How can the channels bleed into one another when the entire layout is dual mono?
The answer, IMO, is that the design is not truly dual mono. If you look at the photos of the preamp at the PL site, especially the photo of the underside, the following things can be seen:
1) The input jacks for both channels are located close to one another, on one side of the preamp.
2) Those jacks appear likely to be wired to a single small printed circuit board, serving both channels. That isn’t entirely clear in the photo, however.
3) There is a single volume control, serving both channels.
4) There is a single input selector switch, serving both channels.
5) The output jacks for the two channels are located close to one another.
6) As a consequence of the foregoing factors several wires can be seen running from one side of the preamp to the other. And in one case, near the control at the top left of the photo of the underside, it appears that wires associated with both channels are physically bundled together. That is good with respect to the neatness and appearance of the underside, but bad with respect to crosstalk.
7) A printed circuit board in the power supply section, the board located toward the rear of the amp that among other things has four large black capacitors on it, serves both channels. Its relatively small size means that circuit points on it that are related to both channels are in close proximity. Also, ground and power "planes" that are probably within that board probably serve both channels.
Undoubtedly there are more such examples. And all of these things create opportunities for crosstalk between the channels, especially at high frequencies.
In any event, though, I agree with the others that 50 db of channel separation at 20 kHz (and >77 db below 1 kHz, per Stereophile’s measurements) is not in itself a significant issue.
Regards,
-- Al
