XLR to RCA Adapters

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

Fine job again, Al. Bill Whitlock and team have documented transformerless input receiver schemes that improve significantly on the all too common poor designs out there in the pro world (with respect to the issues Al has raised); but these would not normally be considered too pretty from the audiophile standpoint. I vote as well for a close reading of the Jensen literature, even if you are not a prospective transformer user (unless you're already a world class expert on grounding issues in sensitive systems -- this stuff is beyond elementary). Check the K & K Audio website for links to Dave Davenport's detailed discussion of similar issues: a second, somewhat different perspective on the same problems can throw some very helpful extra illumination on the important concepts. And again, Mr. Yaniger's piece is on point here.

As Al discusses, perhaps the cables also make a difference. My rca IC's are a balanced (they call them that when both legs are the same) twisted pair type shielded cable from Furutech made from PCOCC copper. There is literally no noticable noise from the speakers at virtually any typical volume level, unless music is playing.

The reason you use balanced lines is lower noise and lower distortion, plus the opportunity to eliminate the sonic role that the cable has in the system. BTW, length (or shortness) of the cables has nothing to do with it!

Now if you are using a single-ended source with a balanced amplifier, you will not be able to realize this advantage. Thus comes the idea of a transformer. You would use the transformer directly at the output of the source, keeping the cables between the source and the transformer as short as possible. However the transformer (including the Jensens, which are excellent) will have a sonic artifact which is easily heard, even if properly loaded. If the transformer is kept as close to the source with the cables to it as short as possible, CMRR degradation issues will be minimized.

Now, if the main interconnect cable can be short, you are better off just making the connection with a single-ended cable. The amp, BTW, is not going to give a hoot if it has differential inputs, and the BAT does.

But if the cable is to be very long, over 20 feet, then there will be an advantage to using the transformer solution (assuming an excellent transformer), despite the artifact it introduces, because that artifact will be *less* than that of a single-ended cable being run long distances like that.

Mari555 asks about the other way 'round, where the preamp is balanced and the amp single-ended. There are several ways to do this. The most elegant is to modify the amp with a balanced input, and even though the amp is single-ended, it is possible to set it up so that the amp uses both the non-inverting and the inverting connections of the XLR.

The second method is to use an input transformer that does the conversion.

Finally, an adapter can be used to good effect, and while this technique is cheap and easy- you get what you pay for- it does not take advantage of the balanced output of the preamp and so the resulting sound will not be as good.

Just FWIW, we built the world's first balanced line preamps. We've been answering these questions for a long time.