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Advice on RCA ground on vintage ARC preamp
Advice on RCA ground on vintage ARC preamp
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Old 23rd April 2018, 10:07 PM   #1
oharchie is offline oharchie
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Default Advice on RCA ground on vintage ARC preamp

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.

Last edited by oharchie; 23rd April 2018 at 10:41 PM.
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Old 23rd April 2018, 10:41 PM   #2
rayma is offline rayma  United States
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Quote:
Originally Posted by oharchie View Post
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.
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?

Last edited by rayma; 23rd April 2018 at 10:48 PM.
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Old 23rd April 2018, 10:51 PM   #3
oharchie is offline oharchie
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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.
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Old 24th April 2018, 12:20 AM   #4
Galu is offline Galu  Scotland
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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.
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Old 24th April 2018, 01:22 AM   #5
Galu is offline Galu  Scotland
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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!
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Old 24th April 2018, 01:54 AM   #6
PRR is online now PRR  United States
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Quote:
Originally Posted by oharchie View Post
...... wouldn't this unite the ground connection of all inputs and outputs and isn't that bad?....
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.
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Old 24th April 2018, 09:21 AM   #7
DF96 is offline DF96  England
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The important thing about grounding is that it needs to be designed. Modifying it only helps when the original design was wrong, or not done at all. Following recipes can work some of the time, but not as good as actual design.
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Old 25th April 2018, 01:17 AM   #8
Speedskater is offline Speedskater  United States
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Quote:
Originally Posted by rayma View Post
Audio jacks are normally isolated from the chassis. ..........................................
Neil Muncy (RIP), Henry Ott, Jim Brown, Bill Whitlock and others, would say otherwise.
__________________
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Old 25th April 2018, 01:31 PM   #9
rmaudio is offline rmaudio  United Kingdom
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Quote:
Originally Posted by Speedskater View Post
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.
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Old 25th April 2018, 01:32 PM   #10
rmaudio is offline rmaudio  United Kingdom
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Quote:
Originally Posted by DF96 View Post
The important thing about grounding is that it needs to be designed. Modifying it only helps when the original design was wrong, or not done at all. Following recipes can work some of the time, but not as good as actual design.
+1
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