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Comment of the week - correct gain and matching for passive EQ

An RP visitor sheds further light on passive EQ. If you don't connect a passive EQ correctly, the results can be unpredictable (but isn't that the fun?)...

The recent article on vintage passive EQ has created quite a lot of envy and desire. However, there is more to connecting such a device into your system than merely plugging it in and hoping for the best.

Pierre from Coresta in France comments...

"Hi, yes, for sure you need a line - amp gain to attack these: A very low impedance source too (average 20 ohms max ) so that they don't detune. These must be charged, as usually at these times, on a resistive 600 ohms load. I have built such "same" correctors from scratch old ORTF parts, the results are great! With good ferrite pots as pictured, you can reach +/- 1dB linearity. But gain must be get AFTER , and don't overload them ... !"

Let me expand on this... Audio tradition has grown up in such a way that each piece of equipment 'conditions' the signal to its own requirements, and supplies an output that is strong and healthy enough to supply anything that follows. In this way, if there is a rogue item of equipment that, say, requires more electrical current than it really ought to, it doesn't matter because the preceding piece of equipment is over-engineered and can supply it.

However, this only applies to active equipment. That is equipment that draws electricity from mains or batteries. Passive equipment that works only on signal power is much less tolerant.

What Pierre is saying here is that a passive EQ such as this must be driven from a source that has a low output impedance and hence can supply plenty of current. He also says that the output of the passive EQ must be connected to an input with an impedance of 600 ohms. 600 ohms used to be a standard in the old days of audio, so it is very likely that the passive EQ unit was designed to give its best performance into this load. Otherwise, the frequency response will be something other than was expected.

One other point that Pierre touches upon is providing gain after the unit and not before. Although this shouldn't make a difference in a purely passive circuit as long as the gain is kept to reasonable amounts, there is another consideration...

The unit may have iron-cored inductors.

Modern equipment tends not to use inductors. An inductor is simply a coil of wire that passes low frequencies easily but restricts the flow of high frequencies. However it is difficult to make inductors to precise values and they tend to be bulky. Hence other circuits such as the gyrator are used to mimic the effect of an inductor.

But a gyrator cannot be made from passive circuitry, so the inductor is a very valuable component to have in a passive EQ.

To achieve high values of inductance however, the coil is wrapped around a core of magnetic material, commonly iron. The core stores energy from the electric signal in the form of magnetism,

However, at high signal levels there comes a point where the core is magnetically saturated. That means it cannot become magnetized to any greater degree. The result is distortion.

Yes, distortion can be produced even in purely passive circuits.

All of this implies that in this particular case it would be well worth experimenting to find the optimum configuration.

However, thinking more generally, perhaps there is a case for more passive equipment becoming available. There's no such thing as having too many options, and passive EQ could be a very useful tool to have around.

Of course, there never will be a passive EQ plug-in. Will there...?

By David Mellor Monday May 1, 2006