G.R.A.S. 14AA Manual de usuario

www.gras.dk

LI0010 – 1 October 2015

Instruction Manual

G.R.A.S. 14AA Electrostatic Actuator Amplifier

LI0010 – 1 October 2015

Revision History

Any feedback or questions about this document are welcome at [email protected].

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.

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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.

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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

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

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.

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.

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

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.

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

GND

AC Input

Overload

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.

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

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

0,1

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

0,2

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

Max cable lenght RG58

10Vrms

30Vrms

100Vrms

Fig. 15. The obtainable high-frequency response as function of cable length and signal level.

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