are some phono stages more resistant to hum?

You have to reference the input voltage to somewhere, and for phono inputs its the shield (-). If you could get a cartridge with a true balance output (like a center tapped transformer), with the split (or center tap) connected to the analog ground/shield, then you can have a true balanced input. Some microphones are wired this way. But phono cartriges are not.

All phono cartridges made today are balanced sources. None are single-ended. You don't need a center-tap to be balanced- that is a common myth. I don't know of any microphones that have such a thing- my Neumann U-67s sure don't. Neither do Shure SM-57s- these are mics at far extremes of cost and performance.

Any tome arm that has 5 connections (stereo signal + tone arm ground) can be run balanced by replacing the interconnect cable between the arm and preamp.

To clarify my comments, phono carts are ISOLATED devices, but are not inherently balanced. A true balanced voltage source would produce a symmetric V+ and V- signal referenced to common.

However, I agree they can be run in a balanced configuration with the correct wiring configuration and true differential input phono stages.

Sorry Lewm if I misread your original post. Its just that true differential phono stages are very rare, and neither of the OP preamps have true differential inputs, so using balanced wiring with these single ended input stages converts the system to single ended with little to no CMR advantages.

The OP asked whether one or another different types of phono stage might be more or less susceptible to hum. In responding, I brought up the fact that true balanced phono stages are much more immune to hum. Then we got off on this tangent. I don't think the discussion was out of line with the intended subject of the thread, but it was prolonged by your insistence that a cartridge is not a balanced source. Anyway, it's all good.

Higher level phono sections like mm carts rather than low output mc carts are less sensitive to EM induced hum from nearby transformers, etc. Shielded cables and/or supplemental shielding around the low level amplification device(s) like a step up transformer helps. See my system listing here on agon for an example. Adding shielding around my phone step up amp allowed me to use my preferred unshielded IC in my case and eliminate significant audible hum.

Lloyd, have you tried temporarily using a cheater plug (a 3-prong to 2-prong adapter) to isolate the safety ground pin on the power plug of the phono stage and/or the turntable? That would break a ground loop between the two components, and enable you to determine if a ground loop is the cause of the problem.

For that matter, is there still a hum if the turntable is unplugged from the AC?

Also, is the hum level affected by whether or not the turntable is on or off, and also by the position of the tonearm relative to the rotating platter and/or the motor?

Lloydc 05-07-12:
Do phono stages have different grounding schemes, making them more compatible with certain tt/tonearm/wiring combos in unpredicable ways?

That strikes me as a very well put question, to which I would answer "yes."

Schematics for your phono stage can be seen by clicking on the two corresponding thumbnails near the bottom of this page. It appears that circuit ground is connected to chassis (and hence AC safety ground) via a 10 ohm resistor (shown on page 2), and a 560 pf capacitor (shown on page 1). Other designs can be expected to handle the interconnections between those grounds differently. Also, differences in the amount of stray capacitance that exists within the power transformer of different phono stages, between the AC line and chassis, can affect ground loop susceptibility.

On the question of cartridges being balanced or unbalanced, I would put it that it could be reasonably argued that a cartridge is inherently balanced, and it could also be reasonably argued that it is inherently neither balanced nor unbalanced, since when unconnected its outputs are floating ("isolated" as DHL said). But it's a moot question, because the design of what they are connected to causes the two signal lines from each coil to be treated in a balanced or unbalanced manner. For balanced operation, the phono stage has to closely match the impedances between each signal line and its internal circuit ground, as well as processing the inputs differentially.

Best regards,
-- Al