Impedance is an important part of audio technology, all the way from plugging your guitar into your amp, your amp into your cab, the cabinet microphone into the preamp and all the downstream connections, both digital and analogue, right the way to your listening experience, from speakers to plugging headphones into your mp3 player. However, how much do we know as producers and music listeners, and how much do we need to?
In all actuality, most of us don’t really need to know that much about it, previous generations of telecom, audio and electrical engineers have addressed a lot of the issues for us and, generally, if we plug something into something else, everything has been designed so that it will just work. There may, however, be a couple of things you may want to consider…
What is impedance?
For those of an electrical persuasion, impedance (Z) represents the total resistance of a particular circuit including, not just the resistors, but also the resistive contributions of other components such as capacitors and inductors. Like resistance, it is measured in Ohms. Voltage (V), current (I) and resistance (R) are related following Ohm’s law: V=IR; power (P) is the product of the voltage and the current so P=VI or P=V^2/R if we do a bit of substituting and rearranging. From that, we can see that resistance/impedance can affect our power output. And that is probably as far as we need to go here.
What does this mean for my hi-fi?
Most hi-fi separates equipment that connects using RCA/phono connectors has been designed with appropriate output and input impedances, so there isn’t usually anything to worry about. However, there might be one area where it pays to think about impedance: you can generally ignore marketing copy that tells you to spend a fortune on hugely expensive speaker cables, for most purposes, using thick, 2-core mains cable will be fine. What you are aiming to do is to reduce the resistance of the cable to ensure maximum power transfer to your speakers. Resistance is proportional to the length of the wire and inversely proportional to the cross-sectional area, the thicker the wire, the lower the resistance. The cheap doorbell cables that come with a lot of mini systems is often quite high resistance, and you lose a lot of the power as heat. Most hi-fi separates equipment that connects using RCA/phono connectors has been designed with appropriate output and input impedances, so there isn’t usually anything to worry about.
What does this mean for my headphones?
Something you may have seen in the specifications list while headphone shopping is the impedance rating. Generally speaking, higher impedance headphones will be quieter for a given volume setting on your device than lower impedance headphones. However, you do need to consider the efficiency of the driver (you may see it in the specifications as a number followed by dB/mW). Generally though, if you have your mp3 player at 100% volume with the included earbuds (possible around 16 or 32 Ohms, you’ll probably deafen yourself, but plug in some 250 Ohm ‘phones, and you may find you’re able to listen quite comfortably. You’ll also draw less current (as V=IR), so the battery might last longer, but remember you’re trading off volume: you don’t get something for nothing.
Higher impedance headphones are often used for more ‘professional’ audio applications. The increased number of turns of wire can lead to a better ‘motor system’ in dynamic headphones, improving the general sound; also, the reduced current draw can reduce distortion in the amplifier. So, for ‘pro’ applications when you’re plugged into a powerful amp, high impedance is the way to go, for portable devices go with low impedance if you like volume.
What does this mean for my guitar?
If you’re recording your guitar DI, then make sure that you are plugged into a high-impedance (Hi-Z) input. A good rule of thumb is an input impedance of more than ten times the output impedance. Guitar pickups usually quote the DC resistance figure (there is a lot of wire when you start to add a lot of turns!) and are also inductive. Direct inputs for guitars are often rated to 1 mega ohm and higher to present a large load. Additionally, make sure that you use a reasonable quality guitar cable, as some cables can exhibit sufficiently high capacitance to cause problems. And what are these problems? A loss of high end frequencies (the output impedance and cable capacitance act as a high-cut filter) and a reduction in the guitarist’s beloved sustain.
The other area the guitarist needs to pay attention is when plugging a speaker cabinet into their valve amplifier. Valve amplifiers can be very sensitive to inappropriate loads on the output transformer. Without an appropriate load on the secondary coil, the energy in the magnetic field of the primary coil can’t be completely transferred, collapses back and sends that energy back to the valve as a ‘flyback’ voltage. This is usually not good. For that reason, use appropriately rated amps and cabs (a number of amps have different rated outputs for connecting to different cabs) and contact the manufacturer if you are unsure.
Fynn's House and Audio Tech Notes 