This is a 9vdc powered LM13700 dual transconductance op-amp based filter very suitable for battery powered circuits. The design is based on an article by R. A. Penfold for maplin magazine but i have adapted the schematic to have manual filter cut-off and Resonance control. The filter self oscillates at high resonance settings (I. E the resistance of VR2 is low) and so can be used used as a basic Voltage controlled oscillator (VCO). This is probably the best sounding simple 9vdc powered filter there is, the resonance peak gives well emphasised cut-off frequency and can oscillates at low and high frequencies.

I Have updated the schematic (may-2011) showing the input and output also R20, C7and D1 (zenner didoe) are only needed if you are powering a 3v audio toy from this circuit I.E you can power a buddha machine or gristleism etc safly from the 9V pp3 battery.