Braided vs shielded wires on input connection

[mpeg2]

I ordered a Crack kit last night for use in my office. While looking at the pictures on the product page, I noticed that the wires connecting the input jacks to the volume pot appear to be braided together. Most of my build experience is with guitar amps - where I've been using shielded cable to connect from the input jack to the remainder of the curcuit (guitar amps usually have the volume pot after a gain stage or two).

Is there any reason why shielded wire is not used in the Crack - it would seem to me to give more resistance to outside signals than simply braiding the input wires & ground?

   Rich

[Paul Birkeland]

Hello Rich,

The signals going into the Crack are generally an order of magnitude larger than the signals coming out of a guitar pickup, so the shielding is less important. 

In addition, the low (compared to a guitar amp) input impedance of the Crack will also mitigate other issues that would otherwise necessitate shielded cabling. 

There's no harm to using shielded cable in your Crack, you can use a piece of shielded Cat5 and just add a hole/solder tab close to the volume control for the drain wire.

-PB

[mpeg2]

PB: Thanks - what you say makes sense. I had to turn my thinking sideways when I went from solid state to tubes - looks like I have to do some more of that moving from guitar to "audio".

   Rich

[Doc B.]

The simple answer is that people don't want to pay for it. The Crack is a bang for the buck kit, and the braided wire less expensive and, as PB points out, because of the relatively high signal level coming into the amp it is just as effective as shielded wire in that application. A typical single coil pup puts out about 250mV max, a humbucker about twice that. But the Crack is going to be fed by a sound card or DAC that probably puts out more like 2V.

[mpeg2]

Doc: THanks. I've got plenty of shielded wire, so I'll use it on my build (as you mention, it may not make much difference, but it feels better & for some weird reason, I like the steps involved in using it).

BTW: I read through the manual last night (pre-build planning) - found it to be very well written...


   Rich

[ssssly]

Or you could shield the wire yourself with copper adhesive tape. One of my new favorite toys.

[mpeg2]

That sounds like an awful lot of work. I use that tape to shield guitars that I build, but can't imagine spiraling it along a wire...

    Rich

[ssssly]

Once you get the hang of it, it really doesn't take much time at all. And it works really well.

[adamct]

I have a fair amount of Canare L-4E6S cable lying around. http://www.canare.com/ProductItemDisplay.aspx?productItemID=53

This is 24AWG quad conductor microphone cable with a braided copper shield. Each conductor has 40 stranded wires, but I think it would be manageable.

1. Is this suitable wire for use in Bottlehead kits (Crack, S.E.X., Quickie, Smack) between the RCA inputs and the volume pot? I would only use 3 of the conductors (or maybe wire two conductors together for the ground).

2. Would I have to attach the copper shield to ground?

3. On both ends or only one end?

4. And how does one go about doing that? Do you strip off the outer insulation, twist the exposed shielding wires together, and solder them to the ground lug?

Best,
Adam

[Paul Birkeland]

This is suitable wire.  Use all four conductors, wiring two to the hots, and two separate conductors for the grounds.

Ground the shield at the cable's load (volume pot).

I will drill an extra hole in the chassis, then install a solder lug that the shield braid can be attached to.

[adamct]

Ugh. I feel dumb. So I need to ask some questions that will probably seem ridiculous to you:

1. When you say "Use all four conductors, wiring two to the hots, and two separate conductors for the grounds", I assume you mean that I use them as follows:

Conductor A: Connects Left hot on the RCA input to the Left input on the volume pot
Conductor B: Connects Right hot on the RCA input to the right input on the volume pot
Conductor C: Connects Left ground on the RCA input to the ground input on the volume pot
Conductor D: Connects Right ground on the RCA input to the ground input on the volume pot (i.e., connected to the same place on the volume pot as Connector C - alternatively, on the volume pot side, twist Conductors C and D together before soldering to the volume pot)

Is that right?

2. When you say "Ground the shield at the cable's load (volume pot).", I assume you mean that I should ground the shield to the solder lug that you mention at the end of your post, right? I.e., I drill a hole close to the volume pot, install a solder lug, and ground the shield to the chassis plate, not to the ground lug on the volume pot, correct?

3. The solder lug mentioned below doesn't need to be attached to anything (other than the chassis plate and the cable shield), right?

Sorry for making such a simple question complicated...
Adam

[Paul Birkeland]

That is all correct.

-PB

[adamct]

Good. Thanks! I feel slightly (but only slightly) less dumb.

[Paul Joppa]

Of course you COULD use the wire the way it was designed, in star-quad configuration. Then it's one channel per cable. You'll have to read up (there's a link on the posted spec page), but star quad is especially resistant to magnetic hum. For what it's worth.

[adamct]

Paul,

If I did in Star-quad configuration, how would that work? The example in the PDF shows how you would wire it for a 3-pin XLR. For RCAs, would I do it as follows?

2 Blue insulated conductors: on RCA-end, twist exposed leads together and solder to Left hot. On the volume pot end, twist exposed leads together and solder to left in.

2 White insulated conductors: same as above, except solder to Right hot and right in on volume pot.

Shield/drain wire: on the RCA-end, separate the strands into two bunches, separately twist each bunch together, and solder one to the Left ground, and the other to the Right ground. On the volume pot end, sold the whole thing to the ground in.

Is that right?