Maximum length of cable for power supply?

Quad Cortex
1

What is the maximum length a power cable can be for Quadcortex. I have a singular cable that runs from the back of my rack to my pedalboard with only my QuadCortex on the board. I am not looking to power other devices just my QC I want it to be all wired from my rack. My mutlicable is 10 metres (30ft).

2

Electrical engineer here. Need more info than just the length of the cable.

What is the power rating of the DC power supply that you’re trying to use?
What is the thickness of the wire in the cable?

Any length of wire will have some amount of resistance in it (however small), and thus there will always be some voltage drop–and power dissipation–associated with it.

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3

The power supply I have is the standard QC power supply however I also have a 12v 4a power supply.
Cable is this.

  • Screening Material: Premium grade lapped copper screening (not foil)
  • Conductor Size: Conductor based on 0.14 sq mm of copper.
  • Screen Structure: Lapped screen comprises 32 strands of 0.12 mm diameter copper.
  • Pair Identification: Each pair is jacketed and clearly numbered for easy identification.
  • Outer Sheath: Heavily sheathed overall with specially formulated rubbery PVC, averaging 2 mm in thickness.
  • Cable Filler: Packed with a cotton filler and cotton tape to retain its shape and resist distortion when wound and rewound.
  • Standard: Manufactured to the ISO9002 standard, ensuring quality and reliability.
  • Available Length: Sold by the meter.
  • Specifications: 8 pair, Diameter 18mm
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Assuming you’re trying to run a DC cable at 10 m in length:

Cross sectional area from .06 mm radius copper → 1.13 * 10^-8 m^2

Resistivity ρ → 1.7 * 10^-8 Ωm

Length of wire → 20 m (power to QC and return path to the power supply)

R = ( ρ * L)/A = 30 Ω for a single strand of wire at 20 m

32 strands of wire in parallel: (32/30)^-1 = 0.94 Ω across the 20 m run

The QC is “officially” rated at 12V, 3A. Many users who have done measurements with the QC typically quote that it actually uses 2A. Let’s look at both cases.

QC pulling 2A: 0.94 Ω * 2A = 1.88V voltage drop, 3.76W power dissipation in the wire

QC pulling 3A: 0.94 Ω * 3A = 2.82V voltage drop, 8.46W power dissipation in the wire

Yikes.

Running a DC power cable that long means you’re dropping voltage to the point where the QC can only get between 9-10 V, not close to 12V.

I would NOT recommend trying to power your QC this way.

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5

Thank you for reply and the calculations, I think you may be right! I may instal an IEC port on the back of my pedalboard and just look at a more tried and tested way of powering it. I am certainly not a fan of the power supply that came with the QC. At least that way I can add some other things if it takes my fancy.

Thanks again :slight_smile:

6

So, just to be sure, your QC’s power supply is in your rack, not in your pedalboard, right? Regardless, you shouldn’t need shielded (“screened”) cable for supplying power. The discription doesn’t state the guage of the hot lead, which is usually comparatively small (20‐24 guage?). Is there a reason you’re using this particular cable? Is it a multiple twisted pair cable so you can run audio too? If you have an extra pair in your cable, could you use the two shields to carry your power? There would still be some voltage drop but not as much as using the small center conductor for one leg. Diffraction, what do you think?

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Unfortunately I can’t remember where I saw it, but someone had a reply from one of the big power supply makers, who said the maximum QC draw they tested in their lab was 1.8A, just FYI.

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Yeah, the QC PSU is in the rack (well that’s where i’d like it lol) and I have one multicore cable already installed taking a signal to the input on the QC from my SLXD wireless, then 2 XLR L&R feeds to FOH and 2 trs to my amp input.

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A lot of people were saying a lot of things a year and a half to two years ago. Some said 2.2A, some said 2.4A.

Anyway, my point is that running a DC cable that’s 10 meters long is a bad idea. There are better ways to achieve what the OP wants.

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10

Keep in mind that I was measuring the 120v AC amperage draw at the outlet, not the 12v DC draw from the power supply. If you wanted to make a short male to female barrel connector pigtail and tap into the positive lead, you could measure the DC amperage draw. If the barrel connector comming from your multi-pair cable has a screw-off insulator, you could also measure the DC voltage at the end of your 30 foot run (with the QC powered on) and see how much voltage drop you have. The line voltage coming from your outlet and the power supply’s regulation capabilities would obviously play a roll. A 30’ run for low voltage DC is pretty long, though

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