[sdiy] More on Xpander modes was Re: HP from LP ?

David Moylan dave at westphila.net
Tue Aug 1 01:48:27 CEST 2017

I'm just finishing up an add on to a Roland IR3109 to do the pole 
mixing, same basic method as the Xpander.  From my early listening tests 
I will say that the high pass modes still seem to have some low pass in 
them.  Can any Xpander users comment?

This is due to the fact that achieving HP responses relies on nulling 
out terms in the transfer function via mixing the different poles.  If 
you're not exact you don't get a full null and the attenuation of low 
frequencies suffers (looks more like a shelf instead of continuing roll 
off of low frequencies and usually has a little dip in the middle).  I'm 
using 1% tolerance resistors, but am about to sub in some 33.2k for 33k 
to get even closer to the theoretically perfect values.

Even if you had perfect resistors I would imagine any mismatching 
between the OTA stages would also cause the attenuation to suffer, but I 
haven't modeled that.

Still, each mode sounds sufficiently different and I particularly like 
the modes with notches in them like 1 pole LP + notch.  I wouldn't 
expect to like a less "dramatic" filter but I love the sound of that one.

FYI, one of the Xpander modes is All Pass / 1P LP or "Phaser" with first 
pole switched out.  The shape has a notch off center and its pole 
weights are 1 3 6 4.  If you change the weights to 1 3 4 2 the notch 
becomes centered and it looks like a 2P band pass with a notch at the 
center and with the first pole switched out it's like a 1P HP + notch, 
mirroring the 1P LP + notch that is already available.  Something 
different to try...  The PDF mentioned in the last post has good 
drawings of the filter shapes which might help make sense of this.

I'm working on this in a Roland GR-300 but the boards could 
theoretically work in any synth that uses the IR3109, like the SH-101 
and MC-202.  Not sure how much room is available inside those, the 
boards are 1.3" x 2.6" using 3 SOIC chips, a small number of 1206 SMT 
resistors and the rest through hole.  Not very difficult to solder.  The 
filter selection is via a pot into a 3 bit ADC, and the first filter 
pole is switched manually. (Haven't had any problem with pops when 
switching that pole.)  That gives the 16 combinations, of which, there's 
one duplicate so you get 15 modes.

If you're interested in a board let me know.  I should have a video on 
YouTube later this week showing it's operation in the GR300.  Will post 

On 07/31/2017 02:28 PM, Tom Wiltshire wrote:
> I’m getting the feeling here that just a simple subtraction of the lowpass response from the input isn’t enough (ahem). If you want better than -6dB/oct, you need to do some more sophisticated summing of the input and the various outputs, and you’d better make sure you’ve got accurate resistor values and signal levels too, since that provides a limit to how m such attenuation you can expect in the stop band off you high pass filter. How good is your signal cancellation? -40db? -60dB? Unlikely to be better than that.
> That’s not to say that it’s not worth it.. As Paula said, the Xpander service manual is the original source, and Oliver Gillet’s documents on the Four Pole Mission board for the Shruthi expand on that:
> https://mutable-instruments.net/archive/documents/pole_mixing.pdf
> Basically, generating a multi-pole highpass from a multipole lowpass response isn't as simple as just taking one off the other. These are both great filters and worth doing.
> HTH,
> Tom
> ==================
>         Electric Druid
> Synth & Stompbox DIY
> ==================
>> On 31 Jul 2017, at 20:50, Julian Schmidt <elfenjunge at gmx.net> wrote:
>> Have a look at this paper on page 64, Multimode Ladder Filter
>> https://www.native-instruments.com/fileadmin/ni_media/downloads/pdf/VAFilterDesign_1.1.1.pdf
>> You can mix the different poles with a simple opamp mixer.
>> input -4 * Pole1 + 6 * Pole 2 -4 * Pole 3 + Pole 4 will yield a funky looking HP filter
>> julian
>> Am 31.07.2017 um 20:10 schrieb Michael Zacherl:
>>> Hi,
>>> I discussed that briefly in one of the last SDIY meetings,
>>> did some experiments but never came to a sound conclusion:
>>> Is there a ‘cheap’ way of turning a low pass filter into a high pass?
>>> How’s that possibly approached?
>>> Just something like subtracting the LP’s output from its input signal?
>>> If so, what about resonance?
>>> My point about doing that cheaply is the (upcoming) release of two synths with Moog ladder filter featuring a switchable high pass mode.
>>> To my knowledge, there was no resonating HP filter from Moog so far in any synth.
>>> Until now.
>>> Since both synths (Moog Mother-32 and Behringer Model D) are on the budget side in terms of money I’d rather expect a simple solution compared to a, say, DSI Pro-2 with two fully featured filters in it.
>>> Any hints?
>>> Thanks, Michael.
>>> --
>>> http://mz.klingt.org
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