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 Forum index » DIY Hardware and Software
comparison of some simple VCFs
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elektrouwe



Joined: May 27, 2012
Posts: 104
Location: Germany

PostPosted: Wed Nov 30, 2016 3:42 pm    Post subject: comparison of some simple VCFs
Subject description: AC simulation of easy to build low pass filters
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in this thread, http://electro-music.com/forum/topic-67769.html&sid=6f45de59470c337e6565a8bb6a3aabd7 I complained that the two filters presented are not very useful as voltage controlled low pass filters. ( they might be nice guitar tone controls anyhow)

A look at the frequency response shows why I dont't like them as VCLPFs :

1) Tim Escobedos well known Quick&Dirty filter ( pic. 5) is in fact a high pass (!) filter with strong resonance
it is nice for wah wah type tone controls, but because it is not a low pass filter you can't sweep it through and have a fat bass the same time

2) the PAIA filter ( pic.4) from the old thread is also not a VCLP filter, it acts as a kind of treble tone control : if CV=0 the diodes are like open switches
and the filter capacitors are just switched of. As you rise CV thes capacitors get more and more grounded and with full CV, when the diodes have a low impedance, you have a 2nd order low pass filter with fixed corner frequency.

pic. 1:
with a small change in topology you can change the Q&D Escobedo filter into a real low pass filter. This is shown in the 1st pic. , simple LED-VCF2.
As suggested in the old thread, I have put 2 LEDs in series, to isolate the impedance of the CV-source from the signa, which is the reason for the small sweep range of the Q&D filter
In fact it is possible to run the LEDs with a true CV (from an opamp buffer) without any series R. This make a very nice exponential CV behaviour (V/oct) as you can see in the frequency response.

pic. 2:
similar to pic.1 , but the 2 LEDs in series have been replaced by 1 transistor - also to isolate the CV-impedance from the signal path. Freq. response is not so nice as in pic.1
but this VCLPF has , like the Q&D filter, a very low component count. This time I've drawn it with a 9V single voltage supply, to make it better comparable to the Q&D filter

pic.3:
now we are getting closer to real (analog synth style) VCLPFs. A Korg style 2nd order LP filter with resonance control. I you can afford a few more components, you
get much more "moogish" filter behaviour copared to the simpler circuits.

(my personal) conclusion : the filters shown I would only use for simple Lunetta style noise circuits with manual tuning. All these filters have terrible CV rejection, which will result in clicks and strange behaviour when controlled by fast LFOs or envelope generators. If you don't want this as a feature (hello Lunetta forum) you should look for OTA or (diode or transistor) ladder filters, which are designed to have a good CV rejection.


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mike page



Joined: Sep 26, 2016
Posts: 56
Location: norwich, uk

PostPosted: Wed Jan 18, 2017 3:20 pm    Post subject: Reply with quote  Mark this post and the followings unread

thanks, v. useful
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moylando



Joined: Dec 11, 2008
Posts: 13
Location: Philadelphia

PostPosted: Fri Jan 20, 2017 7:54 am    Post subject: Reply with quote  Mark this post and the followings unread

Cool. It would be interesting to see the plots with C6/R12 of equal values in LED-VFC and the transistor/opa vcf.
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elektrouwe



Joined: May 27, 2012
Posts: 104
Location: Germany

PostPosted: Fri Jan 20, 2017 1:09 pm    Post subject: Reply with quote  Mark this post and the followings unread

moylando wrote:
Cool. It would be interesting to see the plots with C6/R12 of equal values in LED-VFC and the transistor/opa vcf.


here is LED-VCF vs Trans-VCF. Tried to keep some parts in both filters
equal. Both are 9V single supply now.
If someone is interested in playing with these simulations pls. send PM with email and I will send the zipped ltspice files.


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moylando



Joined: Dec 11, 2008
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Location: Philadelphia

PostPosted: Fri Jan 20, 2017 3:40 pm    Post subject: Reply with quote  Mark this post and the followings unread

Thanks for those. The transistor version looks alot like the resonators used in electronic drums. It looks odd to me to have the collector untied to any DC source, but it seems to work well enough for drum circuits. I wonder if this could also be modified with a diode from V+ to the collector node (maybe with a current limiting resistor). Sort of combing ideas from both filters.

I'm curious enough to see the spice files Smile
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elektrouwe



Joined: May 27, 2012
Posts: 104
Location: Germany

PostPosted: Fri Jan 20, 2017 11:40 pm    Post subject: Reply with quote  Mark this post and the followings unread

moylando wrote:
I'm curious enough to see the spice files Smile

not me Wink will not spend more time in those lousy filters.
Just playing with real VCLPFs and simple through zero VCOs
But I'm curious enough to see/hear your breadboard results Smile
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mike page



Joined: Sep 26, 2016
Posts: 56
Location: norwich, uk

PostPosted: Sun Oct 15, 2017 8:37 am    Post subject: Reply with quote  Mark this post and the followings unread

I built the improved Q&D (trans/OPA) and its pretty cool for the parts count.
Do you recon it would be possible to modify it to self resonate?
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g.gabba



Joined: Nov 29, 2008
Posts: 175
Location: berlin
Audio files: 18

PostPosted: Mon Oct 16, 2017 9:40 am    Post subject: Reply with quote  Mark this post and the followings unread

Hi elektrouwe,
iam very much interested in TZVCO's and how they working, but I have trouble to find schematics.
Could u tell me which ones you use?

cheers
gabbagabi
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elektrouwe



Joined: May 27, 2012
Posts: 104
Location: Germany

PostPosted: Tue Oct 17, 2017 1:56 pm    Post subject: Reply with quote  Mark this post and the followings unread

g.gabba wrote:

iam very much interested in TZVCO's
gabbagabi

other topic, PM
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Ricko



Joined: Dec 25, 2007
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Location: Sydney, Australia
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PostPosted: Fri Oct 20, 2017 5:57 am    Post subject: Reply with quote  Mark this post and the followings unread

You mention terrible CV bleed. Is this bleed linear to the CV, such that it could be mostly nulled out by subracting some of the CV from the output?
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elektrouwe



Joined: May 27, 2012
Posts: 104
Location: Germany

PostPosted: Fri Oct 20, 2017 7:31 am    Post subject: Reply with quote  Mark this post and the followings unread

Ricko wrote:
You mention terrible CV bleed. Is this bleed linear to the CV, such that it could be mostly nulled out by subracting some of the CV from the output?

no, these filters are only useful for slow/manual/foot peda...l CV modulation.
The only practical way to overcome CV bleedthrough is to use a differential/
symmetric design like the moog transistor cascade or similar diode ladders,
where subtraction of the 2 pathes add the signal and cancel out CV
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