Strange Power-Supply(?) Problem with 4-Pole Shruthi (solved)


i just finished building the 4-Pole kit.
Everything seemed to go well and all worked at the first test.

BUT - the Display backlight and LEDs are Flickering at very high speed (low audio range i guess) and there is a kind of filter modulation at the same speed. i hear it without resonance already, and it becomes a lot more distinct with increasing resonance.
sounds like a kind of spacey filter fm at low audible range. a bit like the polivoks with high reso and filter fm=on

the menu and everything are working fine though. i have not checked midi, or audio in, or tuned anything yet.

is this what happens, if the filter caps in ther PSU have bad solder joints and 60hz from the Power line, bleed through???

i have no other explanation.
i will take a break now, because i have been soldering for a few hours now and want to be fresh, when figuring this out.

anybody had some problem like this already?
i can post samples later, if that helps.

cheers Paul

Check for shorts on the connections between the digital and filter board?

i tried power up with battery and the problem is gone.

so i think i was right about the 60hz power-line bleed through.

any ideas, how i can figute out, whats wrong?

Is your PSU AC or DC? only the Shruthi-1 has no AC/DC rectifier.

it is AC DC as it is supposed to be.

you helped a lot anyways:
i tryed another one and all works like it should.
than tryed the bad one with polivoks shruthi and it maked the same problem.

this means, it is time for that vivil after all :slight_smile:

still strange, because i know this PSU has worked with my other shruthis and other gear.
i guess the rectifier broke or something.

It’s supposed to be DC not AC. There is no bridge rectifier in the Shruthi-1.

There’s a regulator which controls the voltage depending on current draw and there’s an inverter chip which converts +5v into -5v. But there’s nothing in there to convert AC to DC, that requires 4 diodes or a bridge rectifier (which is 4 diodes in a smaller package).

DC is what i ment. sorry. i confuse those names all the time.

still only explanation is the Wallwart is not working right any more.
it was a cheap crappy one anyways.

i used it for bending
not bent the wallwart itself off course - i don`t want to kill myself - but stuff that was powered with it.
maybe i had a short between Vcc and GND at some point and it drew to much current and burned something out in it.

Well, the capacitors can start to fail, then you get ripples in the output.

Check how much current your wallwart delivers. If it does not deliver enough current it’ll start “motorboating”, exactly all the symptoms you describe - I got this with fancy crystalfontz LCDs.

White backlight LCD module (esp. with low current limiting resistor) + white LEDs = more current.

the one that gives me that problem it is a “voltcraft” switching power supply
it says primary: 330mA ;secoundary: 1200mA (secoundary is the one that matters here, right?)
i have it set to 9V.

as i sayd, it makes the same problem with my polivoks shruthi, which doesn`t have that white leds. both are the limited edition kit ones and use the displays that came with it. so it should not need that much current i guess.

I’ve seen PSUs rated 300mA that were starting to fail at 150mA.

In German tests 1 out of 3 wallwart PSUs didn’t meet standards. You can’t always trust what is written on them, be it ratings or safety markings.

Hi folks!

These unregulated wall wart PSUs can be most confusing, so here’s a bit of info that may help. The output voltage of any unregulated PSU will only be at the nominal value when at the rated load in mA. This is what I measured with a couple of PSUs:


a) +3 volt setting
5.5 volts with the Shruthi-1 plugged in - amazingly this just about worked, although the output levels were low and a bit warbly, and the LCD display didn’t light up.

b) +4.5 volt setting
7.2 with the Shruthi-1 plugged in - LCD display worked fine. Audio OK but motor-boating (warbly rumble in background) still noticeable.

c) +6 volt setting
9.0 with the Shruthi-1 plugged in - perfect audio performance (ideal voltage for the Shruthi-1, and the setting I now use permanently)

d) +7.5 volt setting
10.7 with the Shruthi-1 plugged in - still fine, but Shruthi-1 7805/7905 regulator chips run slightly hotter than my +6 setting.

e) +9 volt setting
12.5 with the Shruthi-1 plugged in - even hotter regulators.

I then tried another wallwart:

10.8 volts with Shruthi-1 plugged in. Ran perfectly, but its 7805/7905 regulator chips run slightly hotter than they need to

So, if you use the recommended +9 volt @300mA wallwart you should end up on or around +9 volts with the Shruthi-1 plugged in. If you use a higher rated 9-volt PSU your Shruthi-1 will still work perfectly but run hotter. If you use a wimpy one then the voltage will drop below 9 volts and your Shruthi will start to warble. If like me you have several to try, or one with a switchable output, just choose the one that gives 9 volts with your Shruthi-1 plugged in :wink:

Hope this helps!


FWIW, I use a Godlyke Powerall with the red adapter cables. I have been able to run as many as 4 Shruthis at the same time on the same PSU with no warbling, no audio, and no display issues.

Now that’s just showing off! :wink:

Seriously, the Godlyke is a regulated PSU whose output should remain at exactly 9 volts whatever the load. It supposedly has the lowest noise floor of any switched mode PSU on the market, but other switched mode PSUs have been troublesome for adding digital noise to audio gear.

My advice is for the poor folk (like me) with unregulated PSUs.


Noise Wise theres nothing a good Cap couldn’t smooth away. Personally i use 470µF or even Bigger Caps instead of 220µF from the BOM…

Sorry to show off… :wink:

I already had a couple for running long guitar fx chains. I was glad they worked for this and that they came with the correct barrel tip adapters.