Voice cards expand the functionality of the Bullfrog and make it even more versatile. Some voice cards replace patch cables - they connect the "modules* of the Bullfrog internally, while others have built-in mixers, signal generators, and advanced electrical circuits to control the synthesizer, for example, the Sequencer sends CV to control pitch and gate to start events on the synthesizer and the Sampler/Looper emulates reel to reel tape recorder functionality

The HIGHPASS VCF voice card expands the sound treatment capabilities of the Bullfrog by bringing a highpass filter into the game. This allows for exploring highpass and bandpass filtering. The voice card features typical VCF controls – CUTOFF frequency, RESONANCE, and CV LEVEL. The CV source switch allows for selecting between three CV sources – envelope generator 2, Sample & Hold LFO, which is a pulse wave generator in the Sample&Hold “module”. The voice card features internal audio connections that make patching less cluttered.