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

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 this preamplifier ensure reliable operation under humid conditions owing to the heat generated by internal power dissipation.

Dynamic Range

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

Noise

• The electrical circuit in 26AA 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 26AA preamplifier is 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 ½″, ¼″ and ⅛″ microphone capsules respectively. The high-frequency cut-off is determined by the preamplifier’s ability to drive capacitive loads (slew rate), caused by the cable. For large-signals, the effects of these parameters must be accounted for when measurements are performed. The output level is in decibels relative to 1 Volt. Typical capacitance for the cable is 100pF/m (30pF/foot).

Connector

• GRAS 26AA Preamplifier is supplied with 30 cm cable terminating in a 4-pin LEMO plug. It is for use with the GRAS Intensity Probe 50 AI. An extension cable, AA0057, is available with a 4-pin LEMO connector at one end and a 7-pin LEMO series 1B plug at the other end.