Hi All, the subject of phono-pres, specifically 'adapted' to MM came up in some related postings.
IF, and only if, MMs are much to ones liking --- why spend your buck on some 'halve backed' 60dB plus, MC gain requirement, stage? Why not consider put the $$$ into a TOP 40dB gain stage of either SS or tube?
Raul had more thoughs on the subject as he mentioned before, and might share, why he knows that a TOP MM compared to MC stage circuit requirement might NOT be -one suit fits all-.
There could even be a nice argument to fit a tube gain stage only into an otherwise SS only system!?
Again, the $buck saved on the 20dB plus circuitry could be translated into the BEST circuit for an MM. I realise, that most such stages were simply fitted inside some older TOP pre-amps, (e.g. Jadis...). I have not come across a **dedicated** , current 40dB stage neither in nor outside a pre-amp.
I guess this thread is on its last legs, or already dead. I for one would love to hear from anyone who is using a vintage phono stage with an MM cartridge.
That's a review of the Vinyl Reference, their more expensive phono stage, driven by an MC cartridge, so its relevance to the present discussion is limited.
The review is about the more expensive version, the Vinyl Reference. I've encountered more so so reviews on the Vinyl One (the less expensive version) on the internet as well. Somebody told me that the Art Audio Reference is a retrofitted version of the K&K Maxxed out Kit. Going back to the idea of using the MM input of a good preamplifier instead, what about the John Curl MM design that Audible Illusions offered as an option in the AIM3A? Pablo.
>>> ... but I bet it's very good.<<< Apparently if you like it a little bit more 'warm' --- http://www.positive-feedback.com/Issue11/vinylreference.htm
I'm using a V15 III xMR cart going into the 60dB! (40dB sux!!!!) phono-module input (differential) inside the 326S and use unity gain (0dB) setting. It also balances OK with my CD (390S) XLR only needs to go to +6dB upped input level, no sweat.
Right now I can not see that this can be bettered without changing to a different phonolinepreamp, tubes and all - with the trade-off in loosing a LOT of ML convenience (which I truly DO appreciate :-) A.
Did not know that existed. I have never seen one advertised that was MM-dedicated. Thanks for bringing it to our attention; have you heard it? Can you compare it to anything?
From my understanding the Audion Premier is the same as in the Audio Premier Quattro and here is someone that likes it a lot http://www.high-endaudio.com/RC-PhonoStages.html
Hi, I'm still looking for a clue, given that MM carts like the 'simpler' single-ended designs, and some that find: - the fewer components in the signal path the better- it could make for a 'simple' but great MM only design. (Don't forget a tube)
In fact, having mentioned Jadis might just be that, alas too much $$$$ for their flavour of genius...
Next up I'd think of Tim de Paravicini's EAR834P MM only, as some would say the step-up for MC could very easily be bettered. So for MM only - no need.
One thing might be power supply 'simplicity' (no regulation at all?), and some other circuit issues, that sprouted a number of aftermarket mods.
Are there any other such designs we could add to this one?
Thank you very much for the additions Lewm. I've already made an offer for an Allnic H 1200. Let's see if the seller accepts it and, eventually how it sounds with the Soundsmith The Voice. Pablo.
Pablovila, Your list of candidate MM preamps looks like the one I posted on 6/26. I consider it incomplete. Below $1500 we also have the PS Audio GCPH and the Graham Slee Reflex, discussed above. Just to make it more confusing, I wonder how some of the highest end preamps from the "good old days" would fare if one were to upgrade the power supplies and the parts in the signal path. I am thinking of early Audio Research, for one example.
Hi Kirkus, >>> ... assume the "perfect storm" where all the errors just happen to add up in the worst way.<<<
"Monte Carlo" analysis, OK... the inventor must have lost it ALL then - worst case. BUT he did not consider any form of 'synergy'. The importance of synergy has been pointed out over and over, in fact lots and lots of times by Raul himself :-) Must be some concession to this 0.075% perfection.
I worked in pcb, and later semiconductor assembly, that gives me that practical bias what can be done if it gets 'commercially' viable. No way to have much better then 1% components on a tape-fed bonder doing SMDs, as I mentioned earlier. Also no way to start improving tolerances after the stuff (not only SMDs) are on the pcb. (Note: >3x rework = reject)
I have a pretty good idea, that this is where some realism in terms of tolerances has to prevail, meaning ALL hand build, bespoke, and nothing done 'commercially' will qualify --- seriously, that doesn't float my boat yet.
Why? Because a lot of 'bespoke' products have plenty more hidden QA issues then something more 'commercial', where errors have been weeded out to a much higher degree.
As far as SUTs go, yes they need to be matched, well matched! But if done, they have some 'magic' of their own and can make an MC something more full of 'live' and vibrant.
Oh yes, absolutely agreed! Even in a purely technical sense, there are many inherent advantages to transformer-coupling a LOMC cartridge . . . and I obviously chose this approach myself. I simply wanted to give counterpoint for those that feel that SUTs are inherently inaccurate and colored - because I understand how one could come to this conclusion . . . as most implementations on the market IMO don't really realize the full performance possible in the transformer approach.
Not necessarily since there are errors to the (-) and (+) side of the curve, yes? And we are actually talking about observed 'tolerances' just not quite the same as ERROR, in my vocab anyway. Now, tolerances can be additive or subtractive.
From a statistical/tolerance standpoint, yes, this is purely additive error. In electronics design, there is a specific procedure called "monte-carlo" analysis . . . which takes the maximum allowable tolerances of each component and combines them all in the absolutely worst-case situations - and this gives you the performance tolerance of the entire circuit.
In the audio chain, to define frequency-response as a deviation from perfection . . . keep in mind that in this definition we may have no idea exactly what the particular error is for each part, only that they are within certain limits. So while we can hope for that perfect synergy where every error just happens to cancel each other out . . . if we are to truly take responsibility for the performance, we have to assume the "perfect storm" where all the errors just happen to add up in the worst way.
Hi Kirkus, as always, a thoughtful and well laid out response.
As far as SUTs go, yes they need to be matched, well matched! But if done, they have some 'magic' of their own and can make an MC something more full of 'live' and vibrant.
As to this RIAA 'ping-pong' you say: "Well, error is error . . . and it's always statistically additive in this case" Not necessarily since there are errors to the (-) and (+) side of the curve, yes? And we are actually talking about observed 'tolerances' just not quite the same as ERROR, in my vocab anyway. Now, tolerances can be additive or subtractive.
This would also mean that some of this 'synergy' so often mentioned, is just one of the things that make one cart sound better in a given set-up then in another.
I was following the thread the last few days for a different reason than most of the previous participants. I have a Supratek Chenin that has only a MC input and I want to experiment with one of the Moving Irons made by Soundsmith that require a MM input. Currently mi analogue rig is Teres 255/Triplanar VII/Zyx Airy 3 and I am puting together a second analogue rig that would be finished in a couple of weeks. I bought a Townshend Rock III with a Moerch DP-6 and a totally refurbished Lenco L 75 (with a 70 pound plinth) with a Micro Seiki MA 505. I want to install the Soundsmith The Voice in both of those tables and see how it sounds. I can even install the Moerch in the Lenco and see what happens. That is the reason I am looking for a dedicated MM pre-pre. Thank you very much for this very informative thread and the options that you are considering. I will look for the K&K, Whest, Aqvox, Haberman and Allnic here on Audiogon and buy one of them. Regards, Pablo.
1) affordable phono-stage with MM (Reflex?) 2) good stage with MM 3) good stage with SUT and MC (Note: not mentioned headamps i.e. Elevator) 4) top stage with MC
2) and 3) maybe on par, and MC = LO MC > 0.4mV "
Hi Axel - I think its really difficult to make these kinds of general heirarchical statements about performance, price and topology . . . for a few reasons:
- You can't assume that providing the additional required voltage gain is always going to be the biggest challenge when designing a phono preamp for use with an MC cartridge . . . it may be for some designs, but definately not all.
- The usual axiom of "price and performance don't always go together" . . . even though they do frequently, we must always be skeptical here.
- I think that the implementation of an external SUT will always be a compromise - mainly because the transformer will have to be designed in a specific way to provide a "standard" step-up ratio that works well (in terms of voltage gain and output impedance) with a "standard" MM phono input. In a phono stage such as my own, the transformer (a Jensen JT-346) has a turns ratio that is much lower than a typical SUT (thus much higher performance) and provides a perfect En/In match to the electronics that follow it, which in turn have had their gain adjusted to suit the paramters of the SUT. That is, the transformer and electronics have been designed specifically to work together, not as a "universal" add-on solution.
On the subject of accurate RIAA . . . I was actually inspired by Raul's postings to take this matter very seriously in my own design. This isn't an easy thing . . . there are specific challanges to even measuring it to better than +/- 0.01dB across a 40-dB-ish range of signal level, as most test equipment must either generate or measure it through several output/input measurement ranges. For this, I was lucky to have an APx 525 on loan as an extra test set to verify the results against the AP2700's measurements . . . suffice it to say there needs to be some very careful hand-selection of parts to even begin to come close to this level of accuracy.
And there's the question of . . . why should the preamp be so precise, when no cartridge is anywhere close to this? Well, error is error . . . and it's always statistically additive in this case. After all, our CD players and amplifiers should have extremely precise response, even though a loudspeaker can't come anywhere close . . .
Hi Saudio EAR 324, a good one in the context of MM - alas it showed some issues with too little head-room in the 20Hz region. Not a lot going on down there mostly, but still.
The 88PB would be more ===> MC targeted I'd guess.
Hi, Lewm, I didn't offer the Jadis as a reference, Axel mentioned it in his initial post. I checked some prices and one their premium designs (JP80MC) was approaching $20,000 USD. That's when I retracted my suggestion of the Reflex as an example of a high quality dedicated MM stage. Not really fair to compare a $1300 USD phono preamp with one costing more than ten times that. Or maybe it is... '-)
Hi, Axel, it makes sense that newcomers to vinyl would benefit from using MM cartridges initially. And having a high quality, affordable (but dedicated) MM phono stage would be something they could continue to use as they upgraded the turntable and other components. The Graham Slee Reflex and Elevator appears to be one option that fits the upgrade path. I don't have the experience to offer an opinion as to whether it's high end or not.
But I think that for many people starting out, committing to MM cartridges due to a specialized phono preamp is not a comfortable decision. I know when I purchased my first "real" phono preamp, I chose one that had a wide range of gain and loading options because I wanted to be able to experiment with different cartridges without having to purchase additional components; i.e., a SUT or headamp.
But it seems (based on the discussions of this thread) that a dedicated MM phono preamp may not be the best design for adding LOMC capabilities, thus requiring a completely different LOMC preamp to get optimal sound. There's no simple (or affordable?) solution. '-)
Rockinrobin, I am going to guess that your experience is an overall endorsement of the Reflex. If it were not doing a good job, you might not hear a big difference between the two cartridges. Now you should try the Reflex alone driven by a Grace Ruby, Garrott P77, AKG, AT, B&O, or etc. Let us know how such a combo compares to the Elevator/Kontrapunkt source. Maybe some one of us can lend you one of those gems. (Just kidding...., but it would help the cause.)
Tom, what Jadis preamp did you have in mind? The last top end product from them that I am aware of was the JP80. By now there must be newer models. They have been really quiet in the US market, as in "absent".
Tom, re.: Reflex & Elevator... >>> But apparently it's not what Axel considered "top end" when he asked the original question.<<<
Actually I'm not really in a position to be the 'judge' on what a "top-end" MM-stage comprises (other than $$$/hear-say/etc...)
But what I did say earlier on is:
- "What I come up with is, for best sound for the $$$:
1) affordable phono-stage with MM (Reflex?) 2) good stage with MM 3) good stage with SUT and MC (Note: not mentioned headamps i.e. Elevator) 4) top stage with MC
2) and 3) maybe on par, and MC = LO MC > 0.4mV " -
This would be in favour, $$$wise, of using MM with MM-only stage. I think we'll struggle to get an answer to whether a "top-end" MM stage even exists (other than 3160), AND if it makes actually as big a difference in sound, compared to the more affordable ones (e.g. Reflex)
Note: There is absolutely NO DOUBT in my mind, that due to the low output voltage of MCs, only the-best-of-the-best MC-stage will actually be good enough to do justice to an MC's potential.
Maybe, because good MM's, due to the higher output, make for simpler, less costly, yet top performance stage designs?
Like: Single ended, JFET & tube (2 gain stages only), ~ 1dB RIAA error, good clean power supply ---- finito?
If that was so, there'd be plenty to pick from, and it would 'sink' plenty of MC set-ups with only so-so MC-stage. Everyone can get back to vinyl, without another mortgage on the house :-)
Hi, Lewm, it was my first reply to Axel that mentioned the Reflex as a dedicated MM phono preamp. And it was also me that too quickly dismissed the Reflex as not being in the same league as a preamp costing multiples of the Reflex. After seeing where the discussion was going I put the Reflex back in the mix. But apparently it's not what Axel considered "top end" when he asked the original question.
I had, at one time, considered the Reflex as the basis for a phono stage configuration, intending to use a headamp such as the Elevator or a step-up transformer, of which there are many, to run LOMC cartridges. I ended up with a phono preamp that has a relatively simple architecture but handles both MM and LOMC cartridges using jumpers and resistor screw posts.
You might ask over at the Vinyl Asylum about the Reflex and MM cartridges.
I have both the Reflex and the Elevator. I first used the Reflex alone with an Ortofon 2M Black. It sounded great! Then, I added the Elevator and Ortofon Kontrapunkt B. This is an outstanding setup and is highly detailed and refined sounding. To my ears, this combination is transparent across the frequency spectrum, very dynamic, and with no detectable glare, distortion, or other audible anomolies. The Reflex and 2M Black are certainly a nice pairing and I could live happily with them, but adding the Elevator and Kontrapunkt B sounds even better to me and is worth the money. The Elevator is nice in that it has easily accessible switches on the front panel for adjusting MC loading. I have settled on 100 ohms for my MC cartridge. I should also state that Graham Slee himself is very responsive to any questions you may have. You can contact him via the Graham Slee web site's link to their own discussion board. I am very pleased with my Graham Slee units and have no desire to look elsewhere.
In my list of candidate MM phono stages posted 6/26, I neglected to include the Graham Slee Gold Era Reflex (hope I got the long name right). The Reflex was specifically designed for MM and HOMC. Further, it got great reviews from some respected sources, including M Fremer. It's a bit odd in that it uses active RIAA equalization, but Kirkus has espoused that approach also. (The more common gospel is that passive RIAA is better sounding than active.) Earlier in this thread, someone dismissed this or one of the other Graham phono pres as not being "as good as" a Jadis, without naming the particular Jadis preamp he was referrring to. As I have never been a fan of any Jadis product, I am keeping an open mind on the Graham. Has anyone been able to audition this product (the Gold Reflex) in a system driven by an MM cartridge? (There is an optional accessory, the Elevator, which adds the necessary gain for an LOMC. One would prefer to know what the Reflex sounds like without this add-on.) Thanks for any comments.
Thank you for the reply, Raul. You and the others have been kind in answering all my questions. Based on what I've learned here I feel comfortable that I can make more informed decisions in future phono preamp purchases. Compromises abound in audio components to meet the needs of price, performance, and convenience, and I'm getting a better sense of what to look for.
Dear Tom: Yes we use 60 db too in the MC phono stage circuit and we can get, on the whole/overall Phonolinepreamplifier, ( changing the line stage gain ) easy 90 db, I never have the necessity to go to a higher total gain.
Normally 60 db is enough for almost any LOMC cartridge. I own the Ortofon MC- 2000 that is very low output ( 0.05 mv. ) and I can handle very good with very low noise/distortions, yes the Esential attenuators has to be around 3.0 o'clock for around 83-84db on SPL in my system.
Hi, Raul, I was wondering about the design of the 3160: What level of gain is bult into the LOMC side of the preamp? It seems that 60 dB or so is the typical level of gain for many (most?) LOMC stages and I was curious what you use in the 3160. Thank you for your time.
I noticed earlier that someone recommended Juicy Music. They are very very good, but unfortunately Mark has retired but will still service all of his equipment.
Kirkus, since I was rather too curious, I did some back-tracking on the subject in some related A'gon threads. What I come up with is, for best sound for the $$$:
1) 'affordable' phono-stage with MM 2) good stage with MM 3) good stage with SUT and MC 4) top stage with MC
2) and 3) mybe on par, and MC = LO MC > 0.4mV
Raul has his take on what comprises a 'good' stage (3160)and 0.01dB deviation in RIAA --------- other then the one he uses, I've never as yet seen one measured that could do this. So I can at best call my own lot 'affordable'. ML claims +/- 1dB deviation, my previous GCPH claimed 0.25dB deviation. Interestingly, the ML sounds better, and maybe due to using the 326S power supply.
Also some other acclaimed stages do rather poorly in this particular measurement.
Hi Axel . . . well, the first question is understanding as much as possible all of the different factors that change between your comparisons with/without the SUT. I'm speculating that some of them are:
-Cartridge loading is slightly different, i.e. more inductive with the transformer -SUT presents a different source impedance to the phono stage than the cartridge directly -Phono stage loading switches/plugs/resistors/caps are different -Phono stage gain is different, likely affecting noise, bandwidth, and distorion -Of course, the SUT itself has a sonic/performance signature
And some of the likely causes of what you observe:
- more dynamic depth (better hi/low SPL differentiation)
I think this usually corresponds to better headroom, and lower noise floor. The SUT will most likely gives a better En/In match to the cartridge, and better RFI rejection. The phono stage may also have more headroom at the lower gain.
- more powerful bass
Cartridge loading differences, possibly a little bit of low-frequency 3rd-harmonic distortion from the transformer
- more hall/room information, stage depth
I associate this with more high-frequency extension, or different high-frequency phase response. Cartridge loading differences, the transformer's sonic signiture, or better phono-stage performance at the lower gain
Anyway hope this helps a bit, without re-opening the whole SUT/non-SUT debate.
Hi Kirkus, during an earlier part of the MM, balanced vs unbalanced and common-mode rejection posting the SUT subject came up shortly.
It may not fully relate to this thread, but let me try.
You mentioned in your example a 50 ohm SUT input impedance with a 5 ohm DCR MC cart. Explaining some advantages with regard to hum rejection, and how this of course can not be realised in an MM specific phono-pre (as a trannie cannot be used etc.)
Would you share your explanation, why with the use of an SUT (in my listening) some other parameters then hum are notably changed/improved i.e.
- more dynamic depth (better hi/low SPL differentiation) - more powerful bass - more hall/room information, stage depth
I could add some more, but it should do for this example.
Cart parameters: 3 ohm DCR 0.3mV output @ 5cm/sec
SUT parameters: 1:31.6 ratio (30dB, i.e. natural impedance 47ohm with 47k) - primary DCR 1.5 ohm - secondary DCR ~ 65 ohm (as I recall) - primary loading 13 ohm (paral. with 47 ohm nat. imp.) i.e. 10 ohm that the cart sees.
If nothing else, it could high-light how very different a MC stage might just be as compared to an MM stage. So we'd be back at the subject, of sorts.
Lewm, yes, I think this could be right. It again would explain that even some well noted/quoted phono-stage designer(s) like SE. There was Lamm's Vladimir Shushurin? that stated his clear and decided preference for SE -- but I think he then uses a trannie to change the output to balanced. His main argument also was 'noise' and could well be what you just mentioned.
The new PassLabs-X15 seem very well regarded (best ever, etc.) would be most interesting to know what circuit it uses.
Axel, perhaps one reason you don't have "hum" is that the noise that might result from Kirkus' theorem would not necessarily be in the spectrum that we recognize as hum (60Hz or 120Hz, in the US; 50Hz or 100Hz in Germany). The noise he is talking about could be random in frequency (I think) and might not be audible as noise until one took steps to eliminate it and listened for the difference.
Thanks Kirkus, gotta get lucky once in a while in this here Audio game.
But as you mention as well, noise is only one facet of it all. As I noted - tonality is yet another thing. In fact, funny enough it also seem very high on Raul's list of evaluation criteria too :-)
I think he was referring to the generally much higher and more reactive impedance of MMs and the fact that these parameters may not be identical for each phase of the output in a SINGLE channel, using a balanced circuit. This creates a noise that cannot be cancelled by the balanced topology and is instead amplified. Channel balance has nothing to do with it. If I am full of baloney, perhaps Kirkus will correct me.
Actually Lewm, you got it exactly.
Now why do you think, do I have NO hum what so ever with a FULLY opened pre, going balanced into a balanced phono-line-pre?
Well I'd say that you happen to have good synergy between the cartridge, tonearm wiring, and preamp input stage, at least in the sense that its noise-rejection happens to be sufficient to completely eliminate interference from the particular amount of mains-frequency magnetic flux to which it is being subjected.
Now whether or not the ML engineers anticipated/designed for a similar level of performance in other situations or environments, I have no idea . . . but for yours, they got it right.
Lewm, of course you are right! We are talking one channel at a time, sorry. >>> ... he was referring to the generally much higher and more reactive impedance of MMs and the fact that these parameters may not be identical for each phase of the output in a SINGLE channel, using a balanced circuit <<<
I see, but then this would be a GENERAL issue with every kind of MM?! They ALL have a much higher reactive component than any MC.
Now why do you think, do I have NO hum what so ever with a FULLY opened pre, going balanced into a balanced phono-line-pre?
Maybe the answer lies with ML's balanced design then?
Axel, You wrote: "As to hum caused by 'unbalanced' MM cart output impedance. Well, I think that the more expensive carts we are speaking of (Raul mostly), those have always excellent channel balance, often better than more main-stream MCs with an e.g. <1.5dB spec."
I am pretty sure that Kirkus was not referring to differences between channels in his comments on the impedance properties of MM cartridges. I think he was referring to the generally much higher and more reactive impedance of MMs and the fact that these parameters may not be identical for each phase of the output in a SINGLE channel, using a balanced circuit. This creates a noise that cannot be cancelled by the balanced topology and is instead amplified. Channel balance has nothing to do with it. If I am full of baloney, perhaps Kirkus will correct me.
Some would not agree with this necessarily, since other then in digital designs, harmonic distortion is never buried in the noise floor completely.
Sure it is - just about any datasheet-derived single-NE5534 phono stage will do it, for reasonable signal levels and output current. I obviously don't feel that such circuits are ultimate expression of what's possible in a phono preamp, but the only reason to tolerate measureable harmonic distortion in these circuits is if the designer feels that there are benefits to choosing certain types of parts or topologies - and these choices make it impossible to eliminate distortion. I have no problem with that . . . every designer is free to decide what parameters meet their goals - ultra-low distortion just happens to be one of mine.
It seem current understanding that harmonic distortion should rather RISE evenly (even- and odd-order equally) with increasing output, rather than then decreasing with higher output. (Output rise as from cart input rise)
I believe that what you're referring to is a specific type fault that many feel can occur as a result of crossover distortion in Class B power amplifiers. Again, a phono preamplifier can and should be completely free from these types of anaomolies.
I is VERY difficult if not impossible to prevent some potential differences occurring in components (and amongst each other) during all states of operation. So they best possible directed by use of e.g. star-ground schemes. The point is, there are still caps (to ground) involved and caps have power factors, creating a far less 'clean' signal path then the dedicated (-) in a balanced design.
If you re-consider Kirkhoff's laws and look at the signal CURRENT, it always must flow between the power-supply rails, period. The purpose of local bypassing capacitors is twofold - first, to remove the effects of power-supply wiring and traces from the circuit, and second, to prevent different stages' current draw from affecting each other. These functions are necessary in both differential and non-differential circuits, and regardless of whether or not the signal VOLTAGE is defined in relation to ground, at least a portion of the signal CURRENT will always flow through the bypass capacitors . . . and that's the way its supposed to be. It is up to the designer to keep these signal and supply currents separate from each other, regardless of whether or not they flow through a node we call "ground". In fact, it can sometimes be more of a problem in differential circuits, where each side of the circuit has separate bypass capacitors to ground . . . in which the signal current has to flow through a minimum of two capacitors and a ground trace to return to the supply.
Its not that I don't like the differential approach, in fact my linestage is designed this way. But I will confess that I'm especially proud of the bypassing scheme - there are separate, unambiguous AC return paths for both differential signal current and signal current that flows to ground (i.e. from an impedance imbalance in the output cable or the amplifier that follows it).
The trade-off is most always balanced = more dynamic, and 'cleaner' vs. unbalanced = better harmonic completeness, more natural sounding.
I firmly believe that as we improve our art . . . it IS possible to have all of what you describe, without tradeoffs. And not everybody will see it the same way, but hopefully we will all end up with a more fulfilling experience from our recorded music.
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.
Halcro, please understand that I mean no disrespect to Bruce Candy or the namesake of your moniker . . . but unless the Continuum tonearm's wiring scheme is unconventional (in a connection sense, not just in a twisted-wiring sense) or defective (i.e some metal tonearm parts accidentally ungrounded) . . . then the problem is your preamplifier, and the manner in which its input stage is designed.
There seem to be many designers who look at balanced input stages (both line, phono, and microphone) purely as a pair of opposing voltages, not balanced impedances . . . and its amazing how many otherwise top-notch pieces of professional and consumer audio gear are very intolerant of the slight impedance mismatches that I describe. Another example of this would be the work of Douglas Self . . . a designer who I greatly admire and find much of his work invaluable . . . but simply cannot agree with his approch to designing balanced input stages.
But hopefully its just something silly like an a missing/loose ground wire somewhere, and my little diatribe is all for nought.
Kirkus, y.s.: >>> 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?"
Some would not agree with this necessarily, since other then in digital designs, harmonic distortion is never buried in the noise floor completely. It seem current understanding that harmonic distortion should rather RISE evenly (even- and odd-order equally) with increasing output, rather than then decreasing with higher output. (Output rise as from cart input rise)
Two notable designs come to mind by PassLabs (SUSY, Super Symmetry) and the 'cyclotron' (re-invention) by the Thorens TEM-3200 mono blocks have proved that to be correct.
Item: 'ground contamination' as you pointed out is more of a challenge then maybe generally accepted? And we are not talking of major ground loops. I is VERY difficult if not impossible to prevent some potential differences occurring in components (and amongst each other) during all states of operation. So they best possible directed by use of e.g. star-ground schemes. The point is, there are still caps (to ground) involved and caps have power factors, creating a far less 'clean' signal path then the dedicated (-) in a balanced design.
The noise floor of my ML326S is below 90dB (balanced). I think the issue is the 'tonality', influenced by 'combing out' harmonic distortion selectively. It is why even a MUCH noisier single-ended design often will sound 'better, more natural' then does a balanced one. (I speak from experience) The trade-off is most always balanced = more dynamic, and 'cleaner' vs. unbalanced = better harmonic completeness, more natural sounding.
As to hum caused by 'unbalanced' MM cart output impedance. Well, I think that the more expensive carts we are speaking of (Raul mostly), those have always excellent channel balance, often better than more main-stream MCs with an e.g. <1.5dB spec.
I run MMs into my (balanced) ML phono-board and have no discernable noise (ear to the speaker)! This at elevated listing level setting ~ 45 (max = 80, grading in dB steps). Adding 35dB (level 80, that would destroy my ears first, and speakers next!) I can practically NOT hear noise from the listening position (this @ 60dB 'normal' daytime background noise).
I guess, ML is very good at what they do with their balanced designs ---- however, tonality and 'live like' emotionality is not exactly their forte. One always seems to find that with 'noisier' and most always single-ended designs. That's where I see the trade off. Balanced = Hyper-clean and dynamic vs. unbalanced = naturalness, less clean, and better harmonic completeness.
All is of course subject to some generalisations and subject to levels of degree.
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