Basic Analog Audio Mixer Biology Essay

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Audio mixer is a device that uses to mix, and change the level, tone of the sound, and dynamics of audio signals. Audio mixers are also called mixing consoles and sound boards. Mixer is a device that can mix signals no matter it is analog or digital; depend on the type of mixer. It sums 2 amplified audio signals, and mixes the sound. By summing the modified signals, we can produce the combined output signals that can drive the speaker. It can also control volume and other effects.

Audio mixer are used in many applications, including recording studios, public address systems, sound reinforcement systems, broadcasting, television, and film post-production. Simple application is to enable the signals that originated from two separate microphones to be heard through one set of speakers simultaneously.

Audio mixer is made up of 2 input audio, crossfader, 3 band graphic equalizer, inverting amplifier, voltage follower, voltage level indicator and speaker.

Figure 1: Audio mixer block diagram

Figure 2: Audio mixer

Figure 3: Audio mixer simulation

Audio mixer is builds by using 741 op amps.

Operational amplifiers (op amps) can amplify and operate on analogue signals. Applications of op-amp are summing amplifier, voltage follower, comparator and active filter.

Op amp consists of inverting terminal, non inverting terminal, output terminal, positive and negative dc supply terminal, and special terminals for tuning.

An ideal op amp has high, low and high

Figure 4: Op amp 741 and pin assignments

, where is open loop voltage gain.

For ideal op amp, Ao=∞, Rin=∞, Rout=0.

Figure 5: Ideal op amp Figure 6: Practical op amp

Theory:

Crossfader:

A fader is a device that used for fading, like a knob or a button that can slides along a track or slot. A fader can be analogue which can directly control the impedance or resistance to the source; or digital, which can control a digital signal processor (DSP) numerically. Digital fader is like a virtual fader, as they can be viewed on the screen of a digital audio workstation.

Crossfader is a very important component in audio mixer, as it can mix signals from two different channels. It allows to play 2 channels simultaneously and fading between individual channels. It acts like summing amplifier.

Crossfader is important to the Audio Mixer Design due to its characteristics and ability to mesh two audio through fading.

Figure 7: crossfader

3-band graphic equalizer:

Equalizer is designed to alter frequency balance of an audio signal. In middle and high end stereo sound systems, we can commonly see graphic equalizers. In telephone system, graphic equalizer is used to fix the cut down level of high frequencies. They can be use in personal computers for fine tuning of sound. Graphic equalizers are used as a standard practice in live sound reinforcement systems, professional recording studios, and some high fidelity systems, because it can compensate for the room frequency response and room acoustics. Although there are many designs of graphic equalizers, they operate in the same way.

Graphic equalizer can be graphic or parametric.

Graphic equalizers can be use to increase the fixed band audio frequencies, while the parametric equalizers allows any band to boosts up its fc and bandwidth.

Figure 8: Equalizer

The quality factor is use to measure the effects of equalizer band on frequencies. To control a smaller frequency envelope use high Q and will give a narrow shape to the response.

3-band graphic equalizer is a circuit consists of several band pass filter.

Filter is a circuit that can select a specific range of frequencies to pass through it while blocking other frequencies. We can use filter to reduce unwanted noise.

The 4 basic types of filters are low pass filter, high pass filter, band pass filter and band reject filter

There are 2 types of filters which are passive filter and active filter. Passive filter is constructs using passive elements like R, C and L. While the active filter can be construct using op-amps, R, C. It can handle low frequency signals and provides Av on the same time. But it is not reliable in high frequencies due to the BW of amplifier and slew-rate.

.

Passive RC low pass filter Passive RC high pass filter

Active RC low pass filter Active RC high pass filter

Active low pass with Active high pass with

Band pass filter is constructs by combining the low pass and high pass filter. The upper frequency is set using low pass filter while the lower frequency can be set using the high pass filter.

Cascading a passive RC low pass filter and a passive RC high pass filter will produce a passive RC band pass filter.

There are 2 ways to construct an active RC band pass filter. The 1st method is using a passive RC band pass filter with an op-amp. The 2nd method is to cascade a passive RC low pass with op amp and a passive RC high pass with op amp. 2 op-amps better than one op amp because of the seperation of the low pass filter and high pass filter stage.

Active single stage band pass filter Active cascaded band pass filter

Figure 9: 3-band graphic equalizer

Inverting summer amplifier:

Inverting summer amplifier is use to amplify the sum of 2 input signals. It can connect more than one analogue signal to the input, and the summing amplifier will then amplifies each individual signal and add them all together. With the availability of such a system, multiple inputs may be added to a system. So, it is not restricted to a limit of two or three inputs.

In audio mixer, summing amplifier allows us to record songs to many tracks. By using summing amplifier, we can amplify the sound and make it louder.

Figure 10: Inverting summer

Voltage Follower/Buffer:

Voltage follower follows the voltage that is sent in. It have the ability to buffer a high impedance signal can make this a useful little circuit. You can use this to give more power to a long sensor cable run, lower impedance, and protect circuitry from being overloaded.

Voltage follower is useful for circuit isolation applications (circuit act like buffer).

Little power is drawn from source, and this will avoid loading effects. (Loading effects means large load impedance draw small load current, so loading of source is small)

Voltage follower is used to transfer a voltage from a high output impedance level to a low input impedance level. It can prevent the 2nd circuit from loading the 1st circuit inappropriately and interfering with its desired operation. Buffer has unity gain amplifier (=1).

In design of audio mixer, there will be loading effect created by the speaker as the speaker draws much current and power from the circuit. So by adding buffer in the circuit, we can reduce power consumption in the source, distortion from overloading, crosstalk and other electromagnetic interference.

Figure 11: Voltage Follower

Voltage Level Indicator:

Voltage level indicator is a simple application using comparators.

Function of the comparator is to determine which signal is bigger. We can know whether there is any signal that exceeds Vth by using comparator.

Many comparators will share same input and each of them is supplied with different reference or triggering voltage.

LEDs will turn on by the outputs of comparators depend on the input signal voltage.

LEDs will be turned on in succession when the input signal voltage increases.

Audio signal is like the input to the comparator, while LEDs is like a volume indicator. It increase and decreases when the signals of audio are different.

Figure 12: Voltage level indicator

Setup:

Equipment:

Power supply: 1 unit

Function generator: 1 unit

Oscilloscope: 1 unit

Digital multimeter: 1 unit

Components:

Op amp 741: 13 units

Resistor 240Ω: 2 units

Resistor 510Ω: 1 unit

Resistor 1kΩ: 10 units

Resistor 2.7kΩ: 2 units

Resistor 3kΩ: 2 units

Resistor 8kΩ: 2 units

Resistor 10kΩ: 6 units

Potentiometer 10kΩ: 4 units

Potentiometer 50kΩ: 1 unit

Capacitor 0.1uF: 2 units

Capacitor 10nF: 2 units

Capacitor 4.7nF: 2 units

LED 5mm: 4 units

Audio jack: 2 units

Speaker: 1 unit

Circuit construction:

Audio Mixer

Crossfader circuit was constructed. The circuit was tested with function generator and oscilloscope to make sure it can work properly.

3-band graphic equalizer circuit was constructed. It was tested with function generator and oscilloscope to make sure it works properly.

Inverting summer circuit was constructed with 3 inputs, and potentiometer. It was tested with function generator and oscilloscope to make sure it works properly.

Voltage follower was constructed. It was tested with function generator and oscilloscope to make sure it works properly.

Voltage level indicator circuit was constructed. It was tested with function generator to make sure it works properly.

All the circuits were constructed together as in the figure of audio mixer. Make sure the output of the 3-band equalizer each is connected to one of the input of the inverting summer. The audio 1 and audio 2 can be signal from a function generator.

Make sure the speaker's polarity is connected correctly.

Whole circuit was tested with input signal from function generator, and from low frequency slowly to high frequency. Speaker should buzz from low tone to high tone.

When all circuits work properly, it was tested with an audio signal (to audio 1) from computer by connecting audio jack to PC output jack. Circuit was verified by lecture before attempting connection to PC.

If audio 1 works properly, audio 2 was then tested

Audio 1 and audio 2 were tested with two different audio signals from the PC

Crossfader

Circuit with was constructed.

Function generator was set to 1V peak-to-peak sine wave with frequency=500Hz

Power supply was set to -12V and +12V and were connected to pin 4 and pin 7 of op-amp

Function generator was connected to input, Vin1. Signal at V- and output signal, Vout were observed. Potentiometer was turned fully to one direction so that the output waveform is minimum. Input and output waveforms were observed. Step was repeated by turning potentiometer fully the opposite direction so the output waveform is maximize. Minimum voltage gain and maximum voltage gain in dB were calculated,

Vin1 was disconnected. Function generator was connected to input Vin2. Signal at V-and output signal, Vout was observed. Potentiometer was fully turned to one direction so that the output waveform is minimum. Input and output waveforms were observed. Steps were repeated by turning potentiometer fully the opposite direction so the output waveform is maximize. Minimum voltage gain and maximum voltage gain in dB were found.

Function generator was connected to both input Vin1 and Vin2. Potentiometer was turned fully anticlockwise. Signal at V and the output signal, Vout were observed Input and output waveforms were drawn. Step was repeated by turning potentiometer to fully clockwise. Observations were discussed. Minimum and maximum gains were found.

If potentiometer was set to center position, observed results at V-.

3-band graphic equalizer

Circuit of a 3-band equalizer was constructed with R11 = 3kΩ,

Gain of the band pass filter was set to minimum (the feedback resistor=0).

Function generator was set to 1V peak-to-peak sine wave at frequency=100Hz.

Power supply was set to -12V and +12V and was connected to pin 4 and pin 7 of op amp.

Function generator was set to Vin.

Vout1, Vout2, and Vout3 peak-to-peak were measured. Frequencies as in Table 5.1 were repeated.

Each frequency, voltage gain,

Band pass filter was repeated at maximum gain (feedback resistor at 10k ohm). Table 5.2 was filled.

Table 5.1 was observed. Gain vs. frequency (log) graph was plotted.

Gain vs. frequency (log) graph based on Table 5.2 was plotted. Gain provided by the filter was found.

Inverting summer amplifier

and potentiometer were measured using digital multimeter.

Circuit with and was constructed.

Function generator was set to 1V peak-to-peak sine wave with frequency=500Hz.

Power supply was set to -12V and +12V and were connected to pin 4 and pin 7 of op-amp

Function generator was connected to Vin1 and Vin2.

Input signal at the function generator (probe channel 1 of oscilloscope) and output signal, Vout (probe channel 2 of oscilloscope) were observed.

Input and output waveforms were drawn. Voltage gain was calculated

Voltage follower

Function generator was set to 1V peak-to-peak sine wave with frequency 500Hz.

Circuit for voltage follower was constructed.

Power supply was set to +12V (master) in series mode and was connected to pin 7 and -12V to pin 4 of 741 op amp.

Function generator was connected as and was observed.

voltage gain , was calculated for each case using

Voltage level indicator

Circuit was constructed with and

Function generator was set to 400mV peak-to-peak square wave at f=50 Hz and

Voltage to reference voltage of each comparator was measured.

Function generator was connected to Vin. Observe that all LEDs are off. Input voltage was increased slowly and outputs of the comparators were observed when each LED brightens. Peak voltage was written down when each LED lights up. When the last LED is on, it was stopped.

Frequency of the input signal was decreased slowly. The result was observed.

Results:

Crossfader:

When we turned the potentiometer to minimum, the Vin1 has maximum power

Av, min=20 log (Vout/Vin)

=20 log (0.1/1)

=-20dB

When potentiometer is tuned to maximum resistance, much power is absorbed by the resistor leaving very little power for amplification.

Av, max=20 log (Vout/Vin)

=20 log (17/1)

=24.6dB

3-band graphic equalizer:

Table 1: 3-band Graphic Equalizer Response (Rf=0)

60

1

0.00

0.04

0.14

-28.0

-17.2

40

1

0.00

0.20

0.20

-14.0

-14.0

20

1

0.00

0.30

0.34

-10.5

-9.4

15

1

0.02

0.40

0.40

-34.0

-8.0

-8.0

12

1

0.04

0.40

0.44

-28.0

-8.0

-7.1

10

1

0.06

0.42

0.54

-24.4

-7.5

-5.3

9

1

0.06

0.50

0.60

-24.4

-6.0

-4.4

8

1

0.10

0.60

0.60

-20.0

-4.4

-4.4

7

1

0.10

0.60

0.60

-20.0

-4.4

-4.4

6

1

0.10

0.60

0.60

-20.0

-4.4

-4.4

5

1

0.20

0.70

0.70

-14.0

-3.1

-3.1

4

1

0.20

0.70

0.70

-14.0

-3.1

-3.1

3

1

0.20

0.70

0.70

-14.0

-3.1

-3.1

2

1

0.26

0.64

0.60

-11.7

-3.9

-4.4

1

1

0.40

0.5

0.4

-8.0

-6.0

-8.0

0.8

1

0.40

0.44

0.40

-8.0

-7.1

-8.0

0.6

1

0.50

0.40

0.30

-6.0

-8.0

-10.5

0.5

1

0.50

0.34

0.26

-6.0

-9.4

-11.7

0.4

1

0.60

0.26

0.26

-4.4

-11.7

-11.7

0.3

1

0.54

0.20

0.20

-5.4

-14.0

-14

0.2

1

0.50

0.20

0.20

-6.0

-14.0

-14

0.1

1

0.34

0.14

0.14

-9.4

-17.1

-17.1

Freq

(kHz)

Vin,pp

Vout1

Vout2

Vout3

AV1

AV2

AV3

Table 2: 3-band Graphic Equalizer Response (Rf=10k)

60

1

0.1

0.8

1.1

-20

-1.9

0.8

40

1

0.2

1.4

1.8

-14.0

3.0

5.1

20

1

0.3

2.6

3.4

-10.5

8.3

10.6

15

1

0.4

3.4

4.3

-8.0

10.6

12.7

12

1

0.5

4.0

5.0

-6.0

12.0

14.0

10

1

0.6

4.6

5.7

-4.4

13.3

15.1

9

1

0.7

5.0

6.0

-3.1

14.0

15.6

8

1

0.8

5.4

6.4

-1.9

14.6

16.1

7

1

0.9

5.8

6.6

-0.9

15.3

16.4

6

1

1.0

6.2

7.0

0.0

15.8

17.0

5

1

1.2

6.8

7.2

1.6

16.7

17.1

4

1

1.5

7.0

7.2

3.5

17.0

17.1

3

1

2.0

7.2

6.8

6.0

17.1

16.7

2

1

2.8

6.8

5.9

9.0

16.7

15.4

1

1

4.9

4.9

3.7

13.8

13.8

11.4

0.8

1

5.6

4.0

3.0

15.0

12.0

9.5

0.6

1

6.5

3.2

2.2

16.3

10.1

6.8

0.5

1

6.8

2.7

2.0

16.7

8.6

6.0

0.4

1

7.2

2.3

1.6

17.1

7.2

4.1

0.3

1

7.4

1.8

1.2

17.4

5.1

1.6

0.2

1

6.8

1.2

0.8

16.7

1.6

-1.9

0.1

1

4.6

0.6

0.5

13.3

-4.4

-6.0

Freq

(kHz)

Vin,pp

Vout1

Vout2

Vout3

AV1

AV2

AV3

Inverting summer amplifier:

R1=R2=R3=1kΩ, Rf=10kΩ

Vout= (-Rf/R1)Vin1+(-Rf/R2)Vin2+(-Rf/R3)Vin3

= (-10k/1k)1+(-10k/1k)1+(-10k/1k)1

= -30V

Av= Vout/Vin

= 30/1

=30V

Voltage follower:

Vin= 1V

Vout= 1V

= 1/1=1

Voltage level indicator:

Vpp for LED1 to light up is 4.8V

Vpp for LED2 to light up is 2.8V

Vpp for LED3 to light up is 1.2V

Vpp for LED4 to light up is 0.56V

When frequency of input signal decrease slowly, LEDs start blinking.

Problems faced and solutions to it:

The sounds that come out from the speaker are not loud enough. To solve this, we can add a voltage follower before the speaker to boost up the sound.

There are noises in the sound. When play the music, cannot listen to the music clearly, there are many unwanted noise in it.

There is loading effect during construction. This may because the speaker draws much current and power. So, to reduce loading effect, we can add a buffer.

For voltage level indicator circuit, when connect function generator to Vin, all the LEDs should turn off, but what observed is all the LEDs were on. This is because the legs of LEDs are opposite (anode and cathode are wrong in position). To solve it, change the position of the leg.

LED did not light up in sequence, this may due to resistor connection problem, or the connection of wire. To solve this problem, check the connection.

When construct voltage level indicator to other components in audio mixer, LED did not light up when the input voltage increases, maybe this is because the voltage is not big enough to turn on the LED. This problem can solve by adding an extra voltage follower circuit.

Conclusion:

An audio mixer is an essential tool in working with audio. From using a mixer in a live environment as a DJ, to doing production work in a studio, the mixer is often the central and key link in your chain of equipment. Audio mixer controls the routing of audio lines to a destination device for playing or recording. It also controls volume and other effects.

Audio mixer consists of 5 sub circuits, which are crossfader, 3-band graphic equalizer, inverting summer amplifier, voltage follower and voltage level indicator.

We enhanced and improved our circuit by adding 2 voltage follower before the speaker and 1 voltage follower before the voltage level indicator. By adding the voltage follower, the noise was reduced so that we can get better quality sound and the voltage level indicator works properly by turning on the LEDs light in sequence.

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