Preamplifiers for microphone cartridges

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 G.R.A.S. was the first in the world to introduce 1/4” prepolarized microphones and have also released a 1/8” prepolarized microphone.

G.R.A.S. 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 G.R.A.S. 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

  1. Constant current power (CCP)
    Constant current power (CCP)
  2. Traditional power supply (LEMO)
    Traditional power supply (LEMO)
  1. 26CA 1/2'' CCP Standard Preamp (BNC)
    Freq range: 2.5 Hz - 200 kHz
    Noise: 1.8 µV
    Gain: -0.30 dB
    Special feature: General purpose

    G.R.A.S. 26CA 1/2'' CCP Standard Preamplifier with BNC Connector

    The G.R.A.S. ½-inch Preamplifi er G.R.A.S. 26CA is ageneral purpose preamplifi er optimized for use withprepolarized condenser microphones. It is a small,robust unit and uses a G.R.A.S. CCP power supply(ICP ®), e.g. G.R.A.S. 12AL. It has a very low inherentnoise level, a large dynamic range and a frequencyresponse from below 2 Hz to above 200 kHz.

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  2. 26CK 1/2'' CCP Standard Preamp, Very Short (Mdot)
    Freq range: 2.5 Hz - 200 kHz
    Noise: 1.8 µV
    Gain: -0.45 dB
    Special feature: For confined spaces

    G.R.A.S. 26CK 1/2” CCP Standard Preamplifier with Microdot Connector, Very Short

    The G.R.A.S. 26CK ½” Preamplifier, Very Short is a very small preamplifier optimized for use with prepolarized condenser microphones. It is a robust unit that uses a constant current power (CCP) supply. The 26CK has a very low inherent noise level, a large dynamic range, and a frequency response from below 2 Hz to above 200 kHz.

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  3. 26CF 1/2'' CCP Preamp, Gain, Filters (BNC)
    Freq range: 2.5 Hz - 200 kHz
    Noise: 1.8 µV
    Gain: -0.25 / +20 dB
    Special feature: 0/20 dB gain, A-weighting and HP filters

    G.R.A.S. 26CF 1/2'' CCP Preamplifier with BNC Connector, Gain and Filters

    The G.R.A.S. ½-inch Preamplifier G.R.A.S. 26CF is a general purpose preamplifier optimisedfor use with prepolarized condenser microphones.It is a small, robust unit and uses a G.R.A.S. CCP 1power supply, e.g. G.R.A.S. 12AL. It has low inherentnoise level, a large dynamic range and a frequencyresponse from 2.5 Hz to 100 kHz.

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  4. 26CB 1/4" CCP Standard Preamp (Mdot)
    Freq range: 2.5 Hz - 200 kHz
    Noise: 1.8 µV
    Gain: -0.35 dB
    Special feature: General purpose

    G.R.A.S. 26CB 1/4" CCP Standard Preamplifier with Microdot Connector

    The G.R.A.S. 1/4" preamplifier is a general-purpose preamplifier optimized for use with prepolarized condenser microphones. It is a small, robust unit and uses a G.R.A.S. CCP¹power supply, e.g. G.R.A.S. 12AL. It has a very low inherent noise level, a large dynamic range, and a frequency response from below 2 Hz to above 200 kHz.

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  5. 26CC 1/4" CCP Standard Preamp (SMB)
    Freq range: 2.5 Hz - 200 kHz
    Noise: 1.8 µV
    Gain: -0.35 dB
    Special feature: General purpose

    G.R.A.S. 26CC 1/4" CCP Standard Preamplifier with SMB Connector

    The G.R.A.S. 26CC ¼-inch Preamplifier is a general purpose preamplifier optimized for use with prepolarized condenser microphones. It is a small, robust unit and uses a G.R.A.S. CCP power supply, e.g. G.R.A.S. 12AL. It has a very low inherent noise level, a large dynamic range and a frequency response from below 2 Hz to above 200 kHz.

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  6. 26CS 1/4" CCP Standard Preamp, Very Short  (Mdot)
    Freq range: 2.5 Hz - 200 kHz
    Noise: 1.8 µV
    Gain: -0.45 dB
    Special feature: For confined spaces

    G.R.A.S. 26CS 1/4" CCP Standard Preamplifier with Microdot Connector, Very Short

    The G.R.A.S. 26CS ¼″ Preamplifier is a very small preamplifier optimized for use with prepolarized condenser microphones. It is a robust unit and uses a CCP power supply. It has a very low inherent noise level, a large dynamic range and a frequency response from below 2 Hz to above 200 kHz.

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  7. 26CG 1/4" CCP Preamp, Low Frequency (Mdot)
    Freq range: 1 Hz - 200 kHz
    Noise: 1.8 µV
    Gain: -0.35 dB
    Special feature: For low-frequency applications

    G.R.A.S. 26CG 1/4" CCP Preamplifier with Microdot Connector, Low Frequency

    The G.R.A.S. 26CG ¼″ CCP 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 wide frequency response from below 1 Hz to above 200 kHz.

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  8. 26AK 1/2'' Standard Preamp
    Freq range: 2.5 Hz to 200 kHz
    Noise: 1.8 µV
    Gain: -0.25 dB
    Special feature: General purpose

    G.R.A.S. 26AK 1/2'' Standard Preamplifier with Integrated Connector

    The 26AK is a ½” microphone preamplifier with integrated 7-pin LEMO male connector. The CCP equivalent is G.R.A.S. 26CA.

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  9. 26AM 1/2'' Standard Preamp, Cable
    Freq range: 2.5 Hz - 200 kHz
    Noise: 1.8 µV
    Gain: -0.25 dB
    Special feature: General purpose

    G.R.A.S. 26AM 1/2'' Standard Preamplifier with 3 m integrated cable

    The G.R.A.S. 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.

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  10. 26AB 1/4''-1/2'' Universal Preamp, 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

    G.R.A.S. 26AB 1/4''-1/2'' Universal Preamplifier with Integrated Connector

    The G.R.A.S. 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 anda frequency response from below 2 Hz to above 200 kHz.

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  11. 26AJ 1/2'' SysCheck Preamp, Connector

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

    G.R.A.S. 26AJ 1/2'' SysCheck Preamplifier with integrated connector

    The G.R.A.S. 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.

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  12. 26AH 1/2'' SysCheck Preamp, Cable

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

    G.R.A.S. 26AH 1/2'' SysCheck Preamplifier with 3 m integrated cable

    The G.R.A.S. 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.

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  13. 26AK-S1 1/2'' Preamp, Connector, Low Frequency
    Freq range: 2.5 Hz to 200 kHz
    Noise: 1.8 µV
    Gain: -0.25 dB
    Special feature: Low frequency applications

    G.R.A.S. 26AK-S1 1/2'' Preamplifier with Integrated Connector, Low Frequency

    The 26AK-S1 is a ½” microphone preamplifier with integrated 7-pin LEMO male connector and 40 GΩ input impedance.

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  14. 26AC 1/4" Standard Preamp, Cable
    Freq range: 2.5 Hz - 200 kHz
    Noise: 1.8 µV
    Gain: -0.29 dB
    Special feature: General purpose

    G.R.A.S. 26AC 1/4" Standard Preamplifier with 3 m Integrated Cable

    The G.R.A.S. 26AC 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 anda frequency response from below 2 Hz to above 200 kHz.

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  15. 26AR 1/4" Standard Preamp, Mini-connector
    Freq range: 2.5 Hz - 200 kHz
    Noise: 4 µV
    Gain: -0.35 dB
    Special feature: For integration into structures

    G.R.A.S. 26AR 1/4" Standard Preamplifier with Integrated Mini-connector

    The G.R.A.S. 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.

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  16. 26AS 1/4" Standard Preamp, Cable, Very Short
    Freq range: 2.5 Hz - 200 kHz
    Noise: 4 µV
    Gain: -0.29 dB
    Special feature: For confined spaces

    G.R.A.S. 26AS 1/4" Standard Preamplifier with 3 m Integrated Cable, Very Short

    The G.R.A.S. 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 (G.R.A.S. 45BM or G.R.A.S. 45BA) or G.R.A.S. 45CA Hearing-protector Test Fixture. It has a very low inherent-noise level, a wide dynamic range and a frequency response from below 2 Hz to above 200 kHz.

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  17. 26AL 1/4" SysCheck Preamp, 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

    G.R.A.S. 26AL 1/4" SysCheck Preamplifier with 3 m integrated cable

    The G.R.A.S. 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 SysCheck capability. This enables in-situ checks of the complete measurement chain from microphone to analyser. The SysCheck technique works by modulating the microphone polarisation voltage.

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  18. 26HG 1/4" Preamp, Cable, Low Frequency
    Freq range: 1 Hz to 200 kHz
    Noise: 1.8 µV
    Gain: -0.29 dB
    Special feature: For low frequency applications

    G.R.A.S. 26HG 1/4" Preamplifier with 3 m Integrated Cable, Low Frequency

    The G.R.A.S. ¼″ Preamplifier Type 26HG 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 wide frequency response from below 1 Hz toabove 200 kHz.

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  19. 26AA 1/4" Preamplifier for Intensity Probes, Cable
    Freq range: 2.5 Hz - 200 kHz
    Noise: 1.8 µV
    Gain: -0.29 dB
    Special feature: For G.R.A.S. intensity probe handles with 4-pin LEMO connectors

    G.R.A.S. 26AA 1/4" Preamplifier with Integrated Cable for Intensity Probes

    The 26AA G.R.A.S. 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.

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