[sdiy] equalizer

[harrybissell]

cheater cheater wrote:

> Well, what I want is a narrow cutoff (like 24 dB/oct? no idea - hints?)
> Usual equalizers have 6 dB/oct - right? Or is it 3?

Most are 12dB, some are 18dB.. I don't know of any common 24dB designs.

> So at 24 dB/oct:
> 1. The BP would be real narrow and resonant and crappy and shtuff

24dB could still have low, or high resonance.  Q of .707 to 1 would not have
a pronounced peak. BTW the BPF of the state variable is only 1/2 the cutoff
rate of the HPF and LPF.  The 'state variable' design in most commonly a
12dB slope. You 'can' co a 24dB but it gets complex

>
> 2. The HP and LP would already give me the whole spectrum. So the BP
> would only add stuff - am I right? Or would HP+LP=everything-BP? :)
>
> And in other news:
> can anyone recommend a filter that:
> 1. doesn't have a very resonant sound

any one with a Q of .707 to 1...

There are a number of standard filter designs. For memory I will fvck this up
and my listmates will jump my bones. They are Butterworth, Bessel, Chebychev.
One has better flat gain in the passband, one a better cutoff but small peak at the
corner, one has ripple in the passband but even sharper cutoff. They have different
shape of phase shift as well.  Decisions decisions. Most folk use the Butterworth...

> 2. has a pretty narrow cut-off (24? 12? 18 dB/oct? which ones would you choose?)

12dB

> 3. has a pretty uncomplicated circuit (I'm going to have to have a lot
> of these in an equalizer it seems - and even more in a whole mixer -
> woe is me!)

There are a lot of circuits. I'd start with one I like, then copy that first to get
experienced..

4. (a plus but not needed) sweeps don't have the usual cheezy resonant whistle sound

>
> This goes for LP and HP. I'm counting on you synth freaks here!

If you need to sweep the filters, for sure the higher cutoffs will need more
components. Usually a 12dB needs two tuning elements (dual pot, ota etc)...
24dB needs 4 etc.

>
>
> Regarding 3 - am I right to think that SMD components would make the
> [6x2x5=60 (!!!!) (4 bands)x(stereo)x(5 channels)] SIXTY filters easier
> to tune up? Don't they usually fluctuate less? In any case I guess
> it's a must considering space and such...
> I'm *so* buying metal film resistors... and 2% caps.... *cringes*

and good caps are hard to find... especially in SMT

> Also: anyone got the Electrix EQKiller schematic?
> Or the Vestax DCR-1200 Pro?
> I'm very curious what kind of filters they're using, and what order they are.

I'd recommend that you get a copy of the old National Semiconductor "Audio"
or "Audio/Radio Handbook"  (out of print but I think someone sells reprints). The
ICs shown are largely obsolete but the concepts are still quite valid, and it is
written
for the common man (well, common man skilled in the art :^) to understand. It shows a
graphic EQ and a 'room equalizing instrument' that are of interest.

H^) harry

>
>
> Cheers!
> D.
> 8)
>
> On 10/8/05, harrybissell <harrybissell at prodigy.net> wrote:
> > Steven Cook wrote:
> >
> > > A state-variable filter has three simultaneous outputs: lowpass, bandpass
> > > and highpass. I suspect that mixing all three outputs together would
> > > reconstruct the input signal with reasonable accuracy.
> > >
> >
> > True... but the corner frequencies cannot be (individually) controlled... and are
> >
> > really unlikely to be useful in an EQ application.
> >
> > H^) harry
> >
> >