**Audio graphic equalizers**are very common as commercial products (for Hi-fi, car audio and stage use) but circuits for them are very rarely published. I didn't design this one but it's really very simple. The details shown are for a 7 band but the principle can be extended to almost any number of bands - if you can find accurate enough components.

Only one gyrator stage is shown: all 7 gyrators are the same circuit, only
the capacitors change, as shown in the chart. I have shown three of the seven
faders to show where they go.

A gyrator is a circuit using active devices and transistors to simulate an
inductor. In this case the gyrator is the transistor acting with R1, R3 and C2.
It could just as easily be a unity gain op-amp.

The circuit includes three formulae: one which gives f, the the centre
frequency of the band. The second shows how the Q is related to the capacitor
ratio. The third shows the impedance presented by the circuit. Note that this
includes 3 terms, the first purely resistive, the second is the capacitative
contribution from C1 and the third is an inductive term from the gyrator.

If anyone wants the detailed mathematical working out of these formulae, I
might be induced to post it (donations accepted!). The mathematics for active
filters is not as difficult as most tutors tend to make it and I really didn't
understand it properly until I worked it out for myself and found that it wasn't
complicated, I just hadn't been taught how to understand it!

If you do the maths for this you will find the actual frequencies are
actually a little different from the target frequencies shown in the diagram:
that's what comes from using 'standard' values. Audibly they are plenty close
enough.

The rest of the circuit is simply an op-amp. If you consider a 'tuned
circuit' (the gyrator) hanging from the pot slider, it is being connected either
to the positive input or the negative to a variable extent. One will increase
the response at the turned frequency and the other will decrease it.

You must of course chose a good, low noise op-amp: when we manufactured these
we used 741s but we selected low noise ones. The transistors also need to be low
noise, but you can easily change a noisy transistor if you find you have one.

And that's about it. A very simple, effective circuit. The most difficult bit
is going to be sourcing the components - particularly suitable fader pots!.

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