Beigel Sound Lab – Envelope Controlled Filter

BEIGEL SOUND LAB has produced a sound modification device that is truly unique. Combining a parametric Envelope Follower, a versatile Voltage Controlled Filter and an External effects loop; the Envelope Controlled Filter allows so much flexibility that an entire new class of musical effects becomes available. Designed by Mike Beigel, inventor of the envelope controlled filter effect and developer of the famous MU-TRON® product line, this product is for discerning musicians who have the skill to create new and unique sound processes. The Envelope Controlled Filter is uncompromised in every detail of performance and construction. It will expand your universe of musical sound!”

–  From original Beigel Sound Lab ad circa 1980s


Envelope Follower

  • SENSITIVITY adjusts the envelope follower response to the input signal level
  • ONSET adjusts the attack time of the envelope follower
  • DECAY adjusts the decay time of the envelope follower
  • LIN-LOG Sweep Switch changes the way the envelope follower responds to your playing dynamics
    • LIN position gives the widest range of dynamic response, so that playing soft will create much less envelope drive than playing loud
    • LOG position has the effect of compressing the envelope response, much less sensitive to your playing dynamics

Voltage-Controlled Filter Section

  • MODE switch selects the filter characteristics desired for the musical effect
    • LP (Low Pass) passes signals below the filter’s cutoff frequency
    • BP (Band Pass) passes signals in the region of the filter’s cutoff frequency
    • HP (High Pass) passes signals above the filter’s cutoff frequency
  • PEAK varies the amount of emphasis at the filter’s cutoff frequency. The higher the control setting, the greater “peak” in the filter’s audio response. This will produce a more pronounced filter effect
  • PEAK Control Switch allows control of the filter “peak” by either Peak control or the signal envelope
    • MAN position allows manual control by Peak knob
    • ENV position controls the filter peak by the signal envelope. The louder the audio signal, the higher the envelope, the greater the peak
  • SWEEP Controls determine the direction and extent of the filter sweep
    • START: With no envelope (zero signal) the filter corner frequency is determined by the position of the START control
    • STOP: With full envelope (overload light goes on) the filter corner frequency is determined by the STOP control
      • The filter thus sweeps from START to STOP as the envelope goes from zero to its maximum value. If the STOP control is set to a higher value than the START control, the filter sweeps upward. If the STOP control is set lower than the START control, the filter sweeps downward. If the START and STOP controls are both set to the same value, the filter frequency does not change at all – the unit then performs similar to a one-channel parametric equalizer
  • RANGE allows the filter to sweep over two overlapping ranges
    • LO sweeps from 150Hz to 5KHz (START and STOP controls set at 1 and 10)
    • HI sweeps from 350Hz to 12KHZ

Effect Swicthes

  • VCF EFFECT electronically switches the filter section in or out of the audio signal path
  • EXT EFFECT electronically switches an external audio signal processor into the audio path between the input preamp and the voltage-controlled filter
    • VCF and EXT EFFECT switches are independent of each other. An LED indicator for each effect is on when the effect is in the audio signal path. With the switches in the OFF position, both effects can be foot-switch controlled
  • POWER turns the unit ON or OFF
    • Note: To avoid injecting clicks or pops into the audio system, turn the unit ON before turning on processors or amplifiers connected to the output of the Envelope Controlled Filter


  • Maximum Input Signal Level (Bypass Position): 6.3 v RMS (+18dBV)
  • Maximum Signal Gain (Peak Control = 10): 24dB
  • Minimum Signal Gain (Peak Control = 1): 0dB
  • Sensitivity (for full envelope sweep)
    • Minimum: 1.8 v RMS
    • Maximum: 75mv RMS
  • Filter Frequency Rang
    • LO: 150Hz to 5KHz Resonance Frequency
    • HI: 350Hz to 12KHz Resonance Frequency
  • Frequency Response (Bypass Position): 20Hz to 20KHz ± 0.5dB
  • Dynamic Range (Measure at output): 105dB worst case
    • Residual Noise level: -93dBV worst case
    • Maximum undistorted output: +12dBV worst case
  • Envelope Attack Time: 5 milliseconds to 2.5 seconds
  • Envelope Decay Time: 10 milliseconds to 5.0 seconds
  • Envelope Voltage Output: 0 to 10 volts
  • AC Supply Voltage: 115 VAC ± 15%, % VA max
  • Fuse: 3AG  ¼  125 volts

Original Owners Manual:

BSL ECF Owner’s Manual


Soon to come…