VB & VC ???

[Jean-Rock]

I'm looking at the Green bean project schematic and notice that there's VB and there's also VC.

Anyone can tell me what deos it means please ??

Thanx ! 

[kothoma]

Look at the top of the schematic: VC is about +9V and VB is half that, i.e. about +4.5V.
All points with VC resp. VB are connected, of course.

[billstein]

Look at the top of the schematic: VC is about +9V and VB is half that, i.e. about +4.5V.
All points with VC resp. VB are connected, of course.

I hope this makes sense. Would that top part that is separated be the power section of the schematic? The electrolytic caps I assume are the filtering. Could you connect the VB and VC to anyplace in the rest of the schematic that corresponds? 

[rullywowr]

Yes, the top part is the power section. VB is used with the opamp as differential power and vc is used for the transistor. At the top of C11 is vc origin and top of C12 is VB.   For the circuit to work properly,  all vc are connected together as all VC.  It's nicer to have labels on a schematic than it is to have to actually wire them in the diagram.

[Jean-Rock]

Thanx guys. Very helpful.

And the VC means voltage of collectors (I feel stupid to ask)

[billstein]

Yes, the top part is the power section. VB is used with the opamp as differential power and vc is used for the transistor. At the top of C11 is vc origin and top of C12 is VB.   For the circuit to work properly,  all vc are connected together as all VC.  It's nicer to have labels on a schematic than it is to have to actually wire them in the diagram.

Could you explain what you mean by "differential power?"

[rullywowr]

Op amps are a type of differential amplifier.  From wikipedia: "A differential amplifier is a type of electronic amplifier that amplifies the difference between two voltages but does not amplify the particular voltages."

Read all about differential amps here:
http://en.wikipedia.org/wiki/Differential_amplifier

Therefore, the amp needs two discrete voltages (with a difference between them) to control it.  These voltages in this schematic are labeled VB and VC.  You can see in the power section that VC is pretty much +9v.  The +9v source comes in, hits that 47k resistor (for a slight drop), is conditioned and stored by some capacitors, reverse polarity protection is applied, and then you have VC.  Pin#8 on many common dual opamps (4558, TL072, etc) usually takes this voltage as it does here.  VC can also be called V+ or VR or Vcc+ and so forth...depending on the schematic but it all means the same.  Pin #4 is usually ground but can also sometimes be tied to a lower voltage.

A simple resistor network make a voltage divider http://en.wikipedia.org/wiki/Voltage_divider and divides the +9v into about half (or +4.5v).  When the two resistors are equal in value and one goes to ground, you get about half the voltage at the point between them based on Ohm's law.  These two resistors are the two 10k (R18 and R19) in the schematic. This 4.5v is then used with the opamps in various ways to effectively make them work.  Sometimes this voltage is referred to as V- or Vcc- or other labels. 

This is kind of a macro level overview, hope it helps.

[billstein]

Thank you SO MUCH for taking the time to explain that. I'll definitely pour over this to learn.

[kothoma]

Look at it this way:
It you power your op amp with 0V at pin 4 and 9 V at pin 8 then you need to get the audio signal into this voltage range.
Say your audio swings around 0V from -1V to 1V.
If you inject (=add) VB with 4.5V on it, then it swings around 4.5V (3.5V...5.5V).
That's called biasing and I suspect that's where that B in VB comes from.
This DC component is filtered out again at several points with 1uF capacitors.

If you'd power your op amp with +/-4.5V, then your audio signal could stay around 0V and no decoupling caps would be needed.

[Jean-Rock]

This is kind of a macro level overview, hope it helps.

Of course it helps !!! Thank you very much for taking the time to explain !

[rullywowr]

Really happy to share what I have learned the hard way. Just the other day I was troubleshooting a circuit I layed out and was wondering why I didn't get any sound.  Measuring pin 8 on the IC I got almost zero volts. Thinking back to how op amps need power (+9v on pin , I simply placed a jumper from 9v to pin 8 and.... Bam! The effect now worked!  I forgot to hook up V+. 

This is one of the reasons it is very helpful to know and share pin voltages when you are troubleshooting.  I would recommend starting with a breadboard to experiment and if you are so inclined, try to learn Eagle CAD (thanks to the  great tutorials by Jkokura in the Eagle section).

It's one thing to populate a board, but I find you learn so much more when you physically are making the connections (breadboard or drawing in Eagle even).   After a while it becomes simpler. 

[Jean-Rock]

Yeah, I've started learning Eagle couple of weeks ago ! I really like it (better challenge than computer games  )
I'll probably share my first personnal project next week, I'm building the document for DIY community.

It's just a very simple JFET booster, but it's very versatile and it sounds amazing.

I've you seen my proto rig in build report, I really like prototyping on a breadboard.....probably even more than build the unit itself !!

See ya later and thanx again !

[RobA]

Just to clarify one thing about the differential aspect of the op amp. The differential inputs are the + and - signal inputs of the op amp (pins 3 & 4 and pins 5 & 6). If you look at the spec sheet for pretty much any op amp (like this one for the 4558 http://www.mouser.com/ds/2/294/NJM4558_NJM4559_E-218630.pdf it'll have a diagram of the circuit that makes up the op amp. In the diagram (schematic really) you can see the input of the op amp made from a differential pair transistor amplifier. It also shows how the power rails are involved. From there you can see that you have to bias the inputs of the op amp for exactly the same reason you have to bias the base of a BJT transistor. That matches with the explanation that kothoma gave above for how you have to set the input to the amplifier so that the signal can swing completely in the voltage range of the power rails for the op amp.