A question recently popped up – how to go about if I wanted to use PNP in place of a NPN transistor. In theory – it should be a simple enough task. After all, the circuits should be near equivalent. “Near”, just before “equivalent” is the key term in that sentence.
How do we go about it? Let’s see on a simple effect: let’s modify LPB-1 to use a PNP transistor.
Why LPB-1? It’s very simple to breadboard and try out, and this is probably the best circuit for quickly trying out things like this.
If you prefer just seeing the video – go straight to it.
The schematic here is slightly adapted so it is easier to see/explain the differences. Here are the side by side NPN and PNP version of the effect:
Spot the Differences
You need a keen eye to spot the differences because the layout is practically the same. Here are the differences highlighted:
In the above diagram – it is clear that differences are tiny indeed. NPN transistor: 2N3904 in this case, is replaced by a corresponding PNP transistor – 2N3906. Also – the power rails for the amplifier part are swapped.
This is important because lots of times it is said that circuit can be adapted from NPN to PNP transistor by simply reversing battery polarity. In reality, it is slightly more complicated than this.
You only swap polarity for the amplifier – and that’s what I highlighted above. If you’d swap the polarity completely, jacks would be connected to 9V instead to GND and that might not be the healthiest option for subsequent pedals/amp.
Here’s an example of breadboarded circuits:
I organized it so the changes are minimal and clear. Looking at the diagram above, the differences are exactly what I previously highlighted in the schematic. Here’s the close-up:
Replacing the transistor (pinouts are the same) and swapping two wires is all it takes.
Trying it Out
I did a very quick video covering this:
Bonus: Modifying Fuzz Face
In one of the previous builds I did a Silicon Fuzz Face clone. It uses NPN transistors. How do we go about changing this to use PNP transistors (just for fun)?
Here’s a quick diagram showing this:
It is a bit more complex and the diagram is not as well suited for demonstrating this, I’ll do a follow-up with better diagram. But in case you want to try it out, here are the steps:
- Highlighted in green should go to negative pole of the battery (ground)
- Highlighted in blue should go to positive battery pole (+9)
- Highlighted in yellow(ish) – swap NPN transistors with PNP transistors of your choice (minding the pinout)
- Highlighted in red – “rotate” the electrolytic cap so positive end is connected to +9 and negative end to the middle pin of the potentiometer.
Since the circuit is more complex – there are more touch points, but the principle is the same as with LPB-1. One important note is that in here – we do have a component that has a polarity – C2 electrolytic capacitor. It has to be correctly wired up. Because we are reversing polarity – we also need to reverse how the cap is connected too.
If we don’t wire up electrolytic capacitors correctly – they might be damaged or destroyed (and in extreme cases they could explode or catch fire). C2 has to be reversed, but C1 should be left as is – because polarity for it did not change.