Shruthi SMR4 mkII resonance unstable/just disappears

The resonance on my SMR4 mkII seems to disappear over time. When the system goes unused for a while and you first turn it on it’s quite beastly (I imagine like its supposed to be), but then when you use it for a while it just goes silent. It’s really annoying in use because it does still seem to ‘displace’ the sound as it normally would, but then rather than filling the gap with screaming resonance it fills it with silence.

I scoped it out for a while (set oscs to none, cutoff to 63, res to 63, all filter mod sources to 0) with my pc (shruthi audio-out into line-in of my pc and then used Visual Analyser 2011 software), taking some screenshots throughout.
When I started it I had already been messing with it for a bit (10 minutes tops), so it had already gone down a little bit, but during my measurement it went down from:

to this after about 12 minutes:

and then to complete silence/indistinguishable from noise after another 2 minutes.

I noticed that at this point, there was still audible resonance in the higher range of the filter (~100 and upwards), but it was also very soft and was decreasing in volume as well.

I just fired it up again after letting it rest for a while, and while trying different cutoff frequencies I noticed that the frequency of the resonance appeared to be shifting considerably, on some cutoff frequencies more than others. It seems that a cutoff of 127 gives the most extreme example, where the frequency changed by as much as 30Hz per second. The speed at which it changes seems to decrease after a few minutes. Is this normal? I know that analog hardware will never be quite stable, but 30Hz of shift per seconds sounds like a lot (and is actually audible).

Considering that this shruthi is the first notable thing I ever soldered, is it possible that this is the result of shoddy soldering work (thus fixable by resoldering)? Or is it more likely to be some faulty component that is heating up and performing less and less as it does?

Anybody have an idea what might be causing this?

Thanks in advance!

I suspect something heating up may be a possible cause - have you checked the voltage of your PSU isn’t excessive when the Shruthi is powered up?

And how hot do the regulators run?


You appear to have been right on the money. I was silly enough to expect a 9V DC wall wart (old sony discman adapter) to deliver 9V DC, turns out it was more like 10.6V, and yes the voltage regulators got hot. In retrospect it seems kind of obvious that it is an unregulated wall wart and that the shruthi did not draw the 600mA it was rated for. Oh well, live and learn!

So I guess I’m in need of a new PSU. Any recommendations? I figured I need a regulated power supply to avoid more overvoltage, but will that also fix any AC component?

Thanks a lot! You’re a big help :slight_smile:

10.6 V shouldn’t be a problem. Did you measure on load? for example while powering the shruthi?

Yes, I measured it on the bottom of the board on the pins of the power connector while the shruthi was powered on and generating sound. If I measure it without any load I get somewhere between 14 and 15 volts.

Whoahh, this is way too much. Everything above 12 V is definetely not good. Your regulator may handle it, but the last few regulators i got delivered were not from good charges, so it may not…

If it was on load it is a bit high.

Let me be clear:
14 volts was directly from the power supply without any load whatsoever.
10.6V was during shruthi operation measured at the bottom of the board on the connector pins.

Anyway I clearly am not treating it as I should, this is an analog device after all. The quality of the signal I put in is going to directly affect the quality of the signal it puts out. Getting a better power supply is clearly the way to go. I’m just having trouble figuring out what to buy. What should I be looking for in a power supply for the shruthi?

I am using the cheapo ones from Conrad. They are switchable from 6-12 V and regulated. I set them to 9 V and everything is fine…costs are about 13€ and because these are 1500 mA i use them to fire up 3 Shruthis.

I don’t think the power supply is an issue here - there are voltage regulators to do their job!

Just jammed for an hour with a different power supply. Works great. I am really happy that nothing is wrong with the actual shruthi. Thanks a lot for your help guys!

Hey all, this exact same thing is happening to my Shruthi(SMR4 mkII) as well. I first noticed it after trying to tune the filter and thought it was a fluke. I can get it to self oscillate for about 10-15 minutes and then it dies down to almost nothing. I get a little bit of a pluck tone and then a very subtle sine. I tried swapping out my power supply, 9v 1000mA with a 9v 600mA and even tried a 7.5v. The 7.5 did not put enough power out and I got a nasty hum and distorted audio. The other 9v did the same exact thing. Self resonating for about 15 minutes and then poof nothing. The synth sounds great otherwise, but I’m wondering if I should be worried about it. Is this temperature related? Thanks.

Yes, it is temperature related. The simple fix is to replace R21 (18k that goes into pin 2 of IC4) by a 15k.

Easy enough. I’ll give that a go. Thanks.