I answered most of your question here. Concerning why manufacturers would do this, cost is certainly a major factor.
You still get benefit of common mode noise rejection using balanced cables even if amplifier is not fully differential. You also get benefit of better, safer locking connectors. Balanced cables are better, IMHO, but it is matter of your environment (amount of electrical interference) and design of an amp. If amp is designed such way that extra output and input stage can compromise the signal, as Almarg explained recently, then you might get better results with single ended (RCA) cables. Balanced/differential input is often converted to single ended signal (ground referenced) by input transformer. This transformer does not improve sound and is known to produce harmonic distortion at low frequencies. Small distortions at low frequency are not very audible (many great amps have transformers at the input) but still it is a compromise.
Fully balanced design is better but costs more. Common mode noise rejection improves a bit, even harmonics produced by an amp are canceled, power supply gets balanced load (no big ground return current in power amp), each half requires half of gain and the same output power can be obtained from twice smaller supply voltage (bridged output). There might be some issues of gain or phase shift matching so it would be interesting to hear from the amp designers.
Thanks for responding again. I started this thread in the Tech Talk group because I thought it was the more appropriate category for it and I wasn't generating any responses under the Amp/Preamp section. I tried to have the Amp/Preamp thread moved to the Tech Talk category but Audiogon said they could not do that. Wasn't trying to be redundant.
As to cost for fully balanced. I had thought that was the issue but recently have seen some pretty reasonably priced units that are fully balanced. In my case, I do get way better sound by using the XLR inputs and outputs. Was just interested in what I am missing out on. If the incremental improvement in sound in not substantial enough, I can stop obsessing over this!
Well I'm not a manufacturer, but you can still take my comments with a grain of salt. I have heard AND owned fully balanced setups. I am now back using single-ended, so obviously, there IS going back. The main reason for going back to single-ended designs was economic reasons.
That being said, my current system is still very quiet and revealing. I will admit that the noise floor using a fully balanced system was lower, but once you factor in all the other noises in a real world listening enviornment, I can't say that I'm suffering.
My current system is about 20% of the cost of my old big rig days, and to be honest, I'm embarassed by how close it is sonically. To think....all those years of what I was sure were "upgrades"... Sure makes me wonder what I spent so much time and money obsessing over.
Tube/SS, Vinyl/Digital, Balanced/SE, just relax, have fun and enjoy the music.
My preamp manufacturer mentioned to me that "RCA" is much more "musical" than XLR, even though he provided XLRs on the preamp.
For me it was an issue of "gain" in my system. XLR provided me with the necessary gain (twice the volts over the + and i terminals) and I am pretty happy with it. This is from a very recent upgrade from RCA to XLR between the preamp and amp.
Roscoe50, the best way of interfacing a balanced output to an unbalanced input is most likely with a Jensen transformer, as explained in section 3 of this paper. Examples of those transformers are shown near the bottom of this page. They cost around $200. If you want to pursue that approach, give them a call to obtain a recommendation of a specific model.
A less expensive alternative would be an ART DTI, specs shown here, although I can't vouch for its sonic quality.
Another alternative would be to have an adapter cable made up by a cable vendor, that connects XLR pin 2 to the RCA center conductor, and XLR pin 1 to the RCA ground sleeve. In theory a Jensen transformer is a better approach, but as a practical matter an adapter cable MIGHT provide results that are just as good.
The least preferable approach, IMO, although it still might provide satisfactory results, would be to use an adapter plus a separate cable, as you seem to be considering. If you want to try that I think it would probably be best to use an XLR-female to RCA-female adapter at the output of the Fostex DAT recorder, with an RCA cable connected from there to the Denon receiver.
The adapter cable approach and the adapter + cable approach will not provide any of the potential benefits of a balanced interface, regardless of the type of cable that is used, although as I say that might not make any difference in your particular setup.
Be aware, also, that most XLR to RCA adapters short XLR pin 3 (which is usually the inverted signal in the balanced signal pair) to ground (XLR pin 1). I suspect that won't cause any problems with your pro-oriented DAT recorder, but if you pursue that approach you might want to consider purchasing an adapter made by Cardas, which as I understand it do not short those pins together, at least on XLR-female to RCA-female adapters. Pin 3 SHOULD be shorted to pin 1, btw, when adapting an RCA output to an XLR input.
Al, I think I know a better way than that, but it requires a mod to the amplifier. But it is a very simple mod!
Any amplifying device (tube or transistor) has two inputs. On a tube this would be the grid or the cathode. Taking that a step further, you can drive both on a single-ended amplifier. I have done this a lot in the last several years. You just set up the cathode circuit of the amp to accept a signal (add a coupling cap, equal to the value of the cathode bypass cap; if the amp has a cathode bypass cap you use that).
For a tube amp it literally is that easy. With transistors its a little trickier but still easy. IMO this sounds better than a transformer and the CMRR is quite high.