The
621 was our first parametric design. It used a Wien bridge filter with variable feedback to adjust the "Q." They were designed for external power supplies, because I thought that a lot of them would go into console retrofits and would run on one big power supply. This turned out to be far less true than I had hoped
. So we eventually manufactured a supply for them (the 85A if memory serves).
The
622 was a complete redesign. It used a fairly
tricky filter based on an allpass network with variable feedback. It added in/out switches for each band, and (finally!) had an internal power supply.
Both of these used 70's-era opamps and had a couple of
tantalums in the circuit path, so they are open to
modification with better sounding caps (or none at all; just short out the caps and tolerate small DC offsets) and up to date opamps. (Be careful if you replace the opamps in the 622's parametric filters; you can get oscillations if you replace the amplifiers with amps of significantly higher bandwidth or lower phase margin.)
After the 622 was discontinued, there was the
Orban 642, which used better opamps and DC servos instead of caps. It added 18dB/octave sweepable HP filters, 12dB/octave sweepable lowpass filters, and a frequency vernier control that helped the operator center the filter on the desired frequency when narrowband notches were used.
The 642 was a bit pricey for its time (although cheap by today's standards for higher end EQs!) so they're quite a bit rarer than the 621s and 622s.