are some phono stages more resistant to hum?

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

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.

A common method I use to identify a balanced source is what happens if you swap the + and - outputs of the device? Do you get hum or is phase merely reversed? If the latter then its a balanced source which can be treated single-ended or balanced depending on the wiring it drives.

We first started building balanced phono sections (all tube) about 1989. They have XLR connections, and the way a cartridge hooks up is the + output is pin 2, the minus input is pin 3 and the tone arm ground (which has no connection to the cartridge) is pin 1 (apparently we were the first to do this...). In this hookup, you can see that that ground is essentially ignored as the output of the cartridge occurs between pin 2 and pin 3 of the XLR. So there is no need for a separate ground wire like you see in single-ended connections.

Certainly there is no need for a center tap! A center tap can never be perfectly centered in the windings, so the Common Mode Rejection Ratio is reduced if you use a center tap. IOW, you don't want or need a center tap.

Such a connection is immune to hum. The 'shield' connection can be a simple wire and it will work the same.