I mentioned in the circuit introduction that nearly every single component of the circuit can be modified for tweaking the sound. The key is knowing what each tweak does. I’ll just briefly try out several different components and see what we get.
I suppose you need to see the video for the result, but here’s what I’m going to try out: different transistors, using different resistors (or trim pots) in couple of places, adding some capacitors to soften the distortion a bit.
Here’s the experimentation schematic (I removed switch for clarity):
The number of possible combinations is really huge, some do very little, some make a big difference. I’ll try to do it as simple as possible. I’ll try out different transistors first, on the base schematic, end then will go back to BC109C and try out various tweaks.
It is a bit different for germanium version of the pedal due to vast variability in transistor parameters. But when it comes to silicon, nearly any transistor will work. Albeit, they will work with various levels of pleasantness of the sound. I’ll try out the following transistors:
Arguably, the pinout is the most important piece of information from the datasheet 😀. Pinouts might be different for different transistors even if they have the same packaging – so just a bit of care needs to be taken there. If you get them wrong way around, nothing will happen – the effect will not work but the transistor won’t let magic smoke out (at least not the ones above anyway).
Here’s how the pinout looks for some of them (great that at Farnell’s multicomp pro datasheet they reversed numbering, pinout is the same as in On Semi’s one)
I included gain for the transistors and noise figures. Datasheets for BC549C and BC237B state that they are low-noise and great for audio amplification. Others are general purpose amplifiers.
BC109C, BC108C and BC549C are high gain devices, with similar gain range. Their DC current gain is slightly different, and other parameters are also slightly different so they might sound different.
BC237B and 2N3904 have less gain, the latter with the lowest gain range from the bunch. We’ll see how that affects the sound.
Trim Pots and Resistor Alternatives
If you look at the diagram, I’m going to try out different values for R1, R3 and R4.
I have some options for R1 – choosing different resistor values. Higher value gives me some more headroom and higher output, but it also should soften the distortion somewhat.
I said trim pots in the heading, but I’m going to use “normal” pots. They are just way easier to use for breadboarding, but the effect is the same. When you want the pedal to be tweakable when finished, using a trim pot is one way to do it. It does not have a knob but you can use a screwdriver to adjust the sound to your liking before completing the pedal.
I’ve put a normal pot instead of R4. I chose 10K pot. Lower the value – lower the gain of the 2nd transistor and softer distortion. Funnily enough, lower gain causes output level to be higher.
Finally – I can use a pot in place of R3. I chose 100K pot. Lower the value – more feedback and less gain of the first transistor stage. This also impacts input impedance and also lowers the gain for lower frequencies.
If I add a capacitor C4 between collector and base of the 2nd transistor, this will lower the gain for the higher frequencies. This might make distortion sound less harsh. Higher the value, more higher frequencies are cut-off.
I could change other capacitors – changing any of the C1-C3 and using different value caps might give some different audible response but I’ll limit myself to just the above.
Further Notes for Experimentation
There are more possible options to try out. And not just that, any combination of the values is possible. You could try nearly impossible number of combinations – if you find this sort of a thing enjoyable I suppose.
Trying it out
OK, breadboarding, making sure everything works, then trying out all sorts of different combinations. I’m going to prepare two versions of the effect side by side – one with default values and the other with tweakable options. Let’s see how I fared in practice: