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Switched Resistor VCF
MUFF WIGGLER Forum Index -> Music Tech DIY Goto page 1, 2  Next [all]
Author Switched Resistor VCF
Boogdish
I have a new design, which is my first VCF. It uses switched resistors as the means of controlling the cutoff frequency. The only other design I've seen that uses this method is the Maplin 3600/4600, though this design isn't based on theirs. I uses a DG202 analog switch for my design, which has 4 switches per chip, so I made 2 state variable filters with separate inputs, outputs and resonance controls but a shared frequency control.

HERE is the page on my site for this project. That links to the documentation pdf which has schematics, parts list, mp3 samples.

This is a video where I go over the basic features:
https://youtu.be/f7w72HrC2R8

And here is a video where I describe a vocal sounds like patch that uses the filter:
https://youtu.be/wRVVjJsrans
sammy123
Nice videos Michael. I am going to email you right now.
medbot
Hey Michael, the .pdf is all garbled when I download it. Not sure if it's on my end or yours but figured I'd say something. Sounds like a really cool project.
Boogdish
Hmm, I'm not sure what's causing that. I tried re-uploading it but had the same issue. I renamed it and it's working for me now:

http://www.bartonmusicalcircuits.com/srv/documentation1.pdf
medbot
Cool, it's working for me now too.
emdot_ambient
thumbs up APPROVED thumbs up
sammy123
Is there a particular DG202 suitable for this project or does it not matter? The specs seem to vary a bit and I don't see a DG202JN like in the PCB.
Boogdish
I've used the DG202BDJ in my testing
astrosound
Wow, those demos sound really good. Nice work! I have a couple DG202 chips kicking around so I'm sure I'll be ordering a board from you in the near future.

thumbs up

(oh and I have to say, I love the layout of your website. It's like reading a page in the late 90s/early 2000s and I mean that in the most positive way)
whoop_john
Boogdish wrote:
I have a new design, which is my first VCF. It uses switched resistors as the means of controlling the cutoff frequency. The only other design I've seen that uses this method is the Maplin 3600/4600, though this design isn't based on theirs.

The ETI 3600/4600 used a switching HP/BP/LP 12dB filter design. The ETI 3800/5600S used a 24dB LP version.

I have built both of these filters many times over and always thought it was a good approach.

Trevor Marshall's website says: "At the time I held two Australian provisional patents for the technologies I used in the [ETI] Synthesiser designs. One related to the method for generating sawtooth waveforms, the other was for the method of using commutated resistors in voltage controlled filters.

I never made any money out of the designs, or the patents, which have long ago been allowed to lapse.
But it was fun!"

So it looks as if you won't be infringing any patents if you pursue this avenue.
falafelbiels
I'm in!
qfactor
Boogdish

Just wondering if MAX313 chip can be substituted for the DG202?
Have some of these around from fonik's Switch kit. hmmm.....
JimiQ
Does the output not contain some PWM hf energy? Shouldn't it ideally be removed with a 'reconstruction' filter?
JimiQ
(Clever design btw!)
windspirit
Nice sounds for sure, maybe a clarke panel is in order for this one?
sammy123
The cheapest one. Awesome thumbs up

Boogdish wrote:
I've used the DG202BDJ in my testing
yan6
This is lovely, are those dg202 chips hard to come by.
sammy123
Nope: http://www.mouser.com/ProductDetail/Vishay-Semiconductors/DG202BDJ-E3/  ?qs=sGAEpiMZZMtxrAS98ir%252bs0szLGzxh%2fuDn1yLTm%2fz%2fH0%3d
fitzgreyve
looks like this will have a linear CV response ?

Also DG202 - free (samples) if you set yourself up on the maxim website.
Boogdish
Quote:

looks like this will have a linear CV response ?

Yes. I did use a logarithmic tapered pot for frequency control so it would be easier to use.

Quote:
Nice sounds for sure, maybe a clarke panel is in order for this one?

Synthcube are the ones who get panels commissioned by Clarke, so that's a question for them.

Quote:

Does the output not contain some PWM hf energy? Shouldn't it ideally be removed with a 'reconstruction' filter?

Whatever HF artifacts from the switching remain in the signal I haven't been able to detect using either my ears, my scope or spectrum analysis of recordings. I'm not saying that there aren't any artifacts, but if you're just using this to process sounds, the artifacts shouldn't make a difference.

Quote:
Just wondering if MAX313 chip can be substituted for the DG202?

I don't have one to test with, but it should as long as you jumper pins 12 (logic voltage) and 13 (v+). The DG202 has no connection on pin 12, so this is safe.
whoop_john
Did you try with a cheap CD4016B, a similar digitally controlled bi-directional analogue switch? In the ETI circuits, although an SCL version of 4016 was specified, this was not cited as being critical. The highest frequency of gate switching was above 2MHz in that design. I am sure modern components, 40 years later, would be of higher spec.

You are not doing it quite the same way. You are fixing the gating frequency and changing the mark/space ratio, so presumably going nowhere near 2MHz in your wildest dreams.

Does Nyquist's theorum apply here? Chopping frequency must be at least twice the frequency of the highest pitch you pass through the gate?

A 4016 would be a much cheaper and easily available option for people to source.
Boogdish
I know that I'm not doing it the same, the circuit isn't based on the 3600. I just brought it up because it's the only other example of a switched resistor filter that I've come into contact with and I thought it was interesting.

I chose the DG202 over the 4016 because it's designed for a dual rail power supply and I wanted to keep it simple for people to not have to mess with either offsetting their input or making a +/-7.5V power supply on the PCB. The DG202 is still in production and there are other chips that can be swapped in.

I definitely see the argument for 4016, and if this was something designed to be built in large quantities I would side on the 4016 with additional circuitry. But this is something that I imagine people will build one or two of for personal use, so I'm ok with simplifying it.

Also, if someone can't get the DG202 in their country, let me know and I can send you one with a PCB. I do this with the LF398 on my LFO/SH PCB too.
whoop_john
Boogdish wrote:
I know that I'm not doing it the same, the circuit isn't based on the 3600. I just brought it up because it's the only other example of a switched resistor filter that I've come into contact with and I thought it was interesting.

I chose the DG202 over the 4016 because it's designed for a dual rail power supply and I wanted to keep it simple for people to not have to mess with either offsetting their input or making a +/-7.5V power supply on the PCB. The DG202 is still in production and there are other chips that can be swapped in.

I definitely see the argument for 4016, and if this was something designed to be built in large quantities I would side on the 4016 with additional circuitry. But this is something that I imagine people will build one or two of for personal use, so I'm ok with simplifying it.

Also, if someone can't get the DG202 in their country, let me know and I can send you one with a PCB. I do this with the LF398 on my LFO/SH PCB too.


I intended no criticism and you had stated you were doing it differently, as I could see. I totally get your point about the bipolar issue.

I already ordered samples of the DG202 jobbie, so not an issue for me, I was just joining in the debate. Your circuit is simple and elegant, well done.
qfactor
Boogdish

Thanks for the reply regarding using MAX313 as a sub for DG202.
Though on reading the data sheet further I realized that MAX313 is a "normally open" switch while DG202 is "normally closed" (or was it the other way around??

But I any case the MAX is the opposite of the DG.
I guess this may not work then eh?
JimiQ
Boogdish wrote:
Quote:

Does the output not contain some PWM hf energy? Shouldn't it ideally be removed with a 'reconstruction' filter?

Whatever HF artifacts from the switching remain in the signal I haven't been able to detect using either my ears, my scope or spectrum analysis of recordings. I'm not saying that there aren't any artifacts, but if you're just using this to process sounds, the artifacts shouldn't make a difference.

Interesting - thanks.
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