grounding for extension cord

V

viktor80

New member
hi

I have 8 channels audio interface and all channels are constantly used. guitar is in one channel and I never disconnect it or switch it to another, same with keyboard and bass etc not from the interface end, not from the instrument end. but two wires is connected to the interface with female end to use it as extension cable for different instruments and this end is open when the instrument is not connected. it makes a noise and also as I know is not recommended keep wire with open end in the input and every time I have to disconnect it from the interface what is very inconvenient. I tried use TRS both mono/stereo adapter just as a cap, to a certain extent it removes the noise but not completely like if you hold the end with hand what makes grounding. so I'm looking for some cap for this jack extension cable female ends if something exists specifically for this, but if not maybe I can do it myself ?
 
viktor80 said:
hi
I have 8 channels audio interface and all channels are constantly used.

guitar is in one channel and I never disconnect it or switch it to another,

same with keyboard and

bass etc not from the interface end, not from the instrument end.

but two wires is connected to the interface with female end to use it as extension cable for different instruments and this end is open when the instrument is not connected.

it makes a noise and also as I know is not recommended keep wire with open end in the input and every time I have to disconnect it from the interface what is very inconvenient.

I tried use TRS both mono/stereo adapter just as a cap, to a certain extent it removes the noise but not completely like if you hold the end with hand what makes grounding. so I'm looking for some cap for this jack extension cable female ends if something exists specifically for this, but if not maybe I can do it myself ?
Click to expand...

ok, now that I have spread out your post so that I can follow what you are saying the solution is somewhat simple

you need to buy a regular 1/4" plug and wire it so that it connects the tip to the sleeve - this will eliminate the open circuit that is causing the hum (actually your extension cable is acting like an antenna and picking up any emf-radio signals in the vicinity)

to be more accurate in what you should do, can you tell me what your interface is?

knowing this will let me lookup it's expected input impedance and design a small circuit that will provide a good reactive load for the input

if you are feeling adventurous, you could build a small box with a switch on it to switch between the socket and load circuit - simple enough to design and build (just need to drill three holes and solder a few wires together)
 
this is for my reference later
Input Impedance
Input Jack 1-6 (XLR type) ~ 5 k ohms (balanced)
Input Jack 7-8 (XLR type) ~ 10 k ohms (balanced)
Input Jack 1-8 (1/4 inch TRS phone type) ~ 17 k ohms (balanced)
this is for my reference later

now a quick question back to you about your current cables?

are they trs at both ends or just ts or a mix?
 
so regardless of whether you have ts or trs sockets it would probably be better to make up two of the same plug (in case they get mixed up accidentally)

need to make the following circuit using two 1.8M resistors (provides needed load for the input channels on any of the input channels) and trs plug

groundingInputWithLoad.png
 
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bandcoach said:
so regardless of whether you have ts or trs sockets it would probably be better to make up two of the same plug (in case they get mixed up accidentally)

need to make the following circuit using two 1.8M resistors (provides needed load for the input channels on any of the input channels) and trs plug

groundingInputWithLoad.png
Click to expand...

great thank you very much

but one question

if i just twist all inner wires together shield braid included, what is a difference between this solutions?

and if I understand correct I can use it for both mono and stereo
 
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viktor80 said:
great thank you very much

but one question

if i just twist all inner wires together shield braid included, what is a difference between this solutions?
Click to expand...

you short the input hot line of the input directly to ground (whilst the circuit may be designed to cope with this (as in having a standard les paul type guitar volume/tone control setup connected to it), it is not the best solution to the problem, as their is some evidence to suggest that there is a load mismatch which can lead to other noise issues

and if I understand correct I can use it for both mono and stereo
Click to expand...

the design is inherently stable for both unbalanced and balanced lines (it is not stereo in this usage, as you are only ever sending a single signal down both lines with signal phase inverted to promote Common Mode noise rejection)
 
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