B.E.L Electronics BF-20 Stereo Flanger

The B.E.L BF-20 is a stereo flanger rack element. It has separate circuits that first split the input signal into two signal branches, which are fed to a mixing stage with a time delay. The time delay is varied within a small range (1 to 20 milliseconds), resulting in small upward and downward fluctuations in pitch. The superimposition with the unchanged original signal results in interference (comb filter effect). In the 1950s, the flanger effect was produced with tape machines, since the late 1970s digitally. The phaser effect has a similar sound, but is achieved using different technology. (Wikipedia)

"The BEL BF 20 Stereo Flanger is a true stereo unit with separate circuits. Which enables flanging of a stereo mix without upsetting the stereo image. There is a mono switch which feeds the signal of either input into both circuits, used in conjunction with the reverse sweep switch an unusual image shift can be obtained giving a stereo effect from a mono instrument. Thee are three operation modes, manual, envelope and continuously variable oscillator with depth and speed control. In the envelope mode there is a threshold control. Once the signal level exceed that set by the treshold the unit will start to sweep. The rate of recovery is set by the decay control. In this mode channel 2 can be used as a mono flanger and keyed by a signal from an external signal source. There is also a feedback control for increased depth and additional pitch changing effects. When used in the stereo mode the unit sweeps across its left and right inputs, producing a wide variety of unusual stereo effetcts, not just flanging, but also completely random auto pannning created by the haas effect." (BEL product text)