Manley Labs STEREO EQ owner manual Why Passive?, And Why Parallel?

Why Passive? And Why Parallel?

If you hate tech talk, just skip this section - it has to do with electronic parts and circuits and design philosophy.

All EQs use capacitors. They are very easy to use, predictible, cheap and simple. Some sound slightly better than others. Inductors do almost the mirror function of capacitors. Unfortunately, they can be difficult to use (they can pick up hum), they can be difficult to predict (the essential inductance value usually depends on the power going through them which varies with audio), they are expensive and generally have to be custom made for EQs. These are qualities that lab- coat engineers tend to scowl at. Some effort was aimed at replacing the poor inductors and more effort made to bad- mouth them and justify these new circuits. The main reason was cost. All of the "classic" Eqs used real inductors and that has become the dividing line "sought after vintage" and just old.

What the lab-coats didn't consider was that inductors may have had real but subtle advantages. Is it only obvious to "purists" that a coil of copper wire may sound better than 2 or 3 op-amps, each with over twenty transistors, hundreds of dBs of negative feedback along with "hiss", cross-over distortion and hard harsh clipping?

We mentioned the inductance value can change with applied power. This also turns out to be a surprising advantage. For example, in the low shelf, with heavy boosts and loud low frequency signals, at some point, the inductor begins to saturate and loses inductance. Sort of a cross between an EQ and a low freq limiter. The trick is to design the inductor to saturate at the right point and in the right way.

In the mid-bands and bell curves a somewhat different effect happens. The center-frequency shifts slightly depending on both the waveform and signal envelope. This "sound" is the easily recognizeable signature of vintage EQs. It is not a type of harmonic distortion (though it can be mistaken for this on a test-bench) but more of a slight modulation effect.

Inductors in the form of transformers are also a large part of why vintage gear is often described as "warm" whether it was built with tubes or transistors. In fact, the quality of the transformer has always been directly related to whether a piece of audio gear has become sought after. Saturation in this case involves adding odd harmonics to very low frequencies which either tends to make lows audible in small speakers or makes the bass sound louder and richer (while still measuring "flat"). The key is how much. A little seems to be sometimes desireable (not always) and a little more is beginning to be muddy and a little more can best be described as "blat". The number of audio transformer experts has fallen to a mere hand full and some of them are getting very old.

The Massive Passive is a "parallel design" as opposed to the far more common "series design". A few pages back, we mentioned the main reason for going with a parallel design was to avoid extreme signal loss, which would require extreme gains and present the problem of noise or extreme cost. The parallel approach not only avoided this but has a number of advantages as well.

With the series EQ design, if you set 3 bands to boost the same frequency 15 dB each, the total boost will be band one plus two plus three - or 45 dB - but then it would probably be distorting in a rather ugly way. With the Massive Passive, you can dial in 4 bands to boost 20 db near 1K and it still will only boost 20 dB total. If you tend to boost 4 bands at widely separated frequencies (like what happens on two day mixes with sneaky producers), it tends sound almost flat, but louder. Other EQs seem to sound worse and worse as you boost more and more. For some people it will act as a "safety feature" and prevent them from goofy EQ. Occasionally, you may be surprised with what looks like radical settings and how close to flat it sounds. A side effect is that if you are already boosting a lot of highs in one band, if you attempt to use another band to tweak it, the second band will seem rather ineffective. You may have to back off on that first band to get the desired tone. You actually have to work at making the Massive Passive sound like heavy-handed EQ by using a balanced combination of boosts and cuts. In a sense it pushes you towards how the killer engineers always suggest to use EQs (ie gentle, not much, more cut than boost). This is good.

While there may be interesting arguments against any interaction between EQ bands, the reasons tend to be more for purely technical biases than based on listening. In nature and acoustics and instrument design, very little of the factors that affect tone are isolated from each other. Consider how a guitar's string vibrates the bridge which vibrates the sound board, resonates in the body, and in turn vibrates the bridge and returns to the string. What is isolated? The fact that the bands are NOT isolated from each other in the Massivo is one of the reasons it does tend to sound more natural and less electronic. We noticed this effect in a few passive graphic EQs, notably the "560" and a cut-only 1/3 octave EQ.

There is a type of interaction we did avoid. That is inductor to inductor coupling. It is caused by the magnetic field created by one inductor to be picked up by another. It can cause the inductors to become an unexpected value, or if it is band to band, can cause effects that can best be described as goofy. In the Filter Section we utilized close inductor spacing to get some hum-bucking action but avoid magnetic coupling with careful positioning. Some kinds of interaction suck and some are beneficial.

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