[sdiy] Signals leaking into the PSU?

[Tom Bugs]

Yup, this in particular --- precision voltage references make a world of 
difference. LM4040 for example.

On 19/02/2023 19:09, Mike Beauchamp wrote:
> Hi A.M. what VCO are you using?
>
> If the VCO is a DIY design, consider modifying the design so that it 
> isn't dependent on the PSU rails. Any part that is tied directly to 
> the rail should be substituted for a proper reference voltage.. for 
> example, trip points of a schmitt trigger or comparator, and anything 
> else with voltages affecting the pitch. Any tuning potentiometer 
> controlling pitch should have it's voltage taken from your reference 
> voltage sources, not the rails, etc.
>
>
>
> Mike
>
>
>
>
>
>
> On 2/19/23 11:21, A.M. Barrio via Synth-diy wrote:
>> 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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