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Old 2nd November 2008, 03:11 PM   #11
N1ESE is offline N1ESE  United States
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Dumb Question #1 - Should I use shielded twisted pair between the volume pot and the circuit board and RCA jacks?
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Old 2nd November 2008, 05:22 PM   #12
w5jag is offline w5jag  United States
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I just use RG-174/U. That may not be the best choice, but I have a lot of it on hand.

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Old 2nd November 2008, 09:26 PM   #13
n_maher is offline n_maher  United States
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Quote:
Originally posted by N1ESE
Dumb Question #1 - Should I use shielded twisted pair between the volume pot and the circuit board and RCA jacks?

It's hardly a bad idea and I don't see how it can hurt. I know I'll be using shielded wire from at least the inputs to the pot since there's no room in my chassis to use an extension shaft to shorten the signal path.
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Old 2nd November 2008, 10:36 PM   #14
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I used to put shielded wire in between the input jacks and the "signal in" connections on the circuit boards. I did this on a couple of amplifiers, and I think these guys have a slight rolloff on the top end. I wonder if the shield acts as a small value cap in parallel with the wire. Ground one end of the shield, and you might end shorting some high frequency content to ground.

I've since started using just plain unshielded hookup wire and keeping the runs as short as possible. I've noticed improved high frequency response in my recent builds. I need to go back and rework my older amplifiers to finally prove my theory.
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Old 3rd November 2008, 12:28 AM   #15
N1ESE is offline N1ESE  United States
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Quote:
Originally posted by Ty_Bower
[B]I used to put shielded wire in between the input jacks and the "signal in" connections on the circuit boards. I did this on a couple of amplifiers, and I think these guys have a slight rolloff on the top end. I wonder if the shield acts as a small value cap in parallel with the wire. Ground one end of the shield, and you might end shorting some high frequency content to ground.
Yeah, I was wondering a bit about this phenomenon as I've experienced similar stray capacitance with some low power solid-state amplifiers for radio use in the past.

This is primarily going to be hooked up to my iPod and/or computer. I'm just going to put the input connector, 1/8th-inch stereo jack, on the front panel near the pot. Eventually, I will add an input switch and second RCA inputs on the back.
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Old 3rd November 2008, 02:31 AM   #16
n_maher is offline n_maher  United States
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Quote:
Originally posted by Ty_Bower
I wonder if the shield acts as a small value cap in parallel with the wire. Ground one end of the shield, and you might end shorting some high frequency content to ground.
Can you elaborate on this Ty? The way that I've always been taught to use the shield in shielded wire is to ground only one end of the shield and float the other. I don't see, in that case, how the shield could be in parallel with anything acting as a capacitor. And if this were the case, wouldn't you think that people would complain a lot more about shielded IC's since they would tend to behave in exactly the same manner? And wouldn't the signal loss be measurable if it's audible? But my skills lie on the fabrication side of these things, not the technical, so it's quite possible I'm just not seeing it correctly.
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Old 3rd November 2008, 02:50 AM   #17
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Quote:
Originally posted by n_maher
Can you elaborate on this Ty?
I wish I could. I'm probably hooking up the shielded wire wrong. All I know is my amps with shielded wire on the inputs are just a little too soft on the top end. The amps where I use just plain hookup wire sound fine. I keep the runs short (two inches or less, usually) and I haven't had problems. It's not like these are phono level signals anyway.

Maybe I did something dumb like use single conductor shielded wire, and I had to use the shield as the ground conductor? I dunno. As I said, some day I need to go back to those amps and figure it out.
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Old 3rd November 2008, 04:33 PM   #18
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Ok, how's this?

Click the image to open in full size.

On the top I've got a two conductor, shielded hookup cable. The shield is grounded at only one end. Isn't the shield effectively one plate of a capacitor, and the conductor is the other plate? Are my two drawings electrically equivalent?
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Old 3rd November 2008, 04:50 PM   #19
w5jag is offline w5jag  United States
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The capacitance value of quality cable should be specified. It's 31 pF/foot in the case of RG-174/U.

For the run from the input jacks, I have used some crummy "shielded" audio cable that had only about 10% shield coverage, and RG-174/U with about 87% shield, and can't hear any difference, which is to be expected, I guess, since the interconnects running to the amp are just unshielded parallel conductors, of who knows what specification.

I only ground the shield at one end and in the Simple SE, that is at the pcb. Whether it's audio or RF, grounding both ends of a shielded cable invites a ground loop.

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Old 3rd November 2008, 05:11 PM   #20
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31 pF isn't going to do anything in the audible range. I guess my theory is full of it.

Click the image to open in full size.
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