# [sdiy] 1-quadrant multiplier with 2164

David G Dixon dixon at mail.ubc.ca
Fri Jan 21 07:03:15 CET 2022

```Hey cheater,

For my Dome filters, what I do is limit myself to round numbers for the
caps.  So, for example, for the 18-stage filter, I use 4 102s, 4 103s, 4
104s, and 6 105s.  When I have a lot of caps in my inventory, I can make
sets with all the same mantissa (within 0.2%), so, for example, I might use
988pF, 9.88nF, 98.8nF, and 988nF.  Then, I can use 18 different pairs of 1%
resistors in series to get the proper resistance values.  For example, one
of the stages requires an RC of 7.2017ms, and the cap is 104 or 100nF, so
the resistor should be 72017 ohms.  Hence, I use a 71.5k and a 511R, which
gives me (nominally, ignoring tolerances) 72011 ohms, which is within 6 ohms
or 0.008% of the target.  I have written a VB code running behind an Excel
spreadsheet which gives the the correct large resistor from the E96 series
which is just below the resistance target, then it also gives me the smaller
resistor.  I can set a 0 or a 1 to give me the smaller or larger resistor
which gives the sum nearest the target, so I can compare to minimize error.
I also have a plot of the phase displacement across the whole filter, and I
actually look at that and adjust the 0 and 1 to give the best looking
result.

Once I have all the resistor values I need (36 for the 18-stage filter),
then I organize them in zip-lock bags and arrange them in order in a plastic
tub.  Hence, stuffing the filter is just a matter of picking the bags out of
the tub one at a time, in order, and taking out each resistor.  This system
probably saves me a good half hour for each build.

Note that the resistor values don't change, even though the mantissa of the
caps can be anything from about 960 to about 1040.  As long as they are all
the same mantissa, this just changes the frequency range of the filter
slightly (a few percent up or down), and is not noticeable.  Even though
this filter is designed for a frequency range of 0.1 to 10000 Hz, it
actually gives good phase displacement well beyond those limits.

-----Original Message-----
From: cheater cheater [mailto:cheater00social at gmail.com]
Sent: Thursday, January 20, 2022 9:12 PM
To: David G Dixon
Cc: synth-diy
Subject: Re: [sdiy] 1-quadrant multiplier with 2164

[CAUTION: Non-UBC Email]

Thanks for the breakdown, that was very interesting.

I wonder, could it sometimes be a good idea - especially if using smd
- to forego using different value resistors, and just use 1 or 2 resistor
types and build up the required resistances from serial-parallel networks of
the same resistor? It would definitely be easier to pick them out and harder
to mix them up...

With TH it would definitely be way more work than just using the right
resistors.

```