How long can you run +/-24VDC cables?

[JW]

I'm in the middle of giving a really beefy power supply a shot for my mixer. It hums though (mechanical in the room) , and I want to put it in the machine room. I have some three conductor cable that's 18awg but it has no shield. (Same as the mixer's original power cable though)

We're talking something like 20 ft here. What's the length of cable I can run for the main power rails (+/-24V) for a mixing console? (Yamaha PM2000)

Edit:
I have an additional question if you guys don't mind me asking, that pertains to this  project. What is the long term effect of leaving the original power supply unloaded? (I can't seem to dig up the schematic right now. It's 70's era, solid state) I'm disconnecting the +/-24V leads to the mixer, and substituting the ones from the new power supply.  It would be convenient to power the old supply up though, as it has the 48V and 16V leads for phantom and lights.

 

[joaquins]

  Distribution lines run about 1m/V, so, a 33kV line is good for 33km, or the 220V line here in Argentina doesn't run much longer than a couple of blocks. YMMV though

  In you specific case is probably better to calculate as a particular case rather than using a rule of thumb for a completely different scenario. I wouldn't worry about the shielding, and 18AWG 20ft long (no idea about AWG so I just ran an online calculator, bad me) gives around 0.13Ω, that PS runs a 4A switch on 100V mains so at 50VDC wouldn't go close to 8A, at your 0.13Ω it's about 1V, so around 4% of your PS rail, but it won't come to that either... I'd guess more likely half that. No worries there. You also could trim it higher if you'd want, it has internal trimmers.

  The long term effect in leaving the PS unloaded will be few extra cents on the electric bill, I doubt you'll see any other effect of that.

JS

 

[PRR]

You need to know the *current* too. 1mA is fine in the smallest wire you would handle. A big truck starter motor will need BIG cable.

And the acceptable voltage drop. Some "24V" gizmos don't mind 30V or 18V; others may complain outside 24.00 to 23.95V.

Knowing the resistance of wire is also good; tables are out there.

 

[Tekay]

Have you tried to fix the PSU? By changing bad electrolytics I've fixed manny "humming" supplies over the years.

 

[MrDude]

Have you tried to fix the PSU? By changing bad electrolytics I've fixed manny "humming" supplies over the years.

He said it is humming mechanically, maybe the power-transformer or fans installed in it.

 

[ruffrecords]

Apart from dc votage drop, the other possible gotcha is the inductance of the long cable. This could cause crosstalk and instability.

Cheers

Ian

 

[JW]

Thanks everybody. I was actually wrong about the cable gauge. It's 16 awg, so even thicker than I had thought.

BTW, I'm seeing this as a bit of a temporary solution. I already had this huge +/-24V power one supply, so I thought I'd give it a shot while I work on recapping the original supply (not cheap) I wired up the cable yesterday. Voltage drop isn't a problem. The power supply has trimmers to get me exactly +/-24V.

And yes, the humming is caused by the mechanical hum of the transformers. Yes, I've tightened them. All the power one supplies I've ever tried hum like this. Bad luck maybe.

Anyway, Ian, do you mean crosstalk between the +/-24 leads? Can I verify this by sticking a voltage meter on them when the board is fully loaded? They're quite solid with my master channel and a couple input strips right now. Would the symptoms of crosstalk and instability look like the pos and neg rails fluctuating too much?

 

[JohnRoberts]

16ga is around  .013 ohm/meter  so you can calculate voltage drop using ohms law.

If between a console and PS there is probably capacitance at the console end so inductance is probably not an issue

JR

 

[ruffrecords]

Anyway, Ian, do you mean crosstalk between the +/-24 leads? Can I verify this by sticking a voltage meter on them when the board is fully loaded? They're quite solid with my master channel and a couple input strips right now. Would the symptoms of crosstalk and instability look like the pos and neg rails fluctuating too much?

I mean audio crosstalke between  amplifiers inside the mixer . Thre inductance is a common impedance between then that get larger as frequency increases.

By the way. you can eleimiate/compensate for the voltage drop using sense wires.

Cheers

IAn

 

[joaquins]

  If the noise is from the transformer and is already tight, soaking it in some hardening juice usually helps, I use candle wax for the guitar and low level signal stuff, usually for power transformers you need something more serious. The other option is the case where the transformer is in, some iron cases like to vibrate case of the transformer magneto-field, you will know it's that if you take the transformer out and it stops buzzing. The solution for that is mechanically making the case shut up.

JS

 

[Tekay]

Have you tried to fix the PSU? By changing bad electrolytics I've fixed manny "humming" supplies over the years.

He said it is humming mechanically, maybe the power-transformer or fans installed in it.

Fixed several psu's when the too old electrolytics probably loaded the transformer!