G.R.A.S. 14AA Manuale utente
www.gras.dk
LI0010 – 1 October 2015
Instruction Manual
G.R.A.S. 14AA Electrostatic Actuator Amplifier
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LI0010 – 1 October 2015
Revision History
Revision Date Description
1 23 May 2007 First publication
2 4 November 2013 Updated connection diagrams and specifications
3 30 January 2014 AC output specified as V RMS.
4 1 October 2015 Correction to Fig 2.
Copyright Notice
© 2014-5 G.R.A.S. Sound & Vibration A/S
http://www.gras.dk
Any technical documentation that is made available by G.R.A.S. is the copyrighted work of
G.R.A.S. and is owned by G.R.A.S.
The content in this document is subject to change without notice. G.R.A.S. Sound & Vibration A/S
is not liable or responsible for any errors or inaccuracies that may appear in this document.
Trademarks
Product names mentioned in this document may be trademarks or registered trademarks of their
respective companies and are hereby acknowledged.
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LI0010 – 1 October 2015
Contents
Introduction.................................................................................. 4
Setting the Mains Voltage............................................................... 6
Driving a Microphone as Sound Source............................................... 7
Electrostatic Calibration of Microphones ............................................ 8
Technical Specifications.................................................................. 9
The AC Outputs for Microphones and Electrostatic Actuator ............................ 9
Other Specifications ................................................................................. 12
Accessories ................................................................................ 12
Calibration, Warranty and Service................................................... 13
Calibration............................................................................................... 13
Warranty................................................................................................. 13
Service and Repairs.................................................................................. 13
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LI0010 – 1 October 2015
Introduction
The G.R.A.S. 14AA Electrostatic Actuator Amplifier is a high gain, high voltage amplifier. The
output AC signal can be superimposed on either a 200 VDC or a 800 VDC signal. With the
200 V DC polarization voltage a standard measurement microphone as for example G.R.A.S.
40AF 1/2” Ext. Polarized Free-field Microphone, High Sensitivity can be used as a precision
sound source for high frequency signal generation.
The 800 VDC polarization voltage can be used with the G.R.A.S. RA0014 1/2’’ and RA0015
1’’ Electrostatic Actuator for frequency response calibration to accurately measure the frequency
response of microphones.
The input signal to the 14AA is supplied via a standard BNC connector and can be any signal up
to 1V RMS. The input signal is amplified 40 dB (100 times) in the 14AA for a maximum output
signal of 100 V RMS. The 14AA has three output connectors: AC output, AC + 200 VDC output
and AC + 800 VDC output.
Fig. 1. Electrostatic Actuator Amplifier 14AA
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LI0010 – 1 October 2015
Fig. 2. 14AA principle.
The 14AA electrostatic actuator amplifier consists of a 40 dB gain, high voltage amplifier and
two high voltage DC power supplies as shown in figure 2.
The input signal from the BNC connector on the front panel goes to the amplifier stage. The input
signal must be limited to maximum 1 V RMS in order not to overload the amplifier. If the signal
exceeds 1 V RMS, the overload indicator on the front panel will light up.
The input signal is amplified by 40 dB and the output signal is going to the three output connec-
tors : AC Output, AC Output + 200 VDC and AC Output + 800 VDC.
The AC Output is the output from the 40 dB gain amplifier. For an input signal of 1 V RMS the
output signal will be 100 V RMS. This signal can be used for driving prepolarized microphones as
sound sources.
The AC Output +200 VDC is the output from the 40 dB gain amplifier superimposed on a
200 VDC polarization voltage. For an input signal of 1 V RMS the output signal will be 100 V
RMS. This signal can be used for driving externally polarized microphones as sound sources.
The AC Output +800 VDC is the output from the 40 dB gain amplifier superimposed on a 800
VDC polarization voltage. For an input signal of 1 V RMS the output signal will be 100 V RMS.
This signal can be used for driving electrostatic actuators for recording the pressure response of
measurement microphones.
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LI0010 – 1 October 2015
Setting the Mains Voltage
Before conecting the 14AA to the mains power the voltage setting on the rear panel of the 14AA
should be set to the correct local mains power voltage. The voltage setting is indicated in the
Voltage setting indicator on the rear panel, see figure 3. To change the setting, remove the fuse
holder by inserting a small screwdriver in both sides of the fuse holder as indicated in figure 4.
Fig. 3. The voltage setting indicator. Fig. 4. Removing the fuse holder.
Remove the fuse insert as indicated in figure 5a and rotate it 180 degree. Insert the fuse insert
and put the fuse back in. Insert the fuse holder in the 14AA. The voltage setting indicator should
now indicate the new setting of the mains supply voltage.
Fig. 5. Remove the fuse insert from
the fuse holder.
Fig. 6. Rotate the fuse insert 180
deg and insert it again.
Fig. 7. Insert the fuse in the fuse
holder.
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LI0010 – 1 October 2015
Driving a Microphone as Sound Source
The 14AA Electrostatic actuator amplifier can be used for driving a microphone as a precision
sound source, for example for frequency calibration of an an Ear Simulator. When used as a sound
source in accordance with the guidelines laid out in IEC 60318, the sensitivity of the transmitter
microphone can be assumed to be identical to its response as a receiver.
A signal generator is connected to the input BNC connector of the 14AA and the output is con-
nected to a transmitter adapter.
Two transmitter adapters are avialable for 1/2” and 1/4” microphones respectively:
• 1/2” microphone: Transmitter adapter RA0067 and BNC-BNC cable AA0032.
• 1/4” microphone: Transmitter adapter RA0086 and BNC-Microdot cable AA0073-05.
If the microphone is an externally polarized microphone like the G.R.A.S. 40BP 1/4” Pressure
Microphone, the RA0086 should be connected to the AC Output +200 VDC. If the microphone
is a prepolarized microphone like the G.R.A.S. 40BD 1⁄4” Pressure Microphone, the RA0086
should be connected to the AC Output.
Important. Short cable lenghts (0.5 m) between 14AA and the transmitter adapter must be
used. Longer cables will impair the high-frequency performance. For further information, see
“Technical Specifications” on page 9.
An example of the electrical connections needed for a frequency calibration of an ear simulator is
shown in Fig. 8. For a detailed information about how to calibrate an RA0045 Ear Simulator, refer to
the documentation for the RA0045 and the recommendations of IEC 60318.
Actuator Supply
14AA
Powe r
G.R.A.S.
Actuator Supply
Type 14AA
AC Output
AC Output +800VD C
AC Output
On
GND
AC Input
Overload
Equipment for frequency calibration of an ear simulator
Max. 1Vrms
080013
+200 VDC
Ext. pol. microphone
Prepol. microphone
To analyzer from preamplifier
From signal generator
Computer with hardware
and software for signal
generation and signal analysis
RA0086
Transmitter adapter
RA0052
Test Jig
¼”TransmitterAdapter
RA0086G.R.A.S.
Sound source,
e.g 40BP microphone
DUT, e.g.
RA0045 Ear Simulator
Signal generation and analysis
Fig. 8. A setup for frequency calibration of an IEC60318-4 Ear Simulator.
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LI0010 – 1 October 2015
Electrostatic Calibration of Microphones
The 14AA Electrostatic actuator amplifier can be used for driving an electrostatic actuator for
recording the frequency response of measurement microphones. The measurements can for
example be performed with a general purpose frequency analyzer with built-in signal generator.
The output of the signal generator is connected to the input of the 14AA. The electrostatic
actuator is mounted on the microphone diaphragm and connected to the AC Output +800 V DC
output connector. The output signal from the microphone preamplifier is going to the input of the
signal analyzer. The frequency response can then be recorded by a sweep or by a noise excitation
with FFT analysis.
Two different electrostatic actuators are available:
• 1” microphones: G.R.A.S. RA0015 1’’ Electrostatic Actuator for frequency response calibration.
• 1⁄2”, 1⁄4” and 1/8” microphones: G.R.A.S. RA0014 1/2’’ Electrostatic Actuator for
frequency response calibration. (Adapters for 1/4” and 1/8’’ microphones are included with the
actuator).
Fig. 9 shows a block diagram showing a setup using the 14AA for frequency calibration with an
electrostatic actuator.
Actuator Supply
14AA
.
Electrostatic Actuator
RA0014
Adapter
Actuator
Calibration Stand
AL0010
Preamplifier
26AB
(inside
calibration
stand)
G.R.A.S.
¼”-½ ” A dapter
AF 0008
Powe r
G.R.A.S.
Actuator Supply
Type 14AA
AC Output
+200VD C
AC Output
Max. 1Vrm s
+800VD C
AC Output
On
GND
AC Input
Overload
G
.
R
.
A
.
S
.
R
A
0
0
1
4
Microphone
for calibration
Frequency calibration equipment
080013
Computer with hardware
and software for signal
generation and signal analysis
Signal generation and analysis
Fig. 9. Block diagram of a calibration setup using an electrostatic actuator.
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LI0010 – 1 October 2015
Technical Specifications
The AC Outputs for Microphones and Electrostatic Actuator
Under open loop conditions, the AC outputs of the 14AA are linear to over 200 kHz. However,
the cable connected to the output limits the high-frequency performance in the same way as a
low-pass filter. The longer the cable, the lower the cut-off frequency. If you need to measure at
very high frequencies, you must use the shortest cable possible. When the high-frequency roll off
is known, you can use it as a correction factor.
The AC Output (O VDC for prepolarized microphone)
Output Signal, max level peak-to-peak 100 V RMS
–3 dB Low Frequency Cut-off
Open loop 0.35 Hz
1 M Ωload 1.9 Hz
Frequency Response, 1 meter RG-58 cable (~100 pF), 100 V RMS
± 0.1 dB 25 – 50 kHz
± 1 dB 10 – 100 kHz
Max Output Current 5 mA AC
The AC Output +200 VDC (for externally polarized microphone)
Output Signal 100 V RMS
–3 dB Low Frequency Cut-off
Open Loop 0.3 Hz
1 MΩload 1.8 Hz
Frequency Response, 1 meter RG-58 cable (~100 pF), 100 V RMS
± 0.1 dB 25 – 50 kHz
± 1 dB 10 – 100 kHz
Max Output Current 5 mA AC
DC Output Impedance* 5.6 MΩ
* See also schematic drawing in Fig. 2
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LI0010 – 1 October 2015
Typical Frequency Response for the AC Outputs, 0 VDC and + 200 VDC
RG58 Cable, 1 Meter
-0,3
-0,2
-0,1
0
0,1
0,2
0,3
0,4
0,5
0,6
0,7
100 1000 10000 100000 1000000
10V
30V
100V
Frequency [Hz]
Amplitude [dB]
Fig. 13. The Frequency response for the 0 VDC and 200 VDC outputs, 1 meter RG58 cable, 10, 30 and 100 V RMS.
RG58 Cable, 3 Meters
Frequency [Hz]
Amplitude [dB]
-1,6
-1,4
-1,2
-1
-0,8
-0,6
-0,4
-0,2
0
0,2
100 1000 10000 100000 1000000
10V
30V
100V
Fig. 14. The Frequency response for the 0 VDC and 200 VDC outputs, 3 meter RG-58 cable, 10, 30 and 100 V RMS.
RG58, Maximum Cable Lengths
Length in meters
Frequency [Hz]
0,00
5,00
10,00
15,00
20,00
25,00
1000 10000 100000 1000000
Meter
Hz
Max cable lenght RG58
10Vrms
30Vrms
100Vrms
Fig. 15. The obtainable high-frequency response as function of cable length and signal level.
Indice
Altri manuali G.R.A.S. Amplificatore
