Synth DIY: Stompbox Mixer
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. 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.
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. It draws only a few milliamps. 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 Circuit
Here’s the schematic. It can also be downloaded as a PDF here.
How it Works
This is a very simple single-transistor design that uses a generic NPN device. The circuit is a ‘common emitter’ type (a basic description can be found here). Signals are presented to the base, and the output is taken from the collector. 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. 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. I had no trouble mixing three audio test signals.
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. The gain (Hfe) of Q1 together with these other values sets the collector voltage around which point the signals are mixed. The first job is to pick a transistor.
Transistor Selection
A simple way to choose a transistor is to build the test circuit shown here, using just Q1, R5, and R6. Connect power and measure the voltage at the collector. The aim is to get a voltage here a little over half supply, but not too much higher. Something in the region of 5V is fine. I picked a BC108 with Hfe of around 220, which was my starting point for the other component values in the circuit. The 2N3904 is also a good choice, and easy to find. Hfe is not a precise value for any device but a ballpark of 200 will suit nicely.
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.
I should stress here that this design is absolutely a compromise for the sake of simplicity. ‘Close enough’ is fine. The risks are lower headroom and some distortion.
The Rest of the Mixer Circuit
The inputs are brought in via potentiometers and decoupled using small capacitors. Three input resistors mix the signals into the transistor base. Note that the capacitor and resistor in series on each input acts as a low-cut filter to reduce sub-audio content.
In parallel with the 1M resistor discussed before, we add another resistor and capacitor across the NPN’s base/collector (R4 & C4). This does several things, not the least of which is to set the gain of the inputs. 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. 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 pots, by the way, should be log (or audio) taper.
Finally, the output is decoupled so that the signal has no DC offset and moves around 0V.
Powering the Circuit
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. A diode across the input protects against reverse connection, the capacitor helps smooth the incoming supply, and a resistor/LED draws a few milliamps to indicate ‘power on’.
The Passive Multiple
This is simply four jack sockets with their ground/sleeve connectors tied together and their tip connectors tied together. Please note, they are intended to split one signal several ways, not to combine signals.
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… And Finally
Here are some photos of the build. You can see there’s lots of room in that enclosure, but I wanted something that was stable on the desk with a few cables hooked in. It’s possible to make the final unit quite small as the circuit itself takes up very little room. Designing a layout is your only challenge. Of course you can use any kind of box, sockets, and knobs that you like. I hope you find this useful!
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Synth DIY: a White Noise generator (part 1 of 2)
WHAT IS WHITE NOISE?
We’ve all heard white noise in synth patches – it sounds like the wind, adds breath to a pad, rattle to a snare. It’s also a useful source of randomness for modulation, either directly or via a sample and hold circuit.
Technically ‘white’ noise comprises all frequencies at all amplitudes. Despite this sounding complicated, we can generate white noise very simply. It happens naturally in transistors and all we have to do is amplify it.
Once we have our white noise, we can filter it. Different colours represent different frequency content. Many synthesizers only provide white noise, but some also offer pink, which has the high frequencies rolled off. Occasionally you’ll see noise labelled as blue, red, or brown.
METHOD
I decided to make this circuit using discrete components only – no ICs for once! You could use opamps instead for the amplifier stages, but the transistor circuit is compact and runs from a single supply, in this case 9 volts. Battery power is more than adequate.
BASIC WHITE NOISE GENERATOR
Please Note: the R and C numbering in this schematic accidentally begin at 2 rather than 1. This does not affect placement, values, or operation. These identification markers are corrected on the full schematic in Part 2.
Also note the pin arrangement for Q1 will vary depending on your choice of transistor.
Basic discrete transistor white noise generator schematic
HOW IT WORKS
The noise itself comes from the first transistor, Q1. In most circuits, the voltage at the base of an NPN would be higher than that at the emitter, allowing current to flow between the collector and the emitter (transistor basics here if you need them, there’s no shortage of guides on the internet). However, for noise purposes we reverse that – we hold the emitter higher than the base. We also leave the collector unconnected. If the reverse voltage applied is sufficient, it produces noise that we can amplify and use.
Here I’m using a BC182L. This component will require some experiment on your part. Every transistor has a different breakdown voltage (ie., the reverse base-emitter voltage that produces the noise), and every transistor will give different noise quality. I had good results with the BC182L, but I recommend trying whatever NPN devices you have at hand. If you have an oscilloscope, testing each transistor along with just resistor R2 (here I’m using a 1M resistor) is enough to compare a few examples. My selected BC182L with 1M on 9V gave noise levels up to 100mV peak-to-peak. The output was measured at the emitter.
The following image shows a sample from my Rigol 1054z oscilloscope. Horizontal divisions are 1ms, vertical divisions are 20mV. The bright band is the momentary snapshot, the dark band behind it is the signal smoothed out over time. You can see the signal is around 100mV from its highest to lowest point. This is pretty much the strongest result I got from any of my transistor stock.
Oscilloscope display of breakdown noise in a BC182L transistor
I also tried several other silicon NPN transistors – nothing special, just what I had handy. In order to get something in the region of 100mV p-p I had to change the resistor value for each of them. Here’s a quick list of my results:
2N3904 — 200k
These values are a guide only. You should adjust up or down as required – lower value to get a higher output. Something between 100k and 1M should give you useable noise from a broad range of transistors, so don’t worry if what you have isn’t listed here.
BUFFERING THE NOISE
The rest of the circuit around the second transistor Q2 is an amplifier. I won’t describe here how this works (feel free to research common emitter amplifiers), but with these parts the output was around 2V p-p. That should be loud enough for audio testing if you don’t have a scope. You could substitute an opamp stage here, which I won’t detail. Consider it homework ;).
Note the 10pF capacitor. This isn’t essential. In fact, the noise has a higher peak-to-peak level without it (see images below) but it will sound different. This small value capacitor rolls off the harsher top end frequencies, making the basic ‘white noise’ smoother. Adjust, or omit, to your taste.
I recommend prototyping this circuit hooked up to something you can listen with, as well as see the signal on a screen. The component values are not set in stone, and it’s worth experimenting.
White Noise figure with and without 10pF capacitor in feedback of transistor buffer stage
Finally for this stage we can add a capacitor on the output. This will decouple the output from any DC bias when we hook it to something else. You can see the DC bias in effect on the next image. Consider that we are using a single-sided 9V DC supply. The noise has to happen between two positive voltages. Audio signals should be centred around 0V. Any difference between 0V and the centre of the audio signal is the DC offset, and this can cause various problems such as distortion or even speaker damage. The DC offset in the image below is around 4.5V (the dotted horizontal line at the centre of the grid is 0V, the major divisions are 2V).
White Noise output from first stage buffer showing DC offset
The next image shows the same noise signal taken from after the capacitor but measured as an AC signal to remove the offset. See how it is bipolar around the centre point.
White Noise output from first stage buffer with DC offset removed
This is enough for a standalone white noise source, and if you choose your components well the output should be enough for audio. You may wish for more gain if you’re using this with a Eurorack modular or similar. Modular synth levels are around 10V p-p, and we’re not going to reach that with a 9V supply. Feel free to experiment with a 12V supply though. If you want to get a more substantial output, you can use a bipolar supply and an opamp gain stage instead of the second transistor. Alternatively, we’ll be adding an output stage later anyway.
synthnerd 