LC filter design help

[jenks]

Hello, Jenks here.

I'm using an LC filter to affect the frequency response of my input signal in a preamp style. the problem is that the frequecies affected are determined by the equation; Fr=1/(2*pi*SQRT(L*C)).

So for a peak at approx 250Hz my LC filter requires a capacitor of 2mF and an inductor of 0.2mH. I haven't built a circuit like this before but I think these values are impractical as they require crazy capacitor and inductor values (I'm unable to find a 2mF capacitor and I think that a 0.2mH inductor is rather large in size for a stomp-box!)

If anybody has designed an LC filter in this way but using reasonable (and available!) component values please tell me how you did it!

**And another thing... with the above values the reduction (if notch is used) is -110dB at the centre frequency and the rest of the signals amplitude is severly decreased too! (-60dB at 50Hz! & -50dB at 4.3kHz). Grr. Any advice on how to design a preamp that can increase or reduce 250 & 500 Hz by reasonable amounts and without compromosing the level of the rest of the signal would be greatly appreciated!

Thanks.

Jenks.

[Steve Conner]

Why not a 2uF capacitor and a 0.2H inductor? That gives the same resonant frequency. And 0.2H is by no means large: the inductor in a Crybaby wah pedal is 0.5H. Having said that, I've always used gyrators in place of inductors for convenience.

How do you know that the power supply affects it, if you can't even find the components to build it yet?!

[jenks]

Are you sure a 0.2H inductor is not that big?!

I can't find one online anywhere so I'm not sure what dimensions it might be. Have you designed a preamp before Steve? Do you think that a 95dB cut/boost is too much at the centre frequencies, not to mention the 50dB cut/boost elsewhere!

[Albert Kreuzer]

Hi jenks,

a couple of years ago I tried to replicate an Ampeg SVT preamp with transistors. The midrange filter is almost the same as your circuit. The inductor has something like 1.5H, with switchable taps.
Capacitors in the .033 to .15 range, just to give you an idea where to start.

You can use small audio transformers or relay coils for the inductors.

And then stop playing around with simulations and build that dude!

BTW: I found that ANY real conductor I put in there picked up hum like crazy. Ended up using a gyrator.

HTH
Cheers,
Albert
www.albertkreuzer.com

[jenks]

Hey Albert,

I've incorporated a gyrator instead of one of the inductors now but, the peak in the freq response is at 214Hz and the notch is at 1.6kHz! Crazy!

How did you use your gyrators?

[loudthud]

Here are some inductors in the right range. Sit down when you look at the price.

http://www.hammondmfg.com/621.htm

[Steve Conner]

Sure, I've built lots of amps. I used to be fascinated by EQ, I experimented with different graphic and parametric circuits for ages. I finally settled on a hybrid 2-band parametric EQ using a bunch of op-amps and a 12AX7:

http://www.scopeboy.com/peq.html

This circuit goes one step further than a gyrator, it uses a state-variable resonator to simulate the whole LC filter. The inspiration was from a parametric EQ circuit published by John Linsley Hood.

RG Keen shows how to make EQs with gyrators, both the op-amp and discrete kind, here: http://www.geofex.com/Article_Folder...htm#graphic_eq

I guess from the results you're reporting that you're just "building" this thing in simulation? Build the sucker for real! ;-)

BTW, Albert, I bought some BF245A JFETs to build a clone of your preamp, but I got tinkering with them and ended up with a totally different circuit!

[jenks]

Hey Steve, that circuit has all the tonal flexibility I was looking for! thing is though I really wanted to power a preamp/overdrive pedal with 9V batteries and put it inside a stomp box. I tried R.G's graphic EQ and gyrator circuit but I couldn't get it to work how l wanted.

Anyway I've just sent you an e-mail at you scopeboy account. I've attached a few screenshots of the circuits I've been working on. If you have any ideas do get in touch mate. Cheers.