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Old 23rd August 2012, 06:45 PM   #1
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Join Date: Apr 2012
Location: Peninsula San Francisco
Default Wiring advice for internal pre amp inputs

hi

I have an AGI 511 pre amp, which has rear RCA connectors that need replacing (they don't make proper connection). They are mounted on a daughter circuit board and I have not been able to find suitable replacements for them. The daughter input circuit board is connected to then main PCB by a flat ribbon cable and computer type connectors.

My plan is to replace the daughter PCB with individual quality RCA sockets (which will fit into the existing chassis just fine) and replace the ribbon cable with individual wiring per socket to the PCB- although this is likely to be fiddly, I think it can be done neatly.

My question is what is the best wire to use for this purpose, and I assume using a twisted pair approach ( two individual wires, one signal one signal ground) twisted around each other is the desirable approach. Do these then need to be shielded?

Any thoughts and advice greatly appreciated

Regards

peter
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Old 29th August 2012, 11:19 PM   #2
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In a word, no.
rcw
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Old 29th August 2012, 11:25 PM   #3
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Location: California
Quote:
Originally Posted by rcw666 View Post
In a word, no.
rcw
How about actually posting some useful info for this person, who is just asking for some practical advice. Maybe this was supposed to be funny... ???

-Charlie

Last edited by CharlieLaub; 29th August 2012 at 11:41 PM.
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Old 29th August 2012, 11:32 PM   #4
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Quote:
Originally Posted by pwdiya12 View Post
hi

I have an AGI 511 pre amp, which has rear RCA connectors that need replacing (they don't make proper connection). They are mounted on a daughter circuit board and I have not been able to find suitable replacements for them. The daughter input circuit board is connected to then main PCB by a flat ribbon cable and computer type connectors.

My plan is to replace the daughter PCB with individual quality RCA sockets (which will fit into the existing chassis just fine) and replace the ribbon cable with individual wiring per socket to the PCB- although this is likely to be fiddly, I think it can be done neatly.

My question is what is the best wire to use for this purpose, and I assume using a twisted pair approach ( two individual wires, one signal one signal ground) twisted around each other is the desirable approach. Do these then need to be shielded?

Any thoughts and advice greatly appreciated

Regards

peter
Hi Peter,

I assume that this is a metal-case piece of equipment - if so, you don't need to use shielded cable. Is the mains connection three-prong (grounded) or two-prong (polarized)? Does the small PCB that you are looking to eliminate have any passive or active components on it, or is it only there to hold the RCA jacks? Are the ground wires included in the ribbon cable leading back to the main pcb that you want to replace?

-Charlie
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Old 30th August 2012, 12:33 AM   #5
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This post was sitting there for quite a while with nobody responding to.

I then decided to make a remark of some sort, one that seemed to be the consensus amongst those who had viewed it, and when I do I am chided for being flippant, why didn't you offer helpful advice in the first place Charlie?

Generally if it was wired with a ribbon cable then twisted pairs will do fine, but it depends upon the grounding arrangements used.

If for instance you find a common ground then the original used a single wire for each output, this is a common scheme.

Outputs that are low in signal or are to go into high impedance inputs are often connected with shielded cable in the Faraday shield mode, in this the shield is connected at only one end.

I hope this has been helpful.
rcw
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Old 30th August 2012, 12:54 AM   #6
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Location: Peninsula San Francisco
Hi guys

thanks for the input.....

so it is a metal case (nice and thick and heavy
the ribbon appears to be separate signal and earth for each input, it is built to quite a high quality, and yes there are a few minor components on the PCB which I can attach and then earth appropriately.

the gist of my query is should I use (for example) twisted teflon coated pairs for each input/output or a shielded twisted pair (as I used to use for microphones....like u87's in the days when i made "Records"

If I could find suitable RCA plug replacements I would still need to replace the ribbon cable as it is fraying (the protective coating is coming off)

thanks for reading and replying,
the assistance is appreciated...

Regards

Peter
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Old 30th August 2012, 01:44 AM   #7
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Join Date: Mar 2007
Location: California
Quote:
Originally Posted by pwdiya12 View Post
Hi guys

thanks for the input.....

so it is a metal case (nice and thick and heavy
the ribbon appears to be separate signal and earth for each input, it is built to quite a high quality, and yes there are a few minor components on the PCB which I can attach and then earth appropriately.

the gist of my query is should I use (for example) twisted teflon coated pairs for each input/output or a shielded twisted pair (as I used to use for microphones....like u87's in the days when i made "Records"

If I could find suitable RCA plug replacements I would still need to replace the ribbon cable as it is fraying (the protective coating is coming off)

thanks for reading and replying,
the assistance is appreciated...

Regards

Peter
There is a SoCal company called "ApexJr" that sells surplus electronic parts. The owner is Steve, and he is a really nice guy. ApexJr has a permanent thread in the vendor forum here...

Anyway, he sells pretty hefty RCA jacks for $0.99ea, with only a white stripe on them. I have used these and they are good quality. Check out other RCA jacks here:
untitled

Maybe one of those will work for you. In state means fast shipping.

-Charlie
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Old 30th August 2012, 01:51 AM   #8
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Join Date: Apr 2012
Location: Peninsula San Francisco
thanks charlie...excellent link,

I have been usig Mouser and Antique Electronics and occasionally Jameco
btw if you need any tubes let me know..

thanks

peter
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Old 30th August 2012, 11:25 AM   #9
AndrewT is offline AndrewT  Scotland
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Join Date: Jul 2004
Location: Scottish Borders
I suggest you avoid shielded co-ax inside the chassis.
Twisted pair is usually good enough for all line level signals. Even the lower levels from old analogue equipment.
mic and phono leads, outside the chassis, do benefit from additional interference attenuation, not line level.

Look at the possibility of choosing where to ground the input signal returns.
You can do this at the PCB or at the RCA sockets.
Both Leach and Self advise that the RCA ground option as a way that may avoid unwanted interference.

I have found that in practice the RCA grounded end offers a more universal solution to most equipment coupling options.
This is achieved by keeping the RCAs fairly close together and linking all the Grounding/Returns with a thick copper wire soldered to each.
Then connect a further copper wire and wrap it around the signal pairs until it reaches the PCB (but not attached), then take that wire to the Main Audio Ground.
NOTE !!!!!
the grounding wire/trace must not create a LOOP that could Receive interference from other Transmitters.
__________________
regards Andrew T.

Last edited by AndrewT; 30th August 2012 at 11:33 AM.
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Old 31st August 2012, 12:46 AM   #10
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Default grounding

thanks Andrew, I will follow your suggestions and post pictures of before and after

Regards

Peter
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