[sdiy] hi shelving filter

[Magnus Danielson]

From: mark s <djarum11 at mindspring.com>
Subject: [sdiy] hi shelving filter
Date: Fri, 13 Feb 2004 15:16:15 -0500
Message-ID: <4.2.0.58.20040213143131.0225deb8 at pop.mindspring.com>

> greets list

Hi Mark,

> i am building a mic pre based around the SSM2019.  i would like to add a 
> high shelf filter with cut or boost starting at 3 selectable frequencies. 
> 4k, 8, and 16k.
> 
> obviously this is for vocals so it's gotta be pristine-clean and the 
> phase/time-alignment must be good, or it's just not worth implementing.
> 
> 1. bandpass filter from 4k/8k/16k to say 100k ?
> 2. active HPF with make-up gain (i'm not sure how to do this)  ?

First make a highpass filter for the above frequencies (switchable resistors
for frequency). A simple Sallen & Key filter will probably do the job well
enought. Select a Bessel-Thomson or Gaussian responce.

Then, just like in an equalizer, put the output of the highpassfilter to the
wiper of a pot and then the ends to a positive and negative mixing bus. Add the
original signal to the positive mixing bus through a resistor. You are looking
at 4 op-amps including an input-buffer (which is highly recommended unless you
have a good low-impedance source).

> does anyone have experience with inductors?  what are they like in real life?

Reactive? ;O)

Cheers,
Magnus