High Impedance vs Low Impedance

Are low impedance signals balanced? What is the difference between impedance and balanced / unbalanced transmission? In this post, let's take a look at how different rating of impedances will affect your audio signal integrity in the real world.

Before we go into the topic of impedance, I would like to point out that the impedance level of a transmission level does not dictate the type of transmission (balanced / unbalanced). A very simple scenario would be a low impedance microphone connected to an audio mixer with an XLR - 1/4 TS cable (unbalanced cable), thus creating an unbalanced output. Similarly, if the same low impedance microphone is connected through an XLR - XLR cable (balanced cable), the output of the microphone would be balanced. In this simple explanation, we can easily determine that the type of cable used will affect the type of transmission (balanced / unbalanced). (Refer here for more info)

Common examples of a high impedance equipment would be an electric guitar or a karaoke microphone. These equipment are usually seen in consumer products as they are able to provide a relatively higher output signal level as compared to low impedance equipment, hence, needing less amplification or gain. In contrast, low impedance equipment has a lower output level and often require input transformer before preamplifiers circuits in order to set up or strengthen the signal. This is one of the main contributing reason for most consumer devices to adopt high impedance output as in most cases, they do not require amplification after input (negligible) which in turn cut down on overall hardware cost (e.g. preamplifier circuits).

BAE 1272 Microphone Preamplifier, Credits BAE Audio

The main disadvantage of high impedance signal is that they do not perform well over long distance signal transmission (more than 10 meters). In addition, a high impedance line is adversely affected by the inherent capacitance that is present in the cable. This capacitance combines with the impedance of the source create a "low - pass filter"which progressively cuts higher frequencies (the longer the length of the cable, the lower the cutoff of the frequencies). Furthermore, high impedance lines are also more susceptible to the various forms of interference (e.g. high frequencies noise and radio). Therefore, due to these reasons, long cable distance in high impedance output are not practised in production work. Low impedance output, on the other hand, performs well in these situations, retain a higher rating of signal integrity. This is also why many professional grade audio equipment have adopted low impedance output.

An important note, the equipment impedance connected to the cable will affect the transmission line impedance. For instance, a low impedance microphone will lower the impedance of the entire line connected to it. Similarly, a high impedance microphone will have higher impedance line throughout the whole transmission (microphone to an audio console).

In order to preserve signal level, it is important to drive the input of an equipment (input impedance) with a source signal (output impedance) that is significantly lower. If the input impedance of a device is not significantly lower than the source impedance (output impedance), signal integrity will suffer (e.g. low SNR value).

When a signal needs to be split and sent to more than one routes, the impedance of the destinations provide additional paths for the electrical circuits. As a result, reducing the overall load impedance introduced to the signal. As a general rule of thumb, it is advisable to work on 10 : 1 load impedance ratio to help retain signal integrity.  For example, a mixing console output with an impedance of 100 Ω needs to be sent to 4 amplifiers, each with an input impedance of 20 000 Ω, but can the signal be split passively?  Note that we should try to keep loading impedance at the ratio of 10 : 1 or above and there are multiple input destinations.

ARX MSX 48 Mic/Line Splitter (Passive), Credits ARX

Hence, the signal could be split passively as the loading impedance ratio is well above 10 : 1. There are manly ways to split signals, one common way would be to use a passive cable split (“Y” cord cable) or to use a passive isolated mic / line splitter.

Passive "Y-Cord" Mic Cable, Credits Amazon/Hosa