Lunchbox Analog Audio Mixer for Synthesisers

Recently I received a stylish lunch box placed on my desk at work. We all got one after summer. Now, not being ungrateful, it's a good lunchbox. But I already have my trusty stainless tiffin box with side clips to hold things super tight. This is especially important when cycling to work with last nights left over curry. You don't want that stuff leaking all over your corporate notebook! So I wasn't going to use the gift for it's primary purpose. Additionally I'm quite fond of the metal box for personally reasons and plus, admittedly I don't like plastic too much. So what to do with the stylish grey box?

The form and shape of the gifted container seems it would work quite well for a electronics project. A portable battery powered mixer for analog synthesisers? I guess that somehow this is sustainable! at least to re-use the lunch box in a good way.

That was the story and here is the simple instructions to create your own lunchbox mixer.

Specifications

• Simple active 4 channel audio mixer for synthesisers
• Portable size and can be battery powered hence a lunch box.
• Mono inputs which can be panned to L-R stereo channel into the stereo output.
• 3.5 mm inputs and output jacks compatible with Pocket Organizers and Korg Volcas
• Optional headphone amp
• Optional sync splitter for pocket organizers.
• Optional recharge battery

There are other alternatives to build this project, you could buy a Volca Mix. However the Volca Mix isn't battery powered but has a lot features. It all depends what your needs are.

Active vs Passive

This is often a question should I go active or passive ? Passive are fine and dandy their major disadvantage is the gain loss when the signal is summed. Which means you need to crank up your pre-amp or amp. This in turn introduces possibility of unwanted signal noise on the final output.

Of course passive mixer requires no batteries at all.

Part List

• 4 x 100k potentiometer
• 4 x 50k potentiometers
• 100nF / .1 uF capacitors
• 4 x 22pf capacitors
• 16 x 20k resistors
• 1 x Quad op-amp TL084 / TL074 or TLE208x
• 4 x 22pf capacitors
• 2 x 39k resistors
• 2 x 1k resistors
• 7 x 3.5mm input jacks
• 1 x enclosure
• 2 x 1uF capacitors or larger 10/100uF is fine as there on the power line
• 1 x on/off switch
• 2 x 9v battery connectors
• 2 x 9v batteries

The Circuit

A quick search on the internet revealed the website Music from Outer Space. Where upon I found a circuit diagram from Ray Wilson on how to build a simple mixer.

I plan to keep the build as simple as possible. To do this I can exclude the stereo headphone section for the time being.

Instructions to build the panel

The red outlined section is where the components are soldered onto the pots

You will need the following components for this section to build the panel

• Suitable enclosure
• 4 x 100k potentiometer
• 4 x 50k potentiometers
• 100nF / .1 uF capacitors
• 16 x 20k resistors

Layout the potentiometers and drill the holes into your enclosure. Then fix the potentiometers as shown below.

Solder into sixteen 20k resisters as shown in the image below for each pot

Solder in the ground wiring by linking the 100k pot’s right pin to the middle pin of the 50k pot, for each pot. At the end solder a ground wire which will return to the main board. All this is shown in the image below.

Solder the wiring for the left and right channels. The left channel return are the right side resisters while the right channel return is the left side resister. Solder the right resisters together and left resisters. Link each by wire as shown below. (Ensure that none of the bare parts of the wires or component wire touches the other channels or the grounding.)

Solder in the four input capacitors, two returns wires and each of the four input wires to the capacitor’s positive side. All this is shown below in the image.

There should be seven wires coming out of the panel including the ground wire. Then add some insulation tape to fix and protect wiring.

Solder up the input channel jacks using the holes in the box to keep them steady before finally tighten them up.

The Quad OP-Amp chip is solder onto a prototyping or Veroboard. The quad TL084 is powered by two 9v batteries. You can decide to solder in the extra components for the headphone output but check the section below regarding this.

Solder the pins for common ground on the back of board, as shown in the images below. The pins 3, 12, 5, 10 should all soldered together for the common ground.

Solder in the 22pf capacitors and 39k resisters across pings 1 and 2 then 6 and 7. As in the rough diagram, Solder then the 1k registers for the output (L+ R). Then solder in the ground and the two channel wires (marked in yellow).

The the following step please reference to the image below.

1. Take a positive side and negative side from each of the battery connectors and join them together. Solder them to the common ground.
2. The other positive side and the other negative side and solder them to the switch. Out from the switch solder two sets of wires one to power the board and the other to power the LED indicator.

Using 2k2 resister and LED to make a power indicator.

The headphone output of the amp will really serve only high impedance headphones somewhere in the realm between 250 to or even up to 600 ohms. Most headphones used with audio devices such as phones are around 32 to 50 ohms. I would not recommended used them in this circuit unless adding some transistors to the output stage.

For a simple stereo headphone you could use the LM 386 circuit. If you do decide to, try and getting your hands on the LM386N-4 version. Then you can power the chip from both 9V batteries so they evenly depleted. The standard LM 386 can only handle 15V absolute max voltage while the N-4 is 22V absolute.

TIP: I pass the output signal into my Zoom H-1n audio recorder and plug my headphones into the device to monitor the signal.

Two 9V batteries are more than enough to drive the mixer. However if you wish to use one 9V battery you need to do some rail switching or to use LM2577.

A solution would be Lithium Li-Ion Charger Step Up Module

When taking the Stereo output from the Pocket Organizer  it maybe nice to split the signal for audio and sync in the mixer. Maybe for one or two inputs on the mixer. I guess it would be simple enough to do.

Additionally a 6.3mm jack socket could be added for the final output.

Using a CMOS 4016 for audio switching and 4011 for latch switch with some push switch, you could add some Mute functionality for each channel by connecting the centre pin of the 100k Gain pot to ground.

I’m delighted with the final output in terms of sound and look. It’s an awesome addition to anyone’s DAW-less setup!