Often times when mixing multi-track recordings, you can achieve most of the necessary equalization with just some filtering and a bit of tone control. Recognizing this, the design team of of Jens Jungkurth and John Klett created the CurvOmatic console EQ module in 500 Series format, combining some very powerful high and low pass filters with a very forgiving baxandall tone tipper.
Although these circuits are active, they were influenced by passive inductor-based filters on the 1960s, and painstakingly massaged over a substantial period of time until they sounded just as nice. This involved auditioning more than fifty different op amps and a dozen different series of capacitor for each position, and in some cases sourcing New Old Stock components when there wasn’t a better sounding substitute.
One of John’s favorite mix tricks when using an old filter set like Pultec HLF-3C is to remove the 600 Ohm termination resistor, replacing it with a high value rheostat, and followed by a high-Z buffer amp. If high Q, the inductors will ring once undamped, causing a resonant bump at the cutoff frequency. This can be exceedingly useful (i.e. to emphasize the shell resonance and beater click of a close-mic’d kick drum while removing unwanted frequencies below and above), which is why we included resonance knobs for each of the CurvOmatic filters, making filter shape adjustable from 0 dB flat to as much as +15 dB of resonance.