Modern day recording and broadcast is a mixed bag for today's audio engineers when it comes to input signal chains. Microphone electronic technology really hasn't changed that much over the last 50 years. The concept is mostly based around capturing mechanical movement and translating it into electrical energy.
Generally that electrical energy is miniscule and requires significant gain to bring it to operable levels. Figure 1 shows a typical input signal interface between a microphone and the rest of the signal chain.
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| Figure 1. | A typical front-end signal chain between microphone and the preamplifier. |
A phantom power supply is a 48 V DC voltage source used to power any active circuitry in the microphone and set the correct bias for the capsule normally used in condenser microphones.
Usually, the phantom power supply isn't required by any devices further down the signal chain. The microphone preamplifier is usually running from lower value split voltage supplies (±5 V … ±15 V). Many microphone preamplifiers ICs simply aren't designed to accept such a high voltage (48 V) on their inputs. Because of this, DC blocking capacitors are needed to curb the flow of any DC signal going to the microphone preamplifier's electronics. Unfortunately, the phantom power supply has a tendency to find its way through those DC blocking caps and cause some problems.
For those of you who have programmed processors that deal with button presses, mechanical bounce may be something in which you are well versed. When a mechanical switch or relay is activated, the connection isn't always a clean 0-to-1 transition event. The contacts “bounce” together briefly until they settle in place.
Consider for a moment what a +48 V DC signal looks like when you switch it on and off rapidly. The DC blocking capacitors, like those used in our microphone preamplifier see an AC signal, and let the large 48 V straight through.
A good preamplifier front end design includes input protection diodes that will conduct any voltages that are too high or too low away from the device, thus protecting the input IC device. These diodes are the only saving grace we have at the input.
Some designers with little experience in this kind of application simply put regular diodes into the circuit, like 1N4004s or signal diodes as 1N4148. This experience teaches us that designers should really try to use low-capacitance Schottky diodes to conduct excess current up to the power supply.
Diodes used to protect microphone preamplifier devices need to have a very fast turn-on time, so they can react to these impulses quickly while having the ability to conduct a lot of current with low voltage drop. Typically, designers should be prepared for up to 3 A of current.
If the power-supply voltage of the microphone preamplifier circuit is relatively low (like a ±5 V device), then it's critical to make sure that any large voltages that come towards the preamplifier are tapped off to the power rails as quickly as possible.
To stop the power rails from changing significantly, the power supply must have additional Zener diodes to ensure that excess voltage does not raise the power supply of ICs beyond absolute maximum level.
