Filter Bank (og3)

Warning! If you picked up the schematics prior to 08/30/2002, get a new copy! Mistakes have been corrected. Version should say 2.1

Here we present a four channel voltage controlled filter bank. Referring to the block diagram, an input summer summs (and inverts) the inputs and feeds them simultaneously to all four filter channels. The filter channels are voltage controlled state variable types with independently adjustable fc and Q. In addition, a common control voltage is sent to each filter. This way the channels may have their initial frequencies set independently, and the whole filter bank made to track a keyboard. The Q controls are voltage controlled also.

Any of the three filter responses may be selected. They are configured like an equalizer, where the channel's output can be added or subtracted (boost or cut) from the input signal via the Boost/Cut controls.

The outputs of the filter channels are summed in the output summing amp. Also, all or a portion of the input signal can be added also, as determined by the Input Mix control.

• Block Diagram
• State Variable filter channel
• Control Voltage circuitry for filter channel
• Parametric Boost/Cut gain control
• Common Circuitry

• Vocal-like sounds can be produced by setting the center frequencies, Q and boost of the channels to the formant parameters for vowels.
• Setting the channels to maximum cut with staggered corner frequencies, then modulating the common VC input with an LFO will result in a phaser.
• Lowpass, Bandpass and Highpass outputs can be selected independently for each channel. The first channel could be set to a low fc and LP output (to preserve the bass), and the others to BP to provide spectral emphasis/de-emphasis.

The voltage controlled state variable filter is a fairly standard one with simultaneous BP, LP, and HP outputs

OTA U3:A forms the basis for voltage controlled Q.

Boost/Cut functionality is achieved by summing a non-inverted or inverted signal into the Master Mixer circuitry.

Exponential current sources for the OTA's are shown here. These circuits will be repeated for each of the filter channels.

The frequency control exponential current source is temperature compensated. Since the filter channel will double as a sine wave VCO, I wanted them to be more stable than a VCF usually requires. Q1, Q2, and TC1 should be in thermal contact.

Q1 and Q2 are a matched pnp pair. Since matched pnp transistors are pretty hard to find, I hand match a pair of 2N3906's for Vbe.

The frequency control has VC inputs labeled "CV", "Formant CV", "Master CV", "Var CV", and the Initial Frequency pot.

• The CV input goes to an input jack, so the corner frequency can be varied independently of the others
• The Master CV input comes from the Common circuity CV output which is fed to all the channels. This permits all the channels to track a common control voltage, such as from a keyboard.
• The Formant CV input is another CV input I may use later to build a vocal/formant controller
• The Var CV jack takes an input and provides a continuously variable inverted to non-inverted input