Stereo Mixer

Note

This project is a work-in-progress.

Introduction

Mixer is a fun technical term because its meaning varies with frequency. In audio circuits, mixers are devices which add signals, while in radio circuits, mixers multiply signals.

This project is an mixer for audio. The idea is that it takes an audio input from multiple sources and combines them into a single stream. However, since it’s a stereo mixer, there are actually two independent circuits which do the same thing.

System Diagram

A generic mono (single channel) mixer would look something like this:

In my case, I need to replicate this design twice for both channels, and I have some extra things to put in as well. The circuit is entirely analog baseband and can be easily implemented using op-amps.

Rev 1.0 Design

Design Choices

With a large number of potentiometers and audio jacks, I decided that mounting the bulky components on the chassis would be the best way to pursue this design. The components will be connected by wires to the circuit board. I’ve settled on three stereo channels for the board.

The core circuit element will be the LM358 operational amplifier. This is a standard low-cost audio op-amp, which I think will be fine for this circuit. My goal isn’t to make some high-end piece of audio gear and for that reason, I decided not to bother with balanced audio inputs (at least for now).

I would like my circuit to be able to handle both line-level and instrument inputs, so I’ve added two mono channels with a pre-amplifier. This lets passive guitars and microphones to be directly mixed into the audio.

Finally, there are volume controls for each input channel, a master volume output control, and left-right balance control. Finally, if you want to connect a mono input into the stereo channels, there’s a switch for patching the two sides into each other.

System Diagram

With the design choices from above in mind, here’s the system diagram for Rev 1.0.

Chassis Components

The input jacks, volume controls, and the patching switch are all mounted on the chassis. There are 3 wires heading back: left, right, ground. The two potentiometer symbols are actually a single 6-legged stereo potentiometer which allows both channels to be controlled together.

The mono inputs don’t have channel controls, since the volume adjustment will be done on the pre-amp output.

The output channel is just two chassis mounted audio jacks

Preamps

The preamp is a standard non-inverting op-amp amplifier.

The biasing on the non-inverting input allows it to operate on a single power supply. The feedback network sets the voltage gain to 101 (+40 dB power gain) to the input signal.

RV301 provides a volume control for the channel, although it does not actually affect the gain of the preamp. Finally, a buffer is placed to remove variations in the input impedance for later stages.

R305 and C303 form a high-pass filter with a cutoff at 15.4 Hz, which stops the biasing voltage from being amplified. For DC, the circuit looks like a buffer, which retains the biasing for the actual buffer stage without clipping the op-amp.

Mixers

The mixers are implemented as summing (inverting) op-amps. Once again, it’s modified with a biasing network to make it work on a single supply.

The gain is set to unity using feedback, and the output signal is fed into a balance adjust & buffer, followed by a volume adjust & buffer. Since all stages are unity gain, the DC bias is propagated through the circuit, until it’s removed by C411 and sent to the output.

Root Page

Last few things to note is the interconnections between the circuit parts. This is shown on the root page:

The power input is a simple DC barrel jack, with a power switch. I added an LED power indicator using a LM317 regulator connected in constant-current mode. This makes the LED current about the same regardless of what voltage is used to power the circuit.

All components are rated to operate at or above 30V, and the circuits I’ve constructed do not depend on the value of the power rail voltage. Increasing it provides more headroom for the signals, meaning less clipping will occur.

PCB

This is a 3D view of the PCB design. I used a two-layer board with mostly SMD components, but some of the bigger capacitors I got as through-hole electrolytics. The off-board components are simply soldered to the holes.

Rev 1.0 Testing

For my first test, I attached some pin headers to a few of the signal inputs, and shorted all of the potentiometers ports. Using two computers, I inputted two mono audio streams into the board. The output pin is connected to a simple power amplifier I built using a single FET transistor, which is connected to a speaker. My test bench looked like this:

Mixing two mono streams worked quite well! I also tested the preamp by lowering my laptop’s output volume really low until it stopped clipping. I didn’t bother checking both mixers or other channels, since the circuits are identical.

Lastly, I wanted to check if I could actually power the board from 30 V. Increasing the voltage slowly until about 25 V was no problem, but at around 27 V, my board capacitor started letting off magic smoke. Quite unfortunate. I think I’ll stick to using voltages under 20 V, and replace the caps with 50/63 V versions in the next design.