How to build an audio mixer

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An audio mixer, also called a mixing console, is an electronic device for combining, and modifying audio signals. The modified audio signals are summed to produce some combined output signals. Audio mixers can be analog or digital type. Digital mixing consoles use Digital Signal Processing concepts and analog mixers are usually based on op-amps electronic circuits.

In this tutorial, we will show how to build an analog audio mixer using common electronics components, such as resistors, capacitors, potentiometers and operational amplifiers (Op-Amps). We will demonstrate a modular design so that the constructor can decide how many inputs should be provided.

The input audio signals could be anything from microphone inputs, to an audio output of a cd player, to an output of a PC sound card or any other type of analog audio sources. The basic mixer will be able to combine those signals from different signal sources, change the volumes of each input channel, as well as the overall volume of the mixer output. Later, we will add some circuits for audio equalization. In other words, we will add circuits for attenuating or boosting a range of frequencies e.g., bass, midrange, and treble on each audio channel and we will also add a general graphic equalizer to perform general equalization control at the output. We will also discuss about adding a VU meter. After reading this tutorial, you will be ready to build your own analog mixing console at a minimal cost.

Back to basics

Ok, let’s start! The heart of the mixing console is the summing circuit shown in figure 1. This circuit is also known as the summing amplifier. It consists of an operational amplifier, n input resistors (R1, R2 … Rn) and a feedback resistor (Rf).

 Figure 1. The summing circuit

A summing amplifier sums several (weighted) voltages. Its output voltage Uo is given by the formula:

Vo=-(A1*V1+A2*V2+A3*V3+…+An*Vn)

Ax is the voltage gain for the xth input and it is equal to Rf/Rx. If all input resistors, R1, R2, …. Rn and also the feedback resistor, Rf have the same value, then the voltage gain for each input channel becomes equal to the unity and the formula becomes:

Vo=-(V1+V2+V3+…+Vn).

In that case the circuit becomes nothing more than an adder. The minus sign indicates that the summing output is inverted (or otherwise phase shifted by 180 degrees). If the value of the feedback resistor, Rf, is greater than the value of any input resistor, Rx, then the gain for channel x rises above unity.

At this point we must notice that the summing circuit is actually an adder, and an audio mixer is really nothing more than an adder too, so the summing circuit is itself a mixer. This holds true but we must also notice that the summing circuit has not any adjusting elements for adjusting volume (voltage) levels. An audio mixer usually has. Ok, that should be the next challenge.

Let’s build a simple stereo mixer

Let’s make our mixer a little more sophisticated than a simple adder. Let’s add some more circuits according to the block diagram of figure 2. Let’s add a matching circuit at every input to ensure that any signal source is not unduly overloaded. Every matching circuit serves as a preamplifier and a volume level adjuster for one input channel, and from now on we will call it as the” input module”. The outputs of all input modules are all combined in a summing amplifier. There is also a potentiometer in our summing amplifier which is the “master” level adjuster.

 Figure 2. Block diagram of a simple stereo mixer

The input modules

A single input module is a preamplifier and a matching circuit at the same time. As a preamplifier, it must be able to provide some amplification. As a matching circuit, it must have a high enough input impedance to ensure that any signal source is not overloaded. An input impedance of about 47K is considered enough high for audio applications. A circuit which satisfies the above criteria is shown in figure 3.

 Figure 3. Circuit diagram of an input module

The circuit of figure 3 is actually an Inverting Amplifier. An inverting amplifier is a basic op-amp circuit topology. Its basic function is to scale (or amplify) and invert the input signal. The inversion is equivalent to a phase shift and has no audible effect.

Referring to the left-hand channel (the right-hand channel is, of course, identical) and as long as the op-amp gain is very large, the amplifier gain is determined by the external resistors (the feedback resistors R2A and R3 and the input resistor R1). The voltage gain is equal to the ratio of (R2A+R3)/R1. Moreover, the input impedance of the circuit is approximately equal to R1 because the operational amplifier's inverting (i.e., −) input is a virtual ground.

We have chose R1 to be exact equal to 47K in order to ensure that the input impedance is about 47K. We have also chose R3 to be equal to 22K, and R2A can be varied from 0 to 100K, so that the voltage amplification (R2A+R3)/R1 can be varied from about 1/2 to 2.6 (from -6 to +8db). It is obvious that R2 acts as a gain adjuster. C1 is used to prevent high-frequency interference signals from appearing at the output. Together with R2 and R3 it forms a low pass filter which’s cut-off frequency varies from about 130 KHz to 20 KHz and it is inversely promotional to the change in value of R2. The R5 potentiometer operates as an adjustable voltage divider and acts as the volume level adjuster.

The circuit of figure 3 is a good example of an input module of a mixer. Of, course there are many other circuits for the same purpose. Any voltage preamplifier which has adequate input impedance and has also a volume level adjuster can be used at the input stages of a mixer. Actually, there are some additional criteria for the right candidate. The input stages must not produce any noise or distortion, they must have flat frequency response and must also be stable (do not produce oscillations) at the entire audio range (20Hz to 20KHz). Almost everything depends on the right choice of the operational amplifier, the use of appropriate filtering and also the use of as small as possible resistor values.

In the prototype, we use the classic LM833 dual operational amplifier which has been designed with particular emphasis on performance in audio systems. We also use as small as possible resistor values in order to avoid thermal noise (it is well known that any resistor produces some thermal noise power which is promotional to the resistor’s value). Unfortunately, for a single stage circuit as this one, choosing the input impendence to be high enough, somehow limits our choices. Thermal noise power is also promotional to the total bandwidth. Keeping the total bandwidth as small as possible by using appropriate filtering is essential for reducing noise level. Our circuit uses rather simple filtering. We could do better filtering in a more complex circuit.

There is also another important notice regarding the circuit of figure 3. We use DC-coupling in order to achieve flat response even at very low frequencies. This is an advantage as far as the input source does not have any DC-leakage (offset). If there is any input dc-offset, it will be amplified and will pass at the output. In such case, adding an ac-coupling capacitor at the input (in series with R1) will solve the problem. The capacitor should be large enough (the recommended value is about 100uF) otherwise some low frequencies will be attenuated.

The summing amplifier

The summing amplifier of our mixing console, is shown in figure 4. Referring to the left-hand channel (the right-hand channel is, of course, identical), R8A is the feedback resistor and R1L, R2L,....RXL, are the input resistors. The feedback resistor is a 22K stereo potentiometer (R8) which is used to enable the output level to be matched to the sensitivity of the unit to which the mixer is connected. In other words, R8 acts as the master-level adjuster.

 Figure 4. Circuit diagram of the summing amplifier

The input resistors (R1L-RXL or R1R-RXR) have the same value and they are all equal to 22K, so that the amplification R8/R1 can be varied between 0 and 1 (negative infinity to 0db). C8, is used to attenuate high-frequency signals in order to reduce noise level. There is also a series RC network at the output. The purpose of this network is to prevent any DC-offset appearing at the output of the mixer and also obviate any tendency to oscillation caused by a capacitive load (such as long screened cables).

Our mixer is of modular design. We can build as many input channels as we wish and we can also upgrade the design by using some additional modules for equalization (tone control). Such an upgraded design which uses an additional module for tone control at each input channel is presented in figure 5.

 Figure 5. Adding tone control modules

Obviously, we can choose to use tone control modules only at some channels instead of all, and the block diagram can be rearranged as we wish. Usually, the additional module required for tone control is a 2-way or a 3-Way Tone Control circuit. Click on the provided links, and find more details about those circuits and their electronic schematics.

At this point, our mixer uses relatively simple filters for limited adjustments. Graphic and parametric equalizers have much more flexibility in tailoring the frequency content of an audio signal than a simple tone-control module. An audio equalizer is actually a bank of many adjustable filters. Using the modular concept, we could use one graphic or one parametric equalizer at every audio channel of our mixer. However, since an equalizer is a quite complex and expensive circuit, it is more practical to use it only once at our mixer’s output as shown in figure 6. For more details about how to build an analog graphic equalizer, please click here or here

 Figure 6. Adding an audio equalizer

A Vu meter is a device which is used to display a representation of the signal level. Think the Vu meter as a special kind of voltmeter which we can connect directly to any audio signal line of interest.

Most mixers have only one vu meter at their output lines, but some quite expensive mixing consoles offer independent signal level indication for every input channel.

Since we have a modular design we can use as many vu-meters as we like by simply connecting them directly to the signal lines of interest. There is only one restriction; we must use vu meters having enough input impedance otherwise we could overload the mixer circuits. Click here to find complete schematics and details about how to build a high impedance LED Vu meter.

Since now, the input stages are unable to provide significant gain for a microphone source. We could say that our mixer has only “line-type” audio inputs. We can overcome this limitation by adding a microphone preamplifier at any channel we wish. So, we can convert any “line-type” audio input to a “microphone-type” one by simply adding a microphone preamplifier as illustrated in figure 7.  Click on the provided links below, and find more details about some microphone-preamplifiers circuits and their electronic schematics.

A “phono” input refers to a PHONOgraph input. It is a special type input which can accept signals from an analog turntable or a magnetic guitar-pickup (or some specific other types of equipment). A phono input always uses a special circuit to boost the incoming signal and also provides the RIAA equalization necessary to restore the original sound. If you are still enjoying vinyl sound or you have a guitar which uses a RIAA magnetic pickup, you definitely need a phono input in order to be able to connect your classic turntable or your beloved music instrument to the mixer. Again, all you need is to convert one “line-type” audio input to a “phono-type” input by simply adding a phono preamplifier as illustrated in figure 8.

Building the power supply unit

The mixer can be powered from ±15V regulated power supply unit. A current of less than 1A is enough to power up more than 50 modules.

Conclusion

The target of this tutorial was the design of a high quality modular audio mixer. The mixer is formed assembling some main modules that can be varied in number and/or disposition to suit everyone needs.

There is always much room for improvements. If you have any new ideas, some additions, corrections or complaints feel free to provide feedback. I’ m sure that everyone will appreciate any further contribution.