[dmatt]

Hi all,

I'm looking for advice on wire to use to make a pair of balanced runs from my preamp to monoblock amps.

The related equipment:

• Preamp:  ModWright LS36.5 DM; output impedance 110 ohms
• Amps:  Pass Labs XA-100.5; input impedance 30K ohms

The things that can't be changed (already made the investment -- either in money or goodwill with wife):

• The distance run from preamp to each amp is about 40-50 ft.  -- the left and right runs are equal.
• The wire will be in two runs of flexible steel conduit terminating to metal boxes in the wall behind the equipment rack (preamp) and the floor underneath each amp.  The conduit runs are dedicated to the wire carrying the channel signal and a single wire pair carrying the 5V DC signal to turn on the amps remotely.
• The wiring methodology is balanced (differential balanced from preamp to amp.)

My default decision is to use Belden 1800F:
24AWG twisted pair of bare copper in foam high density polyethylene twisted with filler inside a tinned copper French braid shield with copper drain wire.  Outside insulation is PVC.  Capacitance between conductors is 39.4 pF/m; capacitance between conductor and shield is 85.3 pF/m; conductor DC resistance is 77.8 Ohm/km; outer shield DC resistance is 21.1 Ohm/km; inductance is 0.591 microHenries/m; nominal impedance is 110 ohms.

I am also considering modifying the above shielded twisted pair (or any twisted pair I go with) by pulling a tinned copper braid over the Belden 1800F and using that for the pin1 to pin1 connection -- an approach Jon Risch suggested in his balanced cables here:
http://www.audioasylum.com/audio/cables/messages/109148.html
"This geometry and construction should give better sonic results than retail/commercial XLR cable performance, as it isolates the ground from the signal wires a lot more, minimizing pickup of ground loop currents that can flow between chassis to chassis.

The ultimate expression of this geometry would be to use a second braid over the first one, after insulating the outer surface of the first braid. This second braid would replace the 16 ga. wire, and would then make the connection between pin 1 and pin 1. Then another layer of HS or whatever."

What I could use opinion on:

• Any other wire to consider other than Belden 1800F?  Two 50 foot lengths make it hard to consider hand-making my own shielded twisted pair, so I am leaning toward buying the basic wire from a manufacturer.  But I am open to all suggestions.
• Thoughts on the idea of using a second shield over the first to get the ground wire further away (in a symmetrical way) from the signal wires?  It is a pain to construct a 50 ft. run, but if the consensus is that it will help -- why the heck not?
• Thoughts on using the steel conduit and bonded junction boxes as some sort of shield to protect what is inside the conduit from outside radiation?  The conduit/boxes are isolated from other low voltage or mains wires and either pass through or are attached to the wooden structure of the house.  Since they are already there, I figured it wouldn't hurt to ask -- is it simply a matter of attaching a drain wire to the conduit/box system, and if so, where to terminate without introducing other issues?

I thought this might be fun for you guys since the length makes some things more important than if we were just thinking about 1 meter ICs.

Thanks for your thoughts.

Cheers,

DM

[Bob in St. Louis]

I only have a couple seconds to analyze the concept, but the thought of musilux cable and some Parts-Express Neutrix connectors come to mind.
I made some various cables (RCA and XLR and 1/4") cables from these pieces for an insanely low cost.
The only disadvantage is the musilux cable is it's only sold in large rolls. I happened to buy mine from a fellow willing to sell me some from his roll.

Sorry for the half-way answer, gotta go for now.........

[WireNut]

Belden 89207 is another cable to look at. It uses Teflon rather then PVC. It's a fairly stiff cable due to both center conductors being enclosed in an additional Teflon tube, followed by the shield and outer Teflon Jacket. Capacitance is approx 14pf/F.



 

[thingfish]

Hi
Obtain a DBX150x, which is a type 1 DBX compander offering balanced inputs and outputs. The 150x can be run real time - that is not involving a midway recording device. It is wired balanced  as follows:   console In from your preamp, Tape out wired to Tape In, then Console out to your power amps. you will need 8 stereo phone jacks.  Cable should be shielded mic cable. the benefit with the DBX is that it is made for long studio cable runs as it has a very low output impedance drivers. The next benefit is you will gain added dynamic range approx 20db, and it will dramatically improve your audio system.

 

Hope this helps,       

[poseidonsvoice]

DM,

thingfish has some great ideas for sure.

I had a very similar dilemma about 2 years ago, when I built my room. I then decided to keep all my equipment in a separate room and run long speaker cables instead of interconnects (which are all balanced as well). The problem with putting all your equipment in a separate room is you lose line of sight for your remote but there are ways around that dilemma as well.

I'm just wondering if shorter interconnects and longer speaker cables are an impossibility (I know you said it can't be changed but I figure I would ask anyway!).

Best,
Anand.

[Speedskater]

For a balanced cable, 50 feet is a short length.  Many of the best classical music recordings are done with 300 foot mic cables.  The most important parts of a balanced cable are symmetry of the two signal wires to the shield and that the signal wires be identical to each other.  Even a 2 %  difference in the wires insulation diameter's makes a very big difference.  Note that it requires big expensive machines and a good Quality Control dept. to make a good balanced cable. Always use high quality bulk cable brands like: Belden, Canare, Gepco, or Mogami.

While adding extra shielding to an RCA interconnect is a good idea, in your case the only things that it will add is work and cost.

*************************************************
A 2003 AES paper that tests balanced cables by Jim Brown & Bill Whitlock.

'Common-Mode to Differential-Mode Conversion in Shielded Twisted-pair Cables (Shield-Current-Induced Noise)'
http://www.audiosystemsgroup.com/AES-SCIN-ASGWeb.pdf

Other Jim Brown papers and PowerPoints on the subject:

http://www.audiosystemsgroup.com/publish.htm

*************************************************
Several Tony Waldron papers on large studio wiring.

'Tony Waldron's Audio & EMC ranting and ravings'

http://www.fragrantsword.com/twaudio/