[sdiy] Pole Mixing Multi-Mode filter design with SSI2140

[jeff.whitman at icloud.com]

Hi Neil,

Thanks for the input—that’s an interesting idea about adding a single capacitor before the summing op-amp.

It does raise a question I’ve had about how pole-mixing filters behave, and I’ll try to phrase it clearly. Do these two approaches—adding capacitors at the SSI2140 outputs before the summing resistors versus using a single capacitor at the op-amp input—produce different results in terms of filter accuracy across modes?

If the DC offset at each of the four outputs is different (based on the internal design of the SSI2140), wouldn’t that affect how much each pole contributes to the mix? In other words, could it impact the accuracy of the pole mixing itself—which is part of the reason for using 1% resistors?

It’s not immediately intuitive to me that placing a single capacitor at the summing node would yield the same result as blocking DC at each pole output. I’d be interested in your thoughts on whether these two approaches are effectively equivalent, or if there are trade-offs to consider.

Thanks again,

( And yes, I did run my reply through AI so I could try to get the wording correct :). )

Jeff



Sent by voice-transit conductor

> On May 1, 2026, at 1:41 PM, Neil Johnson <neil.johnson71 at gmail.com> wrote:
> 
> Hi Jeff,
> 
> Great to see you continuing on with your project. The issue is caused by small DC offsets being amplified by the high gain on some of the taps. For example, the HP4 config has tap gains of {1,4,6,4,1}. Because the offsets of the individual stages are not the same (see LP1, LP2, LP3 and LP4) then you will get some resulting offset.
> 
> One thing you could try is instead of four capacitors, one for each pole, you can put one capacitor after the mix resistors just before the virtual earth summing node of the mixing opamp. This will block any DC present in the pole outputs, and saves you three large capacitors.
> 
> You do raise an interesting point, so I'll see about including something in the next revision of the datasheet.
> 
> Cheers,
> Neil
> 
> 
> On Fri, 1 May 2026 at 16:13, Jeff Whitman via Synth-diy <synth-diy at synth-diy.org <mailto:synth-diy at synth-diy.org>> wrote:
>> 
>> 
>> Hello there,
>> 
>> I’ve been working on an analog synth project for several years using Sound Semiconductor parts. Their data sheets and design notes have been incredibly helpful, especially for those of us who are not primarily analog designers. I’m a firmware engineer by profession - now retired, with an EE background from a long time ago, so I understand the basics but would really appreciate input from people with more analog design experience.
>> 
>> For this project, I’ve built a “voice” of three oscillator boards using the SSI2131, a filter board using the SSI2140 in the suggested pole-mixing configuration to create a 16-mode multimode filter, a low-pass filter using the SSI2144, and linear-controlled VCAs using the SSI2164.
>> 
>> My questions are specifically around using the SSI2140 for pole mixing. From what I can tell, this design is very similar to the filter used in the Matrix-12, and I’ve spent some time studying that schematic. I’ve also reviewed the AN701 filter design document on the Sound Semiconductor website. I’m including my schematic in PDF form in the hope that some of the very smart analog folks here can offer suggestions about something I’m seeing.
>> 
>> I’ve been using Claude.ai <http://claude.ai/> to help analyze each of my filter configurations and determine whether they are behaving as expected. I’ve also been doing sound comparisons against the Arturia Matrix-12 V modeled synth. One thing I’ve found is that I’m getting DC offset that appears to be coming from the pole outputs of the SSI2140. Depending on the pole-mixing resistor configuration selected for a given filter mode, I’m seeing significant DC offset.
>> 
>> I’ve been feeding Claude scope shots to help analyze this, and so far its interpretation seems pretty consistent with what I’m measuring. As a side note, this has been a very useful application of AI for troubleshooting. For the pole-mixing network, I’m using 1% resistors and also using 1% capacitors, as recommended in the data sheet and AN701.
>> 
>> Here is the table Claude generated from my measurements:
>> 
>> Mode    Mean DC Offset
>> LP1     -12.5 mV
>> LP2     -40 mV
>> LP3     -30 mV
>> LP4     -30 mV
>> HP1     -61.9 mV
>> HP2     -131.1 mV
>> HP3     -284.4 mV
>> HP4     -583.6 mV
>> Claude’s current explanation is that the increasing weighting factors in the pole-mixing network are directly amplifying the DC component.
>> 
>> Things that appear to have been ruled out:
>> 
>> Power supply asymmetry (-11.93 V vs. +12.02 V) — likely negligible
>> Input signal DC component — input is about +117 mV, while U1A shows only -8.33 mV offset
>> TL074 op-amp offset voltage — too small to explain this magnitude
>> Unity-gain buffers — not populated - I have jumpers.
>> The suggested fix is to add four 1 µF film capacitors, such as WIMA MKS2 parts, on the SSI2140 OUT1–OUT4 pins, placed between the OUT pins and the 15k mixing resistors. The idea is that this would block DC before it enters the pole-mixing network, preventing it from being amplified by U1B regardless of filter mode.
>> 
>> I’m curious whether adding these coupling capacitors makes sense, and whether that would be compatible with the intended use of the SSI2140. I’m a little surprised that the data sheet does not discuss the need for them if this is expected behavior. I did find some discussion about this on Mod Wiggler, but I didn’t see a clear conclusion.
>> 
>> I’d appreciate any advice, and hopefully I’ll be able to understand it. Also, if you see any other design issues or possible improvements, I’d be grateful for the feedback — pun intended.
>> 
>> Thanks,
>> 
>> Jeff
>> 
>> 
>> Sent by voice-transit conductor
>> 
>> ________________________________________________________
>> This is the Synth-diy mailing list
>> Submit email to: Synth-diy at synth-diy.org <mailto:Synth-diy at synth-diy.org>
>> View archive at: https://synth-diy.org/pipermail/synth-diy/
>> Check your settings at: https://synth-diy.org/mailman/listinfo/synth-diy
>> Selling or trading? Use marketplace at synth-diy.org <mailto:marketplace at synth-diy.org>

-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://synth-diy.org/pipermail/synth-diy/attachments/20260501/ab024297/attachment.htm>