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Old 4th March 2008, 10:18 PM   #1
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Default An interesting interconnect problem

OK - I built a BrianGT gainclone (non-inverted 3886) and I built a pair of short interconnects to link it with a pre-amp. The initial interconnect configuration was two runs of silver wire insulated with cotton in a twisted pair. I use this interconnect throughout my system without any noise problems.

However, when connected with the amp, it produced a mid-audible buzzing. Mid-audible means it was present on normal listening levels - just enough to be annoying. If the interconnect was replaced by any normal double shielded coaxial cable the noise was gone. I tried yesterday adding a shield to the cables - very heavy copper braid. Tried with a single end connection to the ground and both end connection - the results were the same - the buzz was transformed into slight hum but this time barely audible and almost not influenced by the volume level. Although the results are better I still do not understand what is happening here. Is the amp that sensitive to EMI/RFI pickup? Anything that I can to the amp to stop this? Poly-caps at the input RCAs maybe?
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Old 4th March 2008, 11:25 PM   #2
digi01 is offline digi01  China
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it is grounding issue.

check this thread:understanding star grounding,wish helps.

zang
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Old 5th March 2008, 12:11 AM   #3
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I do not think it is a grounding issue. I have no noise with normal double shielded coaxial cable. It is only my home made interconnect that is creating a problem. I only wonder if there is some way of attenuating the RFI input to the amp or I sould change my interconnect.

... BTW my grounding is done as per your schematics including a ground breaker form eliott sound projects.
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Old 5th March 2008, 12:20 PM   #4
AndrewT is offline AndrewT  Scotland
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I suspect that the chipamp does not have any passive band limiting filters on the input to the power amp nor at the RCA input terminals.

I too suspect that the grounding can be improved.

A buzz is more usually associated with misusing the PSU zero volt connection as the audio star ground
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Old 5th March 2008, 02:04 PM   #5
DcibeL is offline DcibeL  Canada
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Sounds to me like your twisted pair is picking up noise from the AC mains. Is the cable running in any proximity to AC mains? Does the amplitude of the hum change when you move the cable around?

My advice would be to stick with the shielded cable, connecting the shield to ground at one end only.
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Old 5th March 2008, 04:27 PM   #6
AndrewT is offline AndrewT  Scotland
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Quote:
Originally posted by DcibeL
Sounds to me like your twisted pair is picking up noise from the AC mains. Is the cable running in any proximity to AC mains? Does the amplitude of the hum change when you move the cable around?

My advice would be to stick with the shielded cable, connecting the shield to ground at one end only.
is that shielded two core or using the shield as the return?
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Old 5th March 2008, 05:06 PM   #7
DcibeL is offline DcibeL  Canada
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Sorry, yes shielded two core. I have found it to be most effective in removing induced noise and hum.
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Old 5th March 2008, 10:31 PM   #8
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Well - first about the cable(s). The power cable goes into the box in the centre - the input cables connect at the far ends of the box. The power cable is shielded.

Inside - the signal star ground is connected to the input of the ground breaker (diode bridge, cap and 10ohm resistor) - exit from the ground breaker connects to the chassis and the mains ground wire. Signal star ground point is formed by connecting to ground wires from the PCBs. I do not have a direct wire connection of the signal grounds to the star point. I was planning to do this at some stage. PSU zero volt is not connected to the star ground.

Signal input cables are twisted pair and shielded (pseudo balanced)- no difference between single or both end shield connection. At least not in the hum presence - probably yes in overall sound quality (have not had any time to check). Shield is only copper braid (very heavy - at least 96% coverage) but without foil shield. Standard dual shield coaxial had no hum issue but the sound was dreadful.

Just to make things even more interesting. Yesterday hum disappeared. Simply nothing - changed levels, inputs - simply not there. On a shorted input of the preamp - simply dead silent.

Any suggestions?
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Old 5th March 2008, 11:29 PM   #9
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hi,how many signal ground was connected to the ground breaker?
i think it is better to connected all RCA plate(passive signal)together and then use a single wire connected it to the ground breaker.otherwise the interconnect cable will collect noise from outside.
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Old 6th March 2008, 03:46 AM   #10
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Well, honestly I do not see the difference apart from the fact that one chanell (I have dual mono configuration) will have longer ground wire path then the other. This in my opinion ruins the symmetry - apart from slight increase in resistance which is probably not an issue. My amp boards are less then 5cm from the input RCAs. So the wires from RCAs to PCBs are very short. On the PCB they are on the common ground plane and then two wires of maybe 15cm in lenght go to the ground breaker.

I will try as you said and see if it makes any difference. Thanks
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