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

paula at synth.net paula at synth.net
Tue Aug 1 11:03:02 CEST 2017


  sounds interesting, I look forward to hearing that :)


On 2017-08-01 00:48, David Moylan wrote:
> 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 here.
> 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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