More Shruthi-1 SDE discussions

This discussion was created from comments split from: 4Pole Mission, 4.7uf Caps anybody know what roll off this gives? Also do you need 2 sets of headers.

Link to the SDE wiki page

My pleasure - since this thread seemed to have moved permanently to an entirely different subject from your original query I asked Olivier if he would mind splitting it off for clarity, so here we are in a new SDE thread

Thanks Olivier!


Wondering what six way switches you used.

Ok, cool I will check that out. Yes I probably am getting a bit carried away. I’m just putting together an order for the parts for a 4 Pole Mission version, it is looking like it is very good value if you buy the parts yourself, although this may be an illusion brought about by ordering the parts separately.

@shimoda: If you’re referring to the SDE, I used these 2-pole/6-way switches from Doctor Tweek , although they are fairly common elsewhere as well.


Thanks for the diversion, everybody. I’ve been needing to get serious about this stuff again, and I’m glad some of you are talking down to my level instead of letting me flounder. Gotta start studying those schematics properly now!

Clever stuff, I see what you mean by mixing the poles, you literaly take a feed of the different poles. Perhaps something for the future, I did notice there was space on one of the PCBs for clipping diodes. I did think it might be fun to add some LEDs for some switchable distortion.

@Dunk: It’s Olivier who’s the clever one with pole mixing - I reverse engineered some of his 4-pole goodness, but if you’re intrigued by switchable distortion I suspect I’ve taken it further than most on the SDE

Have a listen to my different diode flavour options here


Hi, I really like the “Warm” option. It emphasizes the sweep nicely. It has also been suggested to add a 2/4 pole switch to the SMR4 board, I don’t know where I would add it to the nice laser cut case though. Perhaps round the back next to the sockets.

I’ve just had another read of the wiki page. It’s a great idea to use the 2 pole switch to select both diodes and mix amount. I have made a few of fuzzboxes over the years, mostly for use with synths and drum machines, and one of the most successful was one that was switchable between 1n4148s and Red LEDs. The LEDs were great as when the clipping set in you could see the LEDs lighting up. The clipping sounded very “abrupt”, very heavy.

@Dunk: Be wary of using LEDs for clipping within the Shruthi filter circuitry, since the typical peak to peak output levels are around 0.5V RMS (1.5 V peak to peak), and likely to be lower than the forward voltage of most LEDs (between 1.5 for red and 1.8 for green according to my measurements).

My “Warm” option uses a single 1N4148 diode (forward voltage 0.6V) and a small series resistor to provide the ~10% 2nd harmonic distortion for the Shruthi SDE.


I see, yes the 1.5v wouldn’t be enough to turn on the LEDs. Interesting that my favourite is the bog standard 1N4148, I have cheap tastes I suppose!

I might implement a simple toggle switch with your warm option wired in to one of the poles. I guess all of this distortion is post filter as the input of the circuit is taken from the filter poles. I suppose it would be possible to distort pre filter as well, although this synth has so many waveforms to choose from there probably wouldn’t be a lot of point as one of the waveforms is probably already like a “distorted square”, maybe the wavetables or FM.

Actually the idea of having analogue distortion built in is a very good move, saves having a separate pedal (and associated power supply) to implement it.

The only problem I can see is that the synth looks so neat with a clear case that the wiring will have to be perfect not to spoil the effect!

I’ve realised my comment about the synth having a distorted waveform is a bit naive as I don’t really know what happens when a filter is overdriven. It must be quite complicated.

you could always ask fcd72 to make a broader case for you, if you like to keep the Shruthi form factor.

@Dunk: my distortion flavours are actually part of the filter overload characteristic, and affect the sound and amount of the resonance when it’s turned up high more than the non-resonant sound. Listen in particular to the same Shruthi preset in my Soundcloud demos with FLAT and then SOFT flavours - the resonant sound is completely different!

If you just want analogue distortion then implement my Amp Drive mod, which comes after the filter, leaves the resonance alone, but gives fully variable amp overload.


Hi Martin,
Yes the resonance is much less pronounced on the soft setting. I think if I do any of these mods I need to do some reading up on how the filters work. I understand how you can take a distorted (clipped) signal and then pass it through a filter, I also understand how you can take the output of a filter and pass that through distortion.
What I have realised is that I don’t know what happens when a large signal is passed through a filter, eg if it is designed for 1v peak to peak and you pass 2 v through it.
I have an oscilloscope and that is great for understanding what is going on. I wonder if the external input can be used to experiment. For example passing the output of a signal generator through and turning up the amplitude of the sine wave feeding the filter until it is overdriven?

@yewtreemagic: any link that amp drive mod ? I could find it easily. Note that post distortion is nice but in-filter loop distortion is even better. And pre-filter works rather well too :wink:

Dunk: What I have realised is that I don’t know what happens when a large signal is passed through a filter, eg if it is designed for 1v peak to peak and you pass 2 v through it.

@Dunk: Think of it this way. When you turn up the resonance of a filter you get a peak in the frequency response that means the peak level of the waveform becomes significantly higher. If you then introduce waveshaping (using a diode or whatever) into the filter this resonant peak gets slightly changed in shape, thus changing its sound.

In the case of my SOFT setting, the waveform change is tiny and only really affects the very tips of the waveform slightly. So, the main body of the sound remains largely unaltered, but the sound of resonance still changes.

This is why I was so interested in exploring what distinct flavours I could find for the SDE: they introduce a combination of extra harmonics and alter the type of resonance - a far more interesting combination of changes than simply clipping the waveform. If you want the latter then explore my Output Drive mod, which offers variable clipping while leaving the resonance intact, for some very different timbres.

Hope this helps!


@Mdashdotdashn: the Output Drive mod is an extremely simple one that you can find here

My filter flavours are a progressive combination of in-filter loop distortion and pre-filter distortion, so all three types are available


Thanks for the link, I didn’t think about the wiki ! :slight_smile: