comparison of some simple VCFs

[elektrouwe]

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similar to pic.1 , but the 2 LEDs in series have been replaced by 1 transistor - also to isolate the CV-impedance from the signal path.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]

thanks, v. useful

[moylando]

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

[elektrouwe]

[moylando]

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

[elektrouwe]

[mike page]

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?

[gabbagabi]

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

[elektrouwe]

[Ricko]

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?

[elektrouwe]