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OB-VCF Module

Oberheim type 12 dB/Oct multimode filter. Using CA3080 OTAs as filtering elements. The filter provides three outputs simultaneously (lowpass, hipass and bandpass). A notch output can be created by adding the lowpass and hipass outputs together.

The 12V zener diodes protect the filter against lockup. I use TL074 for the opamps and these can't handle inputs of supply rail voltage. The filter is powered by -/+ 15 Volt, the 12 Volt zeners limit the input 3 Volt below the supply voltage. The 'width' setting changes the balance between the control currents of the OTAs. This is used to ajust for OTA gain spread and capacitor tolerance. The best setting is where the Q factor is the highest. Listen to the filter with high resonance while adjusting this setting. When properly set the filter should be capable of self-oscilation.

The OTA is like a normal opamp, but it enables control of the total amplification by use of a control current. The two transistors convert from a input voltage to the correct control current. This filter is basically a dual RC filter with feedback, where the R's are replaced with OTAs. Changing the control current on the OTAs shifts the cutoff frequency of the two lowpass filters. Creating HighPass and BandPass by use of two lowpass filters is simply basic arithmatic in the frequency domain. Where HiPass=Input-LowPass and BandPass=Input-(LowPass+HighPass) or written diffently BandPass=Input+(-LowPass)+(-HighPass). The resistors scale all voltages to the correct levels. The feedback path creates a resonance peak around the cutoff frequency. The resonance works inverted, turning it up will decrease the resonance effect. Keep this in mind while soldering the controls.

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