I upgraded several of my RCA inputs and added new connecting wires on my heavily modified ARC SP4 pre amp. The old ones were getting old and I was given the advice by an ARC engineer to add better wire between the board and RCA jacks. He provided me with some nice litz wire for the install.
I purchased some cardas RCA female jacks and they come with plastic insulator rings that go between the jack and the chassis. I drilled new holes and installed the rings on both sides... only to discover a terrible hum from the phono stage when put back in service. On closer inspection I noticed that none of the factory installed inputs use anything between the jack and the chassis. Metal to metal on both sides of the chassis on each input and output.
I removed the inner insulator rings from the phono inputs and the hum went away.
But... wouldn't this unite the ground connection of all inputs and outputs and isn't that bad? Since I installed the rings on several new inputs I am reluctant to redo the work to remove the rings until I know which is best, or at least why this is ok.
I may have been reading some incorrect things on other forums so I decided to post here to get it straight. Forgive me if this has been asked time and time again. I searched but is not easy to find a direct match to this topic.
I purchased some cardas RCA female jacks and they come with plastic insulator rings that go between the jack and the chassis. I drilled new holes and installed the rings on both sides... only to discover a terrible hum from the phono stage when put back in service. On closer inspection I noticed that none of the factory installed inputs use anything between the jack and the chassis. Metal to metal on both sides of the chassis on each input and output.
I removed the inner insulator rings from the phono inputs and the hum went away.
But... wouldn't this unite the ground connection of all inputs and outputs and isn't that bad? Since I installed the rings on several new inputs I am reluctant to redo the work to remove the rings until I know which is best, or at least why this is ok.
I may have been reading some incorrect things on other forums so I decided to post here to get it straight. Forgive me if this has been asked time and time again. I searched but is not easy to find a direct match to this topic.
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Audio jacks are normally isolated from the chassis. Anodized aluminum is not conductive
unless the surface oxide is removed, but maybe it was on the inside. Could someone have
removed the isolators before you owned the SP-4?
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No, these are definitely original jacks. The chassis exterior is black anodized aluminum so that makes sense. It's possible the interior is as well but it's harder to tell because it is silver.
Actually that makes a lot of sense as the 3 phono inputs on this preamp (yes 3!) all have an additional inside plate wired to ground.
Since the new main OUT jacks seem fine with the insulator rings I will leave them. I did not know that anodized aluminum is not conductive. Thanks, this is good to know!
The preamp typically plays dead silent so any new hum was bothersome.
Actually that makes a lot of sense as the 3 phono inputs on this preamp (yes 3!) all have an additional inside plate wired to ground.
Since the new main OUT jacks seem fine with the insulator rings I will leave them. I did not know that anodized aluminum is not conductive. Thanks, this is good to know!
The preamp typically plays dead silent so any new hum was bothersome.
Looks like the chassis is intended to be the common ground connection for all the inputs. In that case the barrels of the replacement input jacks should make metal to metal contact with the chassis. If reverting to the factory installed scenario gives a hum free result then all is well. As for whether the grounds of the inputs and outputs are united, perhaps someone else can comment on this after referring to the circuit diagrams in the Audio Research SP4 User Manual.
I formulated my reply before reading that it was an anodised aluminium chassis and that the input jacks have an additional 'inside plate' wired to ground. Looking at the circuit board layout (page 14 of manual), I can see that the input grounds are commoned, not by the chassis, but by a copper ground track on the circuit board itself. Anyway, silence is golden!
Yes, and that's not bad, on something the size of a preamp, if that is how The Designer wanted to do it. Then (presumably) all the rest of the circuitry references that ground-plane.
Relying on anodize Aluminum is not a long-term good idea; Galu says the actual connection is to Copper, which is fine.
Neil Muncy (RIP), Henry Ott, Jim Brown, Bill Whitlock and others, would say otherwise.
Yes but we are dealing with unbalanced 'single wire' connections here and there are opposing considerations.
basically do you want to treat the cable 'screen' as screen or signal - it has both functions here.
An Anodised surface on aluminium will indeed normally be non conductive.
But it is possible to get similar finishes that are conductive.
Scraping the oxide layer off works but may be subject to further oxidisation with time if the connection to it is not 'gas tight'.
If the connection is made via the plates and Copper rod / bar then I don't understand why the insulating washers made a difference...or maybe it's a question of the chassis being grounded via the connectors rather than the other way around ?
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