[sdiy] Signals leaking into the PSU?

[Roman Sowa]

It's not LFO's fault but VCOs.
No filtering will help here, not even that famous $99 powerbus.

Aparently your VCO's frequency is dependent of power rail voltage, and 
that is, without insulting anyone, a bad design.
Take all vaulnerable parts of your VCO out from supply rail, and tie 
them to well regulated voltage reference.
If you need help deciding which ones those are, post the VCO schematics.

BTW, throw away that old LED requiring 330 ohms to light up, and buy new 
one which shines up brightly at 0.5mA. 30mA square curent jumps in power 
rails are troublemakers.

Roman

W dniu 2023-02-19 o 17:21, A.M. Barrio via Synth-diy pisze:
> This is the complete email I wanted to send. There's another thread 
> where the attached files and additional info is missing. My bad, pressed 
> CTRL + Enter by mistake whoops. Please ignore that one! pata at ieee.org 
> <mailto:pata at ieee.org> and mbryant at futurehorizons.com 
> <mailto:mbryant at futurehorizons.com>, thank you for your replies, I have 
> taken them into account :-)
> 
> ------------------------------------
> 
> Hello,
> I'm having an issue where the frequency of my VCO is being affected by 
> the operation of other modules. I'll give a broader explanation:
> 
> I'm a beginner on synth DIY, right now I have my home made case, a PSU, 
> two VCOs, an LFO, a clock module and an amplifier/speaker (built in the 
> case, not externally). Everything has been arranged by me, I haven't 
> purchased any module yet. The clock and the LFO are the latest modules I 
> have built (pretty much both at the same time), and when I tested them 
> on their own, they worked just fine. However I noticed that when they 
> are powered, the frequency of the VCO would stutter following the 
> operation of the clock and the LFO.
> 
> That would be: *without connecting the clock or the LFO anywhere*, just 
> having them powered up, whenever the clock is up or down, or the waves 
> generated by the LFO reset, the frequency of the VCO varies a tiny bit. 
> It gets more noticeable the higher the pitch of the VCO.
> 
> After some head scratching I've realised that the signals generated by 
> any of the modules I have (both VCOs, the clock and the LFO) are somehow 
> leaking into the +12 and -12 rails of my PSU. Measuring any of the rails 
> on my scope (on AC mode, so only the noise is shown), I can see that the 
> waves generated by the modules I mentioned above are there as well in 
> the form of noise (around 20mVpp each of them). That explains the little 
> variations in frequency of the VCO. However I have no idea why this is 
> happening or how to fix it. I have tried with different PSUs I have around:
> 
>   * 11V 0.750A SMPS into DCDC to get +12 and -12
>   * Two 12V 1A SMPS together to get +12 and -12
>   * Linear PSU with 7812 and 7912
> 
> I have the same problemwith all of them. I'm positive there's an issue 
> somewhere with a lack of filtering, but I don't know where, or why.
> 
> I'm attaching a sample of the VCO output where the stutter can be heard 
> and the schematic of the LFO. The way the LFO is set up in terms of caps 
> and isolation can be extrapolated to every other circuit I have made, I 
> always place those 100nF caps between each rail and ground.
> 
> 
> LFO (square).jpg 
> <https://drive.google.com/file/d/1ts1WGcyl2y1Ypp5eP79Ptb5fHXawetZh/view?usp=drive_web>
> 
> VCO stutter.mp3 
> <https://drive.google.com/file/d/1RrUNX_Of_RGLAwn2CyV7I5fxRrlHvfHL/view?usp=drive_web>
> 
> 
> In case you have any idea what could be wrong, your reply will be 
> greatly appreciated. Thanks beforehand!
> 
> Kind regards,
> 
> A.M. Barrio.
> 
> ------------------------------------
> 
> In response to pata and Mike Bryant:
> 
>   * Yes, the current PSU I'm using can provide 8W in total. I'm only
>     powering one VCO and one LFO with some LEDs, power shouldn't be the
>     issue in this case.
>   * I always place 100nF caps between each rail and GND on every
>     circuit/board. However I hadn't heard of placing caps on each IC. I
>     suppose it should be between their power pins and GND? I will have a
>     look at capacitance multipliers as well as I haven't heard about
>     them before.
> 
> 
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