Pultec EQP1A3 Vintage Equalizer at Clear Lake

Passive aggressive
The EQP-1A is a no-loss passive equalizer, which means that the signal level going in to it is reduced (insertion loss around 16dB) and then restored by an onboard amplifier. This means that there is no change in signal level when equalization is switched in or out. The amplifier stage remains in circuit when the EQ is bypassed and adds character to sounds passing through it, largely due to the valves driving the amp. Three valves are used: an EEC-82, an EEC-83 and a 6×4 rectifier. 

Model EQ
The original Pultec EQP-1A has become a studio icon in the same league as Fairchild limiters or even the legendary Neumann U47 microphone. It is undoubtedly the most famous EQ ever made, which is why so many others have taken inspiration from the design – and why original examples in tip-top condition go for silly money. The EQP-1A has been used in all sorts of ways on many thousands of recordings. That’s the beauty of the design: it can be used to process single sources, groups of instruments or an entire mix, although you would need two for stereo, of course. During the US pop boom of the early 60s no self-respecting studio would be without one. Many had several. Tamla Motown made extensive use of them in their studios, so it could be said that the EQP-1A literally shaped the Motown sound. The engineers at the Tamla Studios in Detroit developed a method of carving out some midrange frequencies from the instrumental backing tracks, creating space for the vocals to sit nicely in the mix. A perfect pop technique – one which may well have been achieved by performing the ‘low-end trick’.

The low-end trick? Because the equalizer has separate controls to boost or attenuate the signal, it’s possible to boost and cut the selected low frequency simultaneously. Although this may sound counterintuitive, it works because the respective frequencies at which the boost and attenuation shelves are centred are not exactly the same. For example, if the frequency selected is 100Hz and both the boost and the Atten controls are turned to 7, the EQ curve displays what can be referred to as a ‘midrange droop’ centred on 1kHz. But if the Atten pot is returned to zero and the boost is still applied, the low-end frequencies remain the same but the droop disappears.

This method of simultaneously boosting and cutting the lows can be used to create some very useful EQ curves. It works on a wide range of sources from bass to acoustic guitar – not an instrument usually associated with bottom boost – and, used sparingly, is particularly good on male vocals, providing warmth to the lower registers without clogging up the critical mid band. Interestingly, the original Pultec manual advised that the Boost and Atten controls should never be used at the same time. It’s ironic, then, that the unit’s ability to do just that has helped it to earn its iconic status. 
The Pultec isn’t just about the low end, though. The peaking top boost provides a useful range of frequencies which at the lower settings can add presence;  if a narrower bandwidth is selected, a degree of edge is given to the source material. The higher settings add sparkle, space and air, particularly at broad bandwidth settings.

The high-frequency shelf cut can be used on its own or in conjunction with the Boost control, which is useful when you’re faced with several instruments that occupy similar places in the frequency spectrum (guitars and electric piano, for example, which may need some careful sculpting to remain distinct within the mix).



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