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Instrument Cable In Place Of Speaker Cable?

Discussion in 'Miscellaneous [BG]' started by Stephen S, Apr 12, 2005.

  1. Stephen S

    Stephen S Member

    Apr 10, 2002
    San Bernardino, CA
    I did a stand in for another band and let one of the guys borrow my speaker cable, I have a rehearsel tonight and can't get ahold of the guy, is it safe to use an instrument cable to run my amp to my cab?
  2. Selta


    Feb 6, 2002
    Pacific Northwet
    Total fanboi of: Fractal Audio, AudiKinesis Cabs, Dingwall basses
    Generally no, though I did once for about 15 minutes with no ill side effects.


    Edit: isn't your sig against the rules?
  3. Toasted


    May 26, 2003
    Leeds, UK
    No its not safe :)
  4. paintandsk8

    paintandsk8 Pushin' my soul through the wire...

    May 12, 2003
    West Lafayette, IN
    Generally, no.

    It all has to do with current, resistance, and heat. As current flows through a wire, the wire's resistance will cause a small amount of heat to be generated. The smaller the wire, the more resistance ( and therefore more heat). Different wire guages and conductors have different maximum currents. Then you throw something else into the mix. As a wire gets hot, it's resistance increases, so that makes it get even hotter, which raises it's resistance again, and so forth.

    If you have all the right information you can calculate what guage of wire you need for any particular current.

    You need to know:

    Your cabs impedance ( in ohms)

    Your amps power (in watts)

    The Guage of the cable in question

    So, current = the square root of power/impedance

    Once you know the current, you then must consult a chart for different current ratings.

    Here is a chart: http://www.powerstream.com/Wire_Size.htm

    I'm not sure what guage instrument cables normally are, but hopefully someone can chime in and let us know. (I don't think you want to cut the cable open and find out)

    But 9 times out of 10, the answer is going to be "NO, your instrument cable isn't safe for amp to cab wiring."
  5. jazzbo


    Aug 25, 2000
    San Francisco, CA
    No, it's not safe, as others have mentioned.
  6. Squidfinger

    Squidfinger I wish I could sing like Rick Danko.

    Jan 7, 2004
    Shreveport LA
    I fried a head doing this once.

    It's bad.[​IMG]
  7. SnoMan

    SnoMan Words Words Words

    Jan 27, 2001
    Charleston, WV
    I'm not advocating the use of them but....

    I've always used the same cables. Not by choice...but by ignorance. I had never heard of a difference before coming here....and since I've been here I've been too lazy to buy actual speaker cable. I'm running 350w and haven't seen any ill side effects...it may effect tone, but I have nothing to compare with.

    Hey, speaking of which...where can I buy short speakon cable? I don't really need 25' to go from my head to my cab. Anyone have a good source...I've looked around online for it before, but no luck.
  8. I have never heard of a wire's heating up, changing it's resistance. You can send 10 kV at 10 kA across a paperclip, and while that paperclip will heat up a whole hell of a lot, it will remain a 0 ohm paperclip, theoretically. I say theoretically because few conductors have a resistance of exactly 0 ohms. Most wires will still have some resistance, such as 1-2 ohms.
    Ohm's law states: Voltage across the end of a conductor is directly proportional to the current passing through it, and indirectly proportional to resistance.

    As long as the cable's impedance (cable impedance is caused by the connector at each end, not the wire itself) matches the impedance to your cab, you should be fine. I've had an intrument cable connecting my heads to my cabs for years and never had a problem.
  9. bigbeefdog

    bigbeefdog Who let the dogs in?

    Jul 7, 2003
    Mandeville, LA
    I don't do it (although I have in a pinch), the biggest concern, IMHO, is that instrument cables are cheap, cheap, cheap. I've had so many of them fail on me over the years that I've lost count. So I don't trust them with the business side of my amps.

    If a cheap instrument cable shorts out between your bass and amp input, you change it and move on. If it shorts between your amp's output and cab, though..... blown fuse at best, possibly far worse....

    If you absolutely must do this to get through a rehearsal, select your BEST instrument cable for cab duty, and make sure it does not get jiggled around during the practice.

  10. Sorry but there's a lot of incorrect physics stated here. In fact all of the above statement is incorrect.

    All conductors have resistance, except for superconductors. And this resistance is pretty significant, there's standard published charts available. These charts also show the rise in resistance as temperature increases. Here's the figures for 22 gauge wire, which is standard instrument cable stuff:

    1000 feet of wire at 77 degrees F will have a resistance of 16.5 ohms. The same 1000 ft at 149 degrees F will have a resistance of 19.0 ohms.

    Instrument cables simply cannot handle the current flow. The heat dissipated by the cable, in watts, will follow the formula P equals current squared, times impedance. For speaker cables, the current can be pretty high and even a tiny amount of resistance can be catastrophic. This can often occur at the junctions between the cable and the connector but will also happen in the cable: as the stranded cable gets older, the strands inside may start to break from mechanical fatigue. Eventually there may be a few points where instead of 24 gauge cable, there may effectively be 28 or 30, because of a few broken strands. Now we have a spot that will get hot, and failure can occur.

    Some people may have been lucky, but I advise people to spend five or ten bucks and buy, or make, a real speaker cable. A couple of 1/4 connectors and a few feet of 16 gauge (or better) "zip" cord is all you need.

    Don't believe a few watts of heat can destroy a cable? Try touching the tip of a 15 watt soldering iron. That's only 15 watts, but it'll melt solder. If you use an instrument cable and the internal resistance is dissipating 15 watts or more....get the picture?
  11. paintandsk8

    paintandsk8 Pushin' my soul through the wire...

    May 12, 2003
    West Lafayette, IN

    I just thought I would add that cables don't have impedance. Impedance is defined as the vector sum of resistance and reactance. A device must have reactance in order to have impedance. It is possible that a cable may have a value of reactance, but that value is so small that it is entirely negligible (sp?) in comparison to the resistance. And the relationship between cable resistance and cab impedance doesn't affect anything in it's own right.

    Several guys I know also make their cables from old extension cords. That can be a good low cost solution, but they don't have the greatest conductors in them.

    Hooray for Electricity and all it's little quarks!
  12. Yes, cables actually do have measurable amounts of capacitance and inductance, though very small (at audio frequencies, usually not an issue). By the same token, superconductors actually do have a teeny tiny amount of resistance... :D

    The paperclip analogy (erroneously used earlier) can demonstrate that there is resistance in conductors, by the way. Simply uncoil the paperclip and hold the two ends across the terminals of a 9 volt battery. Ahh, nothing like burnt fingers to demonstrate a point!!! If the paperclip truly had zero resistance, it wouldn't get hot, the current from the shorted battery would pass through it without dissipating any heat.
  13. jive1

    jive1 Commercial User

    Jan 16, 2003
    Owner/Retailer: Jive Sound
    For something like that, I think it's economical to make your own. Since you are using a speakon connector, you shouldn't have to solder anything. Thus, this cable is easier to make than a 1/4" cable. Plus it will cost the fraction of the price of a manufactured cable, and you can make it any length you want.

    www.partsexpress.com is a good source for parts.
  14. Jazzin'

    Jazzin' ...Bluesin' and Funkin'

    Every object will have an amount of ohms. Metals will have lower amounts of ohms. And if a cable (made of metal, or actually anything) is too thin and/or too long, it will heat up and lose energy to heat. The metal will get hotter, and this will make it have more resistance.
  15. paintandsk8

    paintandsk8 Pushin' my soul through the wire...

    May 12, 2003
    West Lafayette, IN

    Oh geesh, Now's he gonna get all technical on us... :D

    (I wonder if stephen S thinks we answered his quesion well enough?)
  16. Tim__x


    Aug 13, 2002
    Alberta, Canada
    Even a cable with zero reactive parisitics would have an impedance. As you said, impedance is the vector sum of resistance and reactance, if a cables reactive component was zero, it's impedance would equal it's resistance.
  17. This is a completely different concept than the point you're trying to make.

    First off, heat isn't measured in watts. Power is.

    Second, try connecting a cable from the positive terminal of your car's battery to the ground, and then tell me that the resistance of that cable is increasing. It's not, the current is EXTREMELY high, due to the difference in potential being high (12V battery to ground [ground = 0V]), and being that this cable is a conductor, with low resistance, the resistance will only go down. Conductor's don't create resistance out of thin air, it has to come from somewhere.

    I'm not trying to create animosity, it's just that in all the training I have received in electronics (it's what I do in the Coast Guard) I have not heard of resistance increasing if a cable/wire heats up. Correct me if I'm wrong...which would mean disproving Ohm's Law.
  18. Passinwind

    Passinwind I know nothing. Commercial User

    Dec 3, 2003
    Columbia River Gorge, WA.
    Owner/Designer &Toaster Tech Passinwind Electronics
    It is. The current will steadily decrease as the wire heats up and more voltage drops across the wire's resistance (assuming the battery doesn't blow up first), and when the wire oxidizes and breaks, the resistance will become theoretically infinite. This is completely consistent with Ohm's law, as far as I can see.

    If you still don't believe that wire changes resistance with temperature, look up power compression as it applies to loudspeakers. Start here, if you like: JBL Audio FAQ

    There's some discussion of speaker wire gauges that might be of interest as well. The graph is labeled incorrectly though. :cool:
  19. Passinwind

    Passinwind I know nothing. Commercial User

    Dec 3, 2003
    Columbia River Gorge, WA.
    Owner/Designer &Toaster Tech Passinwind Electronics
    If the cable opens up from a broken lead, and it's plugged into a tube amp, you can easily cook the ouput transformer. I've seen it happen, and the outcome... :bawl:
  20. BassyBill

    BassyBill The smooth moderator... Gold Supporting Member

    Mar 12, 2005
    West Midlands UK
    Ohm's Law, paraphrased for clarity - "current (which is measured in amps) flowing through a conductor is proportional to the potential difference (in volts) across it, provided the temperature remains constant". Resistance is the ratio of volts to amps (R in ohms=volts divided by amps), and it does depend on temperature. In other words, if your wire gets hotter, its resistance does increase. This is because, to over-simplify a little, as the wire gets hotter, the metal atoms in it vibrate further about their average positions because of the increased energy they have. This makes it more likely for them to be collided with by the electrons flowing in the current as they pass between atoms in the wire, and this increased difficulty in getting past means more resistance. Note that the temperature increase could come from an energy source external to the circuit, or from increased current flow in the circuit (more energy dissipated as more electrons collide with the atoms).

    BTW, power IS measured in watts, but this is defined as the energy transfer per second. Therefore as more power is dissipated in a cable, electricity will be converted to heat faster and the temperature can increase.

    Just thought I'd try to clear up some of the confused physics here, in lay terms as far as possible. In particular, increasing current will not cause a drop in resistance, voltage is not inversely proportional to resistance (quite the reverse!) and none of this "disproves Ohm's Law".

    Those of you guys who know about this, please forgive me if I have over-simplified for the sake of clarity. And no, it is not a good idea to use instrument cable for speakers, for good reasons outlined in earlier posts.

    Bill (university physical science lecturer)

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