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 Forum index » DIY Hardware and Software
IT WORKS! 4-pole filter from two LM13600 (audio demo+schem.)
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beep



Joined: May 05, 2013
Posts: 84
Location: Germany

PostPosted: Thu Aug 29, 2013 11:39 am    Post subject: IT WORKS! 4-pole filter from two LM13600 (audio demo+schem.) Reply with quote  Mark this post and the followings unread

I recently built the ms20 lowpass. the 100k resistors at the control input need to be changed to 10k to increase the frequency range. It's a 12db filter.

I want 24db, so I just added two more filter stages. Will this work? Very Happy


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Last edited by beep on Mon Sep 02, 2013 10:15 am; edited 1 time in total
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beep



Joined: May 05, 2013
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Location: Germany

PostPosted: Mon Sep 02, 2013 10:14 am    Post subject: Reply with quote  Mark this post and the followings unread

https://soundcloud.com/adolfchristus/dif


i breadboarded it today and it seems to work.

resonance is always a bit lower on breadboard,
when soldered the self oscillation goes insane.

should I change the 220k resistors? I found out, the higher the value,
the more self-oscillation occurs. too much.
or just add a resistor at the res pot in the feedback path?


the original description tells about clippers to
avoid distortion when oscillating. where to place them?
in the feedback loop? just a resistor and two diodes
(one to ground and one from ground) ??

had no 680pf caps, so I took 470p ones.

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JingleJoe



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PostPosted: Mon Sep 02, 2013 4:03 pm    Post subject: Reply with quote  Mark this post and the followings unread

Just regarding the oscillation prevention clipping diodes; I used some in a filter I was building today in parallell in the feedback path in my circuit, however here's a similar circuit to yours which I found in a quick google for "roland vcf circuit" (because I knew I'd seen that arrangement you spoke of in roland VCFs) in which the diodes are connected in opposite directions from the wiper of the resonance pot to ground. Smile
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Cynosure
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Joined: Dec 11, 2010
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Location: Toronto, Ontario - Canada
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PostPosted: Mon Sep 02, 2013 4:16 pm    Post subject: Reply with quote  Mark this post and the followings unread

Have a look at the feedback path here:

http://www.schmitzbits.de/rs20.png

The green LEDs cause slight clipping in the feedback path.

In my experiments, I had to adjust the value of the 10k resistor that is parallel to the LEDs. I think I ended using 18k for my circuit, but it was a long time ago and I likely made other changes too.

The Monotron uses 330k for that resistor, but it looks like there are many things that are different with that filter.
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beep



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PostPosted: Mon Sep 02, 2013 4:23 pm    Post subject: Reply with quote  Mark this post and the followings unread

https://soundcloud.com/adolfchristus/24db-oct-lowpass-vcf

I made a more beautiful demo. it's really cool! but the distortion is annoying.

Thank you, JingleJoe and Cynosure!

@JingleJoe: Shall the diodes and the resistor go to vr or to real ground?

@Cynosure: Will the op amp clipper circuit work when I power the op amp from single supply?

edit:
link corrected...lol...tired...Very Happy

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Cynosure
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PostPosted: Mon Sep 02, 2013 4:37 pm    Post subject: Reply with quote  Mark this post and the followings unread

beep wrote:
Will the op amp clipper circuit work when I power the op amp from single supply?

I think it would need a virtual ground.

The filter sounds great already, but yes the resonance is a little untamed.

I recently played around with the JH Wasp filter schematic and changed the resonance to be voltage controlled. I ended up doing something very similar, but with a LM13700 instead of an opamp. I used a virtual ground for it and it worked fine. I needed to add a resistor in the feedback loop to adjust the resonance peak, but I managed to tune the circuit to behave almost identical to JH's version.
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beep



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PostPosted: Mon Sep 02, 2013 4:44 pm    Post subject: Reply with quote  Mark this post and the followings unread

so the resistor to ground on the diode feedback loop will go to virtual ground, right?

A LM13700 in the feedback loop? voltage controlled resistor for resonance CV?

I think I'm gonna stock a few more OTAs before they become unavailable

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Cynosure
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PostPosted: Mon Sep 02, 2013 5:05 pm    Post subject: Reply with quote  Mark this post and the followings unread

beep wrote:
so the resistor to ground on the diode feedback loop will go to virtual ground, right?

Yes. Good luck Smile
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beep



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PostPosted: Tue Sep 03, 2013 3:11 am    Post subject: Reply with quote  Mark this post and the followings unread

Tried both, the passive diode clipper and the op amp one.
both didn't work. the passive clipper seems to do no more than a resistor
in the feedback loop, there is more headroom until it oscillates,
but the same amount of crappy distortion occurs.

The op amp circuit just boosted the oscillation, also with lots of distortion Sad

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Cynosure
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PostPosted: Tue Sep 03, 2013 8:57 am    Post subject: Reply with quote  Mark this post and the followings unread

beep wrote:
The op amp circuit just boosted the oscillation, also with lots of distortion Sad

That is why you need to play with the resistor values. You might need to to tame the amount that the opamp amplifies the signal.
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elmegil



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PostPosted: Tue Sep 03, 2013 10:45 am    Post subject: Reply with quote  Mark this post and the followings unread

I've also discussed using zeners to limit the amount of clipping in resonance, with Dave Dixon. Been too long since that conversation though, so I'm not sure I can reproduce the knowledge he tried to impart to me Very Happy
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beep



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PostPosted: Tue Sep 03, 2013 12:28 pm    Post subject: Reply with quote  Mark this post and the followings unread

I ended up making the feedback loop smaller
(only between the first two poles, adding a resistor between and a passive
diode clipper with a trimmer. It tames the amount of distortion, but also reduces the abilityfor self oscillation.

I adjusted it for a little bit of ringing when reso pot is at 100 percent,
at full oscillation it would still distort too much.

I also added a 1M resistor at the input to get a much lower input signal.
Now I have to amplify it and it gets noisy. I think the shitloads of
wire jumpers (some nice antennas there) are to blame and the oscillators
are on breadboard. I remember someone here told me about
stray capacitance, which is a disadvantage of breadboards and causes
noise/hum.


Tomorrow I will try the opamp circuit again, maybe also with a trimmer.
If nobody says "don't do it" to me, I always use the LM358, it's
my favourite op amp Very Happy Is the 358 okay?

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Cynosure
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PostPosted: Tue Sep 03, 2013 6:48 pm    Post subject: Reply with quote  Mark this post and the followings unread

LM358 is a noisy opamp. I usually use TL07x for audio.

Keep in mind though that the TL07x opamps do not go from rail to rail, so you might need to attenuate the sound on the input pin in order to prevent distortion.
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JingleJoe



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PostPosted: Fri Sep 06, 2013 7:57 am    Post subject: Reply with quote  Mark this post and the followings unread

Don't let that put you off using the LM358 though Smile I hear so many people complain about it but it's output swing at audio frequencies on a single rail supply is un-matched by any op amp of similar price! (but the lm324 of course which is just two lm358s)
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