NTMB+F Technical Details Preamp Module Size:1.76\” [44.7mm] Long, 1.02\” [25.9mm] Wide, 0.54\” [13.7mm] Thick Operating Voltage:9V or 18V - Prewired for 9V. An additional battery clip is included for 18V wiring. Mid:250Hz center frequency can be tuned with a series capacitor. See the wiring diagram for more info. Input Impedance:400KΩ