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

(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

A standard RCA/XLR converter plug may not work, as these typically short XLR pin 3 to ground.... 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.

Actually, that is not correct. The phono stage will see the same signal amplitude whether or not pin 3 is grounded. There will be the same difference in voltage between pins 2 and 3, as generated by the cartridge, either way. The cartridge will not "care" whether those lines are referenced to the phono stage's ground, or are floating.

The difference will be, as I said in my post immediately above, that the impedances of the two signal lines relative to ground will be unequal if pin 3 is grounded. That will severely degrade rejection of common mode noise, but will not affect the amplitude of the signal that is processed through the phono stage.

Essentially, it would convert the balanced input stage into a single-ended input stage, but with no change in signal amplitude.

And again, to be precise, it is one polarity, or one of the two signals in a balanced pair of signals, that is shorted to ground, not one half of a signal. Each of the two signals comprises a complete waveform containing all of the signal information, and the waveforms on the two signal lines are identical except for being inverted relative to each other.

Best regards,
-- Al

I should have added that IF one does use an RCA cable to carry a balanced signal, then one must lift the chassis ground connection on the RCA jacks, for sure. Else, like Dave said, you will lose the negative half of the signal to ground. This equivalent to a 6db loss in signal strength.

Hi Lew,

That's not true in the case of a phono cartridge, as I indicated above. There would be a 6db loss in signal strength in the case of an actively driven balanced pair of signals, such as between powered components (that are not transformer coupled), but that is a different situation altogether.

And again, I suggest that you not refer to "losing the negative half of the signal." That would imply that half of the waveform, and the corresponding signal information, is lost, which as you probably realize is not what happens.

Best regards,
-- Al

Dave, Lew, thanks.

I haven't yet taken the time to go through the thread Dave linked to. But I would expect that a factor that would in some cases be decisive regarding the advisability or inadvisability of using an rca-to-xlr adapter which grounds pin 3 (as opposed to an adapter cable which does not) is:

IF the phono stage and part or all of the subsequent signal path and components in the system are fully balanced designs, then using an adapter which grounds pin 3, in addition to ruining common mode noise rejection capability at the phono stage input, will result in the sacrifice of some of the key advantages of fully balanced design in those subsequent parts of the system. Including cancellation in those components of some forms of internally generated distortion, and, particularly in the case of the power amplifier, reduction of power supply-related noise.

Also, if the entire signal path is balanced, a considerable fraction of the amplifier's power capability (as much as 75%) will probably become unavailable.

It should be noted, though, that grounding pin 3 and thereby in effect making the subsequent parts of the system single-ended in those respects, will NOT degrade the common mode noise rejection capability of balanced interfaces that may be present at points in the signal path subsequent to the phono stage input.

Best regards,
-- Al