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Old 22nd October 2006, 03:03 PM   #1
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Default What type coax to use on chipamp?

I need to purchase some coax to add a attenuating potentiomer to my briangt lm3875 chipamp.

My local electronics shop sells coax with about a 1/8 inch diameter by the foot - I will check the type. Can I use any coax, regardless if impedance is 50 ohms, 75 ohms, or 93 ohms?

Which type of thin coax is recommended?

How to wire the grounded shield?
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Old 22nd October 2006, 03:18 PM   #2
dhaen is offline dhaen  Europe
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Assuming the volume pot is mounted in the amp:
Unless you are forced to run the input wiring past the mains transformer, I would not use screened cable at all, just twist the signal and ground together, and take a short path close to the chassis. Tape it onto the chassis if necessary.
Try also to keep it clear of the speaker wiring.


If external:
Cable impedance is a redundant term for small distances at audio frequencies. The most important parameter is pf/metre. A low value is prefered. RF coaxial cable is not ideal due to it's size. Audio screened cable rarely has impedance quoted anyway.
Use the shield as the signal return - from the "bottom" pin of each volume pot to the star earth point or the input ground on the board. I don't know Brian's board intimately...
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Old 22nd October 2006, 04:08 PM   #3
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The pot will be inside the metal cabinet with the 2 briangt amp boards and power supply. The CD player will connect to the amp case with RCA jacks.

Is the wiring shown in this diagram correct? The diagram reflects use of coax. Are you suggesting as an alternative:

1. I twist the 2 signal wires

or

2. Keep the signal wires separate, but use an adiitional wire with each signal wire, twtied with it and grounded as the coax shield is show grounded in the picture?

This will be a 50k Alps blue velvet pot, audio taper, dual.
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Old 22nd October 2006, 04:19 PM   #4
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Or, if twisting wires instead of coax, is this the way to do it?
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Old 22nd October 2006, 04:41 PM   #5
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Here is a diagram of the amp in its enclosure. In this diagram I show the inputs wired as twisted pairs, no coax.

Is this the best way to wire the inputs? Preferable to uising coax?
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Old 22nd October 2006, 04:52 PM   #6
dhaen is offline dhaen  Europe
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Don't twist the 2 channels together, just each individually with it's signal return wire. This is what you have shown on you last diagram of the layout, but you have swapped the volume pot in and out. - the centre pin goes to the amp module using this arrangement.
Keep the wires close to the metal cabinet/chassis and make sure it's grounded.
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Old 22nd October 2006, 05:17 PM   #7
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Cabinet is tied to star ground. Will keep signal wires twisted to signal ground wires close to/taped to metal chassis, away from toroid.

Is this picture correct then?

Could you explain why twisted pair is better than coax? It surely is easier to assemble and I don't have to find coax, but isn't it a compromise since the shielding is compromised?
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Old 22nd October 2006, 06:43 PM   #8
Nordic is offline Nordic  South Africa
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Be sure to keep signal ground isolated from chassis....
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Old 22nd October 2006, 07:01 PM   #9
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OK good point. to keep the signal ground from contacting chassis. Is that to prevent ground loops?

Is the above wiring diagram (the last one) correct?
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Old 23rd October 2006, 01:12 AM   #10
Leolabs is offline Leolabs  Malaysia
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Why dont use just a thin microphone cable???
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