I used a standard 9V DC barrel jack of the kind seen on many effect pedals – in this case the positive tends to be on the outer sleeve, and ground on the centre pin, Boss-style. The pots, by the way, should be log (or audio) taper.įinally, the output is decoupled so that the signal has no DC offset and moves around 0V. In practice, with these values I found unity gain around 80% of the way around the input level pots, so there’s a little bit of boost available to help balance levels if you need it. The 200k resistor, in tandem with the 100k values at the input, would suggest a gain of 2 (200k/100k = 2) but the real value is somewhat lower. This does several things, not the least of which is to set the gain of the inputs. In parallel with the 1M resistor discussed before, we add another resistor and capacitor across the NPN’s base/collector (R4 & C4).
Note that the capacitor and resistor in series on each input acts as a low-cut filter to reduce sub-audio content. Three input resistors mix the signals into the transistor base. The inputs are brought in via potentiometers and decoupled using small capacitors.
The risks are lower headroom and some distortion. I should stress here that this design is absolutely a compromise for the sake of simplicity. If you find your transistors all giving collector voltages nearer to 4.5V or even lower, and sourcing alternative devices is not an option, try decreasing the 4k7 resistor value – for example, if your Q1 Hfe is nearer 300, a 3k3 resistor will suit better. Hfe is not a precise value for any device but a ballpark of 200 will suit nicely. The 2N3904 is also a good choice, and easy to find. I picked a BC108 with Hfe of around 220, which was my starting point for the other component values in the circuit. The aim is to get a voltage here a little over half supply, but not too much higher. Connect power and measure the voltage at the collector. Transistor SelectionĪ simple way to choose a transistor is to build the test circuit shown here, using just Q1, R5, and R6. The gain (Hfe) of Q1 together with these other values sets the collector voltage around which point the signals are mixed. I won’t go into much detail about all aspects of the design process here, but the core is the transistor Q1, the resistor R6 from collector to 9V, and the resistor R5 from base to collector. I had no trouble mixing three audio test signals. Given that we’re running this from 9V, it’s only really suited to relatively low audio signals, but we still have enough headroom for a small number of mixed inputs. In order for the output to be able to swing up and down (audio signals are AC, don’t forget), the collector needs to sit somewhere a little above half way up the supply when nothing is happening. Signals are presented to the base, and the output is taken from the collector. The circuit is a ‘common emitter’ type (a basic description can be found here). This is a very simple single-transistor design that uses a generic NPN device.
The multiple is entirely optional – it’s purely passive and is just 4 jacks tied together, but it’s a useful addition and you could fit both this and the mixer into the same enclosure. The mixer circuit uses a single transistor and runs from 9V DC, so you can power it from the same supply you use for your pedals. So I designed and built one! It’s a very simple device consisting of a passive multiple and a 3-into-1 audio mixer with input level pots and a single output. Recently I wanted to use two different effect pedals in parallel, but didn’t have anything handy that would easily allow me to split and then re-combine the signals.