[sdiy] problem with 13700 flter upper range

[Tom Wiltshire]

I agree with Simon, Dan. You can safely halve the values of those  
caps or the resistors.


There's a similar 12dB VCF design in Penfolds "Electronic synthesizer  
Construction" book. That uses a +/-12V supply too, but only has 10K  
for the CV resistor. I built the circuit once and put it in a volume  
pedal to make a wah-wah for a friend.
It uses the LM13600 - I don't know if that's a significant  
difference. My understanding was that it didn't matter.

T.



On 27 Jan 2009, at 15:47, Simon Brouwer wrote:

> Dan,
>
> Smaller values of C4 and C5, will give higher cutoff. So if you change
> them go for smaller not bigger. You could try 330 pF for example.
>
> Also, the cutoff will be proportional with the current into pin 1  
> and pin
> 16 of IC4. In this circuit this current will ever reach less than  
> 0.2mA
> (assuming AR and LFO voltage will be at most +9V) while the LM13700  
> can
> handle considerably more (absolute maximum value 2mA, but better  
> not go
> higher than 1mA).
>
> So the other thing you could try is to halve the values of R28, R31  
> and R36.
>
> Dan Snazelle schreef:
>>
>> OK
>>
>> i have gone through a bunch of filters on the breadboard this week  
>> and at
>> first was leaning toward a simple sallen and key with a vactrol  
>> for CV
>> control. however i didnt like having to have the cutoff knob go  
>> through
>> the vactrol...so i moved toward lm13700 based filters.
>>
>> looking at this one in particular ( for low parts count)
>>
>> http://www.musicfromouterspace.com/analogsynth/SOUNDLABMINISYNTH/ 
>> images/synthkit_vcf_vca_001.gif
>>
>> which is the soundlab filter.
>>
>> right off the bat though i ran into problems, the first being that  
>> it's
>> highest cutoff seems to be lower than 12k. whatever i put into it  
>> (drums
>> mainly) gets a ton of high end chopped off.
>>
>>
>> I have reduced the input resistors, tried messing with the filter  
>> caps
>> (using 820 pf polystyrene), etc. BUT i dont know enough about  
>> filters to
>> figure out what might be wrong with my top frequency.
>>
>> I have tried with both CV ins AND the cutoff pot.
>> I dont want to fry the LM13700 so i am afraid to mess with the  
>> resistors
>> on the + and - rails of the cutoff pot too much. (i assume  
>> plugging the +
>> end of the pot straight into +v instead of using the resistor will  
>> bring
>> up the High end a little bit,  in face i tested it for a moment.  
>> but it
>> sill wasnt as high as my dry signal.
>>
>> SO what can i do?? is it just a (feature) of this filter? should i  
>> try the
>> wasp filter instead?
>>
>> or go back to my lowly sallen key?
>>
>> thanks guys!!!
>>
>> --------------------------------------------
>> check out various dan music at:
>>
>> http://www.myspace.com/lossnyc
>>
>>
>> http://www.soundclick.com/lossnyc.htm
>>
>>
>> http://www.indie911.com/dan-snazelle
>> (or for techno) http://www.myspace.com/snazelle
>>
>>
>>
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>>
>>
>
>
> -- 
> Vriendelijke groet,
>
> Simon Brouwer
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