Balanced in phono stages preamp?

Speaking of a phono cartridge, it's just a floating coil with 2 wires. As the coil is floating, you have the choice to operate it as either single-ended or as a true balanced signal source. If you connect the negative terminal to ground, and use the positive terminal as signal, then you have normal single-ended mode.

To better understand the advantage of a cartridge working in balanced mode, I will quote a paragraph from my book "Noise Reduction Techniques in Electronic Systems" by Henry W. Ott: "A balanced circuit is a two-conductor circuit in which both conductors and all circuits connected to them have the same impedance with respect to ground and to all other conductors. The purpose of balancing is to make the noise pickup equal in both conductors, in which case it will be a common-mode signal which can be made to cancel out in the load".

According to this definition, we can confidently say that a pickup cartridge truly acts as a balanced source, because when the cartridge is connected to a normal phono cable (with 2 wires and 1 grounded shield), the impedance of both wires with respect to ground is the same. Further, the noise pickup is equal in both wires, forming a common-mode signal that a differential input stage (or a center-tapped transformer) will easily cancel out.

Please notice that differential mode is not the same as balanced mode. In fact, an input circuit working in differential mode is a pre-requisite for balanced mode. As said, this circuit could be: 1) A center-tapped transformer, or 2) a differential input stage (tubes or solid-state). As you can see, balanced mode has obvious advantages over single-ended mode in phono cartridges.

Everyone here is getting the terms differential and balanced mixed.
Differential signal is when you have the same signal 180 degrees out of phase showing up on two signal legs.
Balanced signal is the differential signal referenced to a ground. Balanced signals are useful in that typically, you get the benefits, i.e common mode rejection, 6db higher signal, you also get disadvantages, the circuitry essentially doubles, you have make sure that the amplification stages of the two opposites of the signal are treated absolutely equally, not just statically, but dynamically and through the aging process that a piece of equipment will go through over years. Anyone want to take a guess at what happens to a volume control or switched attenuator after a few thousand rotation cycles.

Phono Cartridges and loudspeakers are inherently differential devices and a case could be made to treat them in balanced mode at the inputs and output stages of amplification equipment. There are advantages and disadvantages to all topologies and outstanding examples of design for both.

My personal opinion is that the currents involved in the phono (specially mc) stage are small enough and the inertia of the generator is small enough that exceptional results are available with either topology, in the amplifier output stage, for reasons of symmetrical feedback, reduction in ground current etc, it may be that a balanced output is the preferred method of operation. Although my current amplifier output is operated in SE mode (ie the negative terminal is tied to ground) and I am really happy!

That is simply not true. It is not required. A cartridge connected to a single ended input sometimes will hum with the ground connected and sometimes without. Sometimes the ground connection makes no difference.

This is consistent with balanced operation: you may not always get hum when converting to SE. However there is a reason why nearly every turntable manufacturer has a 5 wire system rather than 4, to prevent ground loops (IOW the ground is not mixed with the signal- this implies that the minus output of the cartridge is actually the inverting output, and in balanced systems the inverting signal is often denoted by a minus sign).

Even BSRs and Garrards from the 60s and 70s are set up that way. As a result they can all be run balanced since the ground is not ground looped with the signal.

While it may be true that balanced equipment might have been used in the recording process, the groove in the disc is not balanced.

By that measure, neither is it single-ended! Once the sound is mechanically encoded, you have the same conundrum that you have with actual sound- it is neither SE or balanced- it simply is. It is the way we handle the sound, once it exists as an electrical signal, that makes all the difference (no pun intended :)

Ashly pro audio equipment uses what some would call "pseudo balanced" outputs. (Inputs are true differential balanced circuits).

In the "pseudo" configuration, the wire that would normally (in a single ended interface) be ground is isolated from ground by a resistor of the same value as the output impedance of the active circuit, in my case 100 ohms. This wire is carried to the (-) differential input of the following electronics, along with the signal wire (+), and a ground wire. This scheme will give the same imunity to noise pickup as a "true" balanced output. This approach could be used with a phono signal if the preamp had a true differential input.

it is neither SE or balanced- it simply is

You lost me there. I'll have to think about that one, but I would argue that the record groove as well as sound itself is single ended. I don't think something is balanced unless you have a pair of signals of opposite polarities.

Since nothing in the home stereo meets this requirement unless we create the opposite signal, it is single ended. I'll have to ponder the situation of a cartridge hooked up to the inputs of a diff amp as noise rejection at this low level makes sense. I may try that myself, but I see no compelling reason to go balanced after that.