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MI Ripples theory - Can someone explain the Resonance path?
MUFF WIGGLER Forum Index -> Music Tech DIY  
Author MI Ripples theory - Can someone explain the Resonance path?
Treefingers
Hey everyone,
I recently breadboarded the MI Ripples filter from the schematic provided on their website and have a few questions about how the resonance works.

First, I noticed that the incoming signal gets dropped down to about 1/3rd of it's original volume - and then amplified 3x back to original value after the filter. Why is that? is that just to leave headroom for the resonance or is something else going on there?

I also don't quite understand the resonance path. It seems pretty straightforward, taking the 4p output thru the OTA and bringing it back directly to the input. But the original input signal also goes into the opposite input of the OTA, why? and why thru the 220n cap?

Lastly, I also noticed that the output on the 4p output has a DC bias to it that changes with the cutoff frequency. Shouldn't that be blocked by a capacitor before going to the output, or is it assumed that the next module it is going into will be AC-coupled?

Here's the schematic: https://mutable-instruments.net/modules/ripples/downloads/ripples_v40. pdf
devinw1
Have you checked out AN071 (Filter design for the 2164) yet? Sorry I dont have a link but can post tomorrow if nobody beats me to it!
jakobprogsch
My guesses are:
Treefingers wrote:
Hey everyone,
First, I noticed that the incoming signal gets dropped down to about 1/3rd of it's original volume - and then amplified 3x back to original value after the filter. Why is that? is that just to leave headroom for the resonance or is something else going on there?

The simplest explanation is that this way the input has 100kOhm impedance which is the "pseudo standard" for euro. While using 33k on the intermediate stages is just following the recommendations of the 2164 data sheet.
Quote:

I also don't quite understand the resonance path. It seems pretty straightforward, taking the 4p output thru the OTA and bringing it back directly to the input. But the original input signal also goes into the opposite input of the OTA, why? and why thru the 220n cap?

The feedback signal is out of phase with the input and therefore partially cancels it. This is what causes the "loss of bass/fundamental" people often ascribe to ladder filter resonance. The same thing would happen here as well. By mixing in the input signal you can compensate for that. This compensation scheme also works for ladder filters btw. but since everyone seems to just clone the (mini)Moog filter that doesn't compensate for this, people seem to assume it's a ladder filter inherent thing.
Quote:

Lastly, I also noticed that the output on the 4p output has a DC bias to it that changes with the cutoff frequency. Shouldn't that be blocked by a capacitor before going to the output, or is it assumed that the next module it is going into will be AC-coupled?

By that you mean the output with the VCA I assume? If you read the LM13700 datasheet you will notice that R19 and R21 are often shown as a trimmer with the wiper connected to GND. That trimmer can be used to trim out the offset of the VCA. I guess this was skipped here for simplicity. I can only guess at the reasoning but I would do it the same way arguing that any module that relies on the input not having a DC bias should be AC coupled on the inputs. Being fully DC coupled means Ripples can be used to "filter" slow moving unipolar signals like envelopes etc.

Edit: I guess the last part doesn't make sense since there is actually a coupling capacitor before the VCA? seriously, i just don't get it
paperCUT
jakobprogsch wrote:
My guesses are:


The feedback signal is out of phase with the input and therefore partially cancels it. This is what causes the "loss of bass/fundamental" people often ascribe to ladder filter resonance. The same thing would happen here as well. By mixing in the input signal you can compensate for that. This compensation scheme also works for ladder filters btw. but since everyone seems to just clone the (mini)Moog filter that doesn't compensate for this, people seem to assume it's a ladder filter inherent thing.


The MC-202/SH-101 method works really well, just feed some of the resonance into the output.
forestcaver
Have a read of this:

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

(there are other useful analyses in the shruthi-1 GitHub)
Treefingers
devinw1 wrote:
Have you checked out AN071 (Filter design for the 2164) yet? Sorry I dont have a link but can post tomorrow if nobody beats me to it!


Found it: http://soundsemiconductor.com/downloads/AN701.pdf

Never saw that document before.. it's great. Will start building some of the suggested circuits. As for Ripples.. I think I understand it now. I've messed with some of the resistors to keep even more bass while the resonance is up - it's great to be able to tweak it to taste.
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