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

@oldears  If its not humming I would not worry, but I suspect there is no need for the 'drain' wire, as the preamp is likely electrically grounded via the ground connection of the AC power cord.

We've been making DIN to XLR phono cables for decades; we just tie pin 3 of the DIN (ground of the arm) to the shield which is pin 1 of both XLRs.

@luisma31 Make sure the turntable/tonearm ground is tied to pin 1 of the phono input XLRs.

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.

Gains of both amplifiers have to be exactly the same, otherwise common mode noise will get converted to normal mode signal. It requires some form of cross-feedback to keep gains exactly the same. Phono preamp in addition has RIAA equalization that changes frequency response within audio band by 40dB (100 times). I suspect, that it would be very difficult to match it in both "legs" of true balanced phono stage.

This statement isn't quite correct. To prevent issues with RIAA equalization, you simply do the EQ network in differential mode. This eliminates issues with matching and gain differences.


A bit of audio history since this is relevant: we built the first fully differential phono sections for home use in our MP-1 preamp, introduced in 1989. As such is was also the first balanced line preamp, tube *or* solid state (again, for home use). At the time the XLR connector was the only game in town so that's what we used for the phono input. We had no idea at the time that a product like this should do anything other than support the balanced line standards (AES48) so that is what it did and does.


The correct phono connection is: + and - outputs of the cartridge are pins 2 and 3 of the XLR. The tone arm and turntable ground is the shield of the interconnect cable and ties to pin 1 of the XLR of each channel, so no ground wire is needed. You'd be surprised how many cable manufacturers there are that think the ground wire is still needed :)

This connection is so quiet that we've used unshielded twisted pair with a ground wire (what we've come to call a 'twisted triplet'; but we've also used Kimber braided wire) and its noise-free. But one thing that has been pointed out here that is really important is the CMRR of the input stage. Of course transformers do this really well, but since we're sort of known for transformerless operation, the other way to do it is with an input gain stage that has as high CMRR as possible, combined with the lowest noise.


Obviously opamps work great for this but we wanted to use tubes, so the input circuit is a differential cascode which employs a 2-state CCS (single-stage CCS circuits leave performance on the table). If you want low noise and high CMRR, this type of input circuit (which is common in a lot of solid state circuits as well) is really the only way to go. Because tubes drift and because matching components is tricky, we sorted out the differential mode EQ and as far as I know we're the first to do that by decades prior to anyone else. Differential mode EQ insures that both outputs of the phono section (inverted and non-inverted) will have the same EQ regardless of how much gain imbalance there might be. To prevent common mode noise from becoming differential mode the simple bit is making sure that any ground in the gain circuit is common to both phases- common sense :) In this regard layout plays a role as well, as the designer will find that certain parts in the circuit are best placed as close to each other as possible to maintain noise immunity.


The main downside we encountered was the reluctance on the part of dealers to deal with the balanced input since it meant changing out the tone arm cable; some thought the arm had to be rewired which of course it doesn't :)   The advantage of operating balanced at this point in the signal chain is that as long as the tone arm cable is low capacitance, the cost of the cable isn't important- how its wired is. So you can have a $200 cable sound just like a $2000 cable. Cable immunity was a big incentive for why we developed a balanced preamp.