Over here at Rattlesnake Cable Company and talking with a variety of musicians over the years, I get asked this question a lot. It's a simple question that is easy to scratch your head on. Both types of cables, at first glance, look the same. Basically a cable with a couple of mono phono plugs on each end. Both seem to work either way, so does it matter? Absolutely!
But, let's first talk about the differences of these cables. We need to know what makes a guitar cable or instrument cable different than a speaker cable.
The anatomy of these cables are quite different because they are designed to perform completely different tasks.
With a speaker cable, there are two speaker wires – a pair running through the cable. Typically without RF shielding (RF stands for radio frequency). These speaker wires tend to be heavier in gauge. When talking about gauge we use AWG (American Wire Gauge). The values of these are slightly counter-intuitive, where higher number values are thinner and smaller number values are thicker.
For instance our high-end Speaker Cables use a pair of 12AWG or 12 gauge speaker wire, which is quite thick, but our instrument cable conductor uses a 20AWG or 20 gauge conductor. This pair of speaker wires is used to pass positive and negative current from the amplifier to the speaker or speakers.
Instrument cables are quite different. Instrument or guitar cables have a center conductor that passes a very low or small current which carries the signal. This conductor wire has insulation around it and is then wrapped in a copper mesh (or shielding). This shielding protects the conductor from picking up RF or radio frequencies. This style of cable is also known as a coaxial cable or coax. With instrument cables you do need to worry about cable capacitance along with RF noise. If you're not familiar, definitely check out our article “How to find the best guitar cable”
Besides the cable itself, we also need to talk about the plugs. Instrument cables use standard mono plugs. Generally, the Neutrik NP2 series are very durable and more than acceptable for instrument cables. With the heavier gauge of the Speaker Cable, you will need a beefier solder pad to take the heat and current. That is why we use Switchcraft 188 Jumbo Straight plugs for our Speaker Cables. It's a small detail, but very important to also consider when talking about the differences.
We now understand that both cables basically have two conductors – speaker uses a pair of thick conductors, instrument cables uses a center conductor and shield. Their uses are different, but in reality, "it works" - but is a horrible idea. Let's talk about why.
If you decide to use a speaker cable as an instrument cable, it will work to pass signal and ground (shield contact) and complete the cable circuit. BUT, since the shield, in this case, will just be a wire and not actually wrapping and protecting the conductor, it's wide open to RF (Radio Frequency). It will be incredibly noisy, especially if the cable is long. Cable capacitance will be quite large, so your guitar signal will sound quite muddy and dark – again, the longer it goes.
Where things gets crazy - is using an instrument cable as a speaker cable. Now, we have negative current using the shield and the positive current using the center conductor. Typically a center conductor is 20-24AWG so pretty thin in comparison to 12AWG. That's a lot of current and things can get pretty hot depending on the current you're pushing through the cable. If things get too hot, things can melt. If you have a dead (or shorted) cable at this point, and your amplifier is still trying to push a load and it can't, you'll have a very unhappy amplifier and things can continually go downhill for you - like damage to the output transformers of the amp.
I also have a theory that the variable resistance could be bad for the amp. The shield of the instrument cable has a lot more copper in comparison to the conductor so you would have unbalanced resistance between positive and negative.
I know some of you might read this and say – I've been using a guitar cable as a speaker cable for years without a problem. Then you're lucky and maybe knock-on-wood now. That's like saying, I've been driving a stick and never took it out of 1st gear and no problems with the engine.
During my brainstorming for this article, I thought it would be fun to ask some experts. But I wanted to get a little personal with this by hitting up some builders (legends) I know.
The very first high quality amp I ever bought was a Rivera Knucklehead 100 in 1997/1998. At the time it was like $2000 for this thing, and I saved every penny, sold everything I could to get my hands on one. In fact, that was the very first tube amp I owned. This thing absolutely blew my mind. At Winter NAMM 2020, while at our booth, I actually saw the man himself – Paul Rivera, Sr. of Rivera Amplification, and went over there and shook his hand (pre-COVID days) and thanked him for that amp. So let's hear what Paul Rivera, Sr. has to say about using a guitar cable for a speaker cable.
Transmission of power require low resistance for low loss. Also, high current requires larger cross section of wire. Larger power needs smaller gauge-larger wire. Guitar cables are designed for low noise and capacitance, and tiny amounts of current. If you use them for a speaker cable, large losses in power transmission occur. – Paul Rivera, Sr.
I used that Knucklehead for years. Still own it. Still love it. But in the 2000s I've been dreaming and drooling about the Matchless DC-30 tone. A band mate had one so I had to listen to Matchless tones for years. In 2013, I finally pulled the trigger on the Matchless HC-30 (head version) and still using that as my primary amp since 2013. That thing is an absolutely tone monster, and every time I play it – I feel it. So I thought it would be great to ask this question to Phil Jamison of Matchless Amplifiers
Instrument cables are very different than speaker cables.
Instrument cables should have a small gauge of wire wrapped in shielding so you won't wind up having a large antenna or a loud hum. This is important for any signal path. Positive on the tip and the shield is used as the ground.
Speaker cables should be a very heavy gauge of wire and typically 12 to 16 gauge two conductor, positive and a ground that can handle the output of the amp to speaker. You wouldn't use shielded cable simply because the shield would not be adequate enough to handle the output current.
This might be the best for last. We've worked with Verellen Amplifiers over the years by providing speaker cables and I'm a big fan of his band Helms Alee. I gave him a call to see if I get his thoughts on the subject of using an instrument cable as a speaker cable and he sent over this insane explanation with math. With math people.. with math!
Amplifiers put large amounts of current through the cable between an amp and the speaker. To get an idea of how much current we're talking about... below are some examples of calculating the amount of current that a cable charged with the task of carrying amplifier output current to a speaker load would endure with the amp spitting it's maximum available output power (ie: cranked).....
A word about power ratings.... An amp that is rated by the manufacturer for say...100 watts for example, is telling you that 100 watts RMS (root mean squared) is the maximum clean output power the amplifier can output before the amp begins to distort. Guitar players in practice are often willing to crank the amp to its maxim clean power and beyond, so we'll guestimate a "cranked" amp puts out 20% above it's rated max power (this overshoot will depend from amp to amp).
Now the pesky math....
Some variable nomenclature.....
I : current
R: resistance (or in this case, nominal speaker impedance)
Power formula for current through a speaker: P=I^2/R
therefore via algebra shifting: I = SQRT (P/R)
So.. three specific examples:
100 watt amp (120 watts cranked) : 4 ohm speaker: I=SQRT(120/4) = 5.4 Amps
300 watt amp (360 cranked): 4 ohm speaker: I = 9.5 Amps
15 watt amp (18 cranked): 16 ohm speaker: I = 1.06 Amps
Now you can see that each of these amp/speaker combinations produce very different current outcomes.
"Instrument cables" are designed with very low current in mind and most design effort is around reducing the internal capacitance of the cable as well as shielding out as much potential interference as possible. They're not designed for handling a lot of current, and if they are subjected to more current than they can handle, they will heat up and eventually either fuse (break) or burn through the insulation shorting from one wire to the other possibly causing damage to the amp.
So how much current can a "speaker cable" handle? Depends!
Here is the problem.... cables are made in certain thickness gauges (AWG), the thicker the gauge the more current they can handle, but they are NOT rated for current specifically, so if you have a given wattage amp and a given speaker, and calculate the resulting current, you can't say...grab a 10A speaker cable for your 300 watt 'meatmsoke' amp. You have to do some guestimating. And we haven't even begun to look at cable length, which also becomes a factor for long cable runs (not typical for amp to cabs I know).
So, frustratingly, all of our analysis only gives us a general idea that we want a thicker gauge speaker wire than a typical instrument cable.
One concrete rating that I think is helpful... an IEC (International Electrotechnical Commission) power cable rated for 10A uses 18 AWG wire. So that gives us an indication that IEC testing has led to confidence in an 18AWG wire being able to reliably handle as much as 10A. So our highest current scenario calculated of a "300 watt amp" into a 4 ohm load----> 9.5A, would be acceptable with an 18AWG speaker cable according to to the IEC. And any gauge thicker than that would be added insurance.
Some amp designers especially in the hifi world talk about the audio benefits of using oversized speaker cable. I'll leave that cork sniffing to the individual, but I do think that when it's affordable to overdo something, overdo it. Peace of mind is worth a few extra bucks.
Since powered speakers or powered monitors are "powered", the cables are NOT passing current. So typically, depending on the speakers, you'll either need unbalanced mono cables (instrument cable or guitar cable) or a balanced TRS cable going from 1/4" TRS to XLR (or possibly a straight up XLR cable). All of these cases, they would only be passing signal so we're talking higher gauge (20AWG for example) shielded cable.
In conclusion, I hope this answers the question - "can you use instrument cable for speaker?". Short answer is ‘no', but now you know the why! I want to to say thanks to my amplifier hero builders for contributing to this article on speaker cables!