[sdiy] High pass filter using the 2164

David G Dixon dixon at mail.ubc.ca
Mon Mar 25 18:22:59 CET 2019


This is a good tech note, but it ignores the input+feedback as a fifth
signal.  In fact, using the 5 signals available in the filter, a larger
range of filter modes is possible (at least 31, 22 of which were selected
for the Intellijel Polaris, which uses the 5-input pole mixing scheme).  By
disabling the first stage, one gets that input+feedback as the output of
stage 1, which means that the disabled modes are limited to 3-pole
operation.  There are a significant number of interesting 4-pole modes which
require an enabled first stage.

 

Also, a public and heartfelt thanks to Rutger Vlek for cluing me in to
adding the feedback to the input for that fifth signal.

 

If you want to see the filter modes being utilized on the Polaris, please
consult the Polaris user's manual which is posted on the Intellijel website:

 

https://intellijel.com/downloads/manuals/polaris_frequency-responses.pdf

 

If you're curious, these are the gain formulae for each of the 22 responses
shown in that PDF document (Note, every other value is negative):

 

LPA 0 1 0 0 0  HPA 1 1 0 0 0  BPA 0 1 1 0 0  NA 1 3 6 4 0    APA 1 2 0 0 0 

LPB 0 0 1 0 0  HPB 1 2 1 0 0  BPB 0 0 1 1 0  NB 0 1 3 6 4   APB 1 4 4 0 0

LPC 0 0 0 1 0  HPC 1 3 3 1 0  BPC 0 0 0 1 1  NC 0 3 6 4 0   APC 1 6 12 8 0

LPD 0 0 0 0 1  HPD 1 4 6 4 1  BPD 0 0 1 2 1  ND 0 4 12 16 8 APD 1 8 24 32 16

                              BPE 0 1 3 3 1  NE 1 4 12 16 8

 

So, based on this table, there are three modes where the fifth input is
required.  These are the ones which require both the input signal and the
output from the fourth stage, so HPD, NE, and APD.  The most important of
these is the four-pole highpass mode, HPD.  Also, the double notch, NE, is a
very interesting and useful mode that is only available with the five-input
scheme.  Finally, the four-pole allpass mode APD is important for phasing.

 

Another thing which is cool which was not done in the Polaris, but which I
may do when I finally get around to building a "Doc Sketchy" version of it,
is to crossfade that first input.  You will notice that the first input is
always either 0 (off) or 1 (unity gain).  It turns out that one can smoothly
morph between various modes by fading this input.  For example:

 

NA --> NC

ND --> NE

LPA --> HPA

 

The most interesting of these is the notch pair ND-NE.  When the input
signal is halfway on, the gain formula is {0.5 4 12 16 8}, which is half of
the APD formula {1 8 24 32 16}.  This means that when these two notch
responses are input-crossfaded, they go through the 4P AP response at half
gain.  I don't know, it might sound cool...?

 

Even though they are not listed, there are many other interesting modes
possible when the input+feedback signal is faded in and out against some of
the other modes.  One does have to be a little bit careful about total gain,
though.  For example, if one does this against the APB mode, one gets the
BPA mode at a gain of 4 when the input is turned off.

 

 

-----Original Message-----
From: Synth-diy [mailto:synth-diy-bounces at synth-diy.org] On Behalf Of Neil
Johnson
Sent: Monday, March 25, 2019 7:59 AM
To: Mike B
Cc: SDIY List
Subject: Re: [sdiy] High pass filter using the 2164

 

Hi Mike,

 

> I working on a 4 pole Cascade high pass filter.  My present design uses
Lm13700 and works fine, but I would like to switch to the V2164.  I cannot
find anyone doing this.

> 

> The issue is the input resistor on the 2164 to convert the voltage to an
input current were as the 13700 is a voltage input. We're would that input
resistor go?

> 

> Hopefully this is not a really stupid question.

 

Olivier Gillet published a nice tech note on making filters with the
SSM2164:

https://mutable-instruments.net/archive/documents/pole_mixing.pdf

 

That shows how to make a single low-pass filter element, then combine

four of them in a chain and from that synthesize a range of filter

responses by mixing various tapped outputs together.

 

Cheers

Neil

-- 

http://www.njohnson.co.uk

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