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<p>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. <br>
</p>
<p>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.
<br>
</p>
<p>But I guess the designer didn't know this, or didn't want to use
it. <br>
</p>
<p><br>
</p>
<p>Best,</p>
<p> René<br>
</p>
<p><br>
</p>
<p><br>
</p>
<div class="moz-cite-prefix">Am 07.03.2024 um 02:11 schrieb Mike
Bryant:<br>
</div>
<blockquote type="cite"
cite="mid:CWLP265MB71323B3E28A82DA1B9A93862DB202@CWLP265MB7132.GBRP265.PROD.OUTLOOK.COM">
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class="elementToProof">
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.<br>
</div>
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<div id="divRplyFwdMsg" dir="ltr"><font style="font-size:11pt"
face="Calibri, sans-serif" color="#000000"><b>From:</b>
Synth-diy <a class="moz-txt-link-rfc2396E" href="mailto:synth-diy-bounces@synth-diy.org"><synth-diy-bounces@synth-diy.org></a> on behalf of
brianw <a class="moz-txt-link-rfc2396E" href="mailto:brianw@audiobanshee.com"><brianw@audiobanshee.com></a><br>
<b>Sent:</b> 07 March 2024 00:46<br>
<b>To:</b> <a class="moz-txt-link-abbreviated" href="mailto:synth-diy@synth-diy.org">synth-diy@synth-diy.org</a>
<a class="moz-txt-link-rfc2396E" href="mailto:synth-diy@synth-diy.org"><synth-diy@synth-diy.org></a><br>
<b>Subject:</b> Re: [sdiy] Free SSM2044, SSM2056 chips for
pickup and one question about single supply VCAs</font>
<div> </div>
</div>
<div class="BodyFragment"><font size="2"><span
style="font-size:11pt;">
<div class="PlainText">Just thinking out loud here...<br>
<br>
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.<br>
<br>
Then again, there's the big debate between power planes
versus star ground and star power distribution
techniques...<br>
<br>
Brian<br>
<br>
<br>
On Mar 6, 2024, at 4:36 PM, Mike Bryant wrote:<br>
> 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.
<br>
> <br>
> Never again. <br>
> <br>
> <br>
> From: brian<br>
> Sent: 07 March 2024 00:25<br>
> <br>
> 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.<br>
> <br>
> This basically means that you have to pay attention
to layout so that the ground references aren't a problem.<br>
> <br>
> 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.<br>
> <br>
> 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.<br>
> <br>
> Brian<br>
> <br>
> p.s. Thanks for the link, René. I'll check that out.
Such articles are always appreciated.<br>
> <br>
> <br>
> On Mar 6, 2024, at 6:31 AM, Mike Bryant wrote:<br>
> > 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.<br>
> > <br>
> >> The reason is that the Iabc (the control
current) is created with a<br>
> >> resistor from a voltage referenced to GND.<br>
> >> <br>
> >> The OTAs sinks this current into the
negative supply terminal.<br>
> >> <br>
> >> If you'd run them from +-12V then you need
extra circuitry to translate<br>
> >> the voltage.<br>
> >> <br>
> >> (If you want to read a bit more on OTAs
there is an article at my<br>
> >> homepage, link below.)<br>
> >> <br>
> >> <br>
> >> Best,<br>
> >> <br>
> >> René<br>
<br>
<br>
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