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

Design

All GRAS microphone preamplifiers are based on a small ceramic thick-film substrate with a very high input impedance. The ceramic substrate is shielded by a guard ring to minimise the influence of stray capacitance and microphonic interference.The casing is made of stainless steel for maximum strength and durability. The small dimensions of these preamplifiers ensure reliable operation under humid conditions owing to the heat generated by internal power dissipation.

These preamplifiers are compatible with ½″ microphones as defined in international standard IEC1094 Measurement Microphones, Part 4: Specifications for working standard microphones. The mounting thread (11.7 mm – 60 UNS-2) is compatible with other available makes of similar microphone preamplifiers.

Dynamic Range

GRAS 26AB can handle both single and dual-sided power supplies. The supply can vary between 28 VDC and 120 VDC single-sided or ±14VDC and ±60 VDC dual-sided. When using the high supply voltage (120VDC or ±60VDC), the dynamic range exceeds 140 dB.

Noise

The electrical circuit in GRAS 26AB is built on a ceramic substrate using selected low-noise components to gain very low self-noise. The electrical self-noise is so low that system noise is mainly determined by the microphone capsule’s thermal noise.

Frequency response

The low-frequency cut-off of the GRAS 26AB preamplifieris mainly determined by the input impedance of the preamplifier and the capacitance of the microphone capsule. The capacities 20 pF,6.5 pF and 3 pF equal the typical capacitances of 1/2″, 1/4″ and 1/8″ microphone capsules respectively.

The high-frequency cut-off is determined by the preamplifier’s ability to drive capacitive loads (slewrate), caused by the cable. For large-signals, the effects of these parameters must be accounted for when measurements are performed. Fig. 4 shows the large-signal response for GRAS 26AB for various capacitive loads corresponding to different cablelengths. The output level is in decibels relative to1 Volt. Typical capacitance for the cable is 100pF/m(30pF/foot).

Connector

Preamplifier GRAS 26AB is provided with an integrated 7-pin LEMO connector. For use with GRAS 1/2″ microphones, 1/4″-to-1/2″ Adapter AF0008 is supplied. AF0008 is an inline barrel adapter ensuring minimum influence on the sound field.

7-pin LEMO plug 1B male (ext. view)