Shruthi 4pm volume is low unless I blow air into the filter board

I assembled my shruthi 4pm quite a while ago, but I took it out to make some music recently and I notice that the volume was quite low. This lead me to turn the volume on my amplifier/soundcard all the way to get a decent volume level, but also brought up a lot of noise since the amp is maxed.

I didn’t recall it being like that when I first assembled it, so I thought maybe it had some dust or something in it, I don’t know why, but I blew some air into the case and the volume level fixed itself completely and became normal. Great, except over the course of about a minute, the volume goes from hitting 0dbfs in Ableton Live’s meters like it should, to about -12dbfs again.

This is repeatable, every time I blow on to the filter board (with my breath) it fixes itself for about 1 minute and then gradually becomes -12dbfs quieter. I can hold a note down on my keyboard and hear it do this. It doesn’t matter what patch it is, it’s not any type of modulation or anything like that.

I have disassembled it again, took out all the ICs, redid the tests I did when I first built it, all passing with voltages very close to +5 and -5. Blowing on it or not does not effect these voltages, they are always -5 and +5 like they should be.

Some random observations. The 7805 and 7905 heatsinks are hot to the touch, but this is normal right? Is it me cooling off something by blowing on it that fixes it? No other part of the filterboard is hot. I am powering it with a 12V wall wart I borrowed from my kaosspad3, but I remember actually checking that the voltage was within tolerance when I built it.

Heh, I can’t really use it and record stuff if I need to keep blowing into it :stuck_out_tongue: Can anyone help me figure out what is wrong with it?

Thanks :slight_smile:

The 7805 getting hot is normal as you power it with +12V and it has to dissipate all the energy down to 5V which is pretty much - suggestion would be using a 7,5V supply.
The 7905 getting hot is not so normal as it only has to power some OpAmps which draw next to no Power. My tip would be looking for a short from the negative Rail to GND and/or a bad soldering joint (which can be sensitive to heat, also). Im surprised the LT1054 delivers enough current to heat the 7905 :wink:
The test without chips and digital board are a bit pointless here as you say it is dependent on load/heat…

Well, perhaps the 7905 is not hot, I was trying not to burn my fingers on it, and I assumed they were both hot, one of them is certainly hot.

I am going to try a smaller power supply as soon as I can find one.

I don’t know for sure it is dependant on heat, that was just a guess. Thanks for your reply.

Someone had a weird “resonance gain goes away” problem, and it came from the use of a dangerously high input voltage. I have stop recommending 12V for more than 2 years. 7.5V or 9V is the way to go…

I got a 7.5V AC Adapter and it has fixed the problem, it totally was getting too hot. Thanks for the help you guys.

I will add that I have had some more issues with my resonance self-oscillation dropping out recently.
After reading more than one thread like this, I switched the power supply down to 7.5 volts. However, there were notable changes in the timbre of the resonance. The pitch changed, and the intensity wasn’t as impressive, but it didn’t cut out. A lot of my older patches that used resonance just didn’t sound right. I switched to a HOSA universal power supply set to 9 volts and that seems to work without dropping the resonance now. It also has the same response as before.
I did not expect this as the power supply I have been using worked great for a long time. I would recommend a HOSA as a replacement. Assuming that the higher mA (1200mA vs 300mA) will not affect the Shruthi. I don’t think it will.

No, it won’t. But now you can power up to 4 Shruthis with this power supply and a 1:4 distribution adaptor…

My thoughts exactly. Or the Shruthi and some FX pedals. :slight_smile:

My Shruthi used to drop volume when resonance was turned up … just noticed this and dropped my multi ac adapter down to 7.5v from 9v … wow what a difference it sounds even better now :slight_smile: