[sdiy] Signals leaking into the PSU?

[René Schmitz]

HI A.M.,

On 20.02.2023 18:10, A.M. Barrio via Synth-diy wrote:
>
>   * To David Huss, René Schmitz, Lukas Hazen-Bushbaker: thanks a lot
>     for your thorough explanation on why this is happening. At least I
>     can say I now understand what's going on. David's bucket analogy
>     in particular made it super easy for me to grasp the idea. In the
>     process, though, you also gave me a feeling that these matters
>     might be a little too complicated for me to manage haha! I'm no
>     engineer nor do I have as much knowledge on electronics as I'd
>     like to... I actually do have some, I work as a technician
>     repairing stuff, but the reasons behind my problem and the
>     solutions to it caught me off guard. I was never taught too much
>     on noise issues back when I was studying, I suppose it is explored
>     more deeply in actual EE degrees. As a result I've never been
>     aware of it: there are no 100uF caps anywhere on the entire synth
>     case, my bus board is the crappiest, most DIY thing you can think
>     of and my way-to-go has always been /slap in those 100nF caps
>     between rails and go test it!/ So much for me to learn on this topic.
>
The big caps 100u belong in one place, in your PSU.


>   * To René Schmitz, Roman Sowa: I must note that the schematic I
>     attached on the first email is not the LFO, but the clock, that's
>     my bad again. That's why there's only one LED. However I just
>     noticed that I didn't rectify the signal before plugging it to
>     U1C, so the negative cycles are indeed going nowhere. I will
>     change that in the clock circuit. On the LFO circuit I do have two
>     LEDs per wave, one for positive cycles and one for negative
>     cycles. Took notes about high efficiency LEDs as well, right now
>     I'm using a plain common red LED on the clock and six green (!)
>     LEDs on the LFO, since it has three wave types. I'm still
>     concerned about the LEDs however... a total of 7 LEDs might take
>     around 140mA, would that be correct? Even if I were to correctly
>     bypass all my circuits and properly isolate everything, would the
>     LEDs still suppose an issue in terms of sag/wiggle for a PSU that
>     can supply much more? (all of the PSUs I tried with can supply
>     around 1A max).
>
If you rectify the clock the LED will be always on.

Btw, you can also run regular LEDs at reduced current. 2mA is usually 
sufficient to see them in indirect daylight. No need to drive them to 30mA.


>   * To Mike Beauchamp, Chris McDowell, Roman Sowa: The VCO design
>     comes from Moritz Klein's one. I'm attaching a schematic of the
>     exact circuit I'm implementing. After reading all the responses it
>     is quite obvious to me that it is VERY prone to changes in output
>     frequency: the CV pots are tied directly to the rails, as well as
>     the CD40106 IC in charge of generating the oscillation. Roman, I
>     am taking absolutely no offense from your comment: it is indeed a
>     bad design. I am not willing to improve /this/ particular VCO
>     design as it was just a test and a bad example of me being too
>     excited to have my first panel mounted on my case. Please ignore
>     the wasteful use of the 40106 and the crappy expo converter, much
>     more care will be put into the next VCO I put together. About
>     this: /If you need help deciding which ones those are, post the
>     VCO schematics./ Would you mind pointing them out? I mentioned a
>     couple spots above, but I'd like to make sure I cover them all.
>     That being said, I'd love to hear about the reference voltages.
>     Chris, thanks for the IC examples, I'll check them out. Besides, I
>     have some spare 7805 regulators at home. Although they are quite
>     big, would they do the job for testing before I go ahead and grab
>     some of the examples you mentioned?
>
I guess I'm partly to blame, or could take offense, but won't. :) Moritz 
Klein "borrowed" the expo and summing stage of my VCO4069. And this had 
the same problems.

But hey, I did that over 20 years ago. It really only is a problem when 
you can't know what else you share the power rails with. (you'll see 
this a lot with older synth designs)

You'd be right with powering the 40106 from a well regulated voltage and 
the pots and the expo reference current would need to be made 
independent of the supply rail.



Best,

  René


--
synth at schmitzbits.de
http://schmitzbits.de
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