Dedicated Line Noise Filter?

Thanks for the nice words, Lowrider.

Given that when only the preamp and power amp are connected there is a loud hum when a cheater plug is not used and a lower but still significant hum when a cheater plug is used, it sounds like two problems are present. One being a ground loop between preamp and power amp, and the other probably being due to something internal to the preamp, such as some of the possibilities Davehrab mentioned.

I say "probably" because it is also conceivable that what you are hearing when only the preamp and power amp are connected is the result of having the preamp’s high impedance inputs left unconnected and essentially "floating." Unconnected high impedance inputs often tend to be susceptible to pickup of hum and noise, that may even be coupled into them from elsewhere within the same component. Which would have no relevance to what would occur when a powered up source component having reasonably low output impedance is connected. And if you also hear a low level hum when a powered up source component is connected, perhaps that is due to a ground loop between source component and preamp. (Although if I recall correctly you may have sorted out that possibility some time ago with a Jensen transformer and/or a cheater plug).

But to assure that assessments that are performed with unconnected inputs are meaningful it would be desirable to purchase some RCA shorting plugs. A search for "RCA shorting plugs" at eBay will return many sellers offering those at low prices.

Finally, regarding the possibility of an internal problem in the preamp, the next time you are in contact with Ralph (I recall that you’ve discussed this issue with him) you might ask if the UV-1 uses AC or DC on its tube filaments. While I suspect the answer will be DC, if perchance AC is being used I would speculate that a tube problem may be contributing, unless you've eliminated that possibility via tube substitutions.

Good luck. Best regards,
-- Al

I asked the electrician about code, and he said it meets code and this is how he wired his house for a high-end HT system. He said not having a ground wire back to my circuit box containing old, poor quality wiring would cut down on noise.

We'll see what Jim (Jea48) says, as the expert on these matters, but that doesn't sound right to me.  I assume neutral and safety ground are not connected together at the sub-panel, which would be a definite code violation and would create numerous potential safety hazards, and possibly noise problems as well.  Given that assumption, the main purpose of a safety ground is defeated by the approach you've described.  Especially if there is a significant distance between the ground rods of the two panels, and/or the soil in between them is dry.

If a short to chassis were to develop in anything that is plugged into those dedicated lines, that is not "double-insulated" and therefore utilizes safety ground, the path for the resulting fault current would include the soil between the ground rods of the two panels.  The corresponding resistance will likely be high enough to limit the current to too low a value to trip the breaker in either panel, resulting in a shock hazard.

Best regards,
-- Al
  

The sub-panel is connected by the hot lead and has it’s own ground including a second grounding rod.

Lowrider, my understanding is that a sub-panel should have hot, neutral, and safety ground all wired back to the main service panel, and safety ground (which may or may not be connected from the sub-panel to earth) should only be connected to neutral at the main panel, and NOT at the sub-panel. (Jim/Jea48, correct me if I’m wrong about any of that). Is that how your sub-panel has been wired?

More relevant to your issue, though, have you tried powering the entire system from just one of the dedicated lines coming from the sub-panel? I realize that installing the sub-panel and using its two dedicated lines lowered the noise floor substantially. But using separate dedicated lines for digital and analog components, while certainly beneficial in many cases, in some cases can cause or contribute to ground-loop related noise. And perhaps the improvement the sub-panel installation provided occurred mainly as a result of the new wiring, rather than having separate dedicated lines for digital and analog. Especially if the type of wiring was changed when the sub-panel was installed, as there are several ways in which wiring type can affect noise issues.

Best regards,
-- Al