boji

Fader connections- cable choice
« on: May 11, 2019, 04:48:43 AM »
Hello friends,

Need to extend some P&G faders' funky, conductive-epoxied wirepads?!? * about an extra 7" to get  them over to the channel input cards.

Anyone have an opinion on using  2-conductor, (Mogami W2944)  for fader interconnects?
(would use the shield wire for faders'  Agnd)

Or, would you use 3-wire with a proper**  shield?

As always, thanks for your input!
-Boji


**  On the old 79' Wheatstone, the P&G's 3-wires loosely wandered around to channel card backplanes, with no shielding, no cable  management except end connectors. The runs were all  10"-15" lengths.
So... as long as they remain inside the console frame, everything's peachy?


* Have you ever tried to de-solder P&G fader wires right at the thru hole point on the resistive strips...successfully?  Didn't  short-out any traces while scraping at the silvery blob that looks like solder, but acts like epoxy ? :o
« Last Edit: May 11, 2019, 05:06:11 AM by boji »


JohnRoberts

Re: Fader connections- cable choice
« Reply #1 on: May 11, 2019, 10:32:30 AM »
Hello friends,

Need to extend some P&G faders' funky, conductive-epoxied wirepads?!? * about an extra 7" to get  them over to the channel input cards.

Anyone have an opinion on using  2-conductor, (Mogami W2944)  for fader interconnects?
(would use the shield wire for faders'  Agnd)

Or, would you use 3-wire with a proper**  shield?

As always, thanks for your input!
-Boji


**  On the old 79' Wheatstone, the P&G's 3-wires loosely wandered around to channel card backplanes, with no shielding, no cable  management except end connectors. The runs were all  10"-15" lengths.
So... as long as they remain inside the console frame, everything's peachy?


* Have you ever tried to de-solder P&G fader wires right at the thru hole point on the resistive strips...successfully?  Didn't  short-out any traces while scraping at the silvery blob that looks like solder, but acts like epoxy ? :o
You could easily use 4 conductors or 3 conductors + shield. Doubling up the fader ground leads, allows one to carry the signal current, and the second ground lead provides the fader 0V reference to the following make up gain stage.  Fader kill is an important spec in big dog consoles.

JR
John Roberts
http://circularscience.com
Tune it, or don't play it...

Gareth Connor

Re: Fader connections- cable choice
« Reply #2 on: May 12, 2019, 09:22:15 AM »
1. Soldering directly on to the P&G conductive plastic fader element termination points (silvery blobs).
I have a vague recollection of attempting this, or something similar, some 35 years ago, without success. It was very easy to write-off a fader. As it was already a duff fader and therefore destined for the trash can, it was OK to experiment with. It was an experiement that failed and therefore has not been repeated.
Recommendation: Leave an inch or 2 of original P&G fader wire & extend with screened cable.

2. Mogami 2944 is a 2-core twisted with overall screen.
If you are planning to use this as a combined screened fader send and fader return cable, beware!
High impedance and crosstalk are the enemies.
What is feeding the fader? If it is an amplifier, the fader send will be at low-impedance. Good news :)
The fader return will be at a varying impedance. The fader wiper will be low-impedance at the bottom of the fader travel (ground) and low-impedance at the top (max level & assuming the fader is fed by an amplifier), but along its travel impedance it will be varying.
High-impedance wiring  is susceptible to receiving crosstalk, and the fader send in your twisted core Mogami cable will be having a full-on face-to-face conversation with the fader return wire. Bad news :(
The best solution is to use individually screened figure-of-8 cable, one screened core for the fader send, one screened core for the fader return. You can also then apply John Roberts' advice regarding the use of the return wire's screen as the ground reference for the following circuitry. Do not underestimate the importance of fader kill!




Gareth.

ruffrecords

Re: Fader connections- cable choice
« Reply #3 on: May 12, 2019, 04:36:09 PM »
I can't lay my hands on it right now but I seem to remember a Neve technical note saying for short lengths ( up to about a foot) you can use a twin screened cable but for longer ones you need to use two separate single core screened cable.

Cheers

Ian
www.customtubeconsoles.com
https://mark3vtm.blogspot.co.uk/
www.eztubemixer.blogspot.co.uk


'The only people not making mistakes are the people doing nothing'

boji

Re: Fader connections- cable choice
« Reply #4 on: May 14, 2019, 01:15:37 AM »
Quote
Fader kill is an important spec in big dog consoles.

The best solution is to use individually screened figure-of-8 cable

longer ones you need to use two separate single core screened cable.
So glad I asked!  Thank you everyone.


Edit: I just got why you italicized "plastic" ::)
« Last Edit: May 14, 2019, 05:48:15 PM by boji »

boji

Re: Fader connections- cable choice
« Reply #5 on: May 14, 2019, 11:23:59 PM »
If you don't mind, one last check before purchase:  Is the figure-of-8 pair doing something unique that two, shielded, single core wires running parallel to each other could not accomplish?

For example: https://www.redco.com/Mogami-W2368.html

All I could find in figure 8 was 23ga, so going with the mogami I think...
« Last Edit: May 14, 2019, 11:57:10 PM by boji »

Gareth Connor

Re: Fader connections- cable choice
« Reply #6 on: May 15, 2019, 01:55:19 AM »
Quote
Is the figure-of-8 pair doing something unique that two, shielded, single core wires running parallel to each other could not accomplish?

For example: https://www.redco.com/Mogami-W2368.html

All I could find in figure 8 was 23ga, so going with the mogami I think...

Figure-of-8 is not essential, but it is convenient.
Here in the UK it has traditionally been an easy to find and very good value for money type of cable, so for this application, choice #1. (A red core and a blue core also help with conductor identification :) )

Two runs of the W2368 provides the same solution as figure-of-8.
I am surprised at the (seemingly) reasonable price for the Mogami. Over here Mogami has always been "rudely expensive".

The important thing in all of this is that the send and return signals are not enclosed in the same screen.

Gareth.

boji

Re: Fader connections- cable choice
« Reply #7 on: May 15, 2019, 07:19:47 PM »
Quote
The important thing in all of this is that the send and return signals are not enclosed in the same screen.
Thanks for taking time to explain.  Much appreciated.

JohnRoberts

Re: Fader connections- cable choice
« Reply #8 on: May 15, 2019, 11:57:45 PM »
Thanks for taking time to explain.  Much appreciated.
I have made many with a common screen, the most important thing is to keep the fader current out of the 0V reference line.

Two separate shielded wires is even better, and probably easier to source than 3 conductor shielded.

JR
John Roberts
http://circularscience.com
Tune it, or don't play it...

boji

Re: Fader connections- cable choice
« Reply #9 on: May 16, 2019, 07:48:54 PM »
Thanks JR. 

Please forgive yet another inquiry, but your comment on keeping fader current out of reference has me wondering...would it be a bad idea to route the fader's 0v directly to the backplane agnd bus bar, as opposed to what I planned, which was to have it return to the pcb's fader header that uses an agnd pour?


ruffrecords

Re: Fader connections- cable choice
« Reply #10 on: May 17, 2019, 03:38:42 AM »
For best 'offness' you need to reference it to the AGND of the input fed by the return which as often as not is the board that sent it.

Cheers

Ian
www.customtubeconsoles.com
https://mark3vtm.blogspot.co.uk/
www.eztubemixer.blogspot.co.uk


'The only people not making mistakes are the people doing nothing'

JohnRoberts

Re: Fader connections- cable choice
« Reply #11 on: May 17, 2019, 09:26:47 AM »
Thanks JR. 

Please forgive yet another inquiry, but your comment on keeping fader current out of reference has me wondering...would it be a bad idea to route the fader's 0v directly to the backplane agnd bus bar, as opposed to what I planned, which was to have it return to the pcb's fader header that uses an agnd pour?
You need to reread my chapter about using differential amplifier stages to forward and back reference audio grounds. (Oops I never did write "that" book, but I have posted here on the subject at length).

Real wires have resistance, so the current flowing through the fader, will cause a drop in the wire feeding the fader and the ground wire returning the current. The drop in the feed to the top is inconsequential, but the drop in the ground, can superimpose a voltage on the bottom of the fader, diminishing fader kill.

By using two ground wires (only one carrying current), you can compare the voltage at the wiper, to the actual voltage at the bottom of the fader. As long as there is little/no current in these sense wires you can extract clean audio. This is the same theory behind accurate voltmeters (google "Kelvin 4-wire connections").

Ground is a concept not a voltage, and 0V (another concept, can validly exist in numerous places inside a chassis). Judicious use of differential amps allows us to forward and back reference audio grounds.

In the case of the fader, forget about a brute force ground bus (only one s). Connect the wiper to the + input of the post fader gain stage, connect the ground resistor in the NF network to the fader 0V reference, using proper resistor ratios the ground error will subtract out. In most cases any gain stage can be made differential as easily as adding a couple resistors.. If you study schematics of consoles (especially mine) you will see numerous differential stages sending and/or receiving audio stems around internally. 

JR
John Roberts
http://circularscience.com
Tune it, or don't play it...

boji

Re: Fader connections- cable choice
« Reply #12 on: May 17, 2019, 11:38:03 PM »
Thanks a bunch JR.   If I was closer, I'd come ninja-mow your lawn for ya.   8)

JohnRoberts

Re: Fader connections- cable choice
« Reply #13 on: May 18, 2019, 12:44:51 PM »
Thanks a bunch JR.   If I was closer, I'd come ninja-mow your lawn for ya.   8)
With my big dog  44" zero turn I can cut the grass in 20 minutes or less... then it takes a day or two to do the trim. Mowed yesterday, not looking forward to trim.   :'(

JR
John Roberts
http://circularscience.com
Tune it, or don't play it...


 

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