Flanger BL-20 - General Questions


Do you have questions about the Flanger BL-20?
You will likely find some answers here.

A Flanger is an effect similar to the Chorus, produced by mixing two identical signals.

One signal is delayed by a small and gradually changing amount of time and modulated through an LFO , which produces a characteristic undulating (sweeping) effect with a sound resembling the result of a comb filter.

Speeding up the LFO accentuates the modulation effect.

As mentioned, the effect is similar to the Chorus, but here we use shorter delays and no pitch-shifting.

1. Left panel buttons:

On/Off is a bypass button.

Mono/Stereo is a 2-position switch. When in Stereo, we have full stereo processing. When in Mono, the source signal is "monoized" (Left and Right inputs are summed before processing).

Zero Cr activates the "Zero Crossing Mode". It inverts the polarity of the modulated signal and delays the dry signal of the mean delay curve, so that we have this specific zero crossing effect. The resulting effect is an increase in the undulating characteristic of the sound.

2. Mix - Doppler

Dry/Wet mix control knob.

Default is 50%, but range goes from 0% (dry signal only) to 100% (wet signal only).

3. Regen

Sets the amount of affected signal fed back into the effect, accentuating characteristic comb filter effect.

This effect occurs because the modulated signal is combined with dry signal, causing a change in the phase relations.

4. Graphical Vu-meter

This is a row of "LED lights" that will show the combined "movement" of the effect, according to the way the different controls affect on the modulation.

5. Control Mode

3 switches: Manual, Env (Envelope Follower), and Auto.

All of them can be combined and the resulting modulation will be the sum if each modulation source.

6. Manual

Allows you to control the delay time manually. It can also be controlled by a function set in the Function Generator.

In this case, the function applies a modulation on delay time, which is summed with the Manual knob’s value.

7. ENV

Audio envelope follower. As such, it reacts to audio level, and that way changes flanger intensity.

Threshold sets the detection threshold of the envelope follower.

Decay controls the amount of time the envelope takes to reach zero after being deactivated.

8. Ext env/Int env

Allows you to choose between two modulation sources.

Internal envelope means that the input signal is chosen as a modulation source.
External envelope allows you to have an external signal as a modulation source. Each DAW has its own way of configuring this.


Let the flanger be controlled by the LFO.

Rate controls the speed, while Depth controls the amplitude, and therefore the amount of induced modulation. Rate can also be synced to musical values, which can be handy if we want the Flanger to act in sync with the music.

10 . Reverse Sweep O/P 2

The pedal can be used as an alternate source of modulation.
This button reverses the sweep on the right channel.
This will have an impact in the stereo image of the processed sound, as well as in the resulting effect.


1. 12dB/oct Hi Pass

This filter only affects the wet signal.

If you want to keep your bass and kick drum steady and precise, you may not want your low end to modulate.

By filtering out the low regions of the sound at the input of the unit, only the high frequencies will be modulated.

2. Function Generator

Acts like a step or pattern sequencer, an envelope, an LFO, or anything in between.

It is unipolar but nevertheless, since we can use positive and negative values on the Amount, we can add or subtract to the Manual value.

Find more information about this fun feature in the User Manual.

3. Auto Shape

This button allows you to change the waveform for the LFO (Auto mode).

The original unit's LFO only featured a triangle waveform.

We added the possibility to select a Sine, a Square, and an ascending or descending saw.

4. Stereo Offset

Use for adding or subtracting (+/-) a constant CV offset to the right channel.

This affects the stereo field of the processed signal.

If you have any further questions, feel free to contact us.