Help me develop a bandpass gate!

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Jonachi
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Help me develop a bandpass gate!

Post by Jonachi » Fri Jul 24, 2020 1:25 am

Hi there.

So after 7 years of building I thought I'd try to develop my first own design ( n00b flag waved). Don't know if something like a bandpass-gate exist but I need one. It will be the first circuit in the bigger scheme at the input stages.

So I've always loved Clee's VC resonant gates since it has voltage control over resonance and offset (the resonance part might be extra useful in a bandpass filter I thought).

Original schematics can be found here: http://clsound.com/VC_Resonant_Gate_rev ... ts2-dl.php

At first I thought of exchanging the vactrols for LED/LDR design but that was when I thought I needed 3 of these in my circuit, now I've downsized it a lot and down to just one, might as well use the real thing. I wrote Clee for his advice and he wrote:

"The LPG is based on a Sallen-Key filter with the Vactrols replacing the resistors.

Look here at the Low Pass design and you’ll see the audio path in the LPG Is just a single Filter.

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


Further down is a Bandpass layout. You’d need to have 3 VC resistors instead of 2. In my experiments I didn’t see any difference between driving the LED in a dual Vactrol and 2 LEDs in two singles, so maybe driving 3 LEDs would work.

I don’t know how the 3 would track or what it would sound like, put it could be unpredictable and wonderful."

So I'm just starting out learning how to draw schematics in eagle and right now I'm terrible, I don't really know how to write the bandpass part discirbed above (I think I know what he means though).

Can someone please help me clear this up and help me develop this thing?

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Re: Help me develop a bandpass gate!

Post by Don T » Fri Jul 24, 2020 4:26 am

I can't help much, but I would suggest studying the Buchla 291 filter for ideas

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Jonachi
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Re: Help me develop a bandpass gate!

Post by Jonachi » Fri Jul 24, 2020 5:38 am

I have, but I’m doing a gate, not a filter. Or actually plan b is a vca followed by a regular bandpassfilter.

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Re: Help me develop a bandpass gate!

Post by erroneousbosh » Fri Jul 24, 2020 4:03 pm

Jonachi wrote:
Fri Jul 24, 2020 5:38 am
I have, but I’m doing a gate, not a filter. Or actually plan b is a vca followed by a regular bandpassfilter.
A lowpass gate *is* a filter, it's just a crappy one with 6dB/oct rolloff and a Q of 0.5 because that's what you get with RC filters.

The one you posted in the zip file is a Sallen-Key filter. If you look at how the inputs are wired in the Steiner-Parker Synthacon filter you'll see how to get lowpass, highpass and bandpass inputs :-)

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Jonachi
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Re: Help me develop a bandpass gate!

Post by Jonachi » Sat Jul 25, 2020 3:24 am

Thank you! I beleive I understand what you mean, but wonder if you can be more specific?

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Re: Help me develop a bandpass gate!

Post by SphericalSound » Sun Jul 26, 2020 3:57 am

The steiner multigate sounds like a great idea

Image

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Re: Help me develop a bandpass gate!

Post by Jonachi » Sun Jul 26, 2020 8:33 am

If it is I still need some seriouss help to make it happen! :hmm:

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Re: Help me develop a bandpass gate!

Post by Jonachi » Sun Jul 26, 2020 1:49 pm

ok so in theory what I want to do is:

1. Convert this layout to vactrols, or even better LED/LDR design: https://en.wikipedia.org/wiki/Sallen%E2 ... eneral.svg

2. And when done, convert Clee's design with it.

I dont know where to begin and I've been trying for many days now to try to draw things in eagle. Seems impossible right now.

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Re: Help me develop a bandpass gate!

Post by Jonachi » Sun Jul 26, 2020 2:28 pm

Doepfer beat me two it:

http://www.doepfer.de/a1011_tec.htm

Still need to develop my own thing though.

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Re: Help me develop a bandpass gate!

Post by tele_player » Sun Jul 26, 2020 2:40 pm

Why are you making a distinction between vactrols and LED/LDR? On the schematic they are identical.

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Re: Help me develop a bandpass gate!

Post by J3RK » Sun Jul 26, 2020 4:42 pm

There are several ways that you can approach this. The sound of a XXX-Pass-Gate comes from the mix of amplitude and frequency modulation that occur simultaneously. So you could really mix ANY type of VCA and VCF with a fixed (or adjustable I suppose) mixing circuit. The trick is to get a pleasing and repeatable mix as your result.

You could build a simplified State Variable filter, choose the BP tap, and mix the signal with a VCA. You could mix it in several ways. Parallel signals with a ratio, filter followed by VCA, or even a mix of all of this.

This particular circuit would be higher parts count than some others. It’s just an example of a possible solution.
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Re: Help me develop a bandpass gate!

Post by Jonachi » Mon Jul 27, 2020 11:26 am

Thank you Jerk best and most informative answer yet.

Filter followed by vca is what I will try first. And let them share the same cv in I suppose. What is the most simple vca curcuit one can find?

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Re: Help me develop a bandpass gate!

Post by J3RK » Mon Jul 27, 2020 11:39 am

Jonachi wrote:
Mon Jul 27, 2020 11:26 am
Thank you Jerk best and most informative answer yet.

Filter followed by vca is what I will try first. And let them share the same cv in I suppose. What is the most simple vca curcuit one can find?
The simplest may not necessarily be the best. (you could technically do it with a single transistor :hihi: ) However, you can find discrete circuits that are just a handful of transistors. Or you could do an OTA based one that would only be two chips (and leave a bit left over for possibly the filter side assuming a dual OTA chip). Check out the VCA-1 (from Thomas Henry) but then remove anything you don't need.

Yes, sharing the CV input is good. You'll want to be able to tweak how much of the filter and how much of the VCA takes effect, in order to really tune the sound though. So it might be a good idea while developing this, to have a simple, adjustable summing mixer, and maybe a single attenuator or two. That way you can try parallel or series routings, various mixes that you can then set statically later if you like with resistors.

One other thing you could try. I've been working a lot with the SSI2164 chip lately. You could build the filter and VCA with a single 2164 IC (depending on the type of filter, and whether you want the VCA linearized) and a few op amps. The SSI datasheet has a ton of very helpful examples, ideas, etc. It's a good resource.

The simplest way though, just to get started, is a simple discrete VCA, a two-channel summing mixer (dual op amp and a few resistors) and then the filter topology of your choice. It would be fairly low parts count to do a state variable in its purest form, and only expose the band pass output to your circuit. I think you could probably find a better option, but this is probably how I would approach it if I was doing this. (I just like SVFs :lol: ) You could look at the CGS30 schematic for another idea of how to do this. Or as someone recommended above, the Buchla 291 schematic. The problem with the 291, is that it uses four vactrols. (well, it's not necessarily a problem, but I still consider vactrols to be a component in flux currently, and I've started to avoid using them, unless the circuit doesn't perform as well without them)

You might check out Rene Schmitz' VCA-2 for a simple, discrete VCA, if that's what you'd like to try.
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Re: Help me develop a bandpass gate!

Post by Jonachi » Tue Jul 28, 2020 3:15 am

Thanks. Tried a VCA after my qpas (in mono) last night and it sounded much better than VCA before filter. So I'm going to try this approuch. State variable filter with bandpass only straight into a VCA sharing the same CV. Should be fun!

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Re: Help me develop a bandpass gate!

Post by J3RK » Tue Jul 28, 2020 4:04 pm

One other routing option would be to route the unfiltered audio and filtered audio into two inputs of the VCA, so you could mix a bit of dry signal in. This would get a bit closer to the classic LPG style signal I think.

Routing in series will work too, but it will be a little more like a traditional mono-synth style signal path.

Not better or worse, just different.

You could patch this up pretty easily using a mixer if you wanted to experiment with signal mix ratios.
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