lfleib wrote: You say errors will cancel - what errors? Slight differences between the + and - signals would be cancelled. Couldn't information be lost?
the cancelling of errors that I was referring to, occurs in the differential pair. Any input element (bjt, jfet or valve) has some non-linearity. Now the trick with the differential pair is that what the + branch active element amplifies a pinch too much (for example), is compensated by the brother active element in the - branch that amplifies a bit too little. Now if we only look at the difference between + and - branch of the pair (the principle of a differential amp..), these two distortions nearly cancel each other. And you are right, this happens better if the two active elements are exact twins. In specialist amplifiers you will therefore typically find selected pairs, or dual transistors on the same chip for near perfect symmetry.
By going differential (balanced), some information will indeed be lost. Namely in additional noise: a doubled input circuit means that the effective resistance through which the signal must flow has doubled. Noise voltage scales with square root Resistance, so noise (voltage) goes up by a factor of √2 (= 3 dB). However, no more than that will be lost. So in general, if a balanced amp is adequately designed, you need not fear the loss of information. Actually some of the very best phono stages around are differential. As I mentioned before, I myself am in the process of making a DIY phono stage more or less along the lines of Alan Wright's "RTP 3", which sports a hybrid single stage differential phono circuit for MC. So there you have my preference... By the way: For all of you who want to go deeper into stuff like this and don't hate vacuum valves, Alan has written a phono-minded and good book called "The Tube Preamp Cookbook". It's nicely accessable for the non-technical, and you are in for a good laugh now and then. Available through his enterprise "Vacuum State Electronics".
lfleib wrote: How about resolution, as opposed to linearity?
Resolution of a differential circuit will be at least as good as in a SE circuit. Due to the prabably better linearity of the balanced circuit, one could argue that also its resolving power will be higher. My guess is that fewer by-products (distortion) in balanced operation will give a more complete "unveiled" hearing into the deep down low-level information in the music signal. Of course, this is possible only if the details stay above the noise floor. Therefore the designer must do his/her very best to keep noise as much as possible down in a balanced phono design. The differential input phono circuit is THE area in audio design where special €€ components like dual transistors and non-inductive wire wound or 'bulk metal foil" resistors make sense.
lfleib wrote: When you say "with equal degeneration and bias of the input", equal to unbalanced? This doesn't make sense to me. If the transmission and reception of the signals (+ and -) are not perfectly equal, then the degeneration or cancellation is greater with a balanced input.
Degeneration is an electronics expression for a resistor in series with the 'output' terminal (emitter, source or cathode) of the active element (bipolar junction transistor, jFET or valve respectively). This is often done to tune the 'current out' versus 'voltage in' characteristics of that element. Doing so will add noise (hence the name "degenerate") but improves linearity. Bias, on the other hand, is the term used to describe the voltage and current work-point at which the active element does its job. So, indeed, when I was mentioning "equal degeneration and bias" in a previous post, I meant that the operating conditions for the transistors/valves in balanced respectively unbalanced circuit should be equal, to have a meaningful comparison. BTW In the beautiful under the bonnet pictures of tresaino's amp you see some LEDs used, quite probably, for generating super low-noise bias voltages for some transistors... Bravo, Tresaino!
To conclude some thoughts on the connections and wires. Any noise (hum, radio interference...), or distortion (non-linear contact resistance in connectors, non-perfect dielectrics of insulators,....) introduced before the phono-preamp input can by NO means be separated from the signal itself. That is why the connection between cartridge and phono stage is of so great importance. It is not difficult to do it right, but often some small points are missed....