I know that every signal source that we may feed into an amplifier has a source
ID: 1800625 • Letter: I
Question
I know that every signal source that we may feed into an amplifier has a source impedance. (This is the same as the "Thevenin resistance" of the signal source.) In some cases - like the signal coming from a strain gauge - the source impedance is rather low; perhaps a few hundred ohms. In other cases - like an electret microphone element, or a chemical pH probe - the sensor impedance is quite high; even into the megohm range.Does the signal source impedance make a difference when we try to amplify these signals?
If we know (approximately) what the signal source impedance is, how does this affect the way we design an amplifier for that signal?
Explanation / Answer
The output from most electronic devices in an audio system will be of low impedance in nature, usually 150 Ohms or less. However, the output from many passive devices, such as a high impedance microphone or passive guitar pickup can have much greater output impedance. What's the difference and why is it important to know how to deal with these signals in an audio system? Impedance (Z) is the measure of the total opposition to current flow in an alternating current circuit. It is made up of the sum of two components, resistance (R) and reactance (X). Z = R + X Resistance is essentially constant at all frequencies in an audio circuit and is measured in Ohms. Reactance is the measure of opposition to the flow of alternating current caused by the effects of inductance and capacitance in a circuit. It is also measured in Ohms but it will vary with frequency. The following formula for inductive reactance illustrates how its opposition to current flow increases as the frequency and/or the amount of inductance increases: where F = the frequency in Hertz (cycles per second) and L = the inductance in Henrys. The following formula for capacitive reactance illustrates how its opposition to current flow decreases as the frequency and/or capacitance increases. where F = the frequency in Hertz and C = the capacitance in Farads. These formulas also point out the fact that a specified impedance for an audio device is only going to be valid for a single frequency - the actual operating impedance will vary greatly over the audio frequency spectrum. The Low vs. High Difference A high impedance microphone or guitar will usually output a greater signal (voltage) than a low impedance microphone. This high impedance signal works fine and even has some advantages in a sound system as the mixer or amplifier doesn't need to boost the signal as much. Therefore, any noise on the line is also not amplified as much and this results in an improved signal to noise ratio. Keep in mind however, that the impedance of the transmission line (or cable) is affected by the impedances of the devices that are connected to it. A low impedance microphone will lower the impedance of the entire line connected to it. Similarly, if you connect a high impedance microphone, you will have a higher impedance line all the way from the microphone to the mixer. This can become a problem as the length of the cable increases. High impedance lines are more adversely affected by the inherent capacitance that is present in the cable itself. This capacitance combines with the impedances of the source and destination to set up a filter. As the impedance increases and/or the capacitance per foot increases, the active frequency at which the filter comes into play gets lower. The frequencies above this point actually begin to “short out” across the cable's conductors before they ever get to their intended destination. Keeping impedance low and using quality cables can be important issues for maintaining wide frequency response in long lines. A high impedance line that is interacting with outside electrical interference will act more like an “antenna” than a low impedance line. This problem can get worse as the cable gets longer. This effect is usually insignificant for a guitar or high-Z microphone plugged into an amp with a 15' cord but it can have a big effect if that same signal is sent 100' down a snake. These are reasons why a high impedance signal is almost always converted to low impedance with the use of a Direct Box (DI) before being sent long distances.