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.

Figure 1: A typical front-end signal chain between microphone and the preamplifier
(Click on image to enlarge)

A phantom power supply is a 48V 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 (±5V —> ±15V). Many microphone preamplifiers ICs simply aren't designed to accept such a high voltage (48V) 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 +48V 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 48V 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 3A of current.

If the power-supply voltage of the microphone preamplifier circuit is relatively low (like a ±5V 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.

For more details on input protection, download TI's datasheets for the INA163, INA217 and the PGA2500 at www.ti.com/audio.

About the authors

Dafydd Roche is the Home Entertainment and Professional Audio Marketing Manager for TI's High Performance Analog group. A graduate from the University of York (UK), Dafydd pours his passion and knowledge of audio and music making into his work, doing his part to help enable audio design engineers to make products that end users can't wait to use. In between a hectic life of customer visits, internal meetings and tradeshows, Dafydd still manages to find time to make and record music with fellow musicians in the Dallas area.

Miro Oljaca, Senior Application Engineer at Texas Instruments has over 20 years design experience in the field of Motor Control and Power Conversion. His experience ranges from small fractional to several hundred HP designs. Currently, Miro is responsible for high-precision linear products that focus on industrial applications. Miro received his BSEE and MSEE degrees from the University in Belgrade, YU. With more than 18 international patents in the motor control field, he is a member of AEI, CNI, IEE and IEEE and holds the following titles: Eur Ing, Dott.Ing., MSc and CEng.

Previous installments of this series:

• "SIGNAL CHAIN BASICS (Part 21): Understand and configure analog and digital grounds", www.planetanalog.com/features/showArticle.jhtml;?articleID=210603341, click here
• "SIGNAL CHAIN BASICS (Part 20): Understand the basics of op amps and speed", www.planetanalog.com/features/showArticle.jhtml;?articleID= 210004183, click here
• "SIGNAL CHAIN BASICS (Part 19): Exploring and understanding linear voltage regulators", www.planetanalog.com/features/showArticle.jhtml;?articleID=209900450, click here
• "SIGNAL CHAIN BASICS (Part 18): The op amp as integrator", www.planetanalog.com/features/showArticle.jhtml;?articleID=209101070, click here
• "SIGNAL CHAIN BASICS (Part 17): Hysteresis--Understanding more about the analog voltage comparator", www.planetanalog.com/features/showArticle.jhtml;?articleID=208802817, click here
• "SIGNAL CHAIN BASICS (Part 16): Understanding the analog voltage comparator", www.planetanalog.com/features/showArticle.jhtml;?articleID=208403856, click here
• "SIGNAL CHAIN BASICS (Part 15): Analog/digital converter—dynamic parameters", www.planetanalog.com/features/showArticle.jhtml;?articleID=208401183, click here
• "SIGNAL CHAIN BASICS (Part 14): Analog/digital converter—static parameters", www.planetanalog.com/features/showArticle.jhtml;?articleID=207800114, click here
• "SIGNAL CHAIN BASICS (Part 13): Putting the Bode plot to use", www.planetanalog.com/features/showArticle.jhtml;?articleID=207403561, click here
• "SIGNAL CHAIN BASICS (Part 12): The Bode plot, an essential ac-parameter display tool", www.planetanalog.com/features/showArticle.jhtml;?articleID=207403561, click here
• "SIGNAL CHAIN BASICS (Part 11): Introducing voltage- and power-conditioning circuits", www.planetanalog.com/features/showArticle.jhtml;?articleID=207001505, click here
• "SIGNAL CHAIN BASICS (Part 10): Exploring the Delta-Sigma Converter", www.planetanalog.com/features/showArticle.jhtml;?articleID=206903892, click here
• "SIGNAL CHAIN BASICS (Part 9): SAR Converter Operation Explored", www.planetanalog.com/features/showArticle.jhtml;?articleID=206901015, click here
• "SIGNAL CHAIN BASICS (Part 8): Flash- and Pipeline-Converter Operation Explored", www.planetanalog.com/features/showArticle.jhtml;?articleID=206504089, click here
• "SIGNAL CHAIN BASICS (Part 7): Op Amp Performance Specification--Bias Current", www.planetanalog.com/features/showArticle.jhtml;?articleID=206101908, click here
• "SIGNAL CHAIN BASICS (Part 6): Op Amp Input Voltage Offset", www.planetanalog.com/features/showArticle.jhtml;?articleID=205901111, click here
• "SIGNAL CHAIN BASICS (Part 5): Introduction to the Instrumentation Amplifier", www.planetanalog.com/features/showArticle.jhtml;?articleID=205208593, click here
• "SIGNAL CHAIN BASICS (Part 4): Introduction to analog/digital converter (ADC) types", www.planetanalog.com/features/showArticle.jhtml;?articleID=204803631, click here
• "SIGNAL CHAIN BASICS (Part 3): Analog and the digital world", www.planetanalog.com/features/showArticle.jhtml;?articleID=204400376, click here
• "SIGNAL CHAIN BASICS (Part 2): Op Amp--Basic operations", www.planetanalog.com/features/showArticle.jhtml;?articleID=203101699, click here
• "SIGNAL CHAIN BASICS: Operational Amplifier--The Basic Building Block", www.planetanalog.com/features/showArticle.jhtml;?articleID=202801320, click here