Multiband EQ freq calculations?

[sengo]

Hey guys,

I was thinking about building a 7 band EQ similar to the one at GGG, but I wanted the bands to effect different frequencies. Is there any formula to determine what capacitors to use? I've tried the search function to no avail

Thanks,

Nick

[rnfr]

check this out.

http://www.muzique.com/lab/gyrator.htm

[MusicAudio]

What you're looking for is the "cutoff frequency", it's related to the time constant of the RC circuit.

Below is a link to a Wikipedia article explaining the RC time constant with your equation for cutoff frequency "fc":
http://en.wikipedia.org/wiki/RC_time_constant

Hope that helps.

EDIT: rnfr's link is probably more easily applied to the circuit you want to modify.

[sengo]

Ahh, thank you both for prompt responses! This was probably going to keep me awake tonight. I like both links, one for quick answers and one to understand how it's done, perfect!

Nick

[Rob Strand]

The formula you want is the series resonant circuit formulas:

f0 = 1/(2*pi *sqrt(L * C)

Q = (1/Rs) *sqrt(L/C)

The common gyrator simulates and inductor L out of resistors and Capacitors and an amplifier.

Using the Musique circuit as a reference:

L = R1 * R2 * C2

C = C1

Rs = R2

In reality this form of gyrator doesn't quite form an inductor, the resistor R1 is in parallel with it and that why you see R1 set to high values.

When you have multiple bands the Q of the overall response can be adversly affected.  Even the centre frequency is shifted a bit.

The best way to work out gyrators circuits is to start with the basic calculations then use a circuit simulator, like PSPICE or LTSPICE, to tweak the behaviour of the circuit.