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Selection Guide for Microphone Preamplifiers

This article explains the differences between the two types of preamplifiers and their fundamental characteristics.

Introduction

The output signal from a microphone is very weak and cannot drive cables, so, it is necessary to mount a preamplifier directly on to the microphone or very close to it. The preamplifier does not normally amplify the microphone signal but converts the high impedance microphone signal to a low impedance signal which can drive long cables etc; for this reason, a preamplifier is sometimes also called an impedance converter.

Preamplifier types

Preamplifiers come in two different versions; one is what is normally referred to as "traditional" and the other as Constant Current Power (CCP). Both types are available from G.R.A.S. and are described here.

Traditional preamplifiers

Traditional preamplifiers require an external power supply which delivers a supply voltage of either ±15 to ±60 V DC or +28 to +120 V DC. This type of preamplifier is normally connected to a suitable power supply (for example the G.R.A.S. Type 12AK) via a 7-pin LEMO plug, see diagram below, or to an analyser with a suitable LEMO connector.

7 pin Lemo Plug

CCP preamplifiers

CCP preamplifiers connect to a CCP supply (for example the G.R.A.S. Type 12AL) via a standard coaxial cable and plug (e.g. BNC) or to an analyser with a CCP input. In a CCP preamplifier, the power supply maintains a constant current superimposed on the same wire carrying the signal, therefore, requiring only a standard coaxial cable and connector.

Preamplifier frequency range

All G.R.A.S. preamplifiers cover a frequency range which tops at 200 kHz, whereas the lower limit of the frequency range is determined by the microphone’s capacitance.

Most G.R.A.S. preamplifiers have an input impedance of 20 GΩ which, combined with the capacitance of the microphone cartridge, forms an RC network that determines the low-frequency cut-off.

A typical microphone, such as the G.R.A.S. Type 40AF, has a capacitance of 17 pF which results in an electrical lower-limiting frequency of 2.7 Hz.

Preamplifier Dynamic Range

The dynamic range of a preamplifier can be defined as the range between the highest level the preamplifier can handle and the lowest level it can measure. The highest level is related to the voltage supplied to the preamplifier, whereas the lowest level is related to the noise generated by the preamplifier itself.

Upper limit of dynamic range

The highest level is related to the voltage supplied to the preamplifier so that the peak-to-peak variation in signal levels that can be handled by the preamplifier is slightly less than the voltage supplied. For example, for a supply voltage of ± 14 V DC, the preamplifier can handle peak signals up to 12 V or peak-to-peak signals up to 24 V, see diagram below.

Preamplifier Max Voltage Swing

For a CCP preamplifier, the supply voltage is limited to 24 V which gives a maximum peak-to-peak output signal of 22 V.


The sound pressure levels which can be measured with a particular preamplifier depends on the sensitivity of the microphone used for the measurements, such that a microphone with a low sensitivity can measure higher levels than a microphone with a high sensitivity, see diagram below.

Max SPL vs Sensitivity

Lower limit of dynamic range

The lower limit of a preamplifier’s dynamic range is determined by the noise floor of the preamplifier. A typical preamplifier generates a broad-band noise signal of around 3 µV and will mask the microphone's signal if it lies below this.

The noise spectrum of the preamplifier is dominated by low frequencies as shown in the graph below.

Microphone and Preamplifier Noise

When a microphone is mounted on a preamplifier, the inherent noise of the microphone will be added to the preamplifier noise and the noise floor of the system will be determined by the combined microphone and preamplifier noise.

As the graph above shows, the lower limit of the dynamic range is determined by preamplifier noise at the low frequencies and by microphone noise at the high frequencies.

Copyright® 2004 G.R.A.S. Sound & Vibration A/S 

About G.R.A.S. Sound & Vibration

Gunnar RasmussenG.R.A.S. Sound & Vibration was founded in 1994 by Gunnar Rasmussen and is a 100% privately owned company. The company is located in Holte, Denmark, 20 km north of Copenhagen and produces acoustic front-end products such as microphones, preamplifiers and signal conditioning devices. Gunnar Rasmussen is well known for his numerous contributions to the development of noise and vibration measurement instrumentation. For more than 50 years in the industry he has developed a number of key products such as modern measurement microphones, sound level meters, sound intensity probes, delta shear accelerometers and many others. He has also worked with precision, free-field and pressure reciprocity calibrations, as well as pistonphone and laser calibrations.

For more information, please visit www.gras.dk

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