Manley Labs STEREO EQ - Manual (Page 10)

Beginnings
The very earliest equalizers were very simple and primitive by
todays standards. Yes, simpler than the hi-fi "bass" and "treble"
controls we grew up with. The first tone controls were like the tone
controls on an electric guitar. They used only capacitors and
potentiometers and were extremely simple. Passive simply means
no "active" (powered) parts and active parts include transistors,
FETs, tubes and ICs where gain is implied. "Passive" also implies
no boost is possible - only cut. The most recent "purely passive EQ"
we know of was the EQ-500 designed by Art Davis and built by a
number of companies including United Recording and Altec Lansing.
It had a 10 dB insertion loss. No tubes. It had boost and cut positions
but boost just meant less loss. Manley Labs re-created this vintage
piece and added a tube gain make-up amp for that 10 dB or make-
up gain to restore unity levels. It has a certain sweetness too.
You have probably heard of passive crossovers and active crossovers
in respect to speakers or speaker systems. Each has advantages.
Almost all hi-fi speakers use a passive crossover mounted in the
speaker cabinet. Only one amp is required per speaker. Again,
passive refers to the crossover using only capacitors, inductors and
resistors. Active here refers to multiple power amplifiers.
One of the main design goals of the Massive Passive was to use only
capacitors, inductors and resistors to change the tone. Pultecs do it
this way too and many of our favorite vintage EQs also relied on
inductors and caps. In fact, since op-amps became less expensive
than inductors, virtually every EQ that came out since the mid '70's
substituted ICs for inductors. One is a coil of copper wire around a
magnetic core and the other is probably 20 or more transistors. Does
the phrase "throwing out the baby with the bath water" ring a bell?
Another design goal was to avoid having the EQ in a negative
feedback loop. Baxandall invented the common circuit that did this.
It simplified potentiometer requirements, minimised the number of
parts and was essentially convenient. Any EQ where "flat" is in the
middle of the pot's range and turning the pot one way boosts and the
other way cuts is a variation of the old Baxandall EQ. Pultecs are not
this way. Flat is fully counter-clockwise. For the Massive Passive,
Baxandall was not an option. The classical definition of "passive"
has little to do with "feedback circuits" and we are stretching the
definition a bit here, however, it certainly is more passive this way.
We only use amplification to boost the signal. Flat Gain ! What goes
in is what comes out. If we didn't use any amplifiers, you would need
to return the signal to a mic pre because the EQ circuit eats about 50
dB of gain. Luckily, you don't have to think about this.
We visited a few top studios and asked "what do you want from a
new EQ ?" They unanamously asked for "click switch frequencies",
"character" rather than "clinical" and not another boring, modern
sterile EQ. They had conventional EQs all over the console and
wanted something different. They had a few choice gutsy EQs with
"click frequencies" that were also inductor/capacitor based (which
is why the frequencies were on a rotary switch). Requests like
"powerful", "flexible", "unusual" and "dramatic" kept coming up.
We started with these goals: modern parametric-like operation,
passive tone techniques through-out, and features different from
anything currently available and it had to sound spectacular.
"The Super-Pultec"
Manley Labs has been building a few versions of the Pultec-style
EQs for many years as well as an updated version of the EQ-500
(another vintage EQ). These are classic passive EQs combined with
Manley's own gain make-up amplifiers. Engineers loved them but
we often heard requests for a Manley Parametric EQ with all the
modern features but done with tubes. Another request we had was
for a "Super-Pultec". We briefly considered combining the "best of"
Pultecs into a new product but the idea of some bands only boosting
and some only cutting could only be justified in an authentic vintage
re-creation and not a new EQ.
The next challenge was to make an EQ that sounded as good or better
than a Pultec. With all the hundreds of EQs designed since the
Pultec, none really beat them for sheer fatness. We knew why. Two
reasons. EQP1-A's have separate knobs for boost and cut. People
tend to use both at the same time. You might think that this would
just cancel out - wrong.... You get what is known as the "Pultec
Curve" . The deep lows are boosted, the slope towards "flat"
becomes steeper, and a few dB of dip occurs in the low mids. The
second reason for the fatness and warmth was the use of inductors
and transformers that saturate nicely combined with vacuum tubes
for preserving the headroom and signal integrity.
Could we use a "bandwidth control" to simulate the "Pultec Curve(s)?
The Pultec curve is officially a shelf and shelf EQs don't have a
"bandwidth or Q knob"- only the bell curves. So, if we built a passive
parametric where each band could switch to shelf or bell and used
that "bandwidth" knob in the shelf modes we could not only simulate
the Pultecs but add another parameter to the "Parametric EQ" We
found that we could apply the "Pultec Curve" to the highs with
equally impressive results. This is very new.
The Massive Passive differs from Pultecs in several important areas.
Rather than copy any particular part of a Pultec, we designed the
"Massivo" from the ground up. As mentioned, each band being able
to boost or cut and switch from shelf to bell is quite different from
Pultecs. This required a different topology than Pultecs which like
most EQs utilize a "series" connection from band to band. The
Massive Passive uses a "parallel" connection scheme.
A series connection would imply that for each band's 20 dB of boost,
there is actually 20 dB (more in reality) of loss in the flat settings.
Yeah, that adds up to over 80 dB, right there, and then there is
significant losses involved if one intends to use the same components
to cut and to boost. And more losses in the filter and "gain trim". That
much loss would mean, that much gain, and to avoid noise there
would need to be gain stages between each band and if done with
tubes would end up being truly massive, hot and power hungry.
Instead, we used a parallel topology. Not only are the losses much
more reasonable (50 dB total!) but we believe it sounds more
"natural" and "musical". In many ways the Massive Passive is a very
unusual EQ, from how it is built, to how it is to operate and most
importantly how it sounds.
We designed these circuits using precise digital EQ simulations,
SPICE3 for electronic simulations, and beta tested prototypes in
major studios and mastering rooms for opinions from some of the
best "ears" in the business.
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This section is borrowed from the Massive Passive Manual