PAiA 4730 Multi-Modal Filter Module

The 4730 is a second order state variable filter with low pass, band pass, and high pass outputs.  The filter has a high / low range, resonance, and a sweep/track mode.  The filter should be set to track when the filter is being controlled by a keyboard (e.g. tracking a VCO), and set to sweep when the CV will go to 0V (e.g. a LFO as the CV).  This kit was $37.95 in the 1976 catalog.



4730 PAiA Filter Manual

4730 PAiA Filter Schematic


These are the PCB traces.  The three trimmers were previously converted to multi-turn and the traces repaired on the right.



I upgraded the jacks, sockets and the LM3900 and 748 op-amp, and replaced the electrolytic capacitors.  I used bi-polar for the output capacitors and added 100K bleed resistors. I upgraded the 748 IC to a TL071 and removed the compensation capacitor.  The power cable terminates in a 4 pin MTA connector.



An external CV must be applied in order to set the initial frequency.  Calibration is straight forward but requires a number of passes.  This scope image shows the filter self-oscillating.  It would only self-oscillate with a 5V CV and at maximum control settings.  The waveforms are Input, LP, BP, and HP.


This scope image shows a frequency setting where all three outputs are near-sine shape.


This scope image shows filtering of a square wave with no resonance.


This scope image shows the same control settings with some resonance.


This scope image shows the same settings with maximum resonance.