XLR to RCA Adapters

08-22-11: Bob_reynolds
Mitch2,
Just curious... Why did you select an input transformer when your amps have balanced inputs instead of an output transformer, like Model DM2-2RX?
Wouldn't it be theoretically better to have shorter RCA cables at the preamp and longer XLR cables to the amps?

Bob, that's an excellent question.

I'm not entirely sure of the answer, but I believe it relates to differences in inter-winding stray capacitance (i.e., between primary and secondary) of output transformers vs. input transformers. I've noticed that the schematic representations of the Jensen input transformers that I've looked at all show Faraday shields, which would greatly reduce that stray capacitance, while none of their output transformers that I've seen include those shields. I have no particular knowledge of why that may be; presumably it has some relation to the fact that output transformers have to be able to drive much longer cable lengths.

Lack of a Faraday shield in the output transformers results in very substantial inter-winding capacitance. For instance, the this output transformer is spec'd as having 22,000 pf of capacitance between windings, as measured at 1kHz.

A result of that capacitance will be that the input stage of the amp would see, "looking back" at the output of a preamp + output transformer combination, an impedance which is very unbalanced between the two legs at high frequencies (since one side of the transformer primary is connected to preamp ground, while the other side is not). That imbalance will significantly degrade the cmrr of the amp's balanced input stage, at the high frequencies for which that capacitance becomes significant.

Hopefully someone more knowledgeable about transformers than I am will comment further, but I suspect that the explanation is along those lines.

Best regards,
-- Al

Hi Bob,

Once again you raise excellent questions.

In the absence of a transformer, the cmrr of an active balanced input stage will generally be degraded significantly when driven by an unbalanced source. Good cmrr requires a close match between, among other things, the input impedances of the two legs. Since the output impedances of whatever is driving those two legs are in series with those input impedances, and since an unbalanced source has an output impedance of essentially zero on one of the legs but not the other, an impedance imbalance will result.

A transformer will not be subject to that effect, as I understand it. It will provide an output that (for signals whose characteristics fall within the limits of what it is designed to handle) is simply proportional to the instantaneous voltage difference between its two primary terminals, regardless of source impedance differences.

What may be important, however, if an input transformer is used in conjunction with an unbalanced source, is assuring that pickup of noise, rfi/emi, etc., is as common mode as possible (i.e., as equal as possible between the two legs). That means that even though the source is unbalanced, a coaxial or other non-symmetrical rca cable preferably should not be used. Preferably a shielded twisted pair or other cable that has symmetrical signal and return conductors should be used, terminated with rca connectors, with the shield grounded at the source end.

The degree to which the cable configuration may be important, though, would be dependent on whether the primary contributors to noise are ground loop effects, or rfi/emi pickup. Several of the Jensen writeups suggest that more often than not the former is the more significant problem in home audio systems that have unbalanced interfaces. To the extent that that is true in any given system, it seems to me that input transformers and output transformers, even if connected with non-symmetrical rca cables, may be comparably effective, since either one will break a ground loop. Although in the case of an output transformer, the high inter-winding capacitance I mentioned in my previous post may lessen its effectiveness with respect to ground loop-related noise that is at high frequencies, since that noise may to some extent simply couple through the capacitance.

Best regards,
-- Al

Kirk, thanks very much for the OUTSTANDING explanations, all of which ring true as far as I am concerned, pun definitely not intended :-)

Ralph (Atmasphere), Seakayaker, Tim (Mitch2), thanks for your good inputs as well.

Best regards,
-- Al

Talk2me, several references to your amp that I've seen indicate that it has rca as well as xlr inputs. Is that not the case with your particular amp?

In any event, the reason you didn't find an explicit answer as to the best way of interfacing rca outputs to xlr inputs is that there is no answer that is universally applicable and universally agreed upon. Relevant variables include cable length, component impedances, susceptibility to ground loop effects, rfi/emi environment, deep bass extension of the speakers, etc.

In your case, I found a post somewhere indicating that the balanced input impedance of your amp is an unusually low 2,000 ohms. If so, that probably rules out the Jensen input transformer approach, and limits you to a choice between a Jensen output transformer, such as their model DM2-2RX (the "RX" version is the same as the "XX" version referred to in the data sheet, except that it has rca input connectors), or the adapters you are presently using, or adapter cables. As Mitch2 suggested, you should speak with a Jensen rep before finalizing a choice of one of their transformers.

See section 2 of this paper for further information. Note that for all three of the methods that are illustrated the cable has symmetrical signal and return conductors (i.e., it is not a coaxial unbalanced cable).

Regards,
-- Al