromises.
Kirkus, Would you care to comment in more detail on your earlier statement that a balanced input is per se not ideal for an MM cartridge? At least I think that is what you meant to say.
By the way, thanks largely to Kirkus' input I am beginning to learn some things about MM phono stages and phono stages in general that I did not know. |
Dear Lewm: That's right, if I need a different load impedance then I soldered a new resistor value, no switch. Normaly almost all the LOMC cartridges comes with a very low internal resistance where 100 Ohms can cope all them. The other subject is that due to the Ponolinepreamplifier characteristics you don't need to change the load impedance, only in extreme very extreme cases.
Lewm, one way or the other normaly a switch makes a degradation to the cartridge signal so you have to make a decision on trade-offs between quality or convenience, we choose for quality performance.
Regards and enjoy the music,
Raul. |
I just paid too much for a B&O MMC20CL off eBay. I hope I will be happy. I think that I was looking at that one too . . . glad you got it. If it's in good shape, then you didn't overpay. Stellar cartridge. On the AD797 - this opamp is capable of extremely high performance, but it's definately NOT one to swap into an existing circuit without careful consideration, especially if it originally uses TL072s! The AD797 really shines with low source impedances (not MM cartridges), but on the other hand, it can be limited in its useable output current . . . so it doesn't necessarily do the best job of driving the low-impedance feedback network that suits its low eN characteristics. That's why I used the discrete JFETs as a buffer, and why I used it in the second stage, where it can be driven from the low source impedance of the first. The AD745 is FET-input, so it works great with an MM cartridge . . . but it's not stable at lower gains, so stability and phase margin have to be carefully considered in application. Then there's the fact that the TL072 is a dual, and AD797 and AD745 are singles. For general TL072 replacement, try OP249. |
Raul, Do you mean to say that every time you change from an MC that likes to see, for example, ~100R to one that needs significantly higher R, you actually take the 3160 to the workbench and solder in the required higher R? Or do you use some plug-in system that obviates the need for a switch but still puts mechanical (i.e., unsoldered, pressure-based) contact points in the signal path? Or do you still have a switch for load resistance? (Your post suggests you eliminated switching between input jacks, rather than between load resistors. Even with a single pair of inputs it would be desirable to be able to switch load resistors.)
Kirkus, I just paid too much for a B&O MMC20CL off eBay. I hope I will be happy. The AD797 is one of the ICs I had in mind when I noted that the op amp-based schematic that was posted above might be upgraded with later, better chips. (The schematic used TL072, I think.) |
Daer Axel: I can speak about that switch subject. During our researh and tests on the deign of the Esential 3160 and due that this PHonolinepreamp was/is designed for our ( José and I. ) specifics need in one of the prototypes ( the one battery powered. ) we have six phono inputs through a switch that I use it for several months till I decided to hear the same cartridges with out that switch where every change on tonearm/cartridge means disconnect/connect in manual way each time, well there is a quality improvement with out that switch and we decided to go in this " fashion ".
But the Essential 3160 is not a product thinking on commercial subjects ( facilities for the customer: using switchs instead that manual changes, switchs for impedance changes instead solder resistors, etc, et. ) but with targets of absolut quality performace where less means more and with no trade-offs or trade-offs at minimum. Many times this approach ( no trade-offs on quality performace. ) means that the Essential user has less facilities: switchs.
Regards and enjoy the music,
Raul. |
Hi Lewm - I will add my vote as well to the higher-loading approach, I think that 100K with minimal capacitance is much better (with virtually every cartridge that I've tried) than the common 47K/150pF.
But as far as commercially-available phono stages go . . . I don't really see enough of them anymore to have really strong opinions. I do think that in general, getting the subtle details right makes more difference than any generalisations about whether it uses JFETs, tubes, bipolars, opamps, etc. etc. There are certainly an unlimited number of ways to screw things up, or to avoid doing so.
I'm currently using my Beogram 4002/MMC20CL with a little phono preamp that's an opamp design, made on a Vectorboard . . . and the case and power-supply from a Lehmann Black Cube. Its a two-stage topology like my MC phono unit, with active 318/3180 in the first stage, passive 75uS, then more gain in the output stage. Opamps are AD745 (input) followed by AD797 (output), both of them use complementary discrete JFET buffers for high-current output, within each feedback loop. |
Hi Mark - FWIW, my experimentation with discrete JFET MM stages included both the 2SK389 dual, and a pair of 2SK369 singles. I cascoded them with 2N5089 NPN bipolars, and loaded the collectors with 1K . . . there was also a single 2N5089 used as a simple current source for the tail. Rails were +/- 20V, and I put about 8mA through the tail (4mA through each JFET). VDS across the JFETS was about 5V.
I used this circuit in an unbalanced-input feedback amplifier, in the classic "hybrid-amplifier" (JFET-frontend feeds opamp) topology. Instead of a monolithic, I used a 990 discrete opamp, driving a low-impedance EQ network for the 318uS and 3180uS time-constants, with about 20dB of gain at 1KC. This fed a passive 75uS network and another 990 for 20dB more gain, same as my MC design.
I never really put in the time to fully optimize the circuit, but it was very low-distortion, and CRAZY quiet . . . like greater than -90dBV at the output when looking at a 1K source impedance. I think that the 2SK269s had a little better 1/F noise, and even though the offset on the 2SK389 was much better, it didn't really matter for my application. Of course, YMMV. |
Hi Lewm,
>>> With an MM phono stage, we should all be using a 100K load <<<
Another reason for this thread's subject.
I mean, WHICH phono-pre, of one-serves-all, has a 70k and/or 100k ohm input impedance option I ask?
I know Raul's Essential 3160 has, but that is a non-main-stream item.
As to 'switching degradation' I had a PS Audio GCPH using some pretty convenient switching, and now a pain-in-the-butt ML phono-module using internal binding posts or some jumpers.
But as you say, how could I have tested the GCPH's performance without the switches for gain and loading?
All I know is, that the ML modules sound better --- but because of the switches?
Raul and Co. might have some contribution in sharing their findings, wouldn't they?
Axel |
Tom and Axel, I heretofore have had only one LOMC cartridge, which seeems optimally loaded by a 100R resistor, which I soldered in place myself. But I recently purchased a (third) LOMC that will probably do best with 1000R. I have contemplated installing a switch, so I can change between 100R and 1000R, when I want to change cartridges. Obviously there is a risk of introducing noise at the switch. But in practice, have either of you, or anyone else, actually perceived degradation of the sound with such an arrangement? Many commercial products incorporate such switches in this critical part of the circuit, so most people would not know if they are losing anything in the way of S/N ratio as a result.
But this is off-topic. With an MM phono stage, we should all be using a 100K load, according to Raul. Based on very limited data, I have to agree with him; several years ago, I made a similar accidental observation with whatever MM I owned at that time. It sounded better with a 98K load compared to the standard 47K load, and the diff was not subtle. |
Tom,
I guess 'switching' means loading in your context, yes?
Loading an MC with 100, 500, 1k ohm for example makes little difference as to the phono-pre used as such.
The mechanical issues involved will.
Soldered Rs are best, than clamped in some binding posts, then the 'mouse-piano' i.e. the switching you seem to refer to.
The same would apply (to the phono-pre) using 47k, 50k, 75k or 100k phono-pre input impedance 'switching/changing' for MMs only.
The issue with MCs is rather to have a phono-pre that "can count electrons" (if no SUT is used).
Apparently only few designs are THAT good after all. Mostly noticeable in the difference of those parameters I mentioned that are improved, when an SUT is used with a Low MC.
Often a lack of dynamics is noted if an MCs output is on the low side - use an SUT, and THAT problem is GONE.
When using an MM there are different things at work.
Mainly due to the 'high' cart voltage output --- BUT also because they are a more 'reactive' load, usually 600 - 3.5 ohm DCR, 700mH as ~ 20uH with an MC and compared to only ~ 2 - 6 ohm with LOMCs).
That is why a TOP MM stage would ask for a different design if strictly dedicated to MMs.
No SUT can be used, single ended (SE) MIGHT be better. I'm as little sure about this as Lewm is and it might have to do with the 3-6dB lesser noise in SE design --- which then again might just be made worse by using common ground that can be noisy.
Also 'floating RIAA' as in a differential circuit between +/- is not an option either.
Greetings,
Axel |
Hi, Axel and Lewm. My comment on the need for separate MC phono stages is based on Axel's comment that a SUT degrades the signal, thus my assumption that a dedicated phono preamp (for a specific range of LOMC parameters) would be a better way to handle the amplification and loading requirements. One gain/loading setting probably isn't the best way to handle a wide range of LOMC cartridges, not if optimal performance is desired. And if switches must be used to optimize the LOMC signal, it is not acceptable for a high end preamp, which was the premise of this thread. So, it appears that if the logic is used that a dedicated MM stage is needed (to avoid having additional switching for MC cartridge requirements) then dedicated LOMC stages are needed to avoid having additional switching.
I bring this up because it seems that what is implied, Axel, is that MM is inherently a better cartridge because it doesn't require additional switching or transformers. But a dedicated LOMC phono stage would eliminate the need for switching or a transformers as well. Wouldn't that put a MC cartridge on par with a MM cartridge?
Tom |
Axel and Tom, if you can change load resistors, I see no reason to lust for a separate MC phono section for each cartridge. The LOMCs are not identical to each other, but they are similar enough. Anyway, IF one needed a different circuit for every different LOMC, that is where I would draw the line. I would not ever do that.
Mark, If I understood Kirkus correctly, he maintains that a balanced input is not optimal for an MM cartridge, which was news to me. The Aqvox preamp makers make a similar claim; their product is balanced for MC cartridges and SE for MM cartridges, because the latter are "voltage generators", in their parlance. Yes, MM cartridges are voltage generators as compared to LOMCs, but why does that negate the positive attributes of a balanced input? (Apparently you chose a balanced input for your MM phono stage, for example.) |
Kirkus
Thanks for that, I will have a more careful look at the design calculations for my MM only phono amp. The input stage is currently using 2SK389BL as diff amps with simple resistive loads (balanced inputs, there's no need for a feedback connection to the diffamp 'cos there isn't any feedback).
It will be easy enough to change these to cascode.
Mark Kelly
|
Hi Tom
you say:
>>> it may necessitate a range of dedicated MC phono stages if someone wants to listen to a variety of LOMC cartridges <<<
I can not support that logic as e.g.: all women are people, there for all people are women? :-)
Loading with MC stages and to some smaller degree loading of SUTs (secondary and primary side) takes care of the variations in MC's DCR and output voltage. Of course a mismatched SUT/cart can not be made to work by loading e.g. if it would overload the phono-stage.
Greetings,
Axel |
Lewm, my mistake for not including the intended recipient in my post. (I don't post here at Audiogon very often so am not used to the continuous thread format.) I was trying to get a sense from Axel (and others) if it was worth the added expense of having dedicated phono stages for several LOMC cartridges, each with a unique amplification and loading requirement. That's making the assumption (and the analogy to a dedicated MM stage) that to get the best sound from a phono stage, the design must be specific to the cartridge.
If so, then the question might also be asked: Dedicated phono-pre for (specific) LOMC only?
Tom |
Tom, I did not get that message (that the range of output voltages and inernal impedances of MC cartridges presents a problem per se) from Kirkus' long post. I will have to re-read it, but I think he is saying that the much higher output impedance and reactance of an MM cartridge poses a problem for MM phono pre design.
Kirkus, Given your depth of knowledge of this subject, are there any commercially available phono preamps that seem up to snuff for use with MM cartridges exclusively? |
So it seems that a quality SUT, or, a high quality dedicated MC phono stage, is the key for MC cartridges. Since MC cartridge designs include a fairly wide range of output voltages and internal impedances, and thus gain and loading requirements, it may necessitate a range of dedicated MC phono stages if someone wants to listen to a variety of LOMC cartridges.
Too bad, because that sounds expensive. Is the added expense (of having multiple dedicated MC phono stages) worth the perceived improvements in sound?
Tom |
Hi Mark -- yes we're indeed talking about the same thing, its just that the Miller capacitance is usually much lower. The reason I made the (admittedly rather imprecise) statement about vacuum-tube vs. JFET junction capacitance is from some generalizations about exactly how much this capacitance is actually reflected at the input when these parts are used in a circuit.
That is, 2-3 pF from the interelectrode capacitances of i.e. a 12AX7 are significantly less worrisome than the 30-40 pF or so from something like a 2SK389 set up as a diff-amp. I also think that cascoding JFETs (either with bipolars, or more JFETs) is a simpler, more elegant implementation than the vacuum-tube counterparts, which usually involves bootstrapping and multiple/floating heater supplies.
So I would personally always cascode JFETs in this application, but probably wouldn't with most vacuum-tubes. Just my preferances. |
Kirkus
I'm not sure I understand your point about dynamic capacitance of JFET vs V/T inputs, and if I do I'm not sure I agree with it.
Assuming that you are talking about junction thickness dependent capacitance varying with drain - source voltage, I thought this effect was pretty well analogous to Miller capacitance in V/Ts. In both cases cascoding reduces the variation in drain / source (read anode / grid for V/T) voltage.
Is there a secondary effect in JFETs I need to know about?
Mark Kelly |
Hello Kirkus,
I've printed you notes, and will study them.
There is some most valuable information based on your long experience as I can see on some quick glancing through.
Thank you for sharing!
Axel |
Hello Axel . . . consideration of some of the specific requirements of MC vs. MM cartridges was something I spent quite a bit of time with in my own phono stage design, and I very much feel that the usual "change the gain and loading" approach cannot begin to deliver optimum performance from both types. And while my design is specific to lower-output MCs, I designed, measured, and experimented with some MM-specific approaches.
The most important part IMO is the specific noise/impedance relationship between the the input stage itself (NOT just the loading), and the cartridge. MM cartridges have much higher and much more reactive source impedances than MC cartridges, which means they will require an input stage with lower noise current (hence higher noise voltage), than an MC cartridge, which is exactly the opposite. JFETs and vacuum-tubes are the traditional choices here for such an application, though a lightly-biased bipolar can sometimes work well too.
The second input-stage issue for reactive sources like MMs is capacitance, which can be static or dynamic (changing capacitance with signal level), the latter of which will have a big effect on the ultrasonic behavior of the cartridge. Vacuum tubes are pretty good here, as their input capacitance is usually mostly static (like cable capacitance) for the signal levels we're talking about. With JFETs dynamic capacitance is a big issue, and cascoding a JFET input stage is IMO mandatory to keep this under control for an MM cartridge. Bipolars have such a high transconductance that the capacitance is rarely a problem, but they in turn require special attention to input-bias current so a tiny DC current isn't drawn through the cartridge itself.
For the devices themselves, I think it's silly to turn one's nose up at modern monolithic opamps, especially if a JFET input is what's decided upon. In the past 15-20 years, most of the innovations in JFET fabrication have occurred as part of improved monolithic processes, to the point where JFET opamps can be had that beat the noise performance of the very best discrete JFETs available.
The other question is whether or not to have a balanced input from the cartridge, or a simple unbalanced input. This is even a separate question from whether or not a differential-amplifier is used as the input pair, as the latter is frequently used simply for the feedback capabilities from an unbalanced input. For MC cartridges, I think a balanced input is optimum, especially with a transformer input - this is in part because the low source impedance of the cartridge can make the whole system (cartridge, cable, and input stage) very effective at eliminating magnetic hum pickup.
But the balanced input almost always means a noise penalty of 3-6dB when implemented with active electronics (transformers are unsuitable for MM cartridges). And I'm not sure that with the higher/reactive source impedances of MM cartridges, combined with the low-capacitance cable that they prefer . . . really translates into improved real-world noise rejection when used with a balanced input. At the very least, it greatly complicates some of the decisions required to make the input stage tolerant of common-mode noise, and provide proper protection against accidental overload (30VRMS common-mode nose from a bad turntable ground, anyone?)
Anyway, these are just a handful of the overall-picture considerations that can be very different between a phono preamp design that's specific to MM cartridges. |
Funny that no one has mentioned the new Passlabs XP-15 as yet.
It's SS, but seems to be close in a number of other 'parameters' mentioned. Good price too $3'800...
Any comments?
Axel |
Hi Tom
you ask:
>>> Do you think that a high quality SUT or headamp could provide the gain and loading needed to turn a high end MM stage into a high end MC stage? Or does the use of external transformers or headamps degrade the signal too much to be considered acceptable for a high end phono stage? <<<
To start, a head-amp will be (generally) providing the lesser quality, compared to a good SUT.
(That's why they fell out of favour, years back already --- and today? ~ No need...)
Now to the SUT, and here I guess it will..., become controversial once again?
It is my current experience that a top MM into a good MM(MC?) phono-pre will beat an MC with an SUT into that phono-pre. Given, that we have a top class match between MC and SUT, and a subject all on its own.
Only, and only, if for various reasons of your own, you love that MC, and that low gain phono-pre (and particularly if its a good one!), would I consider the SUT option.
It will not be a bad mix by any means. You'll have more dynamic depth, more bass punch, a bit more harmonic completeness, compared to MC only... so, nothing to scoff at.
Yet a good MM will very likely do it all with a bit more 'EASE', so far what I have found.
I still think, that most (if not all phono-pres just have to try too hard to get most 'everything' from that puny ~ 0.3mV cart voltage into a harmonically complete reproduction OF WHAT IS ON THE VINYL.
Now offer that phono-pre ~ 5.0mV it is just simply easier to get it right, as close as can be. And that in turn applies to the MC plus SUT match, ... with maybe a little less ease.
What will go missing with the MC / SUT setup is some micro detail swallowed up by the SUT's leakage inductance, parasitic capacitance, etc. it can be noticed by a less sharply carved out image presentation -- some would argue is unnatural anyhow (like stuck with your head in the microphone).
Yet, plenty people find that's what's needed, else it does not qualify for 'Hi-End' and I'm not going to argue...
Now all the electronic fundies are gonna jump up, but counting ELECTRONS (in a phono-pre) literally, if you consider micro-information coming from the vinyl by 0.3mV, 5cm/sec @ 1k Hz is actually one MIGHTY tall order, 'cause a lot is a the 'single electron' level before it is 'pumped up' and equlised. One electron gone missing = information lost.
I hope this answers your question.
Greetings,
Axel |
Correction:
I just checked Herron's site and they have discontinued the dedicated MM phono-pre and now have a combination MM/MC with variable loading provided by changeable plugs.
Interesting looking answer to a common concern. |
Herron Audio makes a tube MM Phono Pre-amp. I have not heard it, but I have been using their MC Phono-pre for a number of years (on my second set of tubes), and I have never (yes, never) heard one that was better. As with all things audio, YMMV.
Dave |
Axel, you and I are thinking along the same lines, save for the fact that I would not use a transformer anywhere. The desire for a balanced circuit is a real sticking point as almost none of the commercial products offer that. There is the Aqvox, but as far as I can understand from their very confusing ad copy (where they say in one para that it is balanced and in another that it is SE), the balanced circuit, if present, is used only for MC cartridges. If you use the MM inputs, the circuit is SE. The ad also contains some mumbo jumbo about MMs being voltage generators that prefer a SE circuit topology (as compared to MCs). I don't "buy" that logic. The new K&K phono is I think balanced, and I intend to look into that one.
The IC design is interesting, but the ICs used have been surpassed for audio in recent years; I am wondering whether one can just plug in one of the later better sounding ICs, without a re-design. |
Ah, now we have a reference. What you suggest seems reasonable for a manufacturer to offer for sale. (You would certainly know better than I.) But I think you answered your own question as to why they don't: The lack of offerings is most likely a financial decision, not a design decision. If they don't see the market need for a high end dedicated MM stage they aren't going to bother. I guess they don't read the posts from you and Raul. '-)
A question for you, Axel: Do you think that a high quality SUT or headamp could provide the gain and loading needed to turn a high end MM stage into a high end MC stage? Or does the use of external transformers or headamps degrade the signal too much to be considered acceptable for a high end phono stage?
Tom |
Hi Fap, Tketcham
>>> ..this quite simple to build design is amazing! <<<
OP-Amps, not part of the spec I'm thinking of.
I realize it's maybe time to go into more detail.
Also, the 'Jadis' would be single-ended also not part of the idea, good as it my be.
Rough spec. would be:
- 40dB
- tube gain stage (max. to gain stages)
- 1 JFET gain-stage? Maybe, to get minimum tube-rush/noise
- or e.g. Lundal trannies to get the first 20dB?
- shunt regulated power supply
- differential circuit (XLR out-puts, differential wired RCA inputs)
- no volume control
- floating RIAA (between + and - of the differential circuit)
- one box design (if at all possible, by use of good screening)
- No PCBs, screened silver wire for internal wiring.
- Good,e.g. WBT connectors
That should cover the basics. What tubes to use, trannie, resistors (Tantalum, or?) is dependent on the circuit design and implementation.
Greetings,
Axel
|
Hi Tketcham
>>> To what level of "top" phono stage are you referring? <<<
Simply the best, - second best I already have.
>>> ... is the Jadis the "top" level you were originally asking about? <<<
Raul got this, he is thinking "Essential 3160"...
The Jadis is a still current, just as an example, that's why I mentioned it.
There 'where' other great 40dB stages but ALL gone by now.
I'm really NOT asking about any 'wall-wart' power supply pre's. I thought that was implied. Why even bother to ask then? Like: "Go get one (any one) and call it a day..." :-)
Greetings,
Axel
PS: There are squat (current) 'top' 40dB tube phono-pres out there, and that is already something useful to know. Makes it a proposition to build one's own... a new market gap? |
G'day all, this quite simple to build design is amazing! This is my preferred MM phono preamp. http://sound.westhost.com/project06.htm I have built several of these. Regards, Fap. |
Hi, Axel. Given the replies to the original post, I'll let my first question stand: To what level of "top" phono stage are you referring?
The discussion seems to be about phono stages in the range of $1000 to $2000 USD and thus, the Graham Slee Reflex is an example of what you were asking about. (It's clearly designed for MM cartridges.) The Jadis phono stage reference was a bit misleading. Or is the Jadis the "top" level you were originally asking about?
Tom |
Hi Lew. I can solder but have no ability to design circuits.
I previously used a Threshold FET-10 (a unit I still endorse and a great bargain at $7-800) and an audio buddy built a regulated power supply for that for me. I changed to the JLTi mainly because it offers external gain and loading selection, but when he converted the RPS to my JLTi (which started life with a wall-wart), it offered some audible benefits as well. So yes it is custom and no I don't have a schematic.
The same friend with the EA phono stage just picked up an AT 170(something), one high on Raul's list.
Good luck in finding a few of these "hidden gems". |
The two that come to mind for me are the K&K Audio kit and the original Hagerman Trumpet. |
Hi Pryso, This is the second time someone has recommended EA to me. Since you are a more credible source than the other person, I will look into it. I did also think about buying a basic JLTi, second hand, and upgrading the power supply myself. Did you build the custom supply for yours? If not, do you have a schematic?
Curses, I just lost out on an auction for an AKG P8ES cartridge on eBay. Why? Because I was so busy all day I forgot to increase my bid, which I knew would not withstand the assault of last-minute bidders. i do have my old Grado TLZ, and that is the first MM I will try in the new era. What about the AT150mlx? That seems to be liked by most. |
