XLR to RCA Adapters

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.

How does a transformer work to convert the amp to balanced Atmo? It seems like it would need to be after the input stage. I don't doubt you, I just don't know.

For Atmasphere: Your knowledge, helpfulness, and long experience is much appreciated here. If you follow the set-up in your second paragraph above to the letter, Mr. K.,(and I highlight your spec of short cables in this case), are you better off using one of the Jensen input transformers, for some of the reasons cited earlier in the thread... or should you employ the line output type? I allow that the answer may be different, depending on the preamp in question. Thanks.

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.

Ralph, with all due respect, this solution did not work for me using the Tom Evans Vibe/Pulse 2 single-ended preamp into Clayton M300 (balanced input only) monoblocks. It is also contrary to what I was told by Jensen, who recommended using the input transformers next to the amps with very short balanced cables to the amps, which is the solution that has worked very well.

However the transformer (including the Jensens, which are excellent) will have a sonic artifact which is easily heard, even if properly loaded.

In my case, where my preamp output impedance is very low (12 ohms) and my amp input impedance is 100K ohms, "easily heard" would be an overstatement, better replaced with "barely heard" or perhaps "negligible." Also, the acceptance of this solution can be dependent on the gear. In my case, based on my desire to stay with SS devices, the TEAD preamp with transformers beat every balanced (SS) preamp (without transformers) alternative I have tried to-date.

Al, you're exactly on the right path regarding the performance of input vs. output transformers. Put another way, output transformers require tighter primary-to-secondary power coupling to maintain low output impedance, and the resulting low distortion and low noise figure. A side-effect of this is the necessity of a larger core, and higher leakage reactances between the primary and secondary. Input transformers on the other hand need to transfer very little power, and so can make effective use of Faraday shields and have lower leakage reactances, at the cost of the requirement of needing carefully controlled secondary impedances for good performance. But in both cases, the distribution of these reactances across the two windings can be controlled in the design of the transformer, and frequently an input transformer will work best with its secondary grounded on one side, or the primary with an output transformer.

A huge complicating factor is the fact that the design and performance of "balanced" inputs varies wildly in high-end audio . . . I would divide them into two "worlds", depending on whether the circuit after the balanced input is balanced differential, or conventional unbalanced. Both have myriad potential design issues.

The main issue with balanced-line-to-differential-circuit input stages is that most of them really offer no common-mode rejection at all, that is, a common-mode voltage on the input translates into a common-mode voltage on the output . . . hopefully (but not always) the common-mode voltage gain is less than the differential mode. The result is that any tiny gain or impedance imbalance within the equipment or cables (and in the following equipment, if it's of similar design) will result in the common-mode (noise) voltage becoming differential-mode (signal) voltage. It's also frequently more suceptible to RF interference than an unbalanced input (there are two input paths), and under no circumstances will the circuit work correctly if fed from an unbalanced source. An input transformer can improve things tremendously on all fronts.

The problem with balanced-line-to-unbalanced-circuit input stages is usually that many of the simpler designs have an impedance balance that's maintained by the open-loop gain of the input circuit, and the critical matching of resistors and circuit trace capacitances . . . and since this is never perfect, the CMRR is poor and usually falls as frequency increases. This can be improved by the buffered "instrumentation opamp" topology, but still all of these approaches almost always result in increased noise over an unbalanced input, as a result of the Johnson noise in the resistors forming the differential subtraction. Here again, a high-quality input transformer almost always performs better, especially because input RFI networks aren't required.

When interfacing with source imbalances or an unbalanced output, CMRR is determined by the ratio of the differential input impedance to the common-mode impedance. In the overwhelming majority of both types of input stages, the common-mode impedance is one-quarter that of the differential-mode impedance, making the impedance balance VERY critical, with very little noise rejection from an unbalanced source. There are two ways of dealing with this . . . raise the common-mode impedance, or lower the differential-mode (signal) impedance. Atmasphere advocates the latter with a 600 ohm terminating resistor . . . the obvious disadvantage is that the overwhelming majority of equipment on the market will perform more poorly into the lower impedance load.

Transformers do the opposite, they raise the common-mode impedance . . . which is why they still work well from an unbalanced source. Input transformers will generally have a higher common-mode impedance than output transformers as a result of the lower leakage reactances mentioned above.