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Serge '73 filter adjusting resonance
 
 
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Author Serge '73 filter adjusting resonance
bananeurysm
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PostPosted: Wed Dec 10, 2014 2:53 pm    Post subject: Serge '73 filter adjusting resonance Reply with quote Add User to Ignore List

So I have a '73 filter and did the self-oscillation mod as described by Ken:

"If you wish the filter to self oscillate at high Q settings, the 68k resistor between pins 2 and 6 of the single op-amp (LM748 on the PCB) can be reduced to 27k"

It sounds great and does indeed oscillate at high Q settings.

The problem is - I can't actually turn the resonance all the way down now. There is no trimmer on the board for adjusting resonance.. I assume it's just that resistor.

Is it worth experimenting with other resistor values in between 68k and 27k? Should I even put a trimmer there instead?

OR - should I just put the factory spec'd resistor back.. since my understanding is you can get it to self oscillate simply by patching the bandpass output back to the input. Is that gonna give me just as much utility?

I really like being able to turn the resonance just up to or very very slightly into it's resonance point - so I don't want to lose that facility. But right now the bottom half of my resonance pot travel doesn't do much..

Thanks!

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SpunRaddish
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PostPosted: Wed Dec 10, 2014 3:09 pm    Post subject: Reply with quote Add User to Ignore List

I have two I've got on the bench to be populated - what I considered was a switch to change between the resistor values, so when you want that high oscillation you can just, you know, switch it in. I was going to do it via a push-pull pot; maybe someone else has a better idea, possible like breaking out a trim pot between the two values onto the panel.
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bananeurysm
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PostPosted: Wed Dec 10, 2014 3:17 pm    Post subject: Reply with quote Add User to Ignore List

Is that the right place to put a trimmer? Maybe a 50k trimmer where the 68k resistor goes?
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SpunRaddish
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PostPosted: Wed Dec 10, 2014 5:00 pm    Post subject: Reply with quote Add User to Ignore List

That sounds right. In addition to the 50k trimmer you can put an additional 18k resistor to compensate for that difference, which is already well under the suggested 27k. By breaking it out to the panel you could probably have a bunch of fun finding sweet spots while playing with CV or the input.

I actually had a question for you, though; Do you hear any of the input upon self-oscillation?
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widdly
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PostPosted: Wed Dec 10, 2014 8:49 pm    Post subject: Reply with quote Add User to Ignore List

I would think that reducing the 68k coming from the wiper of the resonance pot would reduce the resonance more as it is turned counterclockwise.

I'm interested to see what you come up with. I've got one of these stock and another waiting for some banana sockets to arrive.
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Chaotic
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PostPosted: Thu Dec 11, 2014 4:16 am    Post subject: Reply with quote Add User to Ignore List

When I need it to selfoscillate I`m only use a patch cord from bandpass to input. hihi
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En_Annan
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PostPosted: Thu Dec 11, 2014 4:31 am    Post subject: Reply with quote Add User to Ignore List

I built mine with mlcc :s, no mods, and it does self oscillate, not a very clean wave, but I can ping it at below full res, and the resonance comes in, leaving a drop like sound. The res at high pitches (över about 5k) is very strong in volume
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En_Annan
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PostPosted: Thu Dec 11, 2014 4:32 am    Post subject: Reply with quote Add User to Ignore List

BTW, I run mine on 15V.
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BugBrand
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PostPosted: Thu Dec 11, 2014 5:16 am    Post subject: Reply with quote Add User to Ignore List

It is a fairly typical 12dB/oct State Variable Filter topography -- so many similar (eg. SEM) but with variance between opamp polarities (depends, for example, if opamp buffers after the OTAs etc).

My usual approach has different phase arrangements, but the key to limiting the resonance amplitude (essential in my view -- who wants normal-usage 10V p-to-p amplitude ramping up to over 20V p-to-p when self-oscillating?!) is adding zener limiting at the BP stage.

You can see what I mean in Dr.Etch'n'Sketch's post about getting a T.Henry VCF self-oscillating - www.muffwiggler.com/forum/viewtopic.php?t=104355


Also, going back to the first question - halving that BP feedback resistor (as suggested, roughly speaking, by Ken Stone) is the standard way.

[again, you can see in the linked SVFilter schem what I mean about opamp phases - see how there's a x2 factor in the feedback path -- though I'd typically do it after the Q-VCA - ie back at the input sum]

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tojpeters
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PostPosted: Thu Dec 11, 2014 10:55 am    Post subject: Reply with quote Add User to Ignore List

Run all 3 outputs trough the buffer.
Add a 4PDT switch as follows.
This mod works very well.


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bananeurysm
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PostPosted: Thu Dec 11, 2014 3:47 pm    Post subject: Reply with quote Add User to Ignore List

Thanks for the suggestions!

So just to clarify - is it likely due to the mod ( 27k instead of 68k) that my resonance doesn't go all the way down to 0?

If I put it back to factory - it sounds like I can get the self-oscillation I want by patching.. but I guess I should remove the buffering I added?

Or would the buffering still help for when I patch it to feedback?

I know I should just experiment - but there are obviously so many ways to go.

I want the option to self-oscillate. I want the option to pull the resonance all the way down. I want my levels to be reasonable.

That's what most of us want right? Do we *need* to put a switch as tojpeters suggests? Because otherwise I'm happy to just patch-program in true serge spirit.

(Also I notice that the input gain is way too high. Is that due to the output buffering I added?)

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J3RK
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PostPosted: Thu Dec 11, 2014 4:37 pm    Post subject: Reply with quote Add User to Ignore List

You're asking for too much!!! hihi

For the resonance thing, I think I'd put in a 22K resistor, and a 25K trimpot in series with it. Or, just trim it to where it sounds the way you want, pull the trim, measure it, and put the closest resistor in to that value. (I think that was suggested already, but...)

The switch is a good idea, but if you're using banana cables, a quick feedback cable is very easy to do.

Sometimes getting the balance between oscillation, and good sounding resonance over the full sweep range can be a bit difficult. I'm running into that with my four-pole filter I'm working on right now. I can make it oscillate, and it sounds great there, but then it doesn't sound quite as good with a signal. If I trim it for the signal, it will only oscillate at certain frequencies before cutting out. I've probably tried hundreds of combinations of value tweaks, diode limiters, altering feedback paths, etc. I'm actually thinking about scrapping it, because I wanted it all to work perfectly without things like switches. (or at least redesigning it again from the ground up, or using different OTA or VCA stages, etc.)

With the Serge though, I bet you could find a good balance just by trimming it up a bit more.

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bananeurysm
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PostPosted: Thu Dec 11, 2014 7:29 pm    Post subject: Reply with quote Add User to Ignore List

Thanks! Yeah - I think I have a game plan now. smile

And also I figure this is worth mentioning - in spite of wanting to improve some of the setup on mine, this really is one of my favorite filters I've ever played with. Really surprised me when I started using it.

I can't recommend it enough to anyone who hasn't built one out yet!

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