[sdiy] problem with 13700 flter upper range

[Tristan]

It is my understanding that the impedance of the darlington buffer in
the LM13600 changes with IABC and that this is preferable for VCO/VCF
circuits. That suggests an LM13600 would be a better choice to
implement this filter cicuit. But if you substitute a FET buffer then
LM13600 and LM13700 performance should be pretty much the same.

/Tristan


Wednesday, January 28, 2009, 6:04:48 AM, you wrote:

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


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

TW> T.



TW> 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
>>>
>>>
>>>
>>> _______________________________________________
>>> Synth-diy mailing list
>>> Synth-diy at dropmix.xs4all.nl
>>> http://dropmix.xs4all.nl/mailman/listinfo/synth-diy
>>>
>>>
>>
>>
>> -- 
>> Vriendelijke groet,
>>
>> Simon Brouwer
>> -*- nl.openoffice.org -*- http://www.opentaal.org -*-
>>
>> _______________________________________________
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>>


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