[sdiy] Module Placement - Dist > Filter or vice versa

[Scott Gravenhorst]

"Roy J. Tellason" <rtellason at blazenet.net> wrote:
>On Thursday 18 November 2004 01:49 pm, Scott Gravenhorst wrote:
>> "Roy J. Tellason" <rtellason at blazenet.net> wrote:
>> >On Thursday 18 November 2004 12:26 am, William Berzinskas wrote:
>> >> i'm building a simple efx unit using a few things lying around..  i've
>> >> got 2 distortion units  selectable via switch..   one is tube overdrive
>> >> sim and the other strait distortion.   i'm also going to add a cd4007
>> >> based filter in there..
>> >
>> >What kind of a filter can you base on a 4007?
>
>> Pretty much like a 4069UB filter.
>
>I don't recall running across that one either,  though I might have and just 
>don't remember...

I don't believe it's a terribly popular idea.  AFAIK, it's only been discussed
in theory so far, but the René Schmitz 4069 VCO proves that the integrator
concept works very well.

>
>> The 4007 contains three inverters which can be used as integrators.  If the
>> input is sufficiently attenuated, you can also use a 4007's N transistors as
>> voltage controlled resistors.  (the attenuation required is no worse than
>> that used for OTA filters).
>
>That sounds pretty nifty.  I'm really surprised that there doesn't seem to be 
>any simple way to control amplitude with voltage,  and hasn't been for as 
>long as I've been looking.  How low of a signal level are we looking at 
>there?  I never did get much of a handle on OTAs though I have a few to mess 
>with here,  both 3080 and 3094...

A quick look at OTA voltage controlled state variable filters reveals a 100:1
voltage divider feeding the OTA input.

I'm looking at a +/- 50 millivolt AC input signal after the attenuator.  This
level satisfies the H11F1 distortion requirements.

>> I'm still sitting on a design myself, 4069UB for the integrators and H11F1
>> for the resistive elements.  Schematic is done, gotta get my ass in gear and
>> breadboard the thing.  I chose H11F1 rather than 4007 or 4069 because the
>> H11F1 eliminates virtual ground problems AND the H11F1 distorts
>> symmetrically (when driven with more than 100mv Vds) according to my own
>> testing.
>
>Sounds interesting,  I'd like to hear more about this once you get around to 
>getting  going with it.

As am I...  (as I kick myself squarely in the butt).

René Schmitz http://www.uni-bonn.de/~uzs159/ has a design for an opamp based VC
SVF filter that uses 4007 N transistors as voltage controlled resistors.  Osamu
Hoshuyama http://www5b.biglobe.ne.jp/~houshu/synth/ has a CMOS synth site that
uses OTAs made from CMOS for an experimental VCF, but the results apparently
are less than wonderful due to nonlinearities that can't be compensated and
these CMOS OTAs can be tricky to implement because resistor values can't be
cast in stone as would be desired for projects that others might build.


---------------------------------------------------------
- Where merit is not rewarded, excellence fades.
- Hydrogen is pointless without solar.
- What good are laws that only lawyers understand?
- The media's credibility should always be questioned.
- The only good terrorist is a dead terrorist.
- Governments do nothing well, save collect taxes.

-- Scott Gravenhorst | LegoManiac / Lego Trains / RIS 1.5
-- Linux Rex         | RedWebMail by RedStarWare
-- FatMan: home1.gte.net/res0658s/fatman/
-- NonFatMan: home1.gte.net/res0658s/electronics/
-- Autodidactic Master of Arcane and Hidden Knowledge.