Parametric EQ biasing

[PBE6]

I came across this parametric EQ schematic on the Elliott Sound Products website (http://sound.westhost.com/articles/state-variable.htm), and I plan to incorporate into a bass preamp pedal:


(I will be making the suggested changes to the capacitor values to give a frequency range of about 110 Hz-725 Hz.)

My question is, how are the opamps being biased? The two opamps in the upper right have their v+ inputs going to ground as is standard for an inverting opamp, and the v+ of the other two make their way to ground eventually. Am I right in assuming that means this circuit was designed for a split power supply? If I'm using a single 9V battery, is it sufficient to change all the ground connections to Vref = 4.5V connections? Are there any ground connections that should be left as is? (Other than the circuit input and output at the bottom of the schematic)

[PRR]

Yes.

Also note that the input impedance may be awful low buy guitar-chain standards. Buffer the front.

[PBE6]

Thanks PRR! That should work out well, I'll be putting an Orange Squeezer in front of this EQ so I can put the other half of the JRC4558 to good use as a buffer.

[mth5044]

There is an updated crossover on the same site (ESP) from 2014 (I think). The one you have is from 2012 (I think).

I had tried simulating the circuit using a single supply and vref but could get it to do its thing compared to the +/-9V circuit. Let us know how you get on.

[duck_arse]

me too ^. I have a parametric pcb designed for dual supply that I tried to convert to single. a miserable failure.

[PBE6]

I just simulated this on Circuit Lab using Vref instead of ground and it works fine, but simulations are notoriously forgiving. I guess it's plausible at the very least, so I'll give it a try in the real world.

[PBE6]

I thought I might have gotten this right on the first try, but sadly no. Will have to continue debugging it tomorrow, but at least it's making some sounds - that can't be all bad!

[PRR]

Verify that every stage's output biases near half-supply.

That's the usual confusion when converting dual-supply to single-supply.

[PBE6]

So it turns out everything was connected as per my Circuit Lab diagram, but my Circuit Lab diagram had the top-left opamp flipped! Wonderful. The simulation apparently doesn't care which input is which, but real life seems to. Hopefully this is the only goof to fix..

[PBE6]

As suspected, it was not the only good. But it's working now! One strange thing about it is that there's slightly more noise and buzz when cutting instead of boosting. It's not so obtrusive that the circuit becomes unusable, but it seems backwards to me nonetheless. Is this normal behavior?

[PBE6]

One other question - I used a 20k pot for the boost/cut control. I know the diagram specified a 10k pot, and that higher values will push the useable range to the ends of the pot. Why is this though?

[Razoumihine]

I tested 100k pot for boost and cut, and I couldn't see any change compared to the 10k pot.
How is the value of this pot affecting the circuit? I was thinking that it may affect the impedance only... But not sure.
Lionel