[sdiy] voltage controlled filter???

[synthplayer88 at spymac.com]

Hi harry,
Thanks a lot for the insight, the reason why I asked is because I am looking to
build an EQ with state variable filter and wondering if there is actually any
point implementing multipliers or OTA to control the Fc.

I guess the only advantage of that is I can use the dual gang potentiometer
method to provide the rev log voltage which alllows smooth sweep as oppose to
rotary switch with limited steps. Other than that, I don't see any benefit for
this application except may be adds the gimmic factor.......  :)

BTW, using multipliers for VCA seems to be the most common method in synth has
anyone tried other VCAs such as from THAT corp the 2180 series or National CLC520
or AD SSM2164, SSM2018, MLT04, AD734, AD835, AD633 or TI VCA810, 4214, 4213,
MPY100, MPY634 or ON MC1496P etc......

I know THAT corp seems to be targeting their VCA as a pro audio thang, is there
really any difference between THAT VCA and the usual multipliers? aside from the
CV is DC as oppose to AC?

Find it very intersting on Ebay people paid some pretty good money for CA3080,
while there are all these ICs available some are as cheap as 50c but some cost
$50USD!!!!!!!

Thoughts?



 

On Wed Jan  5  0:10 , harrybissell <harrybissell at prodigy.net> sent:

>Hiya synthplayer...
>
>synthplayer88 at spymac.com wrote:
>
>> Hi guys,
>> I was wondering if anyone can tell me whats the different betwen using
>> transconductance amp like the CA3080 and the LM13700 or the use of balanced
>> modulators/multipliers to control the cutoff frequency of say a state variable
>> filter, instead of dual rev log pot?
>
>why a dual ?    Also, the reverse log pot is usually a feature of the Moog ladder
>resonance control... the SVF probably does not need the reverse log because they
>work opposite.
>
>Moog filter has maximum resonance when there is 100% feedback   (well high, anyway)
>SVF has maximum resonance when there is 0% feedback (well low, anyway)
>
>> Is is because dual rev log pot are hard to find?
>
>yes.
>
>> In order to use transconductance amp or multipliers don't you need to generate a
>> rev log voltage in order for it to work like a rev log pot?
>
>The reverse log pot is used so that the resonance does not come on suddenly.  You'd
>have to
>experiment if you are using a VCA (or OTA) to see if you like how the 'feel' is when
>you turn
>the pot.   You can generate any curve you want... you could use a linear current
>source and then
>change the shape of the voltage you apply... or use an exponential current source or
>a log current
>source etc...   you need to play and see what you like best.
>
>I'd start with a linear current source, and see if you like it. Its probably the
>easiest to make.
>
>> I am guess people are using dual gang linear pot wiring it like this
>>
>> http://home.comcast.net/%7Esbernardi/elec/og2/partsub_pots.html
>>
>> to get the rev log voltage
>
>yes you can do it that way....
>
>> then control a pair of the transconductance amp or
>> multiplier with it?
>
>A pot has to be turned by hand... the OTA could be controlled by any voltage
>source... LFO,
>Env generator, etc...
>
>OTOH the pot has zero DC  offset voltage, zero distortion, unlimited signal handling
>ability (well
>almost :^) and it is dead simple...
>
>>
>>
>> Isn't it much straight forward to use a dual gang rotary switch  and work out the
>> resistance for cut off frequency?
>
>cutoff frequency and resonance are not the same. If you mean cutoff freq, the SVF
>needs two
>resistors to tune so you could use a dual gang rotary switch or pot...
>
>>
>>
>> I guess my question is, why do people use VCF in synth?
>
>subtractive synthesis. Take a harmonic rich waveform and remove the harmonics you
>don't want...
>
>> why voltage controlled?
>
>how many hands do you have ???   how fast can you move them all... maybe in opposite
>directions
>different amounts at the same time :^P
>
>> For modulation prupose?
>
>> yes.
>>
>> Howcome I don't see people mention the use of balanced modulators, VCAs and
>> multipliers instead of the CA3080 and the LM13700?
>
>when many of these circuits were first built, analog multipliers were VERY expensive
>and
>suffered from a lot of problems.  The OTA had the advantage that it was quite
>inexpensive...
>the only thing that really matters (at the chip level) is that all the transistors
>are matched, something
>that comes almost for free in a monolythic IC.
>
>Some folks DO use Multipliers to control VCFs, Roman Sowa (of our list) for example.
>
>H^) harry
>
>