Proper center frequency for a wah?

[Explorer]

I'm building a circuit based on a state-variable active filter, and I'm really just wondering what frequency I should tune it to, to get some wah-like action. I know it will not be the same as a "real" wah, but it's easy to change between high-pass, low-pass and band-pass.

It's a second-order filter only, meaning I won't be able to get any funny three-pole curves and such.

[brett]

As far as I know, the diffent inductor values and capacitor choices make for quite different centre frequencies.  My wah comes with the standard 0.01 capacitor, which, if my inductor was exactly 500mH, would give Cf around 2250 Hz (Cf = 1/(2*pi*sqrt(L*C) I think).  
(Xl = Xc = 7.07k, Q ~ 0.2, Bw ~ 10kHz)

[Transmogrifox]

I have done some experimenting with the biquad active filter (sounds like what you are playing with is quite similar).

The nasally wah sound that you get at about 3/4 down on a crybaby happens when I use a Q of 10 centered at 1.5 kHz.

I have been working on making this filter able to range from 80 Hz to about 5.6 kHz, but I want it voltage controlled so I can modulate it with a voltage.  I would eventually like to make a phaser out of this filter topology if I can get a dependable VR.  I suppose I should just order some LDR's or CMOS switches or something.  I have been using the whole differential pair equivalent resistance, which works quite well, but is only good for a signal less than 100mV....so I need to do more careful design to get a good resistance range with low distortion....

Anyway, play with a 80Hz to 5kHz sweepable range.  I imagine most wahs don't get above 3kHz, though.

[Explorer]

Thanks to both of you for good and helpful answers.
I found something of interest at geofex too.
http://www.geofex.com/Article_Folders/wahpedl/voicewah.htm

Apparently many of the wovel sounds consist of 2 different frequencies being enhanced, and a typical wah has the ah-sound around 700-1200 Hz, but it will be fun to try out the higher frequencies you specified. Think I'm gonna save that article for later.

I have done some experimenting with the biquad active filter (sounds like what you are playing with is quite similar).

The nasally wah sound that you get at about 3/4 down on a crybaby happens when I use a Q of 10 centered at 1.5 kHz.

I have been working on making this filter able to range from 80 Hz to about 5.6 kHz, but I want it voltage controlled so I can modulate it with a voltage.  I would eventually like to make a phaser out of this filter topology if I can get a dependable VR.  I suppose I should just order some LDR's or CMOS switches or something.  I have been using the whole differential pair equivalent resistance, which works quite well, but is only good for a signal less than 100mV....so I need to do more careful design to get a good resistance range with low distortion....

Anyway, play with a 80Hz to 5kHz sweepable range.  I imagine most wahs don't get above 3kHz, though.

The state-variable filter has seperate outputs for high, low and bandpass, which can be a nice thing. They use the same tuning tho, so frequency and q/damping will be the same for all three outputs. It looks almost the same as the biquad, but the behaviour of the two is very different, as far as I've understood things. (Which is, only theoretical.)

How does the biquad function with the guitar signal? I've only read about it, but I'd guess that the bandwith remains constant while changing frequencies will make it sound a little different than "regular" filters?
If you reduce the input signal, and then amplify it again after the filter, the 100mV limit won't be an issue when using a transconductance amplifier. What is the frequency change/resistor change ratio of the biquad when using two tuning resistors? I know that when using one tuning resistor the ratio is approx. 3:1 change on a 10:1 resistor change, but I can't find anything on using both resistors.

I'm thinking of tuning my filter to 1-kHz, normalized to 10K, using a 100K dual ganged pot to vary the frequency, which will let me play around with the response from about 100Hz to 10kHz. The state-variable filter requires you to keep both tuning resistors the same, unlike the biquad, but I guess that'll be ok. The question is if I'll find a LDR able to go from 1 to 80K resistance or something. If not, I hope large capacitor values are not going to destroy anything..

[Transmogrifox]

The bandwidth change of the biquad depends on the particular biquad you use.  There is a Thomas-Tow biquad, and a...geez...I can't remember the name, but it's the one I'm using, which is the dual integrator loop with the same topology that many switched capacitor filters use (There's an idea, by the way...and SCF).

Anyway, this one maintains constant Q for all frequencies, therefore a changing bandwidth, which means equal power in the pass band for all center frequencies.

I think the biquad sounds great.  I haven't had the chance to use it to its fullest extent yet because I have only gotten as far as a dual pot that I hand crank to change the frequency.   If you set the think up right, you can get the full 80 Hz to 5 kHz with a 50k resistance change or less.

With the OTA I used, I was ranging from 10k ohm to 1Meg (a set parallel resistor).

The reason I don't want to pre-cut, then boost again is that this method is prone to a pretty bad signal/noise ratio.  Any noise in the filter and the OTA used as active resistance will be amplified by the output amplifier right along with the signal, so I want to be able to use a good full 1-3 volt range in this thing so that the signal level is much much greater than the generated noise.

The question is if I'll find a LDR able to go from 1 to 80K resistance or something. If not, I hope large capacitor values are not going to destroy anything..

It seems to me that LDRs usually rest at very large resistances (like 100 Meg) in absolute dark.

[brett]

Many small LDRs (1/4") have lower resistances in light and dark than larger (1/2") LDRs.  Also, because the response to light is negative exponential, even a little light takes the resistance down from several Mohms to 100k or so.  However, you may need something like a superbright LED nearby to get all the way down to 1k.

cheers

[Boofhead]

I'm thinking of tuning my filter to 1-kHz, normalized to 10K, using a 100K dual ganged pot to vary the frequency, which will let me play around with the response from about 100Hz to 10kHz.

That would be OK,  300 to 3k would probably do it.