UAD Pultec Passive EQ Collection Plug-in Analysis


#1

Pultec EQP-1A

Pultec MEQ-5

Pultec HLF-3C

INTRODUCTION
The Pultec Passive EQ Collection provides access to three historical and highly coveted revisions in the Pultec product line.
EQP-1A used on a myriad of recordings from the 1950s to today, the three-band, tube-amplified EQP-1A Program Equaliser has long been a studio staple of recording and mix engineers for its ability to bring out individual frequency ranges without significantly altering neighbouring frequencies. This unit is equally famous for its ability to dial in seemingly dangerous amounts of boost and smooth-as-silk vintage tone. The EQP-1A is also known as the quintessential ‘magic box’ that makes audio simply sound better just by passing through it.
MEQ-5 the Pultec MEQ-5 Mid-Range Equaliser is the richly colourful tube-amplified companion to the EQP-1A. With two bands of midrange boost and one band of midrange dip, the MEQ-5 is designed to enhance and control the ‘power region’ where sound energy is often concentrated, covering the finer tone requirements with an abundance of band overlap and filter interaction.
HLF-3C this plug-in has 12 dB per octave low and high cut filters, providing broad retro-tonal sculpting or bygone-era special effects.

KEY FEATURES

  • Drive the Pultec’s fully modelled transformer and tube-based amplifier sections
  • Give that signature airy, smooth-as-silk vintage tone to your master channel
  • Dial-up extreme levels of EQ boost while remaining natural and musical

HARMONIC ANALYSIS
To analyse the non-linear behaviours of the Pultec collection, I ran a 100 Hz sine wave through the plug-in at various settings.

100 Hz sine wave at -12 dB passed through EQP-1A

100 Hz sine wave at -12 dB passed through MEQ-5

The EQP-1A has a strong second harmonic, the second harmonic is considered smooth and adds body. The EQP-1A also generates the third harmonic, 10 dB down from the second. A strong third harmonic is considered to ad richness and depth. These are qualities intrinsic to the Pultec sound!
The MEQ-5 certainly lives up to its name as the rich and colourful counterpart to the EQP-1A. The MEQ-5 has stronger second and third harmonics with additional low order harmonic components, hense its fatter sound.
The HLF-3C does not generate any additional harmonics, as this unit is completely passive with no tube amplification or transformers like its bigger brothers. The HLF-3C definitely has a smooth sound, though.

EQUALISER CURVES

White noise passed through EQP-1A with maximum cut/boost at 100CPS

As you can see, this produces a 2 dB low frequency shelf boost; with an octave bandwidth 1 dB Bell dip at 1 KHz.

I always wanted to know how this classic Pultec bass boost/cut technique worked. An easy way to remember where the dip will occur is to add the power of ten to the CPS setting. For example, 30CPS equates to a 300 Hz centre frequency dip, 60CPS = 600 Hz dip, and 100CPS = 1000 Hz dip. This information is very useful when sculpting low frequency content, like kick drums and bass.

White noise passed through EQP-1A with maximum boost at 100CPS

It is interesting that the power of ten also applies to just boosting or cutting. As demonstrated in the screenshot above, a 100CPS boost returns to unity at 1 KHz.

White noise passed through EQP-1A Mid at 3KPS, maximum gain and widest bandwidth

This setting approximates to a 2 dB, two octave Bell boost at 3 KHz. This is a far lower gain boost than the gain control range of approximately 12 dB. I am not sure why this is. If anybody can explain this phenomenon, I would love to know. I have attached a screenshot in the ‘Frequency and Phase Response’ part of this article that depicts the range of the gain control.

White noise passed through EQP-1A Mid at 3KPS, maximum gain and narrowest bandwidth

This setting approximates to a 3 dB, one and a third octave bandwidth boost at 3 KHz.
The EQP-1A bands are symmetrical, which means they are identical wether cutting or boosting at the same gain/bandwidth settings.

White noise passed through MEQ-5 Mid Boost at maximum gain

The MEQ-5 does not have a bandwidth control. Maximum boost at 3KCS approximates a 1.5 dB boost with white noise; I could not demonstrate the concave contour of the MEQ-5 in Pro-L but I have created an octave bandwidth as a reference.

White noise passed through MEQ-5 Mid Dip at maximum gain

How interesting, the mid range dip band contour is more typical of a standard one octave bandwidth Bell.

The interaction between these two bands at different gain and/or frequency settings is very musical indeed.

White noise passed through EQP-1A High Atten at maximum Gain

The High Atten shelf also works to the power of ten; attenuation at 20KCS returns to unity at 2 KHz.

White noise passed through HLF-3C with Low Cut at 100CPS and High Cut at 10KCS

These 12 dB per octave shelf filters return to unity at the pass band. I know the screenshot does not demonstrate this for the 100CPS but that is simply due to the vagaries of white noise.

SATURATION CHARACTERISTICS
For this test I ran a synth loop through the Pultec’s at various settings. I have compensated for level so that the synth loop peaks at -1 dBFS for maximum resolution of the waveform in Pro-L.

Dry Synth Loop

EQP-1A at Unity

MEQ-5 at Unity

Very pleasing transient rounding and harmonic saturation from both of these boxes.

HLF-3C with 50CPS and 15KCS Cut Filters

Wow, look at the difference of the waveform, Pro-Q 2 certainly does not do that with same settings. Interestingly, I had to attenuate the output by 2.4 dB so the master did not clip - that is quite a lot of overshoot.

AUDIO EXAMPLES
Dry Synth Loop
Pultec Dry Synth.aif (1.3 MB)
EQP-1A at Unity (level compensated -1.2 dB)
Pultec Synth EQP-1A Unity.aif (1.3 MB)
MEQ-5 at Unity (level compensated -1.2 dB)
Pultec Synth MEQ-1A Unity.aif (1.3 MB)
HLF-3C 50CPS and 15KCS Cut Filters (level compensated) -2.4 dB
Pultec Synth HLF-1C Unity.aif (1.3 MB)

FREQUENCY AND PHASE RESPONSE
Lets take a look at the Pultec’s frequency and phase response at various settings
What I am looking for is the frequency and phase response of the plug-in at unity and how particular settings change this relationship.
Spectrum Colours
Yellow - noise generated by the plug-in analysis software to analyse the plug-in
White - frequency response of the plug-in at that particular setting
Red - phase shift of the plug-in at that particular setting

EQP-1A at Unity

MEQ-5 at Unity

HLF-3C at Unity

There is low and high frequency roll off on both the EQP-1A and MEQ-5, most likely a consequence of output transformers.

EQP-1A Cut/Boost Bass Technique

EQP-1A 3KCS High Band Boost at Maximum Gain and Widest Bandwidth

EQP-1A 3KCS High Band Boost at Maximum Gain and Narrowist Bandwidth

MEQ-5 3KCS Mid Boost at Maximum Gain

MEQ-5 3KCS Mid Dip at Maximum Gain

EQP-1A 20KCS High Atten at Maximum Gain

HLF-3C Cut Filters at 50CPS and 15KCS


#2

Thanks for another amazing analysis. EQP-1 is one of my favorite plugins!
And I also have rare hardware clone built from some vintage soviet parts in my rig (it is one of the first units done by the guys called Magnetec) - it sounds totally amazing while boosting bass or doing boost+cut trick. Compared to hardware plugin is less noisy :slight_smile: but gives same tube mojo and nice bass boost/saturation


#3

That looks so good man!


#4

Thanks, again Paul for the excellent analysis!

i often put an EQP 1A in the mastering chain in the musical EQ section – but lots of the time it’s doing nothing – but of course doing something…saturating, but not eq gain moves.

i think this is an example of UAD modelling at its best. a warm tube/transformer sound that does not sound at all vintage, but works very well in modern soundscapes.

an inflexible musical eq that forces you to make musical choices, like you said…and you can do a lot of gain boosting and get away with it.

thanks again, paul!