The Feedback Master is the best-sounding and easiest-to-use feedback remedy available for acoustic instruments. 
Features include:
- Two proprietary Garrett Null sweepable narrow-band filters easily locate and damp air and top feedback resonances, controlling feedback without sacrificing tone
- Phase inversion switch for additional feedback control
- Adjustable gain for optimum signal level and lowest noise
- All analog pure class A discrete circuitry makes is utterly transparent and quiet
- Works as a standalone buffer-preamp with virtually any active or passive pickup
- Includes mic stand adapter and belt clip
Specifications: 
Size: 4.75 x 2.85 x 1.25
Weight (w/ battery): 7.3 oz
Battery type: Single 9V
Power consumption: 2mA
Battery life: > 250 hours
Class: Pure class A all discrete
Null 1: Off-320 sweepable
Null 2: Off-320 sweepable
Gain:  3dB to  24dB
Signal-to-noise: -92dB, 20Hz-20kHz unweighted
Input impedance: 10 Megohms
Output impedance: 800 Ohms