[sdiy] Fixed filter bank questions

David G Dixon dixon at mail.ubc.ca
Thu May 9 00:50:01 CEST 2019

Thanks, Tom!  That was exactly the sort of response I was hoping for.  I
will definitely consider changing the frequencies as suggested by Rod
Elliott.  I have followed his advice in many of the circuits I have
designed, and have never been disappointed in the results of doing so.
Plus, it did seem to me that many of the knobs on the YuSynth design gave
some pretty hissy results, based on the videos I watched.  Indeed, the most
important knob on the whole panel is the LP (88 Hz) one, which gives a very
pleasant and deeply resonant bass.

Concerning tolerances, presuming that I only build one of these (at least at
first) then I will probably hand-select the capacitors to ensure roughly 1%
tolerances.  I know that the exact frequencies don't actually make that much
difference, but I want them to be as close as possible to the values
advertised on the front panel -- I guess you could call it a matter of
principle.  I also want the specific frequency choices to be based on some
rational pattern and not just semi-random like the Moog ones.  It seems to
me that if the resonant peaks all overlap in the same manner, it should lead
to better performance of the overall filter bank.  (But, I could be smoking

Having said all that, don't ya just love the idea of these filter banks?
The idea that something so electronically simple can have such smashing
sonic results?  This is why I build synthesizers.

-----Original Message-----
From: Tom Wiltshire [mailto:tom at electricdruid.net] 
Sent: Wednesday, May 08, 2019 3:24 PM
To: David G Dixon
Subject: Re: [sdiy] Fixed filter bank questions

> On 8 May 2019, at 22:04, David G Dixon <dixon at mail.ubc.ca> wrote:
> FIRST QUESTION:  Is there anything wrong with any of these concepts?


I'd just flag up that Rod Elliott of the ESP audio pages makes a good
argument that the use of 1000Hz as the "centre" of audio is misplaced.
Instead, he goes for 640Hz as the geometric centre based on a ten octave
range from 20Hz to 20.48KHz:


Having experimented with this, I've found that I agree with him. It makes a
good "mid" frequency. Consider it.

> SECOND QUESTION:  Is there something fundamentally less desirable about
doing it my way than doing it Yves' way?

I don't think so. I'd have done it your way too, for the same reasons. It
seems sensible to reduce the number of expensive caps (these are good
quality close tolerance film caps you're using for something like this,
right?!) and use resistors to set the frequency.
Personally, I wouldn't fret too much about getting the resistor values
absolutely spot-on, unless you're going to buy 1% tolerance caps too. If the
caps are 5%, there's not much point stressing to find an exact resistor
value from the E192 series.


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