Hi,

I've been learning about RC filters. I'm not sure a 2nd order Sallen-Key HPF is going to be good enough, steep enough for my application (80-1000 hz). As I understand it, getting to higher order filters may be easier to achieve with a universal filter IC.
UAF42, I think is only 2nd order.
LT1562, I think is only good for 10k-150kHz
Maxim 74xx series seems promising, but is a switched capacitor filter that requires a clock voltage. Probably beyond my ability.
NS AF100, MF5. Not sure.

What are my options to control the cutoff frequency of a higher order filter with an external pot?

 

It would help if you could actually quantify your requirements better. You mention 80-1000 Hz. which I take as the passband for your intended filter. You also need to specify the width of both the high and the low transition bands as well as the expected attenuation in each of the two stop bands. From those specifications we can tell you what order filter you will need and what topologies have a chance of meeting your requirements.

 

Sorry. I’d like the cut off frequency to be variable between 80 and 1000 Hz roughly, with a pot. So it’s a high pass filter. Everything else above that frequency would pass.

 

So how about attenuation in the passband and width of the transition band between the corner and the stop band.

 

I don’t know the answers to those questions, but I don’t think it’s that important for my application either. It’s a mic’d guitar, lowest string is 82hz. Typical hpf for a secondary internal mic is 250-350 hz. So a range of 80-1000 seems good.

A smooth butterworth type rolloff with a flat band pass and a steep slope I think would work best. The 2nd order that I’m using now just doesn’t attenuate fast enough.

 

I don’t know the answers to those questions, but I don’t think it’s that important for my application either. ...

I guess that is the difference between being a musician and an engineer. We don't even take a dump without a specification. How would we know what done means?

In order to reach your goal, I would design two third order high pass filters, each with identical resistors and a fixed set of capacitors. The corner frequencies would be at 80 Hz. and 1kHz. Then maybe with a 3-gang potentiometer you could achieve your result.

 

I guess that is the difference between being a musician and an engineer. We don't even take a dump without a specification. How would we know what done means?

Fair statement. But I am an engineer, a mechanical engineer. So I get it, but I also know that when it comes to music you have to let your ears be the judge... not the math.

 

Fair statement. But I am an engineer, a mechanical engineer. So I get it, but I also know that when it comes to music you have to let your ears be the judge... not the math.

Yep. In the high-end audio business, I heard it all from the golden ears. Some people believe in factories and some people believe in elves.

 

Here is where I would go to play around with high pass filter ideas:

http://sim.okawa-denshi.jp/en/Sallenkey3Hikeisan.htm

 

The UAF42 filter is a 1st-order lowpass, a 1st-order highpass and a bandpass combination of them.
There is no such thing as an LT1562.

Each "order" of an adjustable frequency filter needs its own pot in a multiple pot device.

Why do you need such a wide frequency range of cutoff of the highpass filter? 8kHz will produce only squeaks and high frequency harmonics.

 

The UAF42 filter is a 1st-order lowpass, a 1st-order highpass and a bandpass combination of them.
There is no such thing as an LT1562.

Each "order" of an adjustable frequency filter needs its own pot in a multiple pot device.

Why do you need such a wide frequency range of cutoff of the highpass filter? 8kHz will produce only squeaks and high frequency harmonics.

I said 80 - 1000 hz.

 

The UAF42 filter is a 1st-order lowpass, a 1st-order highpass and a bandpass combination of them.
There is no such thing as an LT1562.

Each "order" of an adjustable frequency filter needs its own pot in a multiple pot device.

Why do you need such a wide frequency range of cutoff of the highpass filter? 8kHz will produce only squeaks and high frequency harmonics.

UAF42 makes a state variable filter which is 2nd order.

 

Lowest string on a guitar is 80Hz, highest string is 2 octaves above = 320Hz, then there are almost 2 octaves of frets which is 1280Hz, and then there are harmonics above that. What are you trying to filter? If you want a tone control, use the Baxendall circuit https://sound-au.com/dwopa2.htm#s7
As a general rule, op-amps make good enough filters for audio - no need for anything exotic. Use @Papabravo 's link in post #9 to design the filters and plot the responses.

 

A smooth butterworth type rolloff with a flat band pass and a steep slope I think would work best. The 2nd order that I’m using now just doesn’t attenuate fast enough.

I t is not yet clear to me if you are looking for a suitable filter-IC or if you want to build a filter by yourself (based on opamps).
Be aware that is not so easy to realize frequency-tuning capabilities without touching the desired filter characteristics (Butterworth, Chebshev,...) - in particular when you need a filter order higher than n=2.

 

I t is not yet clear to me if you are looking for a suitable filter-IC or if you want to build a filter by yourself (based on opamps).
Be aware that is not so easy to realize frequency-tuning capabilities without touching the desired filter characteristics (Butterworth, Chebshev,...) - in particular when you need a filter order higher than n=2.

This was my experience in trying to come up with a 3rd order filter that had equal resistor values so that a 3-gang pot could be used for adjustment. It is just not obvious to me that this is possible. I could not even begin to speculate how it could be done with a single potentiometer. Maybe if there was a processor involved that took an analog input from a single source and adjusted the separate resistances required by the filter. That solution would be another level of complexity.

 

I think a DSP is the only practical way to get what you are asking for.

The question is why you are asking for what you are? What effect are you trying to achieve? A tunable high-order filter is not all that common.

 

Another level of complexity would be use a single pot to be read by an MCU and do the filtering digitally in software.

 

Another level of complexity would be use a single pot to be read by an MCU and do the filtering digitally in software.

This might be feasible for a low pass application with an analog anti-aliasing filter. This is a high pass application where all the frequencies above the Nyquist limit will be folded back into the baseband.

 

Yes I think you're all pointing me in the right direction. One of the most popular preamps for dual source guitars is a Grace Design Felix. $1200, I owned version 1 a few years ago. Highly regarded, and it uses a 12 db/octave HPF. Other preamps that combine 2 pickups with mic power generally start at $500+, my motivation for building this circuit is to find a cheaper alternative to power a condenser mic inside a guitar.

The 2nd order circuit I'm using below is variable up to 1130 hz. That's almost 4 octaves above low E (82 hz), or close to 48 db attenuation. So when I roll it all the way up to 1130 hz I should DEFINITELY hear a difference on the low E, and I really don't much. So I think I'm doing something wrong. Something with the dual pot I think. I know the preamp side works well, I'll have to troubleshoot why the HPF isn't...

The circuit below was developed in this thread with help from Audio Guru and others.

 

Are the wipers of R10 & R11 adjusted separately or are they mechanically connected (2-gang potentiometer)? Do you or they have a simulation file for this design? You see where I was headed looking for a similar 3rd order design.