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are some phono stages more resistant to hum?

After a tonearm upgrade, which mostly involved "improved" shielded cable, it now hums with tube phono stage (upgraded AR PH3-SE)but no hum with backup ss device (DB Systems). It appears the hum originates with the new wiring, but why would one phono stage be impervious to the hum? Do phono stages have different grounding schemes, making them more compatible with certain tt/tonearm/wiring combos in unpredicable ways? Are ss phono pres less susceptible to hum? Have you ever changed phono pre to cure a hum incompatibility? I see from forums that tt hum problems are common and sometimes difficult to solve. Shouldn't a shielded cable be more immune to hum, not less?
lloydc

Showing 3 responses by atmasphere

atmasphere's avatar
atmasphere
12,074 posts
 
Rtilden, just FWIW, tubes are no more prone to picking up noise from lighting than transistors. That is a power supply problem, not a tube/transistor problem.
atmasphere's avatar
atmasphere
12,074 posts
 
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.
atmasphere's avatar
atmasphere
12,074 posts
 
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.
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