Spectron Dynamic Filter

The Meatball circuit is extremely touch-sensitive and allows control over every aspect of its operation to make it work best with your instrument. However, this also means that there are some combinations of settings that don’t work at all—for example, if the Attack knob is set higher than the Decay knob.

Because of this, it’s important to understand not only what each individual control does, but how each of them interact with the other controls. Many people have built the Meatball circuit and thought at first that it didn’t work, only to realize that the knobs just needed to be set differently.

Potentiometers

• Color sets the resonance of the filter. On higher settings, it can get pretty extreme, especially when the resonant frequency aligns with the fundamental of the note being played on the instrument, so it’s best to keep this control turned down when adjusting other settings and then turn it up to the desired level. Expect some windows to rattle if you’re not careful!
• Intensity sets the depth of the effect by adjusting the brightness of the LEDs. With the trigger switch set to Off, it has a secondary function of tuning the static filter frequency.
  • An expression pedal can be connected to allow the intensity to be foot-controlled. The expression is connected in parallel with the on-board Intensity control, so if the knob on the pedal is turned all the way up then the expression pedal will have no effect.
• Blend blends the output signal between the clean and effect signal. Full CCW is 100% clean, full CW is 100% wet, and at center, the mix ratio is 50/50.
• Attack sets the speed of the envelope’s “rise” to the peak of the filter. It must be set lower than the Decay knob or there will be no envelope effect because the filter will decay before it even starts.
• Decay sets the amount of time that the filter takes to fall back to the resting state (either up or down depending on the Sweep switch setting). This must be set higher than the Attack knob or the envelope will not trigger.
  • There is also an optional expression control for this knob. Unlike Intensity, in this case the expression pedal is in series with the Decay control, so it will have the widest range of effect when the Decay knob is turned down.
• Sensitivity sets the envelope detector’s reactivity to the input signal. The higher the sensitivity level, the lower the signal level needs to be to trigger the envelope. Higher-output instruments such as keyboards or active pickups will work best when Sensitivity is turned low.

Switches

• Sweep (rotary switch) selects whether the effect is in a low state by default and the filter sweeps upward on attack and back down on decay (traditional auto-wah sounds), or whether it’s in a high state by default and the filter sweeps downward and back up for a reverse-wah effect. In the downward sweep mode, the sense indicator LED is on when no signal is applied.
• Bandwidth (3-position toggle) sets the frequency of the envelope detector. It can be either full bandwidth (sensing the dynamics of all frequencies), half bandwidth (sensing the dynamics of high frequencies only, ignoring low frequencies) or the envelope detector can be turned off entirely in the center position. When the envelope detector is off, the circuit acts as a static filter, with Intensity and Color setting the tone.
• Filter (toggle) is an optional modification that applies a Moog-style quality factor to the filter with frequency-variable Q instead of the constant Q of the Meatball.
• Mode (3-position toggle) selects whether the filter acts as high pass, bandpass, or low pass. HP and LP modes are inverted, so there may be some phase-cancellation effects when using the Blend knob in these two modes. This is part of the Meatball sound.
• Range sets the frequency of the filter effect in increments of one octave. Standard guitars will benefit more from the upper two settings while bass guitars will typically use the lower two.