"Funky MF (Multimode Filter)" - LM13700 Multimode Envelope Filter

[frequencycentral]

I've been wanting to build a multimode (highpass/bandpass/lowpass) envelope filter for a while using a LM13700. This thing has been on my breadboard for the best part of 2 months, I finished the build last week and have been making some minor tweaks since. It's pretty close to what I wanted now. Maybe just a few more tiny tweaks. I can't claim to have designed it from scratch - it's actually based on the filter and envelope detector in this circuit but with a number of modifications. It's also fairly close to the Tonepad Maestro FSH but with the addition of the extra modes, but I didn't choose to include sample and hold. As usual - I don't claim to be a whizzbang EE, so this is fully open for hacking/modding/improving. I'll be posting some soundclips in the next few days - hey its a busy time of year! Rest assured, it sounds funky as hell!

[Mark Hammer]

Very nice!  Any thoughts to having a dry/filter blend pot?  That gain stage just ahead of the "range" control seems like an appropriate spot to tap the dry signal, and the op-amp output stage provides an opportunity to mix them together.

Just a thought.

[frequencycentral]

Thanks Mark. A blend control would probably be useful to some users, and would be easy enough to add as you suggest. I didn't really consider one because I was aiming for as 'synthy ' a sound as possible - to this end, I'm using a distortion before this pedal, the filtering is far more pronounced that way than with a clean guitar signal. That gain stage with the trimpot before the envelope detector was added to give the ability to boost a clean guitar signal enough to give a meaninful signal to the envelope detector. To be honest, I was also trying to keep the knob count down, it could have ended up having a frequency control too, and a depth control between the envelope and OTA control inputs.

[theehman]

Very nice!  I may have to build one of these myself.

[frequencycentral]

Very nice!  I may have to build one of these myself.

Thanks theehman - I should also mention I run all my pedals, this one included, at 12 volts. I'll set my power supply to 9 volts and see how it works that way - should be fine though.

[flo]

An idea when using an envelope filter on a guitar signal with a lot of distortion:
Make a separate input for the envelope detector and feed that with the clean guitar signal.
A heavy distorted signal usually has a lot of compression/sustain: A a very slow decay. The clean signal usually has a much faster decay.
This gives a nice "synthy" combination of filtering a signal with lots of harmonics using a snappy decaying envelope.

[frequencycentral]

That's a good idea flo. It did occur to me during the breadboarding phase to maybe add a built-in distortion to do what you describe, as with these type of devices what comes out is largely based on what comes before it in the pedal chain.

I also looked at the Stompboxology article (found on Moosapotamus's site) on envelope detectors. What particularly interested me was the leading edge detector, which would appear to be able to specify the decay irrespective of the input signal. Has anyone done this before?

To my ears, the standout sounds with my build are the autowah type slow attack sounds with a distortion before the filter. A bit of muting is needed to get those fast decay sounds with a distortion before the filter - but they do come more naturally with no distortion. I think that if a new pedal gives you just one great useable sound to add to your arsenal then it's a win.

Hmmm, live and learn! I just know I'm going to do another envelope filter (at some point in the future) implementing stuff I didn't use in this design - blend control, built-in distortion, maybe octaves too. It will probably end up having a lot more knobs than this one!

[Boogdish]

I have a few LM13600s lying around.  Will these work for this build?  I'm still a bit new to the world of transconductance op amps.

[frequencycentral]

I have a few LM13600s lying around.  Will these work for this build?  I'm still a bit new to the world of transconductance op amps.

Those LM13600 will work just fine!

[frequencycentral]

Here's a soundclip/montage featuring my trademark lousy playing style: http://www.aronnelson.com/DIYFiles/up/Funky_MF.mp3

Recorded straight into my soundcard, with my trusty Valvecaster between the guitar and the Funky MF. 

[Boogdish]

That sounds deadly Rick!  I'm definitely going to have to try this.

[Radamus]

How does it sound on bass?

[frequencycentral]

How does it sound on bass?

Sounds cool with bass too - I'll get some bass soundclips together later today.

[Ben N]

That's a good idea flo. It did occur to me during the breadboarding phase to maybe add a built-in distortion to do what you describe, as with these type of devices what comes out is largely based on what comes before it in the pedal chain.

No need to actually build it in. All you would to modify for a sidechain is to wire a switching jack into the circuit right before the envelope detector so that if nothing is plugged in, you are still drawing the envelope signal off the input. It is also probably a good idea to have some kind of buffer/splitter at the front of the signal chain where you tap off the clean envelope signal.
This idea is useful for any kind of envelope-based effects, such as compressors, to let you put the effect in the signal chain where it is most useful, without worrying about added noise or a degraded signal envelope.

[frequencycentral]

How does it sound on bass?

Sounds cool with bass too - I'll get some bass soundclips together later today.

Here's some bass soundclips: http://www.aronnelson.com/DIYFiles/up/Funky_MF_Bass.mp3

My bass is a Washburn Bantam Active RB2500. I found I got the best results running it through my NanoCompressor and then into the Funky MF.

I think if I were planning to use this for bass I would definately add a blend control as Mark Hammer suggested - and always use a compressor before it in the chain.

[Radamus]

always use a compressor before it in the chain.

So it's not volume activated? I see that there's no LED, but does it just automatically run the range of the filter with any sound? Can you vary the length of the decay?

You also said that the sensitivity is set by a trimpot. Can you make that a regular pot? I've made some good sounds with my meatball, but I'm always tweaking the sensitivity based on what I'm playing. I know the filter is different in this. Would having a sensitivity knob be helpful?

Thanks

[Ben N]

Unless it is set on an extreme setting, a compressor may well leave enough dynamics in the signal to trigger an envelope follower. However, this would be another benefit of a sidechain--send a nice, smooth, even, sustainy compressed signal through the filter, while your raw, uncompressed native electric guitar note drives the envelope detector in all its dynamic glory.

[frequencycentral]

always use a compressor before it in the chain.

So it's not volume activated? I see that there's no LED, but does it just automatically run the range of the filter with any sound? Can you vary the length of the decay?

You also said that the sensitivity is set by a trimpot. Can you make that a regular pot? I've made some good sounds with my meatball, but I'm always tweaking the sensitivity based on what I'm playing. I know the filter is different in this. Would having a sensitivity knob be helpful?

Thanks

Yes it is volume activated, I'm just finding that bass has too much dynamic range for the way I have the trimpot set I guess. I only really play bass when recording and I always comp it - maybe has more to do with my 'playing style'! Using the comp tames the bass a little for the envelope follower, without it going ove the top all the time. It's different with guitar.

The trimpot sets the range of the 'Range' control, if you get what I mean. think of the trimpot as coarse tuning, the 'Range' control as fine tuning. Set the trimpot up for the paticular application and then use the 'Range' control for tweaks. The trimpot and opamp form a x10 amplifier, turning the trimpot half its range make this a x5 amplifier etc.

EDIT:

I have to say that it's a lot easier to get good sounds out of the Funky MF with guitar than it is with bass. I did the design with guitar in mind, so it's set up for guitar really. Maybe someone should suggest some bass mods? I suspect it would involve changing the input cap and the cap before the envelope detector.

[Mark Hammer]

There are a couple of things you can do.

One is to make the gain in that first op-amp stage greater for the mids and highs than for the lows, something like what is used in a more extreme fashion in the Rat.  So, stick a 3k9 fixed resistor and a .047uf cap in series and stick that in parallel with the 10k resistor from the inverting pin.  That way, adjustments of the trimpot will result in more gain for the content above 800hz or so, than for content below.

Alternatively, you have the 100k Range pot tied directly to the preceding op-amp's output.  You could stick a parallel RC combo between the op-amp and pot to allow for more attenuation of the signal for the low end than for the mids and highs.  So, a 22k resistor in parallel with a .0047uf-.01uf cap might get you a little more mid/treble drive (relative to bass) coming off the wiper of the Range pot at less-than-max settings.

[Radamus]

Not that this is any huge revelation, but I think most people, when modding for bass, use a 1uf to 10uf capacitor at input and output. Other than that, I'm not sure enough what the circuit does exactly to recommend anything. I would suggest that you make sure you don't lose the low frequencies anywhere, and it's important that the wah sound is present on an open E, or maybe even lower for some of those 5-stringers. It took me forever to tweak my meatball to make an actual wah sound on my open E. I don't know if that means the wah has to cover a range of frequencies that is just above the low E or not. What I mean is that it probably has to be proportional somehow, but I'm no engineer. I'm studying film  . I know that the high pass and bandpass filters on my meatball are only used when I'm showing friends what it can do. Lowpass is all that makes sense because the volume drop is so substantial. It's interesting to have the volume vary on the range of the note, but I think I'd rather have a volume controled volume filter for that (which I'm assuming amounts to a compressor in moderation?).

• Let the bass frequencies in
• Keep the tone quality high at lower frequencies
• You probably don't have to worry about whether or not the highpass and bandpass work on bass unless someone knows how to keep the volume up
• If the dynamic response of basses is different, that might have to be taken into account. I know a lot of basses are active. I'm not sure many guitars are. My bass is passive.

Well, that's all the rambling I have right now. I imagine you probably figured most of that out before I said anything. I don't know any of the math for this stuff, so someone else will have to help you out there.
Good luck. I'm excited to see what you come up with. Always looking for new filters.