Why do most phono preamps lack XLR input even thought cartridges are naturally balanced?

Ralph, very interesting.  As roberttdid said about differential filter "it seem obvious", but it escaped me completely, being stuck on "matching". 

Cable immunity may just be the most underated design aspect in all of hifi land...
I mean we all know cables make a difference...but what if that variable were greatly reduced? More budget is opened up to things like better circuit components and implementation- a very good thing.

We are all told that cables make a difference. Absent knowing what cable or that there was a change, my experience in controlled environments says that is the cable is competent, there may not be a difference. Diminishing returns much quicker for interconnects as well versus speaker cables. Noise rejection is important yes.

How much advantage did you find by adding shielding of the XLR over just using a differential input w.r.t. noise?

Almost none. I ran unshielded tonearm cable in my home system for some years. You could crank up the volume and no hum or buzz from the cable, even if grasping it or moving it around. You do still have to ground the tone arm and it works best if that wiring travels with the signal wires. One advantage here is this allows for much lower capacitance in the cable.

As roberttdid said about differential filter "it seem obvious", but it escaped me completely, being stuck on "matching".

@kijanki  I think a lot of people think of matching 'equal but opposite' circuits when they hear the word 'balanced'. EQ circuits might be the best example of that. I once went to see a demo of the new (at the time) Mark Levinson preamp (IIRC the ML-29) which was their first balanced line preamp. But the phono was single-ended. When I asked why, I was told that matching the EQ was the problem the 'engineers' were worried about weird effects if the parts weren't matched really carefully! That was when I knew they hadn't tried it at all.


At any rate if you want to do balanced right with active balanced circuitry, the way to do it involves differential circuitry. That's how you develop power supply noise immunity (Cross-Mode Rejection) and Common Mode Rejection. (As a side note, the CCS is critical and frankly, most CCS circuits I see are pretty terrible.) But what is less obvious but also important is the simple fact that you should do wiring in differential mode too- common grounds and common power supply points being the most obvious examples, and of course the EQ can be done differentially as well. Now we use passive EQ for our phono (based on the formula of Stanley Lipschitz) so imagine dual EQ networks, one for each phase; this made it go easy.

Those networks should use a common ground of course, but if you think about it, you don't need the ground at all. And if you got that far, then you can see that resistor values are in series as are capacitors- meaning that only one resistor need be used, only one cap (of half the value; you can see where this is going) and now there's one network instead of two. Much, much easier and all you're doing at that point is trimming to the values you need, rather than a Sisyphean task of matching to some extreme; pointless when you have tubes or semiconductors that won't come anywhere close to a similar match. A nice result is you don't have to do crazy amounts of tube matching yet the EQ will be spot on from both phases even as the tubes age. As far as I know, we were the first to do this (1990) and I think it was only about 8 years ago before I spotting any circuits that did the same thing- apparently its not common knowledge even today.

Good you mentioned no advantages with the shield Ralph, I was about to send this in the morning before you even answered in the thread and I think this although been mentioned before it is largely overlooked.
I had 1st a Hifi digital system and then I started getting into serious and educated analog. The digital audio community was introduced to a concept to improve SQ on digital rigs, specifically on DC connectors which basically states that a shield on a cable, not connected to any ground but looped at the ends of the DC cable will improve SQ, John Swenson was the engineer mentioning the concept, if it works or doesn't up to you to decide, I don't take sides, with the exception some people started applying this concept to all sort of cables, USB, digital interconnects and SE and XLR interconnects, some reported improvements, I tried these and notice nothing but since there is shielding involved I thought (at the time) it was great.
Fast forward 1 year, got a new preamp from Ralph (Atmasphere) and there is an accentuated buzzing sound on my tweeters, could be heard from 8 ft away, very annoying, my entire rig is balanced XLR. Started analyzing and graphs showed peaks on 120 Hz, 180, 240 and 300 Hz, AC harmonics of course, some of these 15 db peaks some others 30 - 40 db, very annoying. Turned off the entire house, got into a fight with my wife for it, moved things around, added some power conditioners, buzz was there, called Ralph, he suggested an isolation transformer, got one, some improvement, minor, buzz was still there. Ralph asked me to do a few things and he very nicely sent me some plugs and attenuators (at no cost), attenuators worked taming the buzz but at the cost of power as you could imagine,  because of the gain on his pre and amps is high still got plenty of power on the preamp for my speakers so all good there. But me I can't rest until I figure what was wrong, got my spectrum analyzer and scanned the entire house and even the neighbors, nothing out of the ordinary. Got a long extension cord (daisy chaining 4 extensions) and connected my entire system to a neighbor on the other side of the street (with his consentment), same results.
Talked to Ralph on the phone about something else but me being me (annoying asking things) and Ralph being Ralph (willing to explain and educate you with his great knowledge) we mentioned cables used, I use Mogami which he recommends but the topic stayed lingering on my mind for several days.
Sorry for the long rant, getting finally to my point, playing with the Mogami interconnects I noticed no hum, not even the faintest hum with the XLR in place BUT when unplugged one of the XLR's from the preamp and amp on, disconnecting from the pre there was a nasty hum, this is not supposed to happen, then I remembered when ordering these Mogami's interconnects everyone said better to be shielded and at no cost I got them JSSG (looped shield at the ends), there was one guy don't remember where online which told me before all this, for balanced interconnects never get these shielded but since it was only ONE guy I did not.Replaced the balanced interconnects (between pre and amp) with unshielded ones and when unplugging one end the hum was gone, but the buzz was still there.My source DAC was connected to the pre via XLR shielded, looped interconnects and even with the DAC OFF there was buzz, replaced these and the buzz was gone almost completely, turns out not all the balanced XLR equipment out there is "fully differential" or following the balanced standard, Atmasphere's are but the DAC wasn't.
Summarizing and my conclusions.Shielded cables with tricks like loops etc. have A LOT of capacitance, truly balanced equipment won't care but better to have no shields as there are not needed because of the design of these circuits, single ended equipment will suffer from high capacitance with hum, buzz etc.Of course this is what Ralph's have been saying for decades so not so much new here except that now I don't want any capacitance near any of my cables.I'm sorry Ralph I don't think I told you these findings directly but I just discovered this over the last month or so and since it came up publicly in this thread I thought it was best to share.Luis