vcf that sounds "neutral" when fully opened?

Thomas_H · Joined: Jan 02, 2014 Posts: 2 Location: Munich

hi there, as this is my first post, i would at first like to say thanks to everyone contributing their knowledge on this forum, i have been spent hours of reading here and this forum is just great Smile

ok, to the topic... during the last days i tried especially those two VCFs:

http://electro-music.com/forum/topic-45231.html
sounds nice, but produces high frequency noise when opened
http://picsynth.000space.com/schematic.html
it's the vcf from the picsynth, but has a noticeable "resonance" at the cutoff frequency i could not get rid off...

maybe those filters give some character to synth sounds or guitar pedals, but i wanted to use the filter as an effects unit for e.g. a DJ setup, where the filter ideally does not affect the signal significantly when it is opened. One could achieve that on the circuits above with applying different capacitor sizes, but also the possible frequency range is more limited then..

any suggestions for VCFs which
- only slightly affect the signal when the filter is opened
- have a wide frequency range anyways?

LPF, HPF, everything is fine, in the end it would be cool to implement both.. Wink

BTW: as the goal of my final project would be to control the VCF from a microcontroller, circuits with +-5V supply for the opamps would be preferred..
thanks and cheers from Munich

L´Andratté · Posted: Tue Jan 14, 2014 1:41 pm Post subject:

Hi Thomas!

I don´t know about the fiters you posted, at least the 2nd(twin-t) is very component sensitive.

Have you tried the late Rene Schmitz´ "Late MS-20" design,
from his site/filters:
http://www.schmitzbits.de/

To me, it´s the ultimate beginners filter, and without resonance and opened should be pretty neutral...and with resonance and closed totally awesome!

Only digital filters are totally neutral, I think, and who wants those? Wink

EDIT: 9V battery powered version here:
http://www.jiggawoo.eclipse.co.uk/guitarhq/Circuitsnippets/snippets.html

bubzy · Posted: Wed Jan 15, 2014 4:46 am Post subject:

to electro-music Smile

you can try adding a resistor to the +V side of the cutoff pot, it is quite common for filters to "self-oscillate" when fully opened, by putting a resistor here, you can limit the range of the cutoff pot, (you could also put another pot here to make the limit adjustable) Smile

comrade_zero · Posted: Thu Jan 16, 2014 11:24 pm Post subject:

I just finished building a lp/bp filter from Nicolas Woollaston's schem here at e-m. It is a bipolar design based around the lm13700 dual OTA. It has a very nice, "flat" response with the resonance turned down, but will oscillate with the resonance turned all the way up. Erich Archer has a similar design floating around the web for a single (9v) supply.
How are you planning on interfacing it with your microcontroller? serial DAC, pwm, digipot, LDR/LED? Depending on your microcontroller (earlier Arduinos, for instance) you can power it up to 12v. If you build a simple bipolar wallwart supply you can power the micro from v+ and ground, and still run your filter bipolar. (Your max v out on your microcontroller will still be low, 5v on the Arduino for instance...)
Anyway, check out the Nicolas filter, it's pretty sweet, under the sticky "A collection of circuits from Nicolas.
And welcome, of course, to e-m.
Hope this helps,
c_z

L´Andratté · Posted: Fri Jan 17, 2014 2:15 am Post subject:

He, I love that one, it´s the first filter, I got ever running, with that nice sb-layout! nicolas3141 stuff, simple AND good!
http://electro-music.com/forum/phpbb-files/filter_166.jpg
A pity I did scrounge it soon after... Rolling Eyes

- It could also used with that expo-converter of the MS20 filter.
- why are the linearizing diodes so rarely connected (here via R15/R17 to +V) on other OTA designs, since they decrease the required attenuation at the OTA inputs? Ignorance or are there downsides?
- since the resonance is implemented so nicely, it leaves both filtering caps as possible BP/HP INputs like the steiner VCF, someone tried that?

Thomas_H · Joined: Jan 02, 2014 Posts: 2 Location: Munich

Thanks for all your helpful responses and the good hints!
The reported filters all look very promising. Will definitely try some of the designs as soon as i find time.
In the meantime I simulated the Steiner VCF in LTspice and made some adjustments of the components so that it fits to +-6V / 12V (also had to remove some diodes, only three left now..). The plotted FFT looks promising so far... I hope i find the time to solder it next week and will report then.

@comrade_zero: By now i made the uC interfacing by using pwm at 11.7kHz and a simple RC lowpass afterwards, so that the step response time from 0...1 is around 1ms. Enough for my purposes by now, faster envelopes could be achieved by rising the pwm speed, but with that comes a higher step size. I am currently using a Teensy 3.0 and that gives me 12bits == 4095 discrete steps. Haven't tried a "real" DAC yet...