The different types of diodes

There is a very common marketing point that we see all the time with "boutique" guitar pedals: the famous vintage ultra-rare licorn-made type of diodes! Germanium diodes are often associated with a vintage and warm tone, whereas silicium diode sound harsher and fuzzier...

Time for a blog post about it!

What is a diode? How do they work? Which one should I use when making guitar effects?

What exactly is a diode?

In order to understand the differences between the types of diodes, let's see how a diode work. Do not worry, it is not that complex!

A diode is a semiconductor. Unlike a resistor, they are polarized with an anode and cathode that is represented on their symbol:

As its name implies, a semiconductor does not conduct current in a normal way. I will try to explain it as simply as possible. If you are an electronics-aficionado, the following text is rated R for you and will probably shock you!

A diode is composed of two parts:

• A part poor in electrons (P junction, like "Positive conduction")
• A part rich in electrons (N junction like "Negative conduction")

Practically speaking, there is a part where the electrons feel good (the P-junction), and a part where it is difficult for them to go through (N junction, there are already too many electrons in there)

At first, electrons do not manage to go through the part rich in electrons and accumulate in the P-junction. When there are enough electrons accumulated, they have enough "power" to cross the N-junction: current is flowing throught the diode!

This phenomenon causes what is called the forward voltage (also called voltage drop): bellow a certain voltage threshold, the current cannot go through the diode. The voltage drop is around 0.7V for a classic silicium diode for instance: below 0.7V, the current will not pass!

If we trace a current vs voltage graph for this, we will have something like this:

Below 0.7V, current does not go through the diode. Above it: it passes!
The voltage drop also diminishes the voltage out the diode. If you make 9V goes through a silicium diode, you will have a 8.3V voltage after the diode!

Finally, another very important thing with diodes is that the current can only go through them in one direction! There is often a really small current that can go the other way, named current leakage. If the current applied is too big, the diode breaks passed what is called the breakdown voltage.

That makes a graph that look like this:

You can find this type of graph in any diode datasheet. You can now read it like a pro! 😃

Depending on the type of diode, the forward voltage / voltage drop changes ! 

Finally, last but not least, there is of course a maximum forward voltage current, which is the maximum amount of current a diode can handle. In guitar pedals we do not have this problem because currents are somehow quite low in our little stompboxes, but it can be very important when making amplifiers or high-current-draw electronics.

The different types of diodes

There are lots of type of diodes! Here are the most common ones that you will find in guitar effects.

Germanium diodes

Here are the famous ones! They are used in vintage effects, but also in modern ones for their special characteristics. They are easy to recognize with their glass capsule style. Their voltage drop is low, around 0.35V, but it can change depending on the model.

Here is a D9E germanium diode, used in the famous Klon Centaur:

They are mainly used to distort the signal, but can also be used to correct temperature bias of vintage germanium transistors. Read my circuit analysis of the Tonebender MkIII, there is a germanium diode for that.

Ex: 1n34A, D9E...

Small silicium signal diodes

They are smaller than their germanium counterparts. However, their forward voltage is higher at 0.7V. They are the most used in overdrives and distorsions to generate saturation.

The Big Muff has four of them in its circuit for instance !

Ex: 1n4148, 1n914...

Silicium power diodes

These bulky diodes are a big bigger than signal diodes. Their breakdow voltage is also way higher: they can handle bigger voltages in their wrong polarity. Same voltage drop though, around 0.7V.

They are mainly used to prevent shorts and polarity inversions.

Ex: 1n4001

LED

Often forgotten, they are still diodes and can be used as such!

Their voltage drop is quite high and depend on the LED color :

Color	Voltage drop
Red	1.7V
Orange	2.0V
Yellow/	2.1V
Green	2.2V
Blue	3.2V

Ex: red LED, green LED...etc

Schottky Diode

These diodes, named after M. Schotty, a German physicist, have the lowest forward voltage among diodes : around 0.15V.

Therefore, they are very useful to prevent polarity inversion, because the voltage drop is very limited compared to the 0.7V drop of silicium power diodes.

Molecular diodes

For DIY crazy fools like me! In 2017, the demi-god manufacturer Dr Scientist sold a series of overdrives name Heisenberg using molecular diodes. You can see these diodes here on SIM cards:

What is a molecular diode ? Instead of letting the P-N junction "empty", there is nanometric layer of organic molecules that is added here. That changes the physical properties of diodes. For instance, current can go both ways!

These diodes are still very hard to find in classic retail. They are more a laboratory experiment type of component, but has Dr Scientist is absolutely awesome, he collaborated with a laboratory and made an effect! There is even a published scientific paper.

How do I choose the diodes?

With guitar effects, what is interesting to us is mainly the voltage drop.  Indeed, many distorsions and overdrives work with diodes clipping the signal when it is above or below the forward voltage of diodes, which create saturation!

Depending on the diode choosen and their forward voltage, the saturation will be different:

That will modulate the saturation of the effect! This phenomenom is often called "diode clipping", which you surely heard before. Beware: the more you clip, the more your signal will be compressed and dynamic will be lost. So it is also a matter of choosing the righ clipping amount!

Germanium diode clip at 0.35V more the signal and generate compressed, fat distorsions. Oppositely, a LED with its 1.7V forward voltage will have a more open sound. I use LED on my Dolmen Fuzz to make it sound lighter, almost like a classic drive.

Here is a table to sum it up:

Type	Voltage drop	Distorsion
Schottky	0.15V	Too much, very compressed
Germanium	0.35V	Big, compressed distorsion
Silicium	0.6V	Classic overdrive to distorsion sounds
LED/td>	1.6V	Open and dynamic saturation

But that is not it! You can also combine diodes! Two diodes in series will clip the signal less for instance:

It is also possible to make what is called "assymetrical clipping".  Just use two diode on one side and one on the other. You can also mix the types of diodes.... Here is a new world of experimentations for you!

 Beware! Of course, all of this depend on how the diodes are disposed in the schematic. It works most of the times, but it sometimes can be different. Test it!

Good new is: it is very easy to test diodes with a DPDT switch! Wire it on the diodes pads like I did on my Pharaoh clone:

Here, there is an assymetrical clipping with germanium diodes and a classic clipping using two silicium diodes. But you can try any combination you like !

It is one of the easiest mod to do on overdrive and distortion effects, and very useful to understand how diodes work. So grab a soldering iron and try it!

Protect your circuit against polarity inversions

Diodes can also be used to protect your circuit against polarity inversions... Lets imagine for a minute that a customer of yours invert the battery in your made-with-great-care-and-patience effect.

What can go wrong if I switch the power supply polarity?

If there is no protection diode, there are good chances that a component of your circuit gets damages (especially polarized capacitors...). Or worse, that they heat too much, catch fire or even explode!

To avoid this armagaedon apocalyptical scenario, always add a polarity protection diode. If the polarity of the power supply is inverted, it will not let current pass.

Generally, we use a diode able to resist to a lot of current like a silicium power diode. A Schottky diode can also be a good candidate because they have a very low voltage drop. However they have a bit of current leakage, so check that it is not too much for your capacitors. (should not be in 99% of cases)

It is a very simple circuit that you should always add to your circuits.




There you go! You know all the basics about diodes! Lets experiment now!
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To go further

• Diode basics on SparkFun
• Just Nick video about molecular diodes
• You can buy molecular diodes here! They are expensive though...