DIY attenuator cable for Porta Corda?

[Joe Bloggs]

Ok, looks like I'm going to take on my first DIY project soon... a cable

Ok, so an attenuator cable can either be two resistors placed in series with each channel of the amp (or just one in the ground channel?) or a real voltage divider:

There's two channel inputs and one common ground, so in the case of the attenuator cable I suppose there'd be a mirror image below the picture, for a total of 4 resistors...

Now, which kind of circuit would be better? The series resistor design is much more simple of course, but would require very high resistances. (say I want to attenuate the signal into 1/10 or 1/20 original voltage--I'd need resistors 10 and 20 times the input resistance of the Porta Corda) Does this added resistance translate into output resistance of the pcdp or input resistance of the PC? If the former, does this result in sound quality degradation?

The real voltage divider design allows the use of lower resistances--but is this a good thing at all? Or would the lowered input resistance also cause sonic degradation?

Finally, an attenuator cable is an interconnect and subject to all the regular quality issues of an interconnect. I'd need an intro to DIYing an ordinary interconnect...

Or, does having a proper interconnect (like the Oehlbach cable supplied with the amp) connected to my DIY interconnect eliminate worries about RF and EM pickup and whatever else I have to worry about?

 

[danny]

Joe,

Use the voltage divider method (4 resistors). Do not over engineer the thing, especially until you have the values picked. Your smallest resistor should probably be 1K. (nice round number, and the divider will not load your source) Use 1% or better resistors, or you may add to your imbalance problem. If you can find them use metal film resistors.

 

[JMT]

When you pick your resistor values, let me know. I have a lot of metal films that I would be willing to send you.

 

[Joe Bloggs]

Jan has told me that the Porta Corda has an input impedance of 3 kOhm and suggested the following resistor values for an attenuation of 9.5:

Quote:

4,7 kOhm and 680 Ohm would do fine The 680 Ohm in parallel with 3 kOhm will result in 554 Ohm effective Attenuation will be (4700 + 554) : 554 = 9,5

Now, what is this about metal film resistors? I think I can obtain them in HK, if not I'll pm JMT

Now, what about grounding and shielding and all the other stuff I need to do to make a good DIY cable? Or should I just cannibalize an existing cable and wire in my resistors?

Hmm, and how to insulate the connections and prevent shorts?

I have done a search and got some information about grounding, but none about shielding...

 

[Joe Bloggs]

*Bump*
Help the clueless guy here! ->>

 

[Joe Bloggs]

Found some info about shielding too...

OK, current plan--
1. get connectors *somewhere*...
2. just run the L, R and ground wires straight from one end to the other and not bother about braiding, twisting or whatever (from what I've read so far, seems like it's a toss-up which method is better anyway, and doing nothing is so much simpler than doing anything

)
3. slice up the insulation in the wires in a few places (they're gonna be insulated right??

) and weld in my resistors...
4. got to insulate the exposed wire around those connections somehow right?

5. Wrap the whole lot in aluminium foil for shielding and connect it to one end. (like how?

) That end should be connected to the source output right?

6. Wrap over the foil with plastic tubing of some sort... (should I do this before welding on both connectors?)

Well, you can see that my 'plan' has a lot of gaps... help the DIY newbie??

 

[Joe Bloggs]

Something I just read from Robert Harley's book "The Complete Guide to High End Audio" which Dusty Chalk quoted in an old thread:
Quote:

An example of how a cable's physical structure can affect its performance: simply twisting a pair of conductors around each other instead of running them side by side. Twisting the conductors greatly reduces capacitance and inductance in the cable. Think of the physical structure of two conductors running in parallel, and compare that to the schematic symbol for a capacitor, which is two parallel lines.

This seems to be contradictory to everything I've ever read!!

edit: I'll assume this is just plain wrong, no questions asked...

Another question: how do I apply strain relief to the two connector ends?

 

[PRR]

> Twisting the conductors greatly reduces capacitance and inductance in the cable.

Uh, he's a bit confused.

Twisted pair has more capacitance. Open-wire pair (think of antique telegraph poles) has less capacitance.

Twisting pairs affects inductance, but I'd rather not go there.

Anyway, what does it matter? How long is this cable? 10 feet/3 meters? Plain coax wire runs about 30pF per foot, or 300pF for 10 feet.

Your impedances are, what? 1K? something like that surely. 300pF loading 1K causes 3dB rolloff at 500KHz. This is just not an issue, whether the wire is 10pF/foot open-pair or 50pF/foot micro-thin coax.

Offhand, I just cut-up any handy cable with the right ends on it. Much faster and usually better connections than making my own.

Insulating: I just mash electrical tape on all jounts, and then bundle the whole thing with duct tape. Some say this is inelegant, and only virgin teflon is good enough for audio.

-PRR

 

[Joe Bloggs]

Finally a response...

How do I apply shielding and how do I 'connect' the shielding to the cable? To ground on one end of the cable?

 

[PRR]

> How do I apply shielding

Don't need no shielding. Especially with 550 ohm impedance. Unless you are sitting on power transformers or under a radio tower.

You have a lot of basic questions. Look: get a cheap cable, some cheap resistors, and some duct tape, and just do it. You will screw-up something. Big deal. Nothing will smoke, it will probably work fine, and if it is a total loss you'll only be out $3. Along the way you'll gain a much clearer picture of the process, and see how to do it neater.

If you build it unshielded and find that you really do need shielding around the resistors, build the attenuator into a metal box.

Why do you need an attenuator anyway?

-PRR

 

[slindeman]

Why don't you just change the gain of the Porta Corda? You'd only need to change a few resistors. It's much easier to solder/desolder stuff on a nice pcb than to make a cable.

Anyway, if you want to go the more expensive route, with a professional looking shielded cable, here are some supplies you might want:

1. Wire [cheap PVC insulated hook-up wire at radioshack, or teflon coated wire from places like www.percyaudio.com. My personal choice would be to use the core of Belden 89529 coax, which is 22 AWG stranded copper, insulated with foamed PTFE teflon, and is quite cheap also]

2. Resistors [radioshack]

3. Yellow gas line teflon tape [hardware store, plumbing section]

4. (optional) Braided tinned copper mesh shield [50 cents per foot at www.mcmaster.com and elsewhere. Better yet, borrow the shield from an old discarded beefy coax cable]

5. 1/8" plugs [radioshack or online at www.mouser.com or www.digikey.com and other places]

6. Techflex aka Flexible Braided Polyethylene or Polyolefin Sleeving [www.mcmaster.com and many other places]

7. 3:1 Adhesive or 2:1 Standard Heat Shrink Tubing [radioshack, or online at www.mouser.com or www.digikey.com or www.mcmaster.com and many other places)

Assembly:

1. (optional) Braid wires
2. Solder the resistors to the wires
3. Mummy wrap the wires and resistors with a layer or two of teflon tape for insulation, for strength, to keep the wires from moving, and to space the shield further away
4. (optional) Slide the shield over this
5. Solder the 3 wires to each plug (left, right, ground)
6. (optional) Solder the shield to ground at one plug only
7. Slide techflex over cable
8. Shrink one 1-2" pice of heatshrink at each end of cable, such that half is over the back of a plug, half is over the techflex/main cable. This will keep the techflex in place and act as strain relief.

There you go. Building such a cable should take an evening at the most if you are handy with the soldering iron.

 

[Joe Bloggs]

Well, just because I'm asking all these basic questions doesn't mean I haven't already started work

I thought I was just going to use some aluminium foil as shielding, but I haven't got that far--

I soldered the resistors to the wires and the wires to one plug, then found that the resistors are too big for me to slide the strain relief through

I'd probably have to start over in the holidays, looks like I should solder the wires on one end and screw on the strain relief (it's a screw-on gold plated thingie with a spring attached at the end, I suppose you attach the spring to the wire with tape or heatshrink) before soldering the resistors to the wires.

PRR -- my Porta Corda mistracks a low volumes and Jan recommended me to build an attenuator cable to sort of lower the 'gain' of the PC.

Another thing--I find that I have to use multiple sections of wire to finish the cable--is this bad? Won't I need strain relief at all those solder points as well as at the plug??

If there's some way for me to take off insulation in a section in the middle of the wire without cutting it, I won't need multiple sections... but so far no luck

 

[Joe Bloggs]

And *another* question...

Do you need to attach the strain relief to the innermost wires that you actually soldered to the plugs, or is it ok to attach it to the outer body of the cable? (I would assume the latter, but would there be cases where the outer body is not strained but the wires inside are actually strained?)

 

[tangent]

What I do with strain relieving is clamp it on the outermost shielding. Then, I give the inner wires some slack between the strain relief and the solder point. If you pull on a cable wired like that, only the outer shielding is strained -- at worst, a bit of the slack is pulled out of the inner wires inside the connector.

 

[slindeman]

Quote:

If there's some way for me to take off insulation in a section in the middle of the wire without cutting it, I won't need multiple sections... but so far no luck

If the insulation is just PVC, you can melt it with your soldering iron, then solder the resistor in.