Why Single-Ended?

I'm not knowledgable about how you  configure your circuits but I'm thinking you do not float the signal from input to output so eventually the signals do get referenced to pin one.

We try to avoid referencing signal to ground as much as possible in our preamps as it decreases the Common Mode Rejection Ratio. One area that is tricky though is the volume control. Since two of the five inputs of the preamp are single-ended only line connections (with RCA connectors), the volume control is built with 4 decks (one for each phase of each channel) and it has to have a ground reference to work right with the SE inputs. 

How much potential sonic performance would I lose by using my SE preamp into the unbalanced inputs of a fully differential amp? e.g., my UV-1 preamp into the SE input of an Atma-sphere amp, or using this preamp with a Pass Labs amp.

There’s an impedance difference between fully-balanced and SE inputs, but in what way would sonics be different using unbalanced in a fully-differential design?
Or is it recommended to use a fully-balanced preamp to gain high quality sound?

An addition to my post;
The unbalanced cable runs would be short, 1 to 1.5 meters.

Lowrider, if you were to change from an amp having a single-ended internal signal path to a fully balanced amp I would expect that the differences in the intrinsic sonic characters of the two amps, and perhaps also differences in their interactions with the particular speakers, would most likely greatly overshadow whatever difference may result from providing that amp with an unbalanced vs. a balanced input. Also, my guess would be that finding a fully balanced preamp providing sonics that are as much to your liking as those of the UV-1, for a comparable price, would be a tall order if not impossible.

Regarding your mention of impedance differences, many amps providing unbalanced and balanced inputs connect the center pin of the RCA connector directly to one of the two signal pins on the XLR connector, usually the non-inverted input on XLR pin 2. When the RCA input is used in those cases the unused signal pin on the XLR connector is simply grounded, via either a jumper inserted into the XLR connector or via a switch on the rear panel. And in those cases the doubled impedance that is usually specified for the balanced input, compared to the unbalanced input, simply reflects that the balanced input spec is based on the sum of the input impedances of the two balanced signal lines. So in those cases connecting an unbalanced signal to the XLR input via an adapter, for example, would result in the same input impedance as connecting the unbalanced signal to the RCA input.

Best regards,
-- Al

Al,

"When the RCA input is used in those cases the unused signal pin on the XLR connector is simply grounded, via either a jumper inserted into the XLR connector"

It is correct that some manufactures use a jumper pin from 1 to 3 when a input are configured with two options - XLR and RCA.  IMO there is several issues with this. First one being that pin 1 of a XLR should have Chassis ground (earth) potential - not signal ground which is what the inverting amplifiers input needs connected to when only feeding the non inverting amplifier to get a balanced output. 

Second one being that the non inverting amplifier is connected to ground via the volume control, which means that a varying resistance ( depending on Volume control setting ) is inserted between signal ground and the inverting amplifiers input.

Our preamps are configured so that when a RCA input is selected the inverting amplifiers positive input is connected to signal ground via a relay AFTER the volume control.  

Good Listening


Peter