2, 4 & 8 Band Gyrator EQ

Moving on from the HM2 which obviously has quite a powerful EQ, I thought I could use the circuit to make a handy standalone EQ.  I was going to just use a quad opamp and make this 2 band with 2 of the channels being used as the input buffer and output gain stage, but thought it would still be pretty compact using a dual opamp for the input and output and so settled on 4 bands.

I have included a table showing the example frequencies you can achieve using common component values for R1, C1 and C2, and just altering the value of the R2 resistor.  Another option would be using 4 x 10K multi-turn trimmers instead of the R2 resistors so you could vary the frequency for every band and really fine tune it to the gear you're using.

You can use the Excel calculator I posted here to select your own components and get the frequency and Q you want if you prefer to fine tune the bandwidth.  I also included a trimmer so you can set to unity precisely, or give a slight boost if you like.

And now everything is modular of course, there's no problem in adding another opamp to give you a further 4 bands.

And the baby of the family