RCA to XLR adapters?

Ralph (Atmasphere), one of the excellent papers at your site describes two of the fundamental advantages of fully balanced internal design, for power amplifiers, as being cancellation (or at least substantial reduction) of some distortion products, and presentation of an essentially constant load to the power supply. Won't these advantages be negated if a single-ended signal is put through a balanced amp?

Also, the benefits that a balanced interface provides in terms of reduced susceptibility to ground loop issues, and rejection of common mode noise, will be negated if an adapter is used, respectively because the signal return path will not be isolated from inter-chassis noise currents, and because of the impedance imbalance between each of the two legs and ground.

The two benefits relating to balanced internal design, I believe, can be retained if you were to choose a fully balanced power amp that has rca as well as xlr input connectors. I would expect that in those cases the input stage into which the rca input is fed would perform a single-ended to balanced conversion, resulting in a balanced signal being processed internally.

All of the benefits described above can be retained if you were to use a Jensen transformer between the preamp and power amp, to perform a single-ended to balanced conversion. See this paper. Note, btw, that the adapter cable shown in Figure 2.1 of the paper is NOT configured in the same manner as an rca cable + rca-to-xlr adapter, and the latter will provide considerably poorer noise rejection than the already degraded noise performance that is shown for the adapter cable.

A lot of users here have reported successfully using Jensen transformers in this kind of application with excellent results and no noticeable adverse effects, although I recall Ralph expressing some mixed feelings about them.

Best regards,
-- Al

As soon as the signal is in the amp, it will be in the balanced domain if the amp is really balanced.

But if one side of the balanced signal path is processing a music signal, and the other side is processing 0 volts (as opposed to a signal that is the inverse of what is being processed by the other side), how will any distortion products be cancelled, and how will the load on the power supply not fluctuate with the music (assuming the amp is not Class A)?

I'm not disagreeing, just questioning something that doesn't seem to make sense to me.

Best regards,
-- Al

Thinking a little further about my previous post, Ralph's point does indeed make perfect sense to me, assuming that the input stage within the amp is what is known as a long-tailed pair differential amplifier. That will produce a balanced output in response to a single-ended input, thereby realizing the benefits of fully balanced internal design in the subsequent stages of the amplifier, including the power stages where it matters most.

I assume that is the architecture used in Ralph's designs. So the question becomes whether or not the input stage of all or at least most other "fully balanced" amps can be expected to have been designed in an equivalent manner. I don't know the answer to that question.

Best regards,
-- Al

Kirk, thanks very much for another of your exceptionally knowledgeable big-picture perspectives, which certainly makes a persuasive case for the conclusion expressed in your last paragraph.

The Cardas adaptors we have seen do not do anything with pin 3

It amazes me how so many manufacturers, both pro-oriented and audiophile-oriented, seem to have it backwards when it comes to rca-to-xlr adapters. Rca to xlr-female adapters, which would be used on xlr outputs, should leave pin 3 unconnected, but almost invariably ground it to pin 1. Rca to xlr-male adapters, which would be used on xlr inputs, should ground pin 3 to pin 1, yet here we have an example of a respected high-end manufacturer leaving pin 3 open.

Best regards,
-- Al

12-16-10: Tmsorosk: I think most of the responders here have missed the Question. Bobheinatz wants to use a single ended ( 2 wire cable ) in a balanced set up , Which will require a 3 wire cable . You can use adapters or reterminate , but you won't get a balanced signal with a 2 wire cable.

The discussion has focused mainly on the EFFECTS of putting an unbalanced signal into a balanced amp, so I don't think the question has been missed.

12-15-10: Kirkus: A common example is this notion of a "balanced amplifier" being simply two amps stuck in a chassis wired to different pins of an XLR connector. In this instance (as you correctly postulated) any voltage imbalance between the two amplifier stages will disturb the necessary null for cancellation of even-order harmonic distortion products. And this voltage imbalance will be affected by any impedance OR voltage imbalances in the preceeding equipment, cables, or the input termination resistances . . . not to mention any differences in gain, distortion performance, or bandwidth between the two amplifier stages.

I think it's also worth noting that using an adapter to put an unbalanced signal into that kind of design would result in up to 75% of the amplifier's power capability being unable to be utilized, since the voltage swing capability between the two output terminals would be cut in half.

Best regards,
-- Al

Mikey, as you've gathered from the posts above there are a number of possibilities, and the internal design of the specific components can be an important and unpredictable variable.

My instinct, though, would be to simply use an rca cable between DAC and preamp, and to use a Jensen transformer between preamp and power amp. I'm sure that a call to Jensen will get you some good advice as to selection of a particular model.

I would not recommend using an adapter between DAC and preamp, in part because as I mentioned above most adapters short the signal on xlr pin 3 to ground (most equipment can tolerate that on its outputs, but some cannot), and in part because I doubt that it would provide any benefit compared to an rca-to-rca connection (aside perhaps from saving you the cost of a new cable).

If you want to try an adapter at the preamp output instead of using a transformer, using the adapter in conjunction with your present xlr cables, here are two low priced examples of the kind of adapters that would be suitable. Cardas and others probably offer higher end counterparts, although keep in mind Ralph's comment above that the Cardas adapters he has seen do not ground xlr pin 3. When adapting an rca output to an xlr input, you DO want pin 3 to be grounded.

Regards,
-- Al

Mikey, yes that's correct, if you want to use your existing xlr cables in both places.

However, I would not do that without first either verifying with the DAC manufacturer that it can tolerate having xlr pin 3 grounded, or obtaining an xlr-female to rca male adapter that you are certain does not connect pin 3 to pin 1 (perhaps from Cardas).

See this thread for an example of a problem that was caused by using an adapter on an xlr output.

Regards,
-- Al

My DAC is a Mark Levinson 36. Do you know if this would give a problem?

Don't know, but I found some specs indicating that its output impedance is "less than 6 ohms." That is worrisome, because it would mean that large amounts of current would be drawn from the output stage by a short to ground, unless some protection mechanism kicked in.

The adapter choices you linked to look good, except that I would suggest that you confirm with Cardas that that particular adapter leaves pin 3 open.

Regards,
-- Al