Exactly why is AMP OUTPUT-->GUITAR CABLE-->SPEAKER CAB a bad idea?

Discussion in 'Pickups & Electronics [BG]' started by cazclocker, Sep 20, 2016.

  1. cazclocker

    cazclocker My social skills are rapidly dwindling.

    Oct 24, 2014
    Newton, Kansas
    OK, OK, before you flame me, I already know it's a bad idea to connect an amp output to a speaker using instrument cable. I never do it... I once did a test comparing the two - it definitely sounded different.

    what I'm wondering is electrically, why is speaker cable "better" to handle my amp's output to the speaker cabinet? Hope you guys can tell what I'm asking...are the picofarads divided by the ohms to the millihenrie's power going to blow up if I use instrument cable?
  2. bholder

    bholder Affable Sociopath Gold Supporting Member Supporting Member

    Sep 2, 2001
    Vestal, NY
    Received a gift from Sire* (see sig)
    Not sure it's going to blow anything up, someone will surely correct me if I'm wrong, the difference is primarily that instrument cable is for low level signals, and so is small gauge and shielded. Speaker cable is for high level signals, and tends to be larger gauge and unshielded. The shielding might affect the impedance, not sure, but size of the wires may also be a factor. Speaker cable is cheaper than instrument cable, for what it's worth.
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  3. The main reason is the conductors in a n instrument cable are small 20-22 awg. They will not handle the current and can burn up, short out and take your amp out.
    You can get by with 18 awg for guitar amps but bass needs 14-12 awg as a minimum (my choice-some may use 16 awg and some use larger than 12 awg). The reason the instrument cable will cause it to sound different is it cannot pass enough current to drive the lows in your speaker and the more power you put out the more voltage drop across the cable and heat it up.
    Yes, I am sure impedance mismatch factors in too
    Last edited: Sep 20, 2016
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  4. Johnny Crab

    Johnny Crab HELIX user & BOSE Abuser

    Feb 11, 2004
    In addition to the above, we'll toss in the capacitance of the instrument cable for fun.
    It's also part of the tone-mucking heard before the cable and/or amp decide to let their smoke out.
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  5. cazclocker

    cazclocker My social skills are rapidly dwindling.

    Oct 24, 2014
    Newton, Kansas
    Okay, that all makes sense. You're right, the small size of the instrument cable would pass less current. And I also know cable capacitance is a factor- I recently bought some ELC cables from Lava cable (ELC = extra low capacitance) and they sound better than what I was using before.
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  6. khutch

    khutch Praise Harp

    Aug 20, 2011
    suburban Chicago
    I would not think that an audio power amp would be bothered much by cable capacitance unless it causes it to go unstable. An unstable amp will sound bad. For a few seconds. Right before it blows up! At 20kHz it would take 0.2 uF of cable capacitance to draw even 10% of the current that a 4 Ohm speaker load draws and you are not likely to get that much capacitance with a reasonable length cable run, it would take about a mile of common instrument cable. At bass frequencies the effect on the amplifier will be around two orders of magnitude less.

    If you have a 500 W, 4 Ohm system then you are going to need to deliver 11 Amps to the speaker cab. The current rating of a cable is determined by how high a temperature rise you are willing to allow, how high the room temperature is, and how much voltage loss you are willing to allow in the cable. Obviously if the temperature rise is high enough to melt the insulation then the wires could short and that would be A Very Bad Thing! The current rating of 22 gauge wire for a 35C temperature rise is only 5 A. That is a reasonable temperature rise so if you wanted to use an instrument cable for a speaker cable and it has a center conductor of 22 gauge then you would be limited to 100 W. If you have a 500 W system then you would want to use speaker wires of at least 16 gauge to limit the temperature rise to 35C and 12 gauge if you want to keep the temperature rise to 10C. I am getting this information here, it is on the internet so you know it is true!

    As far as the sound goes the system damping factor is probably the biggest reason to use big, low resistance speaker cables. Damping factor is the speaker cab resistance divided by the amplifier output resistance PLUS the speaker cable resistance. Typical amplifier output resistances are low enough that in many cases the speaker wire resistance will determine the damping factor. Low speaker cable resistance produces a high damping factor and that means the speaker cone motion is well controlled and the audible output is high fidelity. High speaker wire resistance lowers the damping factor and lets the speaker do its own thing, so to speak, which colors the tone of the audible output. You can read more about damping factor here, and again it is on the internet....

    I once read an interesting story about damping factors and tube amp emulators. The author claimed that one of the reasons solid state amps have trouble sounding like tube amps is that their damping factors are too high. Tube amps use output transformers which have winding resistance and this means that tube amps, especially the old classic ones, had fairly low damping factors. His theory was that the "tube amp sound" comes partially from the way the speaker cones of tube amps were free to "ring" at their resonant frequencies, unconstrained by the relatively poor damping factors of vacuum tube power amplifiers. He claimed that by putting a good tube based preamp in front of a modern solid state amp to get that sweet, sweet tubey distortion and then adding resistance to its output to lower the damping factor and let those speaker cones ring anyway they wanted to, he could build amps that no one could distinguish from a full tube amp. This is more internet information of course....
  7. Thanks for another excellent post, Ken.

    The trouble with valve vs solid-state stuff online is that it's usually written by guitarists. Even if it's scientific, mathematics-backed stuff, it's still usually from the perspective of a guitarist. The sound and response of clean guitar and distorted guitar are so foreign to the sound and response of clean bass guitar. Plus we usually use ported cabs with bass drivers, passive cross-overs, piezo drivers etc etc. All very different from typical (mid-range heavy) guitar cabs.

    I've been gigging and recording with an old bassman plus a few modern bass cabs a bit lately and I've actually been wondering about the interactions between output trans, speaker, passive x-over etc. Very thought provoking... Thanks again!
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  8. khutch

    khutch Praise Harp

    Aug 20, 2011
    suburban Chicago
    This is probably true of the article I am referring to. I will see if I can find it again and share the link if I can. On the other hand, if the damping factor theory has any validity I would think you are more likely to hear the effects of speaker ringing on a relatively clean signal. All I really know from experience is that when I engage one of the amp/cab models on my Zoom multifunction pedals there is a noticeable and pleasing change in the sound. I can't quantify what it is but I do hear it.

    OK, here are the links: Part 1 and Part 2.
    Last edited: Sep 22, 2016
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  9. cazclocker

    cazclocker My social skills are rapidly dwindling.

    Oct 24, 2014
    Newton, Kansas
    I like the interesting turn this thread has taken. I suspect there's a lot more to electronic amplification than I ever knew...!!
  10. Passinwind

    Passinwind I know nothing. Commercial User

    Dec 3, 2003
    Columbia River Gorge, WA.
    Owner/Designer &Toaster Tech Passinwind Electronics
    You could start learning a lot more about it in this pretty cool web forum dedicated to that very subject: Amps and Cabs [BG] Yel_wink.gif
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  11. testing1two

    testing1two Supporting Member

    Feb 25, 2009
    Southern California
    The simplest way to understand the difference is this: instrument cable has 1 small, twisted-strand conductor with a loosely braided shield around it that gets tied to ground; a speaker cable has two larger twisted strand conductors with no shielding. If you use an instrument cable as a speaker cable not only are you using a much smaller conductor, you are using the loose braided shield as the other conductor.
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