By Gino Sigismondi
Impedance Matching: In the article, Gino answers five very important questions on impedance.
This month’s myth concerns connecting devices in your sound system, such as the microphone to the mixer (or whatever happens to be the next device in the chain). As Pat Brown is so fond of saying, connecting your sound system is not “plug-and-play” like hooking up a mouse to your computer. Many people with a passing familiarity of hooking up sound systems are at least aware of impedance as a consideration, but mistakenly assume that the impedance of two connecting devices must be “matched” or of an equivalent value. Unfortunately for them, the reality is more or less the opposite! First, a little background:
What is impedance?
Impedance is how much a device resists the flow of an AC signal, such as audio. Impedance is similar to resistance which is how much a device resists the flow of a DC signal. Both impedance and resistance are measured in ohms, the symbol for which is ‘Ω’.
What is the difference between low, medium, and high impedance?
When referring to microphones, low impedance is less than 600Ω; medium impedance is 600Ω to 10,000Ω; high impedance is greater than 10,000Ω.
For audio circuits, is it important to match impedance?
Not any more. In the early part of the 20th century, it was important to match impedance. Bell Laboratories found that to achieve maximum power transfer in long distance telephone circuits, the impedances of different devices should be matched. Impedance matching reduced the number of vacuum tube amplifiers needed, which were expensive, bulky, and heat producing.
In 1948, Bell Laboratories invented the transistor — a cheap, small, efficient amplifier. The transistor utilizes maximum voltage transfer more efficiently than maximum power transfer. For maximum voltage transfer, the destination device (called the “load”) should have an impedance of at least ten times that of the sending device (called the “source”). This is known as BRIDGING. Bridging is the most common circuit configuration when connecting audio devices. With modern audio circuits, matching impedances can actually degrade audio performance.
Are low impedance mixer inputs really low impedance?
Audio mixers often have inputs labeled as “low impedance“. Actually, these inputs have impedances between 1000Ω and 2000Ω in order to properly bridge the low impedance microphone. A low impedance microphone may always be connected to an input with a higher impedance. However, the microphone may not always be able to provide enough signal strength to properly drive the mixer’s audio input. Always compare the microphone’s output level (or sensitivity) to the required mixer input level.
What happens when a high impedance microphone is connected to a low impedance input?
When a microphone is connected to a mixer input with lower impedance, there will be some loss of the microphone signal. To determine the amount of signal loss (or “load loss”) in dB, use the following equation:
LoadLoss = 20 * log (R load/R load + R source)
To illustrate, take a high impedance microphone (if you can find one), which has a source impedance of 10,000 ohms. The load impedance of a mixer designed to accept a low impedance microphone will typically be around 3000 ohms. This results in a loss of almost 13 dB!
Conversely, a typical low impedance microphone is rated at 150 ohms. Using the same equation, the loss is less than .5 dB. To address this month’s myth directly, what happens when we match impedance? Connecting a 600 ohm microphone to a 600 ohm mixer input results in 6 dB of loss. Fatal? No. But I think most of us would take .5 dB over 6 dB of loss any day. So not only is impedance matching not required, it actually makes things worse. Remember, the mixer input impedance should be at least 10 times greater than the microphone.
As I finish typing this, I’ll need to start packing for a few days in San Francisco. If you plan to be at the AES show, please stop by the Shure booth and say hello!