Why Single-Ended?

Also why do reviewers not "call out" the equipment manufacturers, with "fake" balanced connections?

Many amps, preamps, and sources have fake balanced inputs and outputs In that they just put in an extra opamp in the signal path to create a balanced input or output.
Where their single ended input/output is better as that opamp is then not in the signal path.

Cheers George

DBPHD, for the benefit of others who may respond I’ll mention that the manual for the JC2 BP states that it "uses a fully differential balanced circuit and its balanced outputs do not require or use phase inverters." Also, while as far as I can tell from its literature the JC1 does not appear to be fully balanced, it is described as using a differential input stage. And the manuals for both components recommend balanced connections where possible.

So what you might consider doing, at a cost of a bit under $600 plus some additional cabling, would be inserting Jensen transformers between those components and the inputs and outputs of the KUBE. Or, at a cost of around $300 plus some additional cabling, only between the KUBE and the power amps.

Those would convert the unbalanced signals to or from a true balanced pair of signals. They would provide essentially the same noise reduction benefit and reduced susceptibility to ground loop effects that a well designed balanced interface between components would provide. In this particular case, however, I doubt that it’s possible to predict whether the net result would be an improvement, or little or no difference, or perhaps even a slight loss of transparency. Reports here by users of Jensen transformers have generally been very positive, although a few, including Ralph, have provided comments that are a bit mixed.

Suitable models would be the PI2-XR at the input side of the KUBE, and a pair of model PI-RX located near the inputs of each of the amps. Here is a good supplier, although they can also be ordered directly from Jensen, at a slightly higher price in some cases.

Regards,
-- Al

The Tim Paravicini-designed EAR-Yoshino tube circuits are all single ended, the pre-amps and power amps having transformers on the XLR/balanced inputs and outputs.

Some one please explain to me how an SET amp can be fully differential. I see some SET amps that are offered as balanced. Could one of you explain the circuitry in the amp that makes the use of XLR connections advantageous?

All amplifying devices are differential in nature; a tube or a transistor. This is so since if the signal is the same at both inputs to the device (in the case of a tube, the grid and  the cathode) the device will not amplify. This is because it looks at what is different between its inputs.

Most designers don't take advantage of this or don't realize it, so on nearly all SETs the XLR input is not balanced (although some, like the Viva, have input transformers and so can operate either way). However it should be obvious that it is possible to set up the amplifier to use the other input (which will be the cathode of the input tube) by tying it to pin 3 of the XLR while the grid of the tube is tied to pin 2 of the XLR. This technique is not balanced, but it is certainly differential and retains many of the advantages of balanced operation (such as noise rejection). The cathode input is relatively low impedance and some preamps may not be able to drive it (although ours have no difficulty in this regard).

This technique was originally used by George Philbrick who is generally credited with designing the first practical opamps, which were vacuum-tube (as a side note he was not the inventor of opamps although he often gets credit for that too).  

You can feed a single ended signal into a fully balanced preamplifier, ground the negative (inverting) amplifiers positive input - connect the two negative inputs of both amplifiers (non inverting and inverting) together and a balanced signal will appear at the output of the preamplifier and the signal would be fully balanced from here on.

This is certainly true- we do it with our preamps all the time, but its a simple fact that the preamp is accepting the input as a single-ended signal, with the weakness that the cable becomes part of the sound. The preamp then converts the signal (via its differential operation) to a balanced output. So you can see that in this example that the signal was either single-ended or balanced, but never both at the same time.

Phase has to be inverted somewhere, either for TRUE balanced (XLR cable) or FULLY balanced design.  It involves extra circuitry that does not make sound more transparent.  For short connection, in electrically quiet environment, single ended design might be a better choice.  As for the FULLY balance design - it offers slightly* better common mode electrical noise rejection at very high frequencies and cancellation of even harmonics produced by the amps.  I don't care for both since my connections are short, while shields and twisted pair, in my XLR cable, work really well.  Cancellation of even harmonics, produced by the amp, makes it sounding colder, while odd harmonics, responsible for brightness, are left intact.  Also, FULLY balanced amp has practically two amps inside and costs much more.  I could buy much better single ended amp instead. That's why I would never buy FULLY balanced amp ("Fully" is not always "Better").

* In order to provide good common mode rejection two halves of FULLY balanced amp cannot be independent.  Negative cross-feedback has to be used to equalize gains of each half and that might be far from perfect.