All G.R.A.S. microphone preamplifi ers are basedon a small ceramic thick-fi lm substrate with a veryhigh input impedance. The ceramic substrate isshielded by a guard ring to minimise the infl uenceof stray capacitance and microphonic interference.The casing is made of stainless steel for maximumstrength and durability. The small dimensions ofthis preamplifi er ensure reliable operation underhumid conditions owing to the heat generated byinternal power dissipation.
G.R.A.S. 26AC can handle both single and dual-sidedpower supplies. The supply can vary between28 VDC and 120 VDC single-sided or ±14VDC and±60 VDC dual-sided. When using the high supplyvoltage (120VDCor ±60VDC), the dynamic rangeexceeds 140 dB.
The electrical circuit in Type 26AC is built on aceramic substrate using selected low-noise componentsto gain very low self-noise. The electricalself-noise is so low that system noise is mainlydetermined by the microphone capsule’s thermalnoise.
The low-frequency cut-off of the Type 26AC preamplifier is mainly determined by the input impedanceof the preamplifi er and the capacitance of the microphonecapsule (see Fig. 3). The capacities 20 pF,6.5 pF and 3 pF equal the typical capacitances of½", ¼" and %" microphone capsules respectively.The high-frequency cut-off is determined by thepreamplifi er’s ability to drive capacitive loads (slewrate), caused by the cable. For large-signals, theeffects of these parameters must be accounted forwhen measurements are performed. Fig. 4 showsthe large-signal response for Type 26AC for variouscapacitive loads corresponding to different cablelengths. The output level is in decibels relative to1 Volt. Typical capacitance for the cable is 100pF/m(30pF/foot).
The G.R.A.S. ¼" Preamplifi er Type 26AC is provided with a3-m lightweight cable terminating in a 7-pin LEMOseries 1B plug. The cable is only 2.5 mmin diameter and will withstand temperatures from-40 oC to +150 oC. An adaptor (GR0010) for G.R.A.S.½" microphones is included.
7-pin LEMO plug 1B male (ext. view)
Typical applications and use
General-purpose preamplifi er
|Frequency response (±0.2 dB) with 18 pF microphone dummy and 4.7 nF cable load||Hz||2.5 to 200 k|
|Input impedance||GΩ // pF||20 // 0.4|
|Output Voltage Swing, max. @ ±14V/+28V power supply||Vp||10|
|Output Voltage Swing, max. @ ±60V/+120V power supply||Vp||50|
|Noise (A-Weighted) maximum||µV||2.5|
|Noise (A-Weighted) typical||µV||1.8|
|Noise (Linear 20 Hz – 20 kHz) maximum||µV||6|
|Noise (Linear 20 Hz – 20 kHz) typical||µV||3.5|
|Power supply, single||V||28 to 120|
|Power supply, balanced||V||+-14 to +-60|
|DC-offset, minimum, single suppy||V||0.5 x Vs - 1|
|DC-offset, maximum, single suppy||V||0.5 x Vs + 4|
|Temperature range, operation||°C / °F||-30 to 70 / -22 to 158|
|Temperature range, storage||°C / °F||-40 to 85 / -40 to 185|
|Humidity range non condensing||% RH||0 to 95|
|Humidity coefficient @250 Hz||dB/% RH||TBD|
|Connector type||7-pin LEMO (FGG.1B.307)|
|CE/RoHS compliant/WEEE registered||Yes / Yes, Yes|
|Weight||g / oz||8.00 / 0.28|
|Weight with cable and connector||g / oz||TBD / TBD|
¼" to ½" adapter for use with G.R.A.S. ½! microphones
Right-angled (90º) Adapter for½" microphone and ¼" preamplifier
Adapter for ½" microphone and¼" preamplifier
Angled (90º) Adapter ¼! to ¼
Extension cable, 3 metres
Extension cable, 10 metres
Extension cable, 30 metres
Extension cable, 100 metres
Extension cable, XX metres(customer-specified length)
Tripod adapter for ¼" preamplifier
Tripod adapter for ¼" preamplifier with angular adjustment