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 Forum index » DIY Hardware and Software
Discrete Transistor 2040 SVVCF
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Scott Stites
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PostPosted: Fri Apr 12, 2013 5:22 am    Post subject: Reply with quote  Mark this post and the followings unread

Quote:
Glad to know im not going mad!


Well, if you are, I beat you to it. Laughing

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Scott Stites
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PostPosted: Fri Apr 12, 2013 5:29 am    Post subject: Reply with quote  Mark this post and the followings unread

StephenGiles wrote:
Scott Stites wrote:
Hey Dan,

I've been playing with BBDs lately. Lotsa fun, those too.

Cheerio,
Scott


Through zero flanging perhaps? Are you aware that Mike Irwin (where is he now?) designed a TZF using just one BBD?


Hey Stephen,

I missed this - sorry I didn't reply sooner. Is it the phase delay design that Mike based his on?

Mine is through zero, but it uses more than 1 BBD (it uses 8 ). I'm working on a pitch shifting application now.

Cheerio,
Scott

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Scott Stites
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PostPosted: Fri Apr 12, 2013 5:32 am    Post subject: Reply with quote  Mark this post and the followings unread

comrade_zero wrote:
First off, Mr. Stites, a hearty "thank you" for taking the time and energy to share this with us. This filter sounds good (I am a sucker for filters.) Secondly, any idea if or how it would behave on -12/+12v?
Especially if someone is looking to a Euro-format board? Thanks again.
c_z


Dang - how did I miss this one, too? Anyway, I've never tried it with +/-12V - I'm sure it could be made to work at that level if it doesn't out of the box.

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Scott Stites
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PostPosted: Sun Apr 14, 2013 1:19 pm    Post subject: Reply with quote  Mark this post and the followings unread

My THAT transistor version was still on the breadboard - after four years, it still actually works. I recorded a short sample of it and posted it to the THAT FB page just to put a bug in their ear that maybe, some day, the analog resurgence will make it viable for them to make a similar IC as the 2040.

Here's the sample I recorded (LP output):

http://soundcloud.com/scott-stites/that-transistor-svvcf-lp-out

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vtl5c3



Joined: Sep 08, 2006
Posts: 423
Location: PDX
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PostPosted: Thu Jul 11, 2013 10:05 am    Post subject: Reply with quote  Mark this post and the followings unread

Just found out about this thread.

Nice Job, Scott! I'm impressed how fat the filter sounds with only two poles.
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negativspace



Joined: Dec 25, 2010
Posts: 17
Location: Manhattan, Kansas

PostPosted: Thu Jul 11, 2013 12:46 pm    Post subject: Reply with quote  Mark this post and the followings unread

comrade_zero wrote:
Secondly, any idea if or how it would behave on -12/+12v?


It works well on +/-12v with no changes. I imagine someone with a better grasp of transistor circuits could tweak a few resistor values, but based on my first build I don't think it's critical. It certainly sounds great and behaves as expected with no adjustments.

I did change two resistor values: R37 to 39k and R32 to 5.6k. That gives very usable self-oscillation (R37) and eliminates some 'thumping' in the audio that was occurring at high CV values (R32). I also connected the frequency pot between +12 and ground, not +12 and -12, which gives a much more comfortable range.

This really is a thick filter... excellent work, Scott, and thank you for sharing!

For those of you interested in a PCB, I'm working on one now with Scott's blessing. I'll offer it over at Muff's, but I'll do my best to keep this thread updated as it comes to fruition.
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vtl5c3



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PostPosted: Thu Jul 11, 2013 1:09 pm    Post subject: Reply with quote  Mark this post and the followings unread

Awesome! You just saved me a ton of work.

negativspace wrote:


For those of you interested in a PCB, I'm working on one now with Scott's blessing. I'll offer it over at Muff's, but I'll do my best to keep this thread updated as it comes to fruition.
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crashlander42



Joined: Oct 21, 2006
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PostPosted: Thu Jul 11, 2013 10:57 pm    Post subject: Reply with quote  Mark this post and the followings unread

This is awesome, because just a few hours ago I was Googling discrete VCF designs.
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negativspace



Joined: Dec 25, 2010
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PostPosted: Fri Jul 12, 2013 12:24 am    Post subject: Reply with quote  Mark this post and the followings unread

I love it when a plan comes together... Cool
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KnobHell



Joined: Jan 28, 2012
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PostPosted: Wed Jul 24, 2013 9:04 am    Post subject: Reply with quote  Mark this post and the followings unread

Hi Scott

Qestion about the caps.

Should the 1nf be poly?

How about the 2.7nf?

I assume the 220's can be ceramic.
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Scott Stites
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PostPosted: Fri Aug 23, 2013 12:21 pm    Post subject: Reply with quote  Mark this post and the followings unread

I had to think about that answer for a month Laughing

I used 1 nF polystyrene caps; the 2N7 was a mylar cap.

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Scott Stites
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PostPosted: Fri Aug 23, 2013 12:21 pm    Post subject: Reply with quote  Mark this post and the followings unread

And, yep, 220s can be ceramic.
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KnobHell



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PostPosted: Sun Aug 25, 2013 9:01 am    Post subject: Reply with quote  Mark this post and the followings unread

Cool, thanks Scott.

Hey, a month in synth time didn't even give me time to drop my build queue by one... Shocked

Thanks for the follow up.

Len
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oolfur



Joined: Oct 08, 2015
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PostPosted: Sat Nov 14, 2015 9:40 am    Post subject: Reply with quote  Mark this post and the followings unread

i designed an SMD version of the 2N3904/2N3906 schematic - alas - it doesn't work!

i didn't match the transistors, but sourced them off the same tape from mouser - someone mentioned matching wouldn't be absolutely crucial to initial prototyping...

the circuit does pass audio, but it has this wierd "thumping" to it, that is effected by the frequency pot. the resonance pot doesn't seem to do much.

after double, triple and quadruple checking the schematic against my eagle file, i wanted to ask if anyone could point me towards proper troubleshooting stages to go through etc... not sure if scott would be cool with me posting the eagle files, but i'd love to do so if thats okay with him.

if it's possible to get this baby running, it would be one monster of a filter 2/3rds the size of a creditcard!

many thanks,
-úlfur
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oolfur



Joined: Oct 08, 2015
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PostPosted: Sun Nov 15, 2015 5:26 pm    Post subject: Reply with quote  Mark this post and the followings unread

Embarassed everything was wired correctly, i just failed to ground an unused section of my TL074, it was oscillating like crazy!! Embarassed

it's a beautiful filter, WOW!!!! thank you so much for sharing scott!!!

there is some high frequency noise on the highpass output, I wonder where that is coming from - will report back if i figure it out Very Happy
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etheory



Joined: Feb 20, 2013
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PostPosted: Tue Apr 12, 2016 4:26 pm    Post subject: Reply with quote  Mark this post and the followings unread

There is a technical mistake in the original schematic.
R10 and R14 in the original schematic, that provide local negative feedback to the individual filter sections, should be removed in order to make this a true state-variable filter.

With them in circuit, the frequency response is impaired compared to a true SVF.

Then they are present, the filter sections are standard low pass sections. With them removed, they change from low pass sections, to integrators, and then the larger feedback loops from R7 and VR1/R37 can work properly.

The result will be corrected rolloff at higher frequencies for the low pass output, and correct responses for the bpf and hpf outputs.

Happy DIY'ing.
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L´Andratté



Joined: Sep 23, 2012
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PostPosted: Wed Apr 13, 2016 9:33 am    Post subject: Reply with quote  Mark this post and the followings unread

Good find, it´s also in the schematic on the site of René Schmitz, so maybe there´s a practical advantage to it, but maybe it´s just copied from Jürgen Haible´s original filter cells, which are 4-pole cascade...

http://www.schmitzbits.de/rs2040.html
http://jhaible.com/legacy/jh4_vcf_vca_schemo2.gif

I always eyed this one, so if you proceed, please report about this matter!

I think you are also active on GDIY? Very Happy
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etheory



Joined: Feb 20, 2013
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PostPosted: Thu Apr 14, 2016 3:03 am    Post subject: Reply with quote  Mark this post and the followings unread

L´Andratté wrote:
maybe there´s a practical advantage to it, but maybe it´s just copied from Jürgen Haible´s original filter cells, which are 4-pole cascade...

I think you are also active on GDIY? Very Happy


Yep, I was pretty active on GDIY, but not so much at the moment, as I'm working on a lot of analog synthesizer stuff Cool

The links you've provided are for a different filter topology, as you mentioned. It is possible he copied them from there I guess, but he's a pretty smart guy (I've read all his stuff), so I don't think it was an intentional mistake, it looks like an accident.

For example, this is a single low pass pole (one of many possible implementations, this one for an LM13700):

Posted Image, might have been reduced in size. Click Image to view fullscreen.

The local feedback fixes this as a single low pass pole in the context of a multi-pole filter. As such, it has a specific transfer function: 1/(s+1).

In order to make a Variable State Filter (i.e. a filter that solves for multiple transfer functions simultaneously), the basic unit is an integrator, which has a transfer function of 1/s and looks like this:

Posted Image, might have been reduced in size. Click Image to view fullscreen.

Note the lack of local feedback.

In the context of a full SVF, like this classic 2-Pole SVF implementation:
Posted Image, might have been reduced in size. Click Image to view fullscreen.

...note the feedback loops go from after the first integrator to the input, and from the last integrator to the input, only.

This particular arrangement correctly solves for the high, band, and low pass outputs respectively. Which you probably already know.

What is interesting, is that if you add back in the local feedback in this context, you actually change the transfer functions (quite drastically), and with high resonance, the shapes are extremely weird. It could be musically interesting, but it's not a "normal" SVF.

The most notable effects are the band pass doesn't drop below unity gain at low frequencies, and the high pass ends up looking more like a high shelf. It looks quite strange in a simulator like LTSpice.

I've attached the current circuit frequency response simulation (top image), and the version with those resistors removed (bottom image).


VCF_issue.jpg
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VCF_issue.jpg


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maltasynth65



Joined: Jul 20, 2016
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PostPosted: Wed Jul 20, 2016 5:54 am    Post subject: Project BOM
Subject description: errmmm...project BOM :)
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I like the way this filter sounds, so I have elected to build one for my modular after I finish my VCO's. I'm going for the discrete version simply because I have everything at hand to build it, so I have made up a BOM for the project on an Excel sheet. Any one interested into it? It's pretty rudimentary, just the components, no hardware listed, but it's there, basically......

two things......

there are two resistors bearing the R14 denominator, I called the second one R14A.

re the R10/R14 changing the filter's characteristics...in my book, that means versatility, so I am going to switch them in and out of circuit with a DP on/off panel mounted switch...any thoughts on this?
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etheory



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PostPosted: Wed Jul 20, 2016 6:33 am    Post subject: Reply with quote  Mark this post and the followings unread

Ah yes, now I see that (the duplicate R14).

The ones to remove are:

1.) R10, the one sitting between R29 and Q12/Q13

2.) R14, the one sitting between R31 and Q14/Q15

Hope that clarifies.
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maltasynth65



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PostPosted: Fri Aug 19, 2016 2:53 am    Post subject: 2040 svvcf
Subject description: it works it works.......however.....
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today i fired up my strip-board version of this filter.
Everything seems to work properly with r10 and r14 in circuit, however when I remove them, the filter goes nuts into oscillation, I mean from sonar pings to a belfry of mad finches in overdrive!!!! I believe it is called "squegging" and is not conrollable. Any suggestions, please? I am going to try the r37/r32/freq pot mods as noted above and see what happenes... Smile
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etheory



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PostPosted: Sun Aug 21, 2016 2:06 am    Post subject: Re: 2040 svvcf
Subject description: it works it works.......however.....
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maltasynth65 wrote:
Any suggestions, please?


I would suggest that you must have made some kind of wiring error.
The circuit, without R10 and R14 is the standard Variable State Filter configuration, and hence, should be inherently stable.

Maybe try and go over and triple check the way it's been hooked up.
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maltasynth65



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PostPosted: Mon Aug 22, 2016 1:02 pm    Post subject: uncontrollable SVVCF build. Reply with quote  Mark this post and the followings unread

tbh, I was suspecting something on those lines...after all, this is a stripboard build after my converting the paper circuit onto the board myself. I was unsure, but your definitive reply confirmed my suspicions. Mind you, the filter is very useful as it is and the mods suggested elsewhere on this thread gives it a very gritty o/p. I will most definitely look into my own board layout and assembly work when time becomes available thumb up thumb up thumb up . Thank you very much for your valued input......
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