Dumb Question About Gyrator Circuits

[Paul Marossy]

So if you use a gyrator to boost one selected frequency band only, is that electronically like being an adjustable inductor? In other words, is it a resonant circuit, or is it something else?

To my ears, they sound similar, but not quite the same. I like the single band gyrator circuit because it doesn't so much sound like a static wah pedal. Which got me thinking about whether or not a gyrator is a resonant circuit, and hence the dumb question. I have zero experience with the gyrator circuit except that I recently used one on something I am toying with at the moment.

Reference point for question is how gyrators are used in the Boss GE-7 eq pedal.

[CynicalMan]

A gyrator is electrically equivalent (almost) to an inductor in series with a resistor.



A GE-7 puts a gyrator in series with a capacitor, which makes a resonant series LC circuit. Its impedance hits a minimum at the resonant frequency and rises on either side. The sliders on the GE-7 move the gyrator and capacitor from shunting the input directly to ground (frequency cut) to shunting the signal in the op amp's feedback loop to ground (frequency boost).

So, it is resonant.

[Paul Marossy]

Thanks for the info, I thought so. The difference I hear between a gyrator and a wah must be in the Q factor then.

[CynicalMan]

A wah rolls off bass and treble as well as having a resonant boost, whereas something like a GE-7 leaves it flat. The Q is probably bigger on the GE-7 too.

[Paul Marossy]

Yeah, I was thinking that the gyrator Q might be bigger than a wah. They added a resistor in the wah specifically to lower it according to the patent documents. Good point about it rolling bass and treble, too.