How to accomodate a balanced only phono stage ?

The main purpose of the pin 1 ground connection on balanced interfaces between powered components is to keep their respective circuit and chassis grounds reasonably common.

Where one or both of the two components has a 2-prong power plug, or is used with a cheater plug, without that pin 1 connection the grounds of the two components may "float" to very different potentials, due mainly to ac leakage paths and stray capacitances in their power transformers. That potential difference may be large enough to saturate the input stage of the receiving component, or at least to exceed its common mode rejection capability.

If both components have 3-prong power cords, then in the absence of the pin 1 xlr connection the grounds of the two components would be kept common only by the ac safety ground wiring, which is not desirable from a noise standpoint.

None of those factors are applicable to a phono cartridge, which has no ac, no power transformer, etc., and which will happily adapt to whatever the ground potential of the phono stage may be.

That said, it may be somewhat beneficial if instead of using an rca cable into an rca-to-xlr ADAPTER at the phono stage input, a twisted pair shielded rca-to-xlr adapter CABLE were used instead. It would be wired such that xlr pin 2 is connected to the rca center pin at the other end; xlr pin 3 is connected to the rca ground sleeve; and the shield is connected to xlr pin 1. Using a cable like that results in the two signal polarities (not signal "halves") on pins 2 and 3 being wired symmetrically (as opposed to an rca cable where the two conductors are not physically symmetrical). That will mean that any noise picked up by the cable will be essentially the same on the two conductors, and therefore rejectable by the receiver in the phono stage.

Regards,
-- Al

Whew, thanks Al. I didn't dare say that!

Phono carts are all balanced by design...just get adapter from rca to xlr.

To simplify, the two-pin cartridge coil is inherently a balanced signal generator. On one channel map cartridge Red to XLR pin 2 and cartridge Green to XLR pin 3. On the other channel, map cartridge White to XLR pin 2 and cartridge Blue to XLR pin 3. The cable's RFI shield should float unconnected at the the arm and be grounded to XLR pin 1. A separate ground wire may be run from physical tonearm to phono stage chassis.

A standard RCA/XLR converter plug may not work, as these typically short XLR pin 3 to ground and elimininate one phase of balanced signal from circuit. To pass a fully balanced cartridge signal to the phono stage you need a converter plug that maps RCA pin from cartridge positive phase to XLR pin 2, cartridge negative phase from RCA barrel to XLR pin 3, and floats XLR pin 1 unconnected.

As suggested by Lewm, if in an RCA configuration cartridge negative phase and shield to RCA barrel share a common wire, then grounding the shield to pin 1 or chassis will have the effect of shorting out and eliminating one half of the balanced signal.

(I was just about to click "submit" on the following, when I saw Dave's (Dgarretson's) post above. Thanks Dave).

I should have added at the end of my previous post that there is a second reason why it is preferable to use a properly designed adapter cable, rather than an rca-to-xlr adapter.

Most or all commercially available rca-to-xlr adapters connect xlr pins 3 and 1 together (i.e., they ground the "cold" or inverted signal polarity). The resulting difference in the impedance of pin 2 (relative to ground) and pin 3 (relative to ground) will pretty much negate the common mode noise rejection capability of the balanced input.

Regards,
-- Al