Schematic contains 4.5v and 9v ?

[ReeceAblaze]

Hello I was looking at a proco Rat schematic and it has a stage which is unconnected from the rest and contains two different Voltages.

My questions are, Why is this piece of the circuit disconnected and how does it connect?

Also why are there two voltages there?


Thanks

[Kevin Mitchell]

Voltage references are often labeled instead of drawn out to make a schematic drawing more clean and easier to follow. Just as the +9v and ground is referenced - you know they share the same connection.

If you observe the schematic you'll find that the source of the +4.5v comes from a series of 100k resistors between power and ground. Being identical values this effective halfs the supply voltage for it's own mid-point reference. You'll see this in many designs.

[ReeceAblaze]

Thanks I see what you mean. It makes much more sense now.

[aion]

Electrosmash uses the exact voltages in their schematics for clarity, but most other schematics will opt for simplicity by using another label for the reference voltage - usually "VB" or "VR", sometimes "1/2V" - while the main supply is called "+9V" or "VA". But yes, just showing that these all connect to a common point elsewhere in the circuit.

[ReeceAblaze]

Is the half voltage used because of the opamp? Can they only take a max of that voltage or something?

[Kevin Mitchell]

No. It's to balance the headroom. In a way it's emulating a bipolar supply.

Here's one explanation;

AC signals are called "alternating" because they move back and forth, or rather up and down, about a reference point.  Bear in mind that what transistors do is to essentially "permit" current to flow from the power supply so to produce an output that goes above and below some middle point, there has to actually BE a "middle point".  That can be provided by means of a bipolar power supply, OR by using what starts out as a single-ended supply and "faking" a middle point by means of a Vref.  In this use, 0v fulfils the role of the negative supply.

Note that while dividing the supply voltage in half to derive a fake mid-ground provides the most balanced opportunity for headroom (i.e., neither side of the waveform has more "room to move" than the other), that is not always the ideal solution.  In some instances, people will opt for a reference voltage that is essentially asymmetrical.  E.g., a Vref of 4v will mean that the positive half-cycle has more headroom than the negative halfcycle.

[ReeceAblaze]

That is a really good explanation of it actually. Thanks!

[Kevin Mitchell]

That is a really good explanation of it actually. Thanks!

No problem! There's loads of resources here. Just about anything you can think of has been asked and answered at least once before Learning to read schematics is a load of fun. You'll be able to throw just about anything together once you get into it.

Get yourself some breadboards and plug away 

[ReeceAblaze]

I have a breadboard and have so far only built a silicon fuzz face on it . Don't have the money for a bunch of parts to mess with yet. I do want to take a base schematic that I enjoy and try and make it my own but I am still figuring out how at the moment. Its the maths behind all this that hurts my brain, the actual schematic reading and prototyping is fun.

[bluebunny]

Here's a visualisation that may help:



You're skipping in your kitchen.  You "bias" the rope roughly half-way between the ceiling ("9V") and the floor ("ground").  That way you have enough headroom - up and down - to spin the rope.  If you hold the rope a bit high, it hits the ceiling and is "clipped".  Or hold it way down low and it will sound "blatty".

[antonis]

Its the maths behind all this that hurts my brain,

I presume a glass of beer hurts more brain cells than Ohm's Law.. 
(same stands for wine and Kirchhoff's Laws..)

[ReeceAblaze]

Its the maths behind all this that hurts my brain,

I presume a glass of beer hurts more brain cells than Ohm's Law.. 
(same stands for wine and Kirchhoff's Laws..)

What? I don't have a clue what you are on about.

[Kevin Mitchell]

What? I don't have a clue what you are on about.

Pay no mind to them 

[ReeceAblaze]

um ok lol.

Is it worth just experimenting with parts and seeing what sound (if any) eventually comes out?

[Kevin Mitchell]

Well that's up to you.

You could experiment with your breadboarded fuzz circuit and see what changing the cap and resistor values does for you. Who knows. It could be the next big thing. "New" fuzz face pedals come out all of the time. Really it's just minor tweaks to the artist's preference, throw their name on it and the crowd goes wild.

[ReeceAblaze]

I will mess around with some values and see what happens I guess.

[Kevin Mitchell]

I will mess around with some values and see what happens I guess.

Said every electronics visionary