I recommend NE5532, OPA2134 or LM4562 opamps, but others will work too, including very cheap or very expensive types. If decent opamps are used, there should be very little increase in noise, and distortion will remain negligible. Ultimately, virtually all parametric equalisers have lots of opamps, because that's what's needed to get the functionality that users expect. Not that this is a real issue, but it does mean that some people may not be happy with so many opamps in the signal path. If you have a 3-stage equaliser, you have 3 opamps in series. However, each stage is cascaded because you can't make two or more sections behave properly using a single opamp. Despite this advantage, the circuit itself is fairly economical in terms of components. The adjustable circuit described here has the advantage that the Q remains constant, so it covers the same frequency range (in octaves or parts thereof) regardless of the centre frequency. However, its Q changes as the frequency is varied, and that is slightly annoying. I have already described a 'quasi-parametric' equaliser (see Project 28), and it does work rather well. However, there are many places where the ability to vary the Q is not needed, especially for general purpose tone controls.įor musical instrument use a flexible tone control circuit is often a must, and especially so with bass guitar which has some interesting challenges. While other filter types can also be used, most are not as well behaved or as flexible as the state-variable topology. Parametric equalisers are often very complex, because to enable variable frequency and Q requires a state-variable filter. It's not perfect (I don't know of any circuit that is), but it is fairly simple to implement and performs well. The main purpose of this article is to introduce the reader to a flexible equaliser circuit that can be used for hi-fi, mixing consoles, instrument amplifiers (especially bass guitar) or anywhere else that a simple and predictable 'parametric' equaliser is needed.
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