# When reading a schematic, how do you envision an audio signal vs. DC power?

#### Dolmetscher007

Joined Mar 21, 2019
36
I have posted on these forums recently about wanting to build a 100W vacuum tube guitar amplifier. I was sufficiently cautioned to not even attempt anything like that before I have learned a lot more about electronics. So... I have moved on (down) to a guitar effects pedal. I bought the PCB online, and I ordered all the components from Digikey. I have a breadboard, and I am ready to breadboard this thing, see if it works, and then populate the PCB.

And I think I could do all that successfully. However, it doesn't really teach me all that much about how a distortion pedal works. Here is the schematic for the pedal I am building. I sat down this morning to look at it and try to explain (to myself) how the signal flows through the circuit. I could not even get past the "input". LOL!!!

If you look at the schematic, in the lower left-hand corner, you see the word "IN". It appears to me that there are two "INs" since I see two circles with a line. No idea what that's about. (?). But... That "IN" represents a standard mono 1/4" instrument-cable jack that is coming from the output jack of an electric guitar. So the tiny 0.13 mA, 1.2 mV (peak-to-peak) guitar signal enters the circuit... I don't exactly know what happens. It looks like the signal splits in half and goes in two directions, half through a 1 MOhm resistor (R1), and the other half through a a 33KOhm resistor (R2). Then the signal coming from R2 enters a capacitor, while the signal leaving R1... well... I really don't know what happens there, as it looks like it goes down a line splitting off several times to go through more resistors.

I know that I must be missing some fundamental principal here. For example, the input jack... it appears to have two circle/line icons. Is that because a standard mono instrument jack has a positive and a ground? One of those circles = ground? I have also heard it said that guitar jacks have a positive and a neutral. Is "neutral" and "ground" the same thing?

#### AlbertHall

Joined Jun 4, 2014
12,199
For example, the input jack... it appears to have two circle/line icons. Is that because a standard mono instrument jack has a positive and a ground? One of those circles = ground? I have also heard it said that guitar jacks have a positive and a neutral. Is "neutral" and "ground" the same thing?
Yep. You always need two wires to pass a signal. What goes out must come back again. Like a battery has two terminals and if only one of them is connected nothing is going to happen.

On your circuit it is drawn rather unhelpfully. I don't know why there are two circles shown on the input (and output) wire.
The input would be connected between that input connection and the ground wire that runs across the bottom of the diagram. Thee 33k feeds the input into the first stage of the circuit while the 1M resistor is connected across the input connections (between the input and ground).

#### Brevor

Joined Apr 9, 2011
297
Thats a schematic for a Big Muff guitar pedal clone. The 1 M resistor is there to bleed off any DC on the input so the pedal doesnt make a "pop" sound when it is switched into the circuit. The extra circle on the input and output may be there because a DPDT switch is usually put there to bypass the pedal when you dont want the guitar signal to go through the pedal. The circuit is basically just 4 transistor amplifiers placed inline to distort the signal. If you search online for "Big Muff guitar pedal circuit discription" you will probably find a detailed
discription of how it works.

#### MisterBill2

Joined Jan 23, 2018
14,185
To see a whole lot of circuits of all kinds of guitar amplifiers go to the "Schematic for free" website.
There is no reason that you could not build a high powered tube type amplifier, but you do need to learn quite a bit about understanding the symbols first. And the whole thing is perfectly safe until you plug it in and switch on the power. At that point there would be some rather high voltages inside, enough to give deadly shocks. So at that point you need more knowledge of what is safe to touch and what is not safe to touch. I do advise you to get more educated so that you understand what is going on, and what all of the symbols mean.