I HATE GUITARS

Funny enough, in the last post I forgot to describe an important component. When shaping audio signals, it’s vital to be able to actually amplify sound. This can be achieved with transistors. In a schematic, they look like this:

You might recognize these little fuckers from their boring role in computers, where they are used as switches between 0 or 1, off or on. But in audio they make volume go up, and quite linearly so, as I understand it. There is battery-level voltage applied at the collector (C). The audio signal enters the base (B). The voltage from C gets modified by the B voltage and escapes the emitter at its new amplitude (E). When you apply 9 volts at C, while your audio signal just oscillates in the area of millivolts, the resulting wave is of way higher amplitude, but still has the shape of that original audio signal.

Here you can see the input stage of a TubeScreamer, with Q1 being the transistor. Guitar signal goes in on the left; 9V is injected into the collector from the top; stuff goes out at the bottom and rushes towards OUT.

I left the realm of clumsy cable constructions and did my first circuits on a breadboard. Meanwhile I fried 2 red LEDs. I think the first one died because of a somehow missing resistor. The second one because I forgot that you measure current in series and created a bridge around a resistor with my multimeter 🤷‍♀️ – Anyway, here you see some LEDs. Current flows each time I push the button. The green one signifies the capacitor being loaded (it’s in series before it). The red one (yeah, I bought some new ones) is lit by the energy saved by the capacitor each time the switch closes the second circuit. Unfortunately, I was dumb enough to push the button roughly in rhythm with the music. Now everything happens too fast and it’s not as instructing as it could be.

I also built some simple sound circuits, where voltage moves a piezo ‘speaker’. This was so unimpressive that I didn’t even take a picture.

Apart from that I tried to read some pedal circuits before sleeping. I had a small revelation when I stumbled upon the clipping stage of an MXR Distortion +, which is about as simple as it gets:

I couldn’t quite understand it. My thoughts were somewhat along the lines: “This is weird. Those diodes are placed somewhere where there’s not a lot of resistance, but all the beautiful power goes down the drain [earth]! How does it even get there, doesn’t it just wanna flow towards the exit, when RV-OUTPUT is wide open and there’s no resistance there at all?!” WRONG. Turns out, I thought too linearly. Kind of: Energy goes in at IN and out at OUT or maybe, if it wants, elsewhere. Rather like programming, input-output, it’s that simple. But in electronics I guess you have to think a lot more relational. What happens in the diode-equipped part that goes to ground, influences nonetheless what happens at OUT. You can’t see it as absolute.

For certain waves, on the upper or lower cycle of the AC, D1 and D2 are permeable and without resistance. That means, that part of the signal then enters this area and goes to ground, while the rest moves towards OUT and forms our new signal. We don’t force current through the diodes (as is the case in other circuits), but we steal some parts of waves and trash them, which shapes the remaining waves via difference. Likewise with the capacitor. As we learned in the last post, capacitors let higher frequencies pass along easier than bass frequencies. This way higher frequencies rather take the path into the ground. Thereby extreme treble harmonics in the now-distorted waveform get shaved off.

Another realization was the constant confrontation with the fact, that we always have two different circuits in a pedal. For one, we have the AC from the guitar pickup, at mV scale. Then we have the 9V circuit from the pedal (battery or power supply). That’s why a lot of circuits have some measures of blocking DC (via capacitors) or why you bias the current of a transistor by e.g. 4.5 volts: To give it enough operating space to operate with its DC on the audio wave’s AC. Both circuits only touch each other in active components, like aforementioned transistors. Both use the same ground.

I hope I can show you my first pedal circuit next time. I will build a very simple fuzz circuit. I’ve already got the parts. Stay tuned.

I’m gonna paint my mental picture of the matter so far. I’m a total beginner, so feel free to mail me if I got something wrong.

When you hit the string of an electric guitar metal vibrates over a magnetic pickup. The magnet acts as an inductor and the electrons in the guitar’s wiring start to move. This wave, current, moves towards the cable and makes its way through the pedals. Later it hits the amplifier and is converted into mechanical energy by the speaker. The result meets our ears. As I understand it, the electrons themselves don’t bother to move far, just across neighbor atoms. It’s the collective small steps that create a big wave, and for once the result is quite wholesome. The potential to make those waves is called voltage.

Circuits

When electricity happens, there’s a circuit. This is the case with the guitar and the pedals as well. – “What, a circuit, that sounded pretty much like a line or something!” – Well, like us humans, electricity has a tendency to return to earth. This is called ground, or earth. An electric guitar seems to be grounded through the cable. Pedal circuits on the other hand are grounded by their connection to the negative pole of the battery or the negative pole of the power adaptor. This ground has to exist for electricity to occur.

The sounds coming out of the guitar consist of alternating current (AC) – the string moves left and right, after all – but guitar pedals are powered by direct current (DC). This means we inject DC into the pedal circuit to modify an AC signal – make the sound louder, distort it, whatever, basically manipulate the ‘sound wave’. As I understand it, earth acts as a kind of zero calibration for this whole circuit. As the voltage which drives the current is difference, you need something to differ from. The positive pole of the battery does differ by 9V from ground, or something like that.

All the tasks pedal circuits perform deal with manipulating those differences. The following paragraphs summarize my understanding of the most important components for audio signal manipulation.

Resistors

Current likes to take the easiest path. If there’s resistance, current says: I would prefer not to. You can use resistors to scare current away. Move on, nasty current! You can direct or split voltage this way. Only a little resistance, some current might actually take that path and voltage is divided accordingly. Crazy resistance, no way.

A famous example for a resistor in an audio circuit would be a volume potentiometer. The potentiometer does some wiping when you rotate it and a resistor changes its resistance in the process. If there’s shitloads of resistance, the current is not in the mood to move on towards the output of the circuit. Where does it go then? I don’t know yet. Maybe into the ground? Or maybe it gets converted into heat, like in a normal resistor? – But if the potentiometer is turned up all the way and there’s little resistance, current is cool with moving on to the next pedal.

Diodes

Franzi tried to sell me the idea that diodes are kind of a one way street. Imagine you build a complicated network of different voltages with resistors in all sorts of places. Some current might be tempted to move backwards. We don’t want that. A diode seems to allow current to flow only one way.

Now, in pedals, diodes are usually utilized for clipping == distorting a signal. It seems you can take advantage of the fact that diodes only become restrictive when a certain threshold is passed. Remember, our guitar signal is AC, but we’re in a DC circuit in our pedal. We feed the guitar signal into a diode which via its direction likes to block positive voltage. This only happens after a certain threshold is passed, so a portion of the positive voltage gets through, the rest gets clipped. This sounds fucked up, as in pleasurably fucked up. Here’s an illustration:

For ‘traditional’ Overdrives we don’t want to clip only one half of the waveform. Here’s the clipping circuit of a TubeScreamer. There are 2 parallel wired diodes (D1 & D2) to clip the positive as well as the negative voltage cycles of the AC, respectively.

The diode challenge from Franzi’s starter kit somehow didn’t work, it involved rotating the motor to load a capacitor (see below) by hand and prevent current from flowing back with the diode. But I completed the experiments with the other kind of diodes in the kit, LEDs. I can confirm they really don’t work if installed the wrong way round. Diodes have a resistance of their own, so it seems, and while regular diodes resist via converting current to heat, LEDs create light, as you might know.

Capacitors

Capacitors have the ability to store electrical energy for a while. If you feed them, they charge. If you connect them to another circuit, they release their storage into this circuit. If you put a resistor behind them, it regulates the speed of the discharge. Higher resistance means: Saved current isn’t that eager to travel on and likes to take baby steps. Lower resistance means: Current is flowing towards the exit like a crazy person. Gonna show you a video where I charge a capacitor through a circuit with a green LED and then discharge it into a circuit containing a red LED. Franzi says, if you choose a larger resistor the light would be less bright but shine on longer. I like my lights bright.

While resistors are used to control the amplitude of the signal and diodes are used to manipulate the shape of the signal, capacitors mess with the frequency of the signal. While it might be obvious that they seem to fuck around with the time domain somehow, the exact ongoings were rather not intuitive for me. Big thanks to Flo for giving me a good explanation.

Charging a capacitor means hitting it with positive voltage. Discharging means hitting it with negative voltage. AC (our signal) has positive and negative voltage cycles. So if some current with negative voltage hits the capacitor this means the capacitor gets discharged immediately. As I understand it, the current just continues to do its thing, whereas a ton of DC actually breaks the circuit because the capacitor sucks up all it can and then has no way to release it again. As Flo phrased it, capacitors simply break the circuit for DC while they let AC pass somehow.

Now you have to remember that low notes have way larger cycles in the signal (they’re called low frequencies after all), so they don’t change as often from positive to negative as high notes (== high frequencies) might. This means, a capacitor has higher resistance to low frequencies, letting higher frequencies pass easier, thereby filtering the tone 🤯. At least in principle, exact workings seem to be dependent on the properties of the respective capacitor.

New Items From Franzi

As I’m almost done with the starter kit, I bought two more kits from Franzis. The first one is an electronics kit coming with a breadboard. I expect it to be the sequel to my starter kit. The second one is for learning how to solder.

I was pleased to learn that the stuff you solder components together with is literally called “Das Lot” in German. So I recorded a little song called “Das Lot” to glorify solder. I’d like to join JHS Pedals’ tradition of short demo snippets and this shall be the first entry.

Edits

03/27/2022: Differentiate voltage & current

Amidst my recent pedal buying spree I shortly woke up from the comforts of a newly gained habit and contemplated. Where to go next? What’s left to discover, now that I played the goddamn intro to My Iron Lung the way it’s meant to sound? So I began to explore the depths of my soul the way reasonable people do: I started a 3-day YouTube binge.

I came to observe: A crapload of people are building their own guitar pedals. And they pretty much sound like the real thing, if that’s what you’re after. I thought: I might want to build one myself, you know, going for depth instead of breadth. Plus, it would be nice to create stuff with my own hands, instead of just buying something from The Internet. Then I watched a lot of videos like this one, where people solder themselves a pedal in 5 minutes and the like. But soon I realized I must have wiped every memory of physics lessons from my brain.

After watching some theory videos on electronics I could not really connect the dots. I decided to get into this stuff for real. So I bought a starter kit:

The first experiment consisted of pushing the contacts of a resistor (the stripey things, and boy, are they small) against the poles of the battery casing. Resistor was supposed to get warm, or even hot, as a warning said. Which makes sense, as resistors resist by transforming power into heat. I sure didn’t overdo it and called it a pass when I had the slightest sensation of warmth.

The second experiment involved a motor. Now I had to close the circuit with some provided cables, none of which is yellow or green, btw, Das Franzis Lernpaket box designers. Worked as expected, the motor did stuff. Then they instructed me to reverse the flow (red cable on minus, black cable on plus), so the motor would rotate the other way round. To make direction assessment more fun, I mounted a… kind of rotor onto the motor. This is my first creation as an electrical engineer. I call it The Helicopter. It even flew a bit. Actually wants to fly so hard, I had to shoot the video a few times because it took off.

Anyway, maybe I’m onto something here. Tomorrow, an LED will be lit up. Diodes! You can distort shit with diodes. This guy did. This person teaches us how to modify that fuzz to make it sound worse. Maybe you could make it sound better, who knows, seems like you really can do anything.