[sdiy] Re: LVM, or not

[harrybissell]

Gene Stopp wrote:

> I remember being a member of a group of production line repair techs in an
> all-digital company, and then a new product came along with some op-amps
> (around a modem chipset). They could not figure out how a device could have
> a complex output with ground at both inputs!

That is REALLY funny. As a new (ly employed) tech... I worked on a repair that
had
NO signal at the base of an input transistor.  An hour later it dawned on me
(after
careful consideration) that I was looking at a discrete OPAMP with negative
feedback....

H^) harry

>
>
> - Gene
>
> -----Original Message-----
> From: owner-synth-diy at dropmix.xs4all.nl
> [mailto:owner-synth-diy at dropmix.xs4all.nl]On Behalf Of Tim Parkhurst
> Sent: Tuesday, November 16, 2004 3:49 PM
> To: 'john mahoney'; Roman
> Cc: synth-diy at dropmix.xs4all.nl
> Subject: RE: [sdiy] Re: LVM, or not
>
> > -----Original Message-----
> > From: john mahoney [mailto:jmahoney at gate.net]
> > Sent: Tuesday, November 16, 2004 2:18 PM
> > To: Roman
> > Cc: synth-diy at dropmix.xs4all.nl
> > Subject: Re: [sdiy] Re: LVM, or not
> >
> > > sure I'm with you! I just thought 5V modular wasn't so popular.
> >
> > Cool! My comment was kind of stupid, anyway. :-/
> >
> > There seems to be some interest. I'm just collecting people's thoughts
> > on it, looking into where the problems might be (e.g., transistor
> > ladder filter), etc. Hopefully I (we) can start designing LVM modules
> > in Spring 2005. I've got plenty going on for the next few/several
> > months, though.
> >
> >
> Just in case anyone's interested: I think I've figured out how the LVM CV
> summer should work. This may seem simple to some (and it is, once I had
> thought it through), but I had to sit and think about this for a minute (any
> gurus out there are more than welcome to critique or correct my assumptions
> here). This is kind of lengthy, so be warned.
>
> FIRST: My understanding of basic op-amp behavior...
> Basically, an op amp summer works by keeping the output equal to the SUM OF
> the inverting and non-inverting inputs. If we ground the non-inverting
> input, apply a voltage through a resistor to the inverting input, and (this
> is important) take the output and feed that back through another resistor to
> the inverting input, THEN the output will adjust as necessary to keep the
> voltage at the inverting input AT THE SAME LEVEL AS THE NON-INVERTING INPUT.
> If, as in my example here, we've stuck the non-inverting input to ground,
> then the op amp will try to keep the voltage at the inverting input at 0
> volts. It's not ground in the strict sense, but it acts just like it, hence
> the name 'virtual ground.'
>
> Am I correct on this so far?
>
> So if we run 1.5V into the inverting input, the output will go to -1.5V in
> an attempt to keep the voltage at the inverting input at 0V (remember we're
> summing the 1.5V input and the -1.5V feedback through resistors to the
> inverting input). We can add more input resistors and sum more voltages to
> the inverting input. The output will still adjust as needed to keep the SUM
> OF THE VOLTAGES at the inverting input at 0V (until it runs into the supply
> rails, of course).
>
> Right?
>
> SO, FOR A LVM INPUT SUMMER: We want the modulation based around 2.5V. A
> signal less than 2.5V will produce negative modulation (lower frequency,
> lower gain, etc), and a signal greater than 2.5V will produce positive
> modulation. THEREFORE, all we need to do is change the non-inverting input
> on our op amp summer and put it at 2.5V instead of ground! Again, this will
> seem dirt stupid basic to many of the experts out there, but I was rather
> pleased with myself for having FINALLY figured this out! Now what we get is
> that any input (to our summing node at the inverting input) LESS THAN 2.5V
> produces a positive voltage at the output. If we put 1.5V at the input,
> we'll get +1V at the output (1.5V + 1V = 2.5V which means our the inverting
> and non-inverting inputs are equal again). By the same token, any voltage
> greater than 2.5V produces a negative voltage at the output. Now we have a
> CV response centered around 2.5V instead of 0V. At this point, we either
> apply this to an expo converter with an inverted response (our summer
> inverted things, remember), or just stuff it through another inverting
> op-amp and THEN to the expo converter.
>
> SO, BASICALLY: Changing the 'center point' of our CV modulation from 0V to
> 2.5V is relatively simple. Just change the 'virtual ground' point, and
> you're off and running.
>
> Well, if you've read this far, thanks! Like I say, this is really a basic
> concept, but I wanted to explain it 'out loud' so that 1)I can get corrected
> if I'm wrong and 2)I can share my explanation and hopefully help others
> understand some basic electronics.
>
> Okay, and now... back to work!!!
>
> Tim (out loud!) Servo