XLR to RCA Adapters

Hi guys, I just had a chance to check in and see Bob's question about the input transformers. I believe these minimized any impedance differences, as long as you keep the balanced IC's from the transformers to the amps very short. Jensen says 1M or less, but shorter is better. Of course the ideal approach would be to mount the transformers internally with RCA inputs to the amps.

Between talking with Jensen and going over their literature, I remember that the input transformers were clearly the better approach for the rca/XLR interface. Virtually no sonic penalty with regards to measurements (I believe some very small input loss - like 1.5 dB) which I cannot detect even when comparing to balanced preamps. They work with loads as low as 10K ohm and I believe the impedence issues are somewhat related to the load, and to the transformer to amp cable length. My highish 100K ohm input amps result in an optimal impedance situation. The transformers also provide noise rejection benefits. Here is a link to read,[url]http://www.jensen-transformers.com/datashts/pi2xx.pdf[/url]

The guys at Jensen and the associated documentation do a better job of explaining than I can. Compared to giving up equipment you like the price of trying the transformers make them a stupid easy decision. I remember initially trying the output transformers based on someone's suggestion but they were noisy and didn't sound good, I called Jensen and they told me "you want the input transformers." They swapped me for the input transformers and I later purchased their mono version to keep cables very short - mine are like 2 feet long from the transformers to the amps. Call Jensen, they are very friendly and knowledgeable.

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.