[sdiy] Signals leaking into the PSU?

Sun Feb 19 19:50:40 CET 2023

Hello and Welcome to synth diy!

What you have here is a non-steady power consumer paired with a "soft" 
power supply.
Your devices see a variation in voltage due to internal resistances: 
Composed of the finite output resistance of your PSU and the wiring and 
connectors.
As the current varies, there is a voltage drop at this parasitic 
resistance. This changes the effective supply voltage and/or ground 
potential seen by each individual module.

If the consumption of your LFO were independent of the signal then no 
modulation would occur. (Well a fixed one, but you won't notice that...)

The advice to add bypassing is not bad per se. But it is not a panacea.

At LFO frequencies even 10uF does next to nothing.
When working into 10ohms the corner frequency is 1.59kHz.
It might remove clicks due to the higher frequency content but will 
still move your VCO up and down a little.

I'd suggest to also look at the other factor of the root cause, and that 
is the varying current flow:

First, I'd be looking at temporary disconnecting the LED from your LFO. 
Maybe that makes a difference already?
To me this is the most promising candidate, how the LFO signal could 
affect the PSU (likely both GND and +12).
You run several mA between +12 and GND there.
If confirmed, go for a high efficiency LED, and raise the 330 to 
something much larger.
Depends on the LED of course. But less current will result into less 
modulation of your PSU.

Second: Why not use the triangle signal for indication instead. That 
makes for less abrupt signal to begin with.
And hopefully this is then less noticeable. I've been there, and had 
several situations where the square wave created problems with LEDs, due 
to the higher dV/dT.

Third, your Opamp U1C might not like it that it is run open loop for 
half of the cycle. A 1N4148 in reverse (or a second LED) will easily fix 
that.
This will then create a more continuous current flow, which is less 
noticeable.

And finally:
Also look into your VCO design, it seems it has sensitivity to the power 
supply voltage. I would look into ways to reduce that. (Voltage 
references for pots etc.)

Best,
  René

On 19.02.2023 17:21, 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 and 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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