A.P. Circuit Corp AP-401 Variable Band-Pass Band-Reject Filter 1Hz-10kHz

A.P. Circuit Corp AP-401 Variable Band-Pass Band-Reject Filter 1Hz-10kHz

The A.P. Circuit Corp AP-401 is a benchtop variable band-pass and band-reject (notch) filter covering a frequency range of 1 Hz to 10 kHz. It provides simultaneous band-pass and band-reject filtered outputs on separate BNC connectors, allowing the operator to use both filter responses from the same input signal without switching. The Q factor is continuously adjustable from 1 to greater than 100, enabling narrow or broad filter bandwidths to suit different measurement needs. DC offset trimmers and a null adjustment control are provided for zeroing the output.

Specifications & What's Included

• Manufacturer: A.P. Circuit Corporation, New York, NY
• Model: AP-401
• Filter type: Variable Band-Pass and Band-Reject (notch)
• Frequency range: 1 Hz to 10 kHz
• Frequency multiplier settings: x0.1, x1, x10, x100, x1K
• Adjustable Q: 1 to greater than 100
• Outputs: Dual — separate Band-Pass output (BNC) and Band-Reject/notch output (BNC)
• Input: BNC connector on front panel
• Front panel controls: On/Off toggle switch, Q knob, frequency dial, gain control, notch gain control, null adjustment trimmer, DC offset trimmers
• Power indicator light on front panel
• Form factor: Benchtop portable with integral carry handle
• Enclosure: Blue steel case
• Includes: Unit with attached power cord

Applications

Variable band-pass and band-reject filters of this type were used in analog signal processing and electronic measurement applications requiring selective frequency isolation or rejection. The band-pass function passes a narrow range of frequencies centered on the dial setting while attenuating everything above and below — useful for isolating a signal of interest from noise or interference. The band-reject (notch) output does the opposite, attenuating the center frequency and passing the rest, which was commonly applied to remove a specific interference frequency such as 60 Hz power-line hum from a signal chain. With Q adjustable above 100, the AP-401 could produce very narrow, sharp filter responses suited to precision signal analysis, vibration testing, audio measurement, and laboratory signal conditioning work. This style of continuously tunable analog filter was a staple of electronics labs and test benches from the 1970s through the 1980s, before digital signal processing and FFT-based analysis became widespread. Units like the AP-401 are still sought by engineers working on analog test setups, vintage electronics restoration, and educational lab environments.