Achieve a higher Q on Sallen Key filter?

[stonerbox]

Good eve.. day, morning? Ahh, what's so good about it anyways... Hey everybody! 

I'm attempting to breadboard a npn germanium fuzz face with a lowpass filter in the middle of the two stages. I've built the square wave parades Simple Resonant Low Pass Filter except for the upper filtering and voltage divider part, I've also removed the 100k pot at the input and subbed the dual 500k for two 500k pots. Once I find the sweet spot all the pots will be replaced with resistors.



Now, it does what it is supposed to but I would like increase to a higher Q in order to really push the overdriven Q2 of the fuzz face. Go slow I'm sort of a virgin when it comes to active filters.

[stonerbox]

Hold on.. Is that filter supposed to run off a bipolar power supply?

[Scruffie]

Look at the opamp big muff, good high Q filter for fuzz already done for you.

[Keppy]

Mismatch the caps, with the cap to ground smaller than the feedback cap. If I remember right, 1/1=sub-Bessel, 2/1=Butterworth, bigger ratios make higher Q. I'm currently finishing up a synth that uses 100n/470p in an envelope filter.

And yes, that's for a bipolar supply. Just replace V- with ground and replace ground with Vb.

[stonerbox]

Mismatch the caps, with the cap to ground smaller than the feedback cap. If I remember right, 1/1=sub-Bessel, 2/1=Butterworth, bigger ratios make higher Q. I'm currently finishing up a synth that uses 100n/470p in an envelope filter.

And yes, that's for a bipolar supply. Just replace V- with ground and replace ground with Vb.

Wouldn't that affect a lot more then just the Q?

> Just replace V- with ground and replace ground with Vb.

Replace ground with Vb?

[Keppy]

Mismatching the caps will affect the cutoff frequency as well. If you want to see the effects, check out http://sim.okawa-denshi.jp/en/OPseikiLowkeisan.htm. The bottom calculator matches your topology. I read your post backwards before and thought you were using a dual gang pot. Mismatching the resistors should work just as well as mismatching the caps, so focus on those since you're already using separate pots.


Vb=bias voltage=1/2 the supply voltage=4.5v.

The top half of the schematic show you how to do this, but the points on the right should be labeled:
+
Vb (not ground)
Ground (not -)

The two 100k resistors form a voltage divider from 9v to 0v, so the point in the middle where they meet is 4.5v. 100k is a pretty big resistor for this purpose. I'd use 10k resistors instead.

In the bottom half of the schematic, replace the middle ground point with Vb, and connect the opamp (-) supply to ground. Add coupling capacitors in series with the input and output to block DC, so the 4.5 volts this circuit is idling at doesn't mess with circuits before and after.

[R.G.]

"The Active Filter Cookbook"

[Transmogrifox]

... http://sim.okawa-denshi.jp/en/OPseikiLowkeisan.htm The bottom calculator matches your topology.

+1.  That online calculator allows you to quickly try things before you commit it to breadboard.

You may also want to consider one of the bridged-T or multiple feedback filter types for higher Q.  Morley power wah circuit may be the ticket for some inspiration.

[stonerbox]

Thanks guys!!