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Old 17th August 2005, 01:08 PM   #1
Sherman is offline Sherman  United States
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Question Multiple Inputs in One Shielded Cable?

I'm building a stereo SE EL84 amp with a pair of inputs near the back. The input tube is at the front so I'll run the signal up there in shielded cable. I did the same thing in my KT88 monoblocks and it works great. No radio stations or hum even with several inches of wire between the jack and the tube.

So now with stereo I have two jacks at the rear. I have some five conductor shielded cable. So- am I likely to run into problems if I put both inputs in one cable for the run up front? I'll have one wire unused in the cable? I haven't pulled the wires out to check but they appear to be gently twisted, maybe one twist every two inches.

I may just give it a try since I'm at the point of soldering it up but if there are problems it may help to know what to rip out first!
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Old 17th August 2005, 02:18 PM   #2
kmtang is offline kmtang  Canada
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The individual wires should be shield separately as well. If not, it will cause cross talk between the wires.


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Old 17th August 2005, 02:27 PM   #3
Sherman is offline Sherman  United States
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Quote:
Originally posted by kmtang
The individual wires should be shield separately as well. If not, it will cause cross talk between the wires.


Johnny
I thought that might be the case. How significant an issue do you think the cross-talk would be? I do have plenty of two conductor shielded cable so I could run two separate lines easily.

This amp is a learning experience for me. I've now built kits, a couple scratch-builts using schematics from others and now this with my own schematic. I don't expect it to be perfect but I would like to avoid gross mistakes (and explosions/fire/electrocution etc.).
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Old 17th August 2005, 02:40 PM   #4
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It should work okay,but you may have a bit more "cross-talk" between channels,as opposed to using a single shielded conductor for each channel.
It may or may not be a problem. Give it a try.
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Old 17th August 2005, 02:44 PM   #5
PRR is offline PRR  United States
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> How significant an issue do you think the cross-talk would be?

Depends entirely on the source impedance.

In this case, I would just use two cables.

FWIW, I do have installations running hundreds of feet of cable, some of it not even shielded. But the source impedance is very low and the source separation is low to begin with. I did the math at the time and was happy for that situation. But I don't think I would do it that way inside a hi-fi amp if I could afford and fit two cables (one per signal).
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Old 17th August 2005, 05:26 PM   #6
Sherman is offline Sherman  United States
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Quote:
Originally posted by PRR
> [i]...In this case, I would just use two cables...

Thanks for the reply! I've decided to go with 2 cables; easy to do and eliminates one potential source of noise problems up front.

One of these days though I think I'd like to build an amp with a lot of stuff done "wrong". Then test it and make one change I'll probably never get to though.
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Old 17th August 2005, 07:38 PM   #7
PRR is offline PRR  United States
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> I'd like to build an amp with a lot of stuff done "wrong".

I've done that. It can be fun. Many sacred cows don't stand up to test.

Still, on a good all-round power amp, two hunks of coax is cheap and ensures total separation.
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Old 17th August 2005, 10:33 PM   #8
amperex is offline amperex  United States
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Default Crosstalk

The wires next to one another act as a low value PF capacitor. They will pass high frequency noise plus the higher registers of the audio. The stereo seperation will be limited & in general sound poor compared to seperate cables for each channel.
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Old 18th August 2005, 01:21 PM   #9
Sherman is offline Sherman  United States
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Well I started soldering yesterday and went with 2 separate shielded cables for the inputs. Thanks to everyone for the advice. I think it was one of those questions where I really knew what I should do but needed confirmation.
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