Why do most phono preamps lack XLR input even thought cartridges are naturally balanced?

Luisma31 7-9-2020

... playing with the Mogami interconnects I noticed no hum, not even the faintest hum with the XLR in place BUT when unplugged one of the XLR’s from the preamp and amp on, disconnecting from the pre there was a nasty hum, this is not supposed to happen, then I remembered when ordering these Mogami’s interconnects everyone said better to be shielded and at no cost I got them JSSG (looped shield at the ends), there was one guy don’t remember where online which told me before all this, for balanced interconnects never get these shielded but since it was only ONE guy I did not.Replaced the balanced interconnects (between pre and amp) with unshielded ones and when unplugging one end the hum was gone, but the buzz was still there.My source DAC was connected to the pre via XLR shielded, looped interconnects and even with the DAC OFF there was buzz, replaced these and the buzz was gone almost completely,

... I described hum with a shielded balanced cable when unplugged from the pre side, when an unshielded cable was used there was no hum, my understanding is the shield adds capacitance and IMO is not needed and undesirable.

Kijanki 7-9-2020
You keep saying "unplugged from pre side" but you probably mean that shield was disconnected, otherwise what are you listening to if nothing is connected to pre?

@luisma31 @kijanki

If I am interpreting correctly (and I’m not sure that I am), I believe Luis is saying that completely disconnecting the "looped," shielded, and balanced preamp-to-amp cable from the preamp (not just disconnecting the shield) resulted in hum and buzz. While doing the same thing with an unshielded cable resulted in buzz but no hum. And replacing the "looped," shielded, and balanced DAC-to-preamp cable with an unshielded one essentially resolved the remaining buzz.

Also, I believe that when Luis refers to a shield being "looped" he simply means that it is not connected at the corresponding end.

If those interpretations are correct I would not attribute the differing results to differences in capacitance. Consistent with one of Kijanki’s comments I cannot envision a means by which high interconnect cable capacitance (within reason of course, and the capacitance of Mogami, even in stock form with the shield not looped, is certainly well within reason) could result in hum or buzz. But on the other hand I can certainly envision that a cable of any type hanging off of the input of a component, while not being connected to a signal source, could result in hum as a result of EMI effects, depending on the design of the specific component and the specific cable, as well as on the surrounding EMI environment. And likewise when the cable is connected at both ends but the source of the signal is turned off (assuming the component providing the signal is not one of the relatively few designs in which the output is grounded via a relay when the component is turned off).

Also, while Luis describes the shields as being "looped" I’ll mention FWIW that in a conventionally designed balanced cable (i.e., one in which the shield or some other kind of conductor connects XLR pin 1 at one end to XLR pin 1 at the other end), the resistance of that shield or other kind of conductor can significantly affect the degree of hum and buzz that may result from ground loops. That would also depend on whether the components connect pin 1 to chassis (as they should) or to circuit ground (as is often incorrectly done), as well as on the relation (i.e., the impedance between) circuit ground and chassis ground within the components.

The bottom line: As I and others have said in past threads it is often ***very*** easy in audio to attribute an observed difference to the wrong variable. And that is especially likely to be the case when the observations involve a very limited number of components, cables, and circumstances. I believe that in this case differences in cable capacitance were not the cause of the observed differences.

Regards,
-- Al

Thanks for the mention, Kijanki. To add further technical elaboration to this erudite discussion of esoteric matters, IMO the main reason "most phono preamps lack XLR input even though cartridges are naturally balanced" (quoting from the subject line of this thread) is simply what Lew said earlier:

... it’s a longstanding tradition to offer only RCA inputs.

:-)

And of course traditions often tend to be self-perpetuating.

It’s perhaps also relevant that not too many decades ago I believe a considerable majority of high quality turntables had pendant (non-detachable) phono cables terminated with RCA plugs.

Finally, regarding RIAA equalization Ralph’s (Atmasphere’s) MP-1 and MP-3 preamps are of course fully balanced, provide transformerless balanced XLR inputs for their built-in phono stages (RCAs can be added as an option), and are spec’d as having RIAA accuracies of 0.07 db and 0.1 db respectively! And given especially that it is Ralph who has provided those specs, I have no reason to doubt them. I’m not in a position to elaborate on how he accomplishes those numbers, of course, other than pointing out that he uses triode-based differential stages rather than separate signal paths for the two legs. But as far as purely technical considerations are concerned his designs certainly speak to the practicability of accomplishing what the OP has asked about.

Best regards,
-- Al