Bruel & Kjaer 1621 Band Pass Filter

1621 Front Panel

Introduction Inspired by Hainbachs use of test equipment I bought a 1621 Band Pass Filter in May 2021, to add a different sound into my analog synthesizers, especially the Big Moog. The 1621 was in good condition and was made in the 1980’s. It is a tunable single pole Butterworth filter with switchable bandwidths, with the addition of external digital control of the frequency.

You can download a very nice user manual here.

How it Works The filter is a more complex and precise device than a regular analog synthesizer band pass filter. It contains two Integrators which work as a tunable oscillator. The circuit starts oscillating when the input frequency is the same as the tuned-in frequency.  The feedback circuit controls the amplitude so the output is the same as the input amplitude. For an input frequency lower than resonance, the amplification will be very high feedback which increases the input to the first Inverter and thus stops the oscillating.

Inside the 1621

Frequency Range There are five frequency ranges selected by a front panel rotary control, with the large frequency control sweeping between the lower and upper points in each range. With a range of between 0.2Hz to 20kHz, the filter works perfectly in analog synthesis. Here are the ranges:

• 0.2Hz  to 2Hz
• 2Hz to 20Hz
• 20Hz to 200Hz
• 200Hz to 2,000Hz
• 2,000Hz to 20,000Hz

The band pass bandwidth is switchable from 3% or 23% ( 1/3 octave), and the filter can be put in bypass mode (called Linear) whilst powered on. The filter has two voltage ranges and I have used the larger 1V RMS which is selected on the rear panel.

Nice filter capacitors

Power Supply The 1621 can be powered from a range of external power supplies (+/-14V or a +12V car battery). However I rather like the internal battery option as it makes the device truly portable, initially I powered it from 12VDC using a mains adapter and 7-pin DIN plug. A nice red LED comes on when the power is switched on with the front panel control knob. This knob also selects the bandwidth of the filter, either narrow at 3% or broad at 23%.

Connections The 1621 uses BNC connectors which I have added phono connectors to, then patched it into my 984 Quad Mixer in the Big Moog. The external power and the digital control both use DIN connectors.

Digital Control When placed in Auto mode the 1621 can be controlled by an external serial clock stream that sweeps the selected frequency range of the filter, through 1024 steps. The circuit is a discrete 10-bit DAC which transforms the serial pulses into a ramp generator that sweeps the frequency. There is also a start pulse which goes out from the 1621 when the START switch is momentarily depressed.