# Understanding circuit diagrams

Where is the 5V going from the 7805 regulator in this guitar amplifier schematic?

This comes across to me as a dead end part of the circuit.

I know the grounds are all linked but I am still struggling to work out what the purpose of this voltage regulator is and where the 5V is elsewhere in the circuit.

• "...where the 5V is elsewhere in the circuit." You didn't show the whole circuit! If you're going to update it, please crop the superfluous white part on the top and black part on the bottom. – Huisman Apr 30 '20 at 9:52
• Please don't post blurred images that are illegible. – Andy aka Apr 30 '20 at 10:39
• Apologies, hopefully this is more clearer. – Andy03436 Apr 30 '20 at 10:56

Look at R1 it has a value of $$\\color{red}{\text{330/1W}}\$$

Compare with R39 - it has a value of $$\\color{red}{\text{[100/5W]*}}\$$ - why the square brackets and asterisk you might ask?

In fact all the component values associated with the 5 volt rail are enclosed in square brackets followed by an asterisk.

In my book this means "not fitted".

There's probably just a +5 annotation missing in that schematic. (compare the +15 and -15 above).

Also, wild guess, this is an old amplifier circuit that you got from a service manual. Especially if this is a guitar amp: There's been a few schematics that have intentionally altered by the manufacturer to still be useful for servicing the device, but would make it at least harder for someone not employing competent engineers to produce a clone of the device. Fun fact: that seems stop quite a few counterfeiters of things that are already produced with cost optimization in mind.

• Hi Marcus, yeah that makes sense, it is a guitar amp schematic. Thanks for your answer! – Andy03436 Apr 30 '20 at 10:38

It seems to me as if the +5V rail is not used at all. All the opamps are powered by either +/- 15V or +/-24V. Maybe the 7805 is just not populated: if you have a closer look, the regulator, all its capacitors, D8/9, R39 and J12 are enclosed by square brackets. Another possibility is that we don't see the complete schematic.

The 5V reg and it’s associated components are not loaded - it’s a stuff option. Probably, the designer had that part laid out and late in the process, then deleted the section that required 5V.

When they did that, the PCB or schematic DRC tool probably flagged the unconnected 5V, so they deleted that from the schematic too.

Or it’s just simply an oversight that doesn’t matter.