Ladder Filter for Shruthi

I am curious to hear, how a Ladder filter in Shruthi sounds.So i start to redesign a ladder Filter for Shruthi.
Heavy to get it work with +/- 5V power supply and also the log converter was not so easy to build.
But now it sees that it works.
Specs from my prototype.
25 Hz up to 16 KHZ , 0,025 V pro octave
Power supply +/-5 V
Maybe i made a PCB for this.


Here a Video with soundexamples:


More sounddemos coming soon on Soundcloud.

Strange, that was one of the train of thoughts in my head 0345 this morning when I couldn’t sleep. How does it sound?

The funny thing is: one comment I have often heard from people casually listening to Shruthi-1 sound samples was: hey this does not sound like a clone of –insert synth manufacturer– , it sounds more like a Moog. Got this kind of comment for instance on the Yellow Magic.
I mean apart from the Shruthi sounding like a Shruthi and the filters just recreating the sound of the filter not of the complete synth, and WTF anyway, it still funny that people comment that way.
Anyway, I’m also quite interested in hearing how it sounds… but if you have a Moog laying around you could just pass the raw oscillators from the Shruthi into the Moog filter and hear how it souds :slight_smile:

Yo Andre, would you like to pass schematics?
And: It would be great to have the first Non MI Filter Board

Yo, i agree to Frank…this would be some interesting thing !

Hi, i have build first a prototype , The final schematics isn’t ready yet.

It is on 5 different Schematic plans.One for the Log converter, the other tor the cascade and some test i have simulate.
I will make a comlplete Fiilter board with Power supply so that you connect it direct on the Shruthi Motherboard. Next i solder it on veroboard, Than i test if all works fine. At least i will complete the schematics and than i post it.
Maybe i make a PCB , The plan is to set the Volume a little higher , so that the filter makes also soft distortion’s
Hopefully it will work.



great plan.

there is a quite low parts count on that perfboard. is that with or without a vca?

I’m all for more filter boards, and somebody have to do the 1st non-MI board. I would love even more along the line of SEM-filters. The 4PM is just awesome with the notch and resonance flavours. Not sure that I’m so intrigued by just Moog though.
But hey, just go for it! All developments are interesting.

One thing worth keeping in mind (and a bit counter intuitive), is that the signal coming out of the OSC pin is PWM - so it is largely unaffected by non-linearities - any transformation of the signal will just change the high and low levels, but the waveform information is not encoded in the amplitudes themselves. You’ll probably need to have a basic 1 pole or 2 pole filter to make it look again like an analog signal, then the distortion, then the filter.

Bonus point if the pre-filter distortion can be controlled by one of the CV1 or CV2 outputs :slight_smile: And if this leaves you with a half LM13700 or SSM2164, you can use the other-half for a post-filter distortion as it is done on the new sub phatty.

I was going to have a play with this, it’s nice that someone else is faster! If you add pre- and post-filter distortion this will end up having up lots of character to explore.

Just add an Arp Quadra phaser and it’s good to go. I kid I kid, that would eat up too much PCB estate, but a guy’s gotta daydream :slight_smile:

Hi Olivier, the basic layout are some of your circuits. The VCA Section is the same, Q is the same> but this i had to figure out at first. The LOG converter works positive and i use IC 1 A for the LP Filter.
In plan is a try with 2 antiparallel diodes in the resonancy feedback path.
But at firts i must bring it complete to work. And only the sound counts. If it doesn’t sound good enough i trash it.
There is a lot to do…Yesterday i have figure out how the control-signals out of the Shruthi mainboard work. And than i had simulate it to look, how i can get the schematics to work.
For control the feedback i will take the LM13700. This i must do at first.


Looks quite exciting. Hope it will become a DIY kit :slight_smile:

Hi, the first test with the Shruthi oscillator. I simply connect IC 6 pin 3 (remove it before)to the ladder filter and play a little.
Seems it sounds not bad.



Rooooaring :slight_smile:

very fat

I just spotted you are round the corner from here (globally seen)…can i have a look?

fcd72, why not, contact me

I know now why R.M use matched transistors. If in the ladder the transistors don’t have exact the same curve , i become by LFO modulation - without a signal- an DC modulation. Interesting, the filter works with standard unselected Transistors, but if you control it with an ADSR or LFO it ’ pumps DC.
Oh it will be a never ending story…So i first had to select the Transistors.


You definitely need at least the top and bottom pairs of the ladder to be perfectly matched - which might be easier to achieve if you use the matched pair of a transistor array IC ?
Inside the ladder I think you need them reasonably matched but doesn’t need to be perfect.
My understanding is that if you buy modern transistors like BC549, BC 560 etc from the same hFE grade ( e.g A B or C ) and all from the same place then there is a very good chance they will all have very close characteristics already.
In practice it seemed to be the case last time I had to match some transistors.

Hi, i have on the bottom a CA 3046 transistor array. The rest are 2N3904.
But without matching, no chance. AMD had Transistor arrays but cost 5€ and is hard to find a distributor.
I will take a try and look for good TR pairs.