Phono Pre-amp with XLR

Prochk3, I looked at the phono pre ad and saw it claimed the unit was fully balanced input to output. But it has RCA instead of xlr inputs.

If the RCA is grounded as it usually is, the noninverting or plus signal is still going to the noninverting input of the balanced differential phono amps circutry. Its called a dif amp because it ampliflies the difference between the noninverting and the inverting inpts. So even if the inverting input is grounded you still get a signal at the output because of how the diff amp works. Thus you have a the xlr for the output of the balanced phono pre.

Now the outside of the RCA input jacks may be floating, not grounded, and thus the inverting signal from the cart still makes it to the iverting input of the bal diff amp.

That is not the ideal way to handle a balanced signal but the positive/negative signal from the cartdrige does make it to noninverting/inverting input of the bal diff amp.

I do not know if there is a significant difference sonically.

Atmasphere, what is your take?

Phono preamps can be misleading because some are single ended but add an extra balanced circut at the end. That is undesireable if you want to keep gain stages to a minmum.

TD

Atmasphere, with a true balanced phono pre amp, does it make a significant sonic difference if the pre amp is designed with RCA inputs where the inverting signal from the cartdrige is grounded at the RCA jack leaving only the noninverting signal going into the balanced diff amp?

Thanks, TD

RCA jacks can be used as an input to a balanced preamp. How it works is that the RCA jack is isolated from the chassis ground, so that the 'ground' connection of the RCA can function as the inverting input to the balanced preamp.

There are some problems with this approach- you still have to use a ground post, perhaps no problem if the ground connection is supplied. However, the interconnect between the tonearm and the preamp **must not use the minus (-) output of the cartridge as a shield for the plus (+) side!** This would result in hum.

Now assuming that such a cable issue is dealt with, you still have the problem of the fact that at the RCA connection itself, the 'minus' (inverting) connection is shielding the 'plus' (non-inverting) connection. This damages the overall Common Mode Rejection Ratio of the system; IOW, it makes it susceptible to hum.

You can set up the RCA so only one of the inputs of the preamp is driven and the other side is at ground, but again the CMRR of the preamp is not available to the user, although you don't have some of the hum susceptibility of the scenario in the prior paragraph.

So, on a very practical basis, the XLR is a superior connector. Any use of the RCA for a balanced connection is a sort of kludge approach.

Atmasphere, nice explanation of the cable and connector issues.

And I agree that using RCA connectors with a balanced input is to be avoided.

Lets assume a phono pre has RCAs with the inverting lead grounded and only the noninverting signal feeding the balanced diff amp. What does that mean to the bal diff amp vs having both the noninverting and the inverting signals in a proper xlr configuration.

Going beyond CMR and such, it seems like there might be some difference in feeding half of the diff amp vs feeding both inputs. But maybe not. Still the same gain?

Thanks again, TD

Tdaudio, If you read Ralph's (Atma-sphere) post and then read mine from yesterday, you will find a remarkable confluence. I have been using an MP1 for about 14 years now. I think some balanced phono stages offer RCA phono inputs because, as Ralph indicated, the world has not beaten a path to his door. Some other manufacturers fear that offering XLR inputs will frighten off persons who have typical RCA-terminated phono cables. I like that Ralph is a man of principle and won't compromise his design by offering an inferior connection to a balanced circuit.