[sdiy] ladder filter questions?

[Dan Snazelle]

Thank you so much!!!! Wow


This is going to help a lot.


I decided to tear my circuit apart and go backwards from a known working design....it Was my caps


And now, with your email, your paper, tons of filter schematics , and a breadboard, I think ill figure it out!!



I bit off too much too fast!!




Sent from my iPhone

On May 21, 2011, at 6:08 AM, "Tim Stinchcombe" <tim102 at tstinchcombe.freeserve.co.uk> wrote:

> Hi Dan,
> 
> This... 
> 
>> the odd thing was that it was working when i had nothing more 
>> than a pot to +v and -v hooked up with a 1k resistor to the 
>> emitters (for cutoff) but when i switched to a real cutoff 
>> circuit the filter stopped working and I had to change all my 
>> resistor values.
> 
> ...is most likely linked to this:
> 
>> I am also just sort of randomly deciding where to put 
>> coupling caps (do they have to be on the inputs? do they have 
>> to be on the outputs into the differential opamp, etc)
> 
> ...which with this:
> 
>> i fear the thing i dont understand well is how the  gain 
>> structure/feedback works. it seems if i put too much feedback 
>> into the - side of the ladder, i get no or little sound, and 
>> if not enough, i get thin resonance.
> 
> first leads me to say that designing a filter from the ground up, even if
> taking an existing circuit and playing with resistor values etc., can be a
> tricky prospect - how some people manage to do it mostly by trial and error
> eludes me, and suggests they have a more intuitive grasp of what is going on
> than I can ever hope for. Me, I stick to the maths, as (for me) it yields
> results more surely than any other method, though simulation helps here too!
> 
> So here is a quick whip round the function of key (coupling) caps in a
> typical diode ladder filter - I'll work from this, as it is easily
> referencable:
> 
> http://www.timstinchcombe.co.uk/synth/diode_18_24/tb303_vcf.gif
> 
> First up the frequency control. This stems from Q9/10/11: to work correctly
> the emitters at the bottom of the ladder, Q12, must be high enough so as not
> cause any problems with Q11 collector. In turn, this is (hand-waving)
> 'somewhere above Q11 emitter' = Q9 emitter, a diode drop above approx 2.6V,
> the 5.3V ref halved by R64/65. This means Q12 bases must be a diode drop
> higher still than the emitters: R68/70 set these at the 5.3V ref voltage,
> ample to eliminate any problems back down to Q11. This leads to several more
> points: first we have to ensure that any DC in the input signal applied
> through R62 doesn't mess with the 5.3V level at Q12 base - this is why C17
> is needed; the input is attenuated by R70/R62, and we need to prevent the AC
> of the input signal upsetting the 5.3V ref set at Q12 bases - this is the
> function of C16, which lets the AC through, and so gives the 'AC ground'
> point for the ladder. The same applies for the feedback side, where cap C23
> is again 'decoupling' the DC level of the feedback signal (whatever it is)
> from the 5.3V set for Q12 bases. (The attenuation here is by R97/R98, and
> you'll notice it is much less than the input side.) In a transistor ladder
> you'll see R68/C16's function taken by a large cap somewhere up the whole
> resistor chain biasing the ladder; and the 5.3V set at the ladder inputs is
> also a bit 'extreme' - for a simpler expo converter, the output collector
> may only need to be a volt or two above ground so as to avoid trouble.
> 
> The voltages gently increase up the ladder, where we have similar problems
> with the DC levels in the differential 'take off' amp formed around Q21. It
> has DC biasing levels set by R116/112/109, again with C29 preventing the
> ladder signal from upsetting these. Caps C25 & C27 allow for the DC
> difference between the diff amp and the signals picked off the ladder. The
> diff amp output at R110 is referenced to the 12V supply, but is buffered by
> emitter-follower Q19/R98 and then C14 takes the 'unwanted' DC out so that
> the signal is now swinging about ground (this being the actual filter
> output). This is then fed through the resonance pot (clipped from that
> schemo), back through *another* decoupling cap, C15, because there are more
> level-shifts around the buffer Q18/R94 - I leave these as the dreaded
> 'exercise for the reader', in the hope that you are now starting to get the
> hang of it!
> 
> The TB303 circuit is perhaps more complicated than others, due to no op amp
> involvement, which in general terms lead to quicker understanding of what is
> going on (certainly I find it much easier to work out the gain from any
> given op amp set-up than I can for a transistor-based diff amp such as
> here). And it is also a bad example from the gain setting point of view, as
> there are many places around the feedback loop which are having an impact,
> and so which could be a reason for why the resonance isn't what anyone who
> builds it would like it to be - there is no substitute for understanding won
> at the expense of a lot of hard grafting!
> 
> Tim
> __________________________________________________________
> Tim Stinchcombe 
> 
> Cheltenham, Glos, UK
> email: tim102 at tstinchcombe.freeserve.co.uk
> www.timstinchcombe.co.uk
> 
> 
> 
> 
> 
> 
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