Voltage divider for ground

[GibsonGM]

The bias voltage (aka 'reference voltage'...Vr)  is applied TO the incoming AC signal; they mix (superposition).  That signal then rides on a DC level (4.5V)....it's elevated 4.5V ABOVE zero. This is so that we can amplify both the positive AND negative portions of the signal...otherwise, the opamp (or other active device) would clip off that negative half of the wave, like a rectifier.   We're making the zero-crossing AC signal able to be worked on, by lifting it up.  We 'put it back down' at the end of the process by blocking the DC with an output capacitor (or coupling cap, depending on what we're doing to the signal).

It doesn't change the ground reference a bit. The opamp is working using DC - the ground is still zero volts, to it.   It's easier to get this if you look at something like Dist +, which uses input and output caps - that BLOCK the DC voltage. Either side of them is an AC-only signal.  Inside their 'loop', you have the AC signal riding on that DC level.    Vr replaces the AC's original zero-crossing point, but that's not ground...some call it "AC ground", but that's separate from DC ground (lol).   Vr is NOT connected to GROUND...look at Dist +....it has a cap that does go to ground, but DC does NOT pass a capacitor (that cap filters off noise - a different topic).  There is still a resistor between Vr and ground in the bias network....   

[GibsonGM]

Here's some awesome artwork for you.  Top, how the AC looks coming from your guitar.  Sort of.
Bottom, jacked up by 4.5V so it doesn't cross zero anymore, and a DC-driven opamp can work with it.
Sorry it sucks, I can't draw with a mouse...

[antonis]

Sorry it sucks, I can't draw with a mouse...

And you dare to call yourself "professional painter", Sir..??

[augggh]

Looking closely at the Distortion+ is helping a lot, thanks  The drawing is helpful, I think what I struggle with is understanding is the ending, how pull-down capacitors and resistors actually succeed in their task. If it makes you feel any better, I submitted a drawing of similar quality made on a trackpad for part of a music theory final last semester 

[Dormammu]

You obviously didn't read or understand exactly what I wrote.

Someone has an irresistible urge to overcomplicate simple things.
Occam's razor is hopelessly dull ?   

[antonis]

I think it makes sense, from "ground" point of view ONLY..!!
(circuit needs mods like input cap, caps in series with 1k feedback resistors, 100Ohm (R?) value, etc..)

[GibsonGM]

There are no pulldown caps   Pulldown resistors just reference something to ground rather than an arbitrary floaty thing like "10 mV".    Don't worry about that just yet...I suggest getting your mind around what happens when we input an AC signal into an opamp....how the opamp does its job.    Looking at the Dist+ makes it much more clear (from my POV anyway).     

The R's and C at the end of the Dist+ schematic (power supply) just give the 4.5V needed to jack up the input so we can work with it.     It's separate from the actual 'power supply'.  No conflict with a power supply jack being used.

You can see input and output caps, that 'keep the DC inside this section' for the opamp to use.  That is a next step after the basic ground questions

Antonis, my lines become much more straight when my arm is amplified!  There is insufficient bandwidth when using a mouse!! 

[Rob Strand]

Sorry for the late reply! Here's the original design and here's an edit I made where I have a "second ground" using a 100uf capacitor to ground. I know that with a battery there's no issue, but I'm worried that if it were to be applied in the context of multiple pedals with one power supply it could cause a short circuit.


I think it makes sense, from "ground" point of view ONLY..!!
(circuit needs mods like input cap, caps in series with 1k feedback resistors, 100Ohm (R?) value, etc..)

The original circuit works, and you might see that configuration in battery powered stuff.  However, when you start doing the same thing with effects pedals on a power board local ground will short to -V soon as you connect more than one pedal to the same power supply.

The more common config like antonis' circuit doesn't have that problem - but you need input output caps (and the most of the ground connections should go back to Vref (the orange dots)).

[GibsonGM]

^  This is what I'm nudging him towards, LOL.  First let's sort out "ground"   

[niektb]

It ís possible to 'fake' a bipolar supply from a uni-polar DC input but you need some kind of amplifier to drive this ground. For example like this: (this image shows a battery but a DC adapter works also from experience)

An opamp can get pretty hot though as it'll need to sink all the current you circuit consumes