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Microphone Month 5

Speaker cables, guitar cables and impedances.

Q: “I noticed in a recent inSync Tech Tip regarding matching impedance between head and cabinet that you emphasized using speaker cables instead of guitar cables to connect head to cab. The both look the same to me, so what’s the big deal?”

A: First, let’s consider the instrument cords that connect your guitar to your amplifier, chain together your stomp boxes, and so on. These cords are operating in a high-impedance, small-signal environment. That simply means that they are “feeding” an amplifier or other device that has a high resistance (many thousands of ohms) path to ground. Furthermore, they are carrying a tiny current that is measured in thousandths of an ampere, and a very small voltage. All of these things make them quite susceptible to noise from electromagnetic interference (EMI). Also, because they are most often connected to a device that has fairly high output impedance, such as an electric guitar, cords with too much built-in capacitance will bleed off high frequencies badly, especially over long runs. All shielded cords have some capacitance; the goal is to get wire having as low a capacitance as possible.

Critical factors for a guitar cord or other instrument patch cord:

– Low capacitance – to prevent high frequencies from being lost.
– Good shield coverage – to block noise from EMI.
– Physically rugged – because they typically take a lot of abuse.

“But wait!” you say. “What about low resistance? I don’t want to lose signal volume in the cord!” Remember we said that these cords are feeding a high resistance? Even the cheapest, crummiest, tiny-gauge copper wire you can find is only going to have a few ohms of resistance over a run of 100 feet or less. In practical terms, there is no difference between a few ohms of resistance and zero resistance when you are feeding an amplifier with input impedance that is many thousands of ohms. (Of course, that extra-cheap wire is going to be unsuitable for other reasons – it will probably have very high capacitance and poor shielding.)

Now, let’s consider the speaker cables. Speaker cables operate in an environment that is exactly the opposite of that for instrument cables. Speaker cables connect the low impedance output of an amplifier with the low impedance of loudspeakers. Furthermore, they carry a relatively high current and voltage. Consequently, these cables aren’t very susceptible to noise from EMI but the circuit is very intolerant of high resistances in these cables.

Critical factors for a speaker cable:

– Low resistance – to prevent signal loss.
– Physically rugged – because they typically take a lot of abuse.

“But, what about shielding and capacitance?” you ask. Neither is critical in this low impedance circuit. Only an incredibly powerful EMI source can make an impression on the high-current, low impedance main signal. (It’s not utterly impossible, though.) As for capacitance, even a fairly significant capacitance will still have an impedance that is outrageously high compared to the output impedance of the amplifier, and thus will have little impact on the high frequencies.

So there you have it; with a little knowledge under our belt, we can easily see why it is important to use speaker cable to connect head to cab.

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