Yes, that's all pretty vexing.
An intermittent phono cable connection could conceivably affect the noise floor in several ways, although one of those ways would affect it in the opposite direction from what you observed.
First, if the connection opens up, the low impedance of the cartridge coil would no longer be connected across the phono stage input. Under normal conditions, the low impedance of the coil would tend to "short out" noise that would otherwise be generated or introduced at that point, and be amplified by everything downstream. That is the opposite of what you observed, though.
Second, if the cable has an intermittent short, when the short is present it would tend to kill noise as well as signal at the phono stage input.
Third, if the cable has an intermittent open, noise that may be picked up by the cabling "ahead" of the point where the problem exists would not make its way into the phono stage when the open is present.
Given that you are not yet 100% certain that the problem follows a channel swap at the phono stage input, though, I would also be suspicious of the possibility that a mute circuit in a downstream component may have an intermittent problem. If so, given that you've already changed phono stages, the line stage would seem to be the likely culprit.
As to why a "pop" type signal might have affected a marginal connection, I suppose the answer would have to be mechanical vibration resulting from the sound coming from the one speaker that was functioning at the time.
Also, I see that your turntable is located such that heat from your tube equipment will raise its surrounding temperature significantly. Perhaps that is why the problem only appears near the start of a listening session. Eventually the temperature rise fixes the marginal connection, or whatever else is causing the problem.
Finally, it may be helpful to use a multimeter to perform checks for continuity between the rca plug at the phono stage end of the cable and the corresponding headshell leads, and also for the possibility of a short between the signal pin and the ground sleeve of the rca plug, while jiggling various points on the cabling. DO NOT, however, use a multimeter while the headshell leads are connected to the cartridge, as you don't want the test voltage supplied by the meter to be applied to the cartridge.
Hope that helps,
-- Al
An intermittent phono cable connection could conceivably affect the noise floor in several ways, although one of those ways would affect it in the opposite direction from what you observed.
First, if the connection opens up, the low impedance of the cartridge coil would no longer be connected across the phono stage input. Under normal conditions, the low impedance of the coil would tend to "short out" noise that would otherwise be generated or introduced at that point, and be amplified by everything downstream. That is the opposite of what you observed, though.
Second, if the cable has an intermittent short, when the short is present it would tend to kill noise as well as signal at the phono stage input.
Third, if the cable has an intermittent open, noise that may be picked up by the cabling "ahead" of the point where the problem exists would not make its way into the phono stage when the open is present.
Given that you are not yet 100% certain that the problem follows a channel swap at the phono stage input, though, I would also be suspicious of the possibility that a mute circuit in a downstream component may have an intermittent problem. If so, given that you've already changed phono stages, the line stage would seem to be the likely culprit.
As to why a "pop" type signal might have affected a marginal connection, I suppose the answer would have to be mechanical vibration resulting from the sound coming from the one speaker that was functioning at the time.
Also, I see that your turntable is located such that heat from your tube equipment will raise its surrounding temperature significantly. Perhaps that is why the problem only appears near the start of a listening session. Eventually the temperature rise fixes the marginal connection, or whatever else is causing the problem.
Finally, it may be helpful to use a multimeter to perform checks for continuity between the rca plug at the phono stage end of the cable and the corresponding headshell leads, and also for the possibility of a short between the signal pin and the ground sleeve of the rca plug, while jiggling various points on the cabling. DO NOT, however, use a multimeter while the headshell leads are connected to the cartridge, as you don't want the test voltage supplied by the meter to be applied to the cartridge.
Hope that helps,
-- Al