How to make this LPF an HPF?

[Cabezahead]

So I stumbled upon Eric Archer's "DIY Analog VCF" (at http://ericarcher.net/wp-content/uploads/2011/03/9v-analog-diy-lpf.pdf) while poking around for various 9V filters.  I've kind of gathered that some other filters that use this chip (LM13700 OTA) are pretty easily switched to high-pass, but I haven't been able to wrap my head around HOW, exactly.  I'd love to build this thing with a switch that bounces between HP and LP - or maybe a box that has a LP and HP.   Any insight would be greatly appreciated as the theory is pretty far beyond me at the moment.

Also - Hey!  Haven't been around here for about eight years or so. :-)

[WhenBoredomPeaks]

http://www.diystompboxes.com/smfforum/index.php?topic=85279.0

additional goodness here: http://www.diystompboxes.com/smfforum/index.php?topic=97062.0

btw while we are at filters, does anybody know a transistor/diode ladder lp filter which does not lose it's bass frequencies when the resonance gets high? the sound of my minimoog vcf thins out at high resonance which is normal for that design but are there "non-lossy" 24db filters out there?

[Cliff Schecht]

So I stumbled upon Eric Archer's "DIY Analog VCF" (at http://ericarcher.net/wp-content/uploads/2011/03/9v-analog-diy-lpf.pdf) while poking around for various 9V filters.  I've kind of gathered that some other filters that use this chip (LM13700 OTA) are pretty easily switched to high-pass, but I haven't been able to wrap my head around HOW, exactly.  I'd love to build this thing with a switch that bounces between HP and LP - or maybe a box that has a LP and HP.   Any insight would be greatly appreciated as the theory is pretty far beyond me at the moment.

Also - Hey!  Haven't been around here for about eight years or so. :-)

To switch from lowpass to highpass with an LM13700 is easy. Switch the cap and OTA so that the cap is the series element (thus blocking DC) and the OTA acts as the resistive shunt element.

I won't go into the circuit details because there is a problem with doing this. The resonance circuit used for a multi-pole lowpass filter (read: NOT a state variable type topology, that's different) doesn't directly translate into what's needed for the highpass implementation. I played with this for weeks on the bench and in sim before giving up, and I only gave up because of a conversation I had with a buddy of mine who is a DSP whiz. He had apparently tried to do the exact same thing before (take a multi-pole lowpass filter and turn it into a highpass) and found that the feedback circuit for the resonance doesn't translate directly.

I've seen this type of circuit implemented in a digital manner before (see the attached image below) but not in an analog sense, at least not using 13700's and op amps. Maybe I will look into this further tomorrow, I'll be stuck in front of a computer anyways and it's fun putting synth circuits together in simulation .

[Cabezahead]

Hrrrmmmm.. Thanks guys!

Yeah - I found all of the stuff of people explaining the theory but have never found anything where someone says "I did it and it worked!"  I guess what I need to do is bite the bullet and learn SPICE.  I just hate breadboards with such a deep passion...

[R.G.]

... I've kind of gathered that some other filters that use this chip (LM13700 OTA) are pretty easily switched to high-pass, but I haven't been able to wrap my head around HOW, exactly.  I'd love to build this thing with a switch that bounces between HP and LP - or maybe a box that has a LP and HP.   Any insight would be greatly appreciated as the theory is pretty far beyond me at the moment.

It's not the chip, it's the math that the chip implements. That said, I'll skip the math and do only the insight.

As Cliff says, highpass and lowpass are duals of one another. That kind of means that you can transform one into the other by swapping one reactance (cap or inductor) for the opposite, or in R-C filters, swapping R and C. That takes care of the nominal AC response, then you have to fiddle with getting the DC conditions and feedback to do the right thing to get it to work with AC again.

The state variable filter, which Eric's page notes the filter you reference is, is a slightly different animal. The state variable filter is an electronic implementation of the math of a pendulum. One of the quirks of this math is that to do this, you use two integrators and some feedback from a couple of places, and then some gain. The math naturally produces a high pass filter output and a low pass filter output from different places in the electronics/calculation at the same time. So you always get both a high pass and a low pass, as well as a bandpass output, if you do a state variable filter. One additional opamp stage added on produces a notch filter. All the filter outputs have the same turnover frequency, and if you use an OTA as the frequency determining element, all tune in unison as you change the control current on the OTA.

The state variable filter is really, really useful  because you can get a second order filter with minimal chips, tune it incredibly well, get multiple output types, and have complete and independent control of frequency, gain, and resonance.

The OTA (an LM13700 is a dual Operational Transconductance Amplifier) can be used in many, many other filter uses, but the math of those filters may or may not let you pick high or low pass simply.

[Cabezahead]

Thanks, R.G.  It seems like I need to take a few steps back and do some serious reading, as I was looking for more of a "Change this to this and it will make all of your wildest fantasies come true" sort of answer.  Any recommended readings that take someone from the shallow end to at least a doggy paddle in the realm of VCOs and VCFs?

[R.G.]

Actually, yes. If you have any electronics understanding at all, get "The Active Filter Cookbook". I took courses in the math of active filters, passed the tests, but this book is what I pull out when I wonder "... now how did that work??"

[Cabezahead]

Fantastic!  I'm at the point where I can tell all of the types of fuzzes and distortions and passive filtering by looking at schematics.  This is exactly what I was looking for.  Thanks so much.