[sdiy] Free SSM2044, SSM2056 chips for pickup and one question about single supply VCAs

René Schmitz synth at schmitzbits.de
Thu Mar 7 14:15:20 CET 2024


There is a solution to this: A PNP common base circuit. That does 
translates a current into (near) GND into one that runs to -12V.

I've used this numerous times with OTAs (and discrete long tail pairs), 
no problems with the actual value of the negative supply or noise, 
because the collector of the PNP is producing a current.

But I guess the designer didn't know this, or didn't want to use it.


Best,

  René



Am 07.03.2024 um 02:11 schrieb Mike Bryant:
> Another way of thinking about it is to assume that all power lines are 
> poorly regulated, so just choose one of them as your reference and 
> stick to it for everything.
> ------------------------------------------------------------------------
> *From:* Synth-diy <synth-diy-bounces at synth-diy.org> on behalf of 
> brianw <brianw at audiobanshee.com>
> *Sent:* 07 March 2024 00:46
> *To:* synth-diy at synth-diy.org <synth-diy at synth-diy.org>
> *Subject:* Re: [sdiy] Free SSM2044, SSM2056 chips for pickup and one 
> question about single supply VCAs
> Just thinking out loud here...
>
> If you want or need to use -12V as a reference, then you'll have to 
> consider a couple of things. First, I assume that many -12V supplies 
> are poorly regulated. If that's the case, then you probably can't get 
> too far. Second, if the -12V supply is well-regulated, then you might 
> be able to use it as a reference if you run a low-impedance trace all 
> the way back to the supply input. So long as all of the high-current 
> -12V current takes a different path, most of the supply noise will be 
> on different traces.
>
> Then again, there's the big debate between power planes versus star 
> ground and star power distribution techniques...
>
> Brian
>
>
> On Mar 6, 2024, at 4:36 PM, Mike Bryant wrote:
> > Agree.  And just once in my career of over 50 years I didn't check 
> the ground to a minor socket wasn't free from noise.
> >
> > Never again.
> >
> >
> > From: brian
> > Sent: 07 March 2024 00:25
> >
> > Good point, but a ground reference isn't guaranteed to be a perfect 
> solution. Not every "ground" reference is free from noise, because if 
> enough current flows through that "ground" reference and if there's 
> enough resistance along the path then the Voltage will rise above 
> ground. If the current is both both substantial and has a noise 
> component, then the error on the "ground" reference will show up as noise.
> >
> > This basically means that you have to pay attention to layout so 
> that the ground references aren't a problem.
> >
> > Granted, many of the parts will only pull power from +12V and -12V, 
> but if your circuit induces enough current through the "ground" then 
> there can be noise.
> >
> > The converse of this is that you can create a relatively noise-free 
> -12V if you take the same amount of care, although the current will 
> probably be higher. I understand that +12V almost always uses more 
> current than -12V, depending upon the circuit.
> >
> > Brian
> >
> > p.s. Thanks for the link, René. I'll check that out. Such articles 
> are always appreciated.
> >
> >
> > On Mar 6, 2024, at 6:31 AM, Mike Bryant wrote:
> > > And if you translate the control voltage to the -12V line you'll 
> probably inject PSU noise as well. Ground referencing is always best 
> for control voltages if you can manage it.
> > >
> > >> The reason is that the Iabc (the control current) is created with a
> > >> resistor from a voltage referenced to GND.
> > >>
> > >> The OTAs sinks this current into the negative supply terminal.
> > >>
> > >> If you'd run them from +-12V then you need extra circuitry to 
> translate
> > >> the voltage.
> > >>
> > >> (If you want to read a bit more on OTAs there is an article at my
> > >> homepage, link below.)
> > >>
> > >>
> > >> Best,
> > >>
> > >>   René
>
>
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-- 
--
synth at schmitzbits.de
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