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Volume 2, Number 6, July 2004
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Analog Obsession: Pultec Plots

With the release of the Pultec Pro plug-in, there has been renewed interest from the public as to what curves the Pultec EQs are imparting on their tracks. Whether analog or digital, the Pultecs have some unique qualities, which I will show via the plots provided.
The MEQ-5 is a three band rotary EQ, with a selectable frequency for each band. For each band the plots represent measurements taken at each numerical value on the boost or cut controls (1-10). The numbers don't necessarily represent a numerically equivalent dB value, although they are sometimes close. Let's look at what's going on with the MEQ-5, from left to right:

Low-Mid Peak
The low-mid peak behaves similar to a constant Q style filter, as the bandwidth remains the same as the "skirt" of the filter is increased. Depending on the frequency selected, the width can narrow or widen.

Mid Dip
The midrange dip (my favorite part of the MEQ-5) is the same circuit design as the low-mid boost and the high-mid boost. Interestingly, in extreme settings the range of the cut control loses sensitivity; 8-10 does are virtually the same setting.

High-Mid Peak
Like the others, the high-mid peak behave similar to a constant Q style filter. The high-mid peak's Q retains its general shape regardless of gain boost, but the width changes even more dramatically than the low-mid peak depending on frequency.

Now let's look at the EQP-1A-- a program equalizer with three bands of EQ.
From Left to right, the first band is a shelving filter, the next a fully parametric, the last band another shelving filter.

Low Boost and Attenuation Shelf
The first band of the EQP-1A is a shelving EQ. Once again, we plotted the EQP-1A at each numerical value on the boost or attenuation; but as you can see, the numbers don't correlate with a dB value.

One of the really beautiful idiosyncrasies of the EQP-1A is the ability to boost and cut the low shelving band simultaneously. This is due to the fact that it is a passive EQ with no buffering between the boost and cut stages; this allows the boost and cut filters to interact with each other, and the cut becomes slightly resonant. This has the sonic effect of simultaneously tightening and boosting bass frequencies.

High Frequency Parametric
The EQP-1A's high frequency parametric is limited to boosting only.

High Attenuation Shelf
The final band is attenuation only shelving filter.


A great resource for learning more about different EQ technologies/ filter types:
http://www.rane.com/note122.html

-Will Shanks

Questions or comments on this article?