Shielded DC cables?

CarlGuitarist

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According to Mason it's much preferable to use shielded (coaxial) cables for DC power because of superior noise rejection. Is there any merit to this at all? If so, are companies like Truetone (who do not use coax cable) simply unaware of this? It sounds very strange to me. Anyone with technical knowledge who can chime in?

The link should start at the appropriate time so you don't have to watch the whole thing.

Edit: He says there may be some "nominal difference in noise rejection", did he mean to say "minimal"?

 
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The reason signal-wire is shielded is because the signal itself is very low-Voltage, maybe 1 Volt.
The reason speaker-wire isn't shielded is because the amp's output is higher-Voltage & not susceptible to noise the same as the lower-Voltage signal of the amp's input.

A precaution is to keep signal-wire away from power-wire, and if they must cross, do so at a 90-degree angle, never in parallel.
 

CarlGuitarist

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The reason signal-wire is shielded is because the signal itself is very low-Voltage, maybe 1 Volt.
The reason speaker-wire isn't shielded is because the amp's output is higher-Voltage & not susceptible to noise the same as the lower-Voltage signal of the amp's input.
Sure, but he's talking specifically about dc cables here. I've never heard that there's any benefit with shielded dc cables.
 

Chandyland

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2,346
Theoretically you could get some AC signal noise in your DC cables, but that wouldn't necessarily manifest as audible noise.

It would basically cause (extremely) small voltage fluctuations to your pedals' DC inputs. If you have pedals that are actually that sensitive to power fluctuations, they probably don't sound very good to begin with.

I'm also not aware of anything at all that's that sensitive.

It is vastly more critical to have good quality power supplies to begin with because if you're not sending a consistent, filtered DC signal, then unshielded cable is a moot point.
 

cbm

Gold Supporting Member
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6,897
Any environmental radiated noise would have a very hard time affecting a change on an extremely low impedance / high current DC power cable. Even if some noise was somehow imposed on a DC power cable, any well designed pedal will have filtering on the DC input.

A DC power cable should not radiate anything itself.
 

lefort_1

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Any environmental radiated noise would have a very hard time affecting a change on an extremely low impedance / high current DC power cable. Even if some noise was somehow imposed on a DC power cable, any well designed pedal will have filtering on the DC input.

A DC power cable should not radiate anything itself.
The few times I've seen where shielding the power-cables was handy were

- when the load had a big inductive 'kick' (e.g. a big motor starting) .. Shielding other things around it from the noise that could be broadcast by a longer power cable.

- in a handful of bio-medical applications where they're looking at peak-to-peak signals in the low, single-millivolt range. if the sensor is using a remotely powered 'head' at the patient, then having the power cables shielded isn't a bad idea.

The noise that we get on our hungry-digital-devices (if the device is designed with insufficient bulk and low-inductance/high-freq bypass caps) can be passed down the +9 cables, BUT they usually don't act as much of a radiating source due to their physical distance from other power lines (crosstalk reduces with distance between/increases with increasing parallelism)

Now, if you have a noisy digital pedal ON the same power as a fuzz, expect to be sad. But that's not a situation that shielding would improve.




But in THIS application, until proven otherwise, I'ma gonna post this little visual aid:

 
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andrekp

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It is true that some pedal power circuits are not sufficiently filtering. In those situations, noise can enter on the power cable. A common thing is tick from pedals with LFOs that share power with the unfiltered pedal.

but generally, this isn’t really an issue necessitating coaxial power cables.

oh, and in my above example, which is the most common case (noise from an adjacent pedal) shielded cable will not change things.
 

Timtam

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2,324
Edit: He says there may be some "nominal difference in noise rejection", did he mean to say "minimal"?
He says "nominal". So what he's effectively saying is that there may be a small difference with using shielded DC cables, so why not use them if you have the choice. But he's implying that it may not matter much.

This thread is overthinking a brief remark. I probably would not have used "much better job", without some direct evidence.

But like lots of guitar electronics, we don't have easy objective ways of testing these things in real practical environments. So we often do things that "make some sense", based on some theoretical principles (but you have to have the correct principles), even if we can't prove it. That's all that's happening here. The problem is that that uncertainty also allows others who spout pseudo-science or outright voodoo, that sounds vaguely plausible, to also get some traction with players. In many cases, manufacturers could provide evidence for their claims, if only the market demanded it. Failing that, learn the science behind these things, so you can make your own decisions.
 
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lefort_1

Nuzzled Firmly Betwixt
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Exactly what is radiated by DC in a cable? If anything worth considering, wouldn't power supply manufacturers be well aware of it? How many recommend or supply shielded DC cables? Too bad that Berkeley Engineering school shut down….
Well, you can have considerable noise on a DC line, most often caused by a noisy load (in our case, it's those big digital pedals)… problems caused by this noise is usually limited to conducted emissions problems with other pedals on the same daisychain.
The noise CAN radiate, BUT it is rarely a problem.
A DC-line of sufficient length CAN act as a receiving antenna, but it is generally terminated at the PSU end with a very low impedance and is also rarely the site of a problem.

The BIGGEST problem with a shielded cable is that standard PSUs do not offer a dedicated, low-impedance connection to chassis ground available at the cable-connections for each tap. Even if they did, these cables would have to be implemented with single-ended shielding, which can act as an inductor (antenna) and/or ESD-concern (before a fast-rising 10 kV ESD-spike might be dissipated down a long/inductive cable-shield, it may punch through the 600v insulation to one of the power conductors inside).

I dunno… just some random thoughts.
;)
 




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