The Bel BF-20 Stereo Flanger was a British professional analogue effects processor introduced in 1978. Designed by Bel founder Mick Barnard, the Bel BF-20 was developed for studio installations at a time when dedicated electronic flanging remained relatively uncommon. Bel offered mono, true stereo and customised versions, although the stereo BF-20 became the best-known configuration.
Rather than generating a stereo output from a single processed signal, the Bel BF-20 Stereo Flanger employed two separate flanging circuits for independent processing of the left and right channels. Its analogue signal path used SAD512 bucket-brigade delay devices to produce the short, continuously variable delays required for electronic flanging. The delayed signals could be combined with the original audio to create moving comb-filter effects without first reducing a stereo source to mono.
The front-panel Mix control adjusted the relationship between the original and delayed signals, while the Regen control returned part of the processed output to the delay circuit. Increasing regeneration strengthened the repeated phase cancellations and could also produce more pronounced pitch-changing effects. Effect and bypass switching allowed the processor to be compared directly with the untreated signal.
Three operating modes were provided. Manual mode allowed the flanging position to be set directly, making it possible to use the Bel BF-20 Stereo Flanger as a fixed comb filter or move the effect manually. Envelope mode caused the sweep to respond to the level of the incoming audio. A Threshold control determined the signal level required to initiate the sweep, while the Decay control governed how quickly the sweep returned after the triggering signal fell below the threshold.
The automatic mode used a continuously variable internal oscillator to create repeating sweeps. Separate Rate and Depth controls adjusted the speed and range of the modulation. This allowed the Bel BF-20 Stereo Flanger to produce continuous movement without requiring a trigger signal or manual adjustment.
A Mono switch routed either input through both processing circuits, allowing a mono source to be converted into a moving stereo effect. The Reverse Sweep function caused the two circuits to travel in opposing directions, with one channel sweeping upwards as the other swept downwards. This produced a changing stereo image and could also generate automatic panning effects through differences in arrival time between the two channels.
The envelope circuit could be keyed from an external signal source, allowing the movement of one signal to control the flanging applied to another. This made it possible to trigger the effect from percussion, rhythm tracks or other programme material without processing the trigger source itself. Voltage-control connections were also included for integration with external control equipment.
The original Bel BF-20 Stereo Flanger was supplied in a 483mm rackmount chassis and was available with standard unbalanced connections or optional balanced audio connections. Bel specified a noise level better than -65 dB for the original version.
Features
- True stereo processing with separate left and right flanging circuits
- SAD512 bucket-brigade analogue delay devices
- Manual, envelope-controlled and automatic operating modes
- Continuously variable oscillator with rate and depth controls
- Threshold and decay controls for signal-triggered flanging
- External envelope keying from a separate audio source
- Mono and stereo input operating options
- Reverse Sweep function for opposing channel movement
- Mix and regeneration controls
- Standard unbalanced or optional balanced configurations