Preamplifiers for microphone cartridges

The output from a condenser microphone is a very high-impedance signal and is therefore very sensitive to the capacitive loads of cables. This makes it necessary to introduce a driver with high input impedance and low output impedance. Such a driver is called a preamplifier.

The frequency range of a preamplifier is determined by its electronic circuit and is typically more than 200 kHz at the high end and 1-10 Hz at the lower end. The lower end is determined by the input impedance of the preamplifier and the capacitance of the microphone. High microphone capacitance gives a low cut-off frequency.

The dynamic range of a preamplifier is defined as the range between the highest level that the preamplifier can handle without distortion, and the lowest level it can measure. The highest level is related to the preamplifier’s supply voltage, whereas the lowest level is related to the electrical noise generated by the preamplifier itself.

There are today two different preamplifier principles in the world of acoustics. One is the traditional type for externally-polarized microphones often referred to as the “LEMO” type because of its 7-pin connector and has become an industry standard. It is voltage driven and can handle high voltage signals up to 50Vpeak.

The other principle uses a Constant Current Power (CCP) supply and was introduced around 1996 to the world of high-precision acoustics. Before that, the quality of CCP preamplifiers was not as good as the voltage driven LEMO types, but this is not the case today. A CCP preamplifier uses a Constant Current Power supply, which must be between 2 mA and 20 mA (nominally 4 mA), to produce a constant nominal voltage level of 12 Volt DC (referred to as the bias voltage). The output signal from the microphone superimposes fluctuations around this DC level. The great advantage of CCP preamplifiers is that they use a two-wire system, where the signal is superimposed on the wire through which the current is kept constant.

This means that simple co-axial cables can be used instead of the more complex 7-core cables used with the voltage driven LEMO types. This is traded off by accepting a lower upper limit in dynamic range (due to the lower driving voltage of a constant-current source) which limits the maximum output signal to approximately 8 Vpeak, and the necessity of having to use prepolarized microphones. The range of available prepolarized microphones is still not as wide as for externally-polarized microphones, although GRAS was the first in the world to introduce 1/4” prepolarized microphones and have also released a 1/8” prepolarized microphone.

GRAS microphone preamplifiers are all small, robust units optimised for acoustical measurements with condenser microphones. They are all compatible with measurement microphones as defined in the international standard IEC 61094 “Measurement Microphones, Part 4: Specifications for working standard microphones”.

All GRAS preamplifiers are built around a small, thick-film precision amplifier with very high input impedance. The casings are made of stainless steel for maximum strength and durability with minimal sensitivity to vibration and microphonics.

They will work within their specifications up to a temperature of 70°C (158°F). Special versions for use at temperatures up to 120°C (248°F) are available on request. The only effect of elevated temperature is a slight increase in inherent noise level. This will change the lower limit of the dynamic range of the microphone/preamplifier combination, thus limiting the ability to measure very low sound pressure levels.

CCP (Constant Current Power) is the same as IEPE (Integrated Electronic Piezo-Electric) and CCLD (Constant Current Line Drive) and is compatible with many other constant current driven products such as Deltatron®, Isotron, etc.

DeltaTron is a trademark of Brüel & Kjaer

GRAS 26AA 1/4" Preamplifier with Integrated Cable for Intensity Probes

Freq range: 2.5 Hz - 200 kHz
Noise: 1.8 µV
Gain: -0.29 dB
Special feature: For GRAS intensity probe handles with 4-pin LEMO connectors

GRAS 26AA 1/4" Preamplifier with Integrated Cable for Intensity Probes

The 26AA GRAS ¼″ Preamplifier is a small robust unit optimized for acoustic measurements using condenser microphones. It has a very low inherent noise level, a wide dynamic range and a frequency response from below 2 Hz to above 200 kHz...

GRAS 26AB 1/4''-1/2'' Universal Preamplifier with Integrated Connector

Freq range: 2.5 Hz - 200 kHz
Noise: 1.8 µV Gain: -0.29 dB
Special feature: For both 1/2" and 1/4" microphone cartridges

GRAS 26AB 1/4''-1/2'' Universal Preamplifier with Integrated Connector

The GRAS 26AB 1/4" Preamplifier is a small robust unit optimised for acoustic measurementsusing condenser microphones. It has a very low inherent noise level, a wide dynamic range and a frequency response from below 2 Hz to above 200 kHz...

GRAS 26AC-1 1/4" Standard Preamplifier with Integrated Miniconnector

Freq range: 2.5 Hz - 200 kHz Noise: 1.8 µV Gain: -0.29 dB Special feature: General purpose

GRAS 26AC-1 1/4" Standard Preamplifier with Integrated Miniconnector

The GRAS 26AC-1 1/4" Preamplifier is a small robust unit optimised for acoustic measurements using condenser microphones. It has a very low inherent noise level, a wide dynamic range and a frequency response from below 2 Hz to above 200 kHz...

GRAS 26AH 1/2'' SysCheck Preamplifier with 3 m integrated cable

Freq range: 2.5 Hz - 200 kHz
Noise: 1.8 µV
Gain: -0.35 dB
Special feature: SysCheck

GRAS 26AH 1/2'' SysCheck Preamplifier with 3 m integrated cable

The GRAS 26AH ½-inch Preamplifier with SysCheck is a small and robust unit optimised for acoustic measurements using condenser microphones. It has a very low inherent noise level, a wide dynamic range and a frequency response from below 2 Hz to above200 kHz...

GRAS 26AI 1/2'' Preamplifier with Integrated Connector, Low Frequency

Freq range: 1 Hz to 200 kHz Noise: 1.8 µV Gain: -0.29 dB Special feature: Low frequency applications

GRAS 26AI 1/2'' Preamplifier with Integrated Connector, Low Frequency

The 26AI is a 1/2" microphone preamplifier with integrated 7-pin LEMO male connector and 40 GΩ input impedance...

GRAS 26AJ 1/2'' SysCheck Preamplifier with integrated connector

Freq range: 2.5 Hz - 200 kHz
Noise: 1.8 µV
Gain: -0.35 dB
Special feature: SysCheck

GRAS 26AJ 1/2'' SysCheck Preamplifier with integrated connector

The GRAS 26AJ ½-inch Preamplifier with SysCheck is a small robust unit optimised for acoustic measurements using condenser microphones. It has a very low inherent noise level, a wide dynamic range and a frequency response from below 2 Hz to above 200 kHz...

GRAS 26AK 1/2'' Standard Preamplifier with Integrated Connector

Freq range: 2.5 Hz to 200 kHz
Noise: 1.8 µV Gain: -0.25 dB
Special feature: General purpose

GRAS 26AK 1/2'' Standard Preamplifier with Integrated Connector

The 26AK is a 1/2" microphone preamplifier with integrated 7-pin LEMO male connector. The CCP equivalent is GRAS 26CA...

GRAS 26AK-S1 1/2'' Preamplifier with Integrated Connector, Low Frequency

Freq range: 2.5 Hz to 200 kHz
Noise: 1.8 µV Gain: -0.25 dB
Special feature: Low frequency applications

GRAS 26AK-S1 1/2'' Preamplifier with Integrated Connector, Low Frequency

The 26AK-S1 is a 1/2" microphone preamplifier with integrated 7-pin LEMO male connector and 40 GΩ input impedance...

GRAS 26AL 1/4" SysCheck Preamplifier with 3 m integrated cable

Freq range: 2.5 Hz to 200 kHz
Noise: 1.8 µV Gain: -0.29 dB
Special feature: In-situ sensitivity calibration of 1/4" microphone/preamplifier set

GRAS 26AL 1/4" SysCheck Preamplifier with 3 m integrated cable

The GRAS ¼" Preamplifier Type 26AL is a small robust unit optimised for acoustic measurements using condenser microphones. Type 26AL hasa very low inherent noise level, a wide dynamic range and a frequency response from below 2 Hz to above 200 kHz. It is similar to Type 26AC, buth as a built-in SysC..

GRAS 26AM 1/2'' Standard Preamplifier with 3 m integrated cable

Freq range: 2.5 Hz - 200 kHz
Noise: 1.8 µV
Gain: -0.25 dB
Special feature: General purpose

GRAS 26AM 1/2'' Standard Preamplifier with 3 m integrated cable

The GRAS 26AM ½-inch Preamplifier is a small robust unit optimised for acoustic measurements using condenser microphones. It hase a very low inherent noise level, a wide dynamic range and a frequency response from below 2 Hz to above 200 kHz...

GRAS 26AR 1/4" Standard Preamplifier with Integrated Mini-connector

Freq range: 2.5 Hz - 200 kHz
Noise: 4 µV Gain: -0.35 dB
Special feature: For integration into structures

GRAS 26AR 1/4" Standard Preamplifier with Integrated Mini-connector

The GRAS 26AR 1/4" Preamplifier is a small robust unit short enough for use in confined spaces. It has a very low inherent-noise level, a wide dynamic range and a frequency response from below 2 Hz to above 200 kHz...

GRAS 26AS 1/4" Standard Preamplifier with 3 m Integrated Cable, Very Short

Freq range: 2.5 Hz - 200 kHz
Noise: 4 µV Gain: -0.29 dB
Special feature: For confined spaces

GRAS 26AS 1/4" Standard Preamplifier with 3 m Integrated Cable, Very Short

The GRAS ¼" Preamplifier Type 26AS is a small robust unit short enough for use in confined spaces, e.g. for binaural measurements with the KEMAR Manikin (GRAS 45BB or GRAS 45BC) or GRAS 45CA Hearing-protector Test Fixture. It has a very low inherent-noise level, a wide dynamic range and a frequency ..