Why does unplugging/replugging TT leads from tube phono pre-amp reset dead channel?

Jmfawdofile 12-22-2017
Suggestions on loading to address this issue? It sounds fine as it’s currently set.

Joe, the main reason I suggested that you determine what the loading is set to is that it might help in determining the cause of the problem. I’m doubtful that any change you might make to the setting would constitute a solution, although I suppose it’s possible.

My original point regarding loading was that when you disconnect and reconnect the cables you are not only physically manipulating the cables and connectors, but in addition when the cables are disconnected you are temporarily changing the impedance presented to the input circuit of the phono stage from a very low value (essentially the cartridge’s impedance) to a significantly higher value (whatever the loading is set to). And it seems conceivable to me that doing so might be allowing the input stage to recover from some abnormal condition.

As you can see in the manual resistive loading is set at the factory to 47K, but DIP switches are provided in the unit which can put resistors of 100 ohms, 1K, and 10K in parallel with the 47K, in any combination. There is also provision for paralleling a resistor supplied by the user, which a previous owner may have done if you are not the original purchaser.

So the overall load resistance may be somewhat less than 100 ohms, or it can be as high as 47K, or it can be at various settings in between. If it is at the low end of that range, the likelihood that my hypothesis accounts for the problem is minimal, since whether or not the cables are connected would make a relatively small difference in the impedance presented to the input stage. If it is at the other extreme, 47K, the likelihood of my hypothesis being relevant would be significantly greater.

But to answer your question, if there is any possibility that changing the loading might be a solution, my guess is that the 47K setting would stand the best chance, if it is not already set that way. If it is not already set that way, changing to 47K would change the impedance presented to the input stage during normal operation in the same direction (i.e., making it higher) as disconnecting the cables has been doing, albeit to a smaller degree.

Good luck. Regards,
-- Al

Lew, not sure you saw that shortly before you posted the OP indicated that he tried connecting the RCA plugs to the BAT’s XLR input connectors via adapters, with the problem nevertheless presenting itself subsequently.

Joe, no problem re not having done the measurements I suggested. I made that suggestion mostly because I thought it might provide me with some insight into how the input circuit is implemented, which in turn might have triggered further ideas.

At this point, if you haven’t already done so I would suggest that while music is playing you very gently wiggle and tug on the cables, near the turntable end as well as at the phono stage end. If that doesn’t precipitate a dropout, and given that the problem has occurred with both the RCA and XLR inputs, I would have to think that the cables and connectors are exonerated, and the culprit is something in the circuitry of the phono stage. As to specifically what that may be, I’m at a loss at this point.

Per one of my earlier suggestions, though, it might prove useful to know what the resistive loading is set to, since as I had mentioned the act of disconnecting the turntable cables changes the impedance presented to the input stage from essentially the cartridge’s impedance to whatever value the loading is set to. I realize that in order to determine that you would have to open up the phono stage. But who knows, when you do that you might spot a burned resistor, a leaking capacitor, or some other visual evidence of something that might account for the problem!

Regards,
-- Al

Thanks for the additional info.  It doesn't sound like a grounding issue, as there's no hum involved.

If by any chance you have a multimeter, it might be helpful if you could perform the following experiment:  Remove one of the XLR shorting plugs.  On that plug, measure the resistances between pins 1 and 2; between pins 1 and 3; and between pins 2 and 3.

Depending on whether all three pins are shorted together, or just pins 1 and 3 (or possibly 1 and 2) are shorted together, or if there is some significant resistance between the pins, I might have some further ideas.  Such as perhaps trying the XLR inputs using RCA-to-XLR adapters.

If you don't have a multimeter, you might ask BAT if using an adapter, such as this one, would be ok.  Like most such adapters, it connects XLR pins 1 and 3 and the RCA ground shell together, while connecting the RCA center pin to XLR pin 2.

Good luck.  Regards,
-- Al
 

Layniesdad83, thanks, but both the description and the manual of the VK P-10SE, as well as various rear panel photos that can be found online, indicate that it provides RCA as well as XLR inputs.  And the manual states the following in bold-face type:

When connecting your cartridge to the VK-P10SE RCA inputs, it is necessary to install the supplied shorting plugs into the unused XLR inputs.

Regards,
-- Al

Some questions, that might be relevant if the problem turns out to not be a connection intermittency as the others have suggested:

1)Assuming the turntable is connected via RCAs, are the XLR input shorting plugs that were originally supplied with the unit installed?

2)What do you have the resistive loading set to? The reason I ask is that when you disconnect one of the turntable cables you are changing the impedance that is presented to the input stage for that channel from a low value (essentially the cartridge’s impedance) to a higher value (whatever the input loading is set to).  And perhaps that change is allowing the input stage to recover from some abnormal condition.

3)When the dropouts occur, do you hear a relay clicking?

4)When you said "it actually is out when the system powers up" are you referring to before or after the approximately 45 seconds following turn-on during which the phono stage is designed to mute its output?

Regards,
-- Al