The PEQ-2A Vintage Program EQ - In Detail
The PEQ-2A is a Vintage Pultec inspired EQ, available as an LV2 plugin for linux Digital Audio Workstations. Separate boost and attenuate controls allow simultaneous boost and cut at low frequencies to produce the classic Pultec sound. Passive filters and a transformer-coupled valve amplifier emulation further enhance the vintage qualities.
Emulating Vintage Hardware
Original Pultec EQs can be heard on many classic recordings and are very much sought-after and highly regarded by engineers fortunate enough to have used them. Several aspects of the design contribute to their desirable sound. With reference to schematics and data from original units, the PEQ-2A models the filter and control interaction, to replicate some of these characteristics.
Passive Filters
The Pultec EQ uses passive filters, followed by a valve (tube) amplifier to make up for the insertion loss of the circuit. This results in an EQ with relatively gentle filter slopes, and while this may not lend itself to more 'surgical' corrective equalization, it is responsible for much of the 'warmth' and 'open' sound of these designs.The PEQ-2A has a similar response, combined with innovative DSP technology employed in the mid / high frequency peak filter, which provides a more analogue characteristic than conventional digital filters, as can be seen from the graph.
In a conventional digital peak filter, the gain is constrained to 0dB at the upper limit set by the sample rate.
The de-cramped filter used by the PEQ-2A allows the gain to remain above 0dB at this point providing a more symmetrical curve on either side of the centre frequency. This more closely emulates the behaviour of conventional analogue circuitry.
Transformer-Coupled Tube Amplifier
A passive filter causes attenuation of the signal passing through it. In order to overcome this, the original EQs used a tube (valve) amplifier to provide gain after the filters. This transformer-coupled amplifier also contributes to the desirable tone of the circuit by its subtle effect on the frequency response, and the saturation which occurs at high signal levels.This is also emulated in the PEQ-2A and, just as in the Pultec design, the amplifier remains in circuit even when the EQ is bypassed so there is always a pleasing and subtle effect on the sound even when the filters are not selected.
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