<> catalogue

Chapter I : design requirement
Chapter II : Overall thinking
Chapter III : Specific circuit design
1,MIC Amplifying circuit
2, Power amplifier circuit
3, Sine wave generating circuit
4, Square wave generating circuit
5, Additive circuit
6,Line-in Circuit
7, Audio adjustment circuit
Chapter IV : summary
Chapter V : appendix

<> Chapter I design requirement

1, Basic Requirements
• Functional requirements : Microphone amplification , Volume control , Mixing function
• Rated power :1W( Distortion degree THD≤5%)
• Load impedance :8Ω
• frequency response :fL≤50Hz,fH≥20kHz
• Input impedance :20kΩ
• Voice input sensitivity :5mV
2, Improve requirements
Tone control characteristics :1kHz Gain at 0dB,
125Hz and 8kHz There are ±12dB Adjustment range of .
3, Play part
Some additional functions can be designed and implemented by ourselves

<> Chapter II Overall thinking

Basic requirements :

The sine wave and square wave generation circuit in this design is an independent module , Not involved in audio amplification circuit . Sine wave and square wave can be used to verify the correctness of adder . Audio signal passes primary MIC After amplification, power secondary amplification is connected , Achieve basic audio output . On the basis of single channel audio output , add Line-in signal ,Line-in Signal and MIC Amplified signal access adder , Power amplification after mixing .
Overall block diagram :

<> Chapter III : Specific circuit design

1,MIC Amplifying circuit
Simulation diagram :

principle : In the figure, the function generator is used to simulate the generation of microphone mv Level signal ,C1 Play the role of filtering , Filter out DC noise .R1 And R4 Provide bias voltage for microphone .R3 And R2 Determine magnification ,Av=1+R3/R2,R3 Potentiometer , Realize adjustable magnification .
simulation result :

Due to the problem of simulation software , The position of the bias resistor is modified , DC component introduced , However, it can be seen from the waveform diagram that the waveform is Vpp Approximately equal to 62V
, Input is amplitude 1V Sine wave signal of , The magnification in the figure is 31 times , The simulation result is correct .
Actual effect :
Enter a valid value of 10mv Sine wave signal of , Change frequency , The following results are obtained

stay 10hz~50khz Within the bandwidth of , The waveform is undistorted , And the attenuation amplitude is less than 3dB.
2, Power amplifier circuit
Simulation diagram :

principle :C3 Play the role of filtering ,Av=1+R6/R3
simulation result :

In the simulation diagram Av=20*0.35/0.68=10.3, Input in the waveform diagram 100mv, The output is approximately 1.03V, Correct simulation .
Actual effect :
Enter a valid value of 200m, Input sine wave signals with different frequencies respectively , give the result as follows

Frequency at 10HZ~50KH The internal wave shape is distorted , have 8*0.707=5.6 knowable , In input 10HZ Hour , Attenuation greater than 3dB, Others are less than 3dB.

This design requires that the valid value of input is 10mv Signal of , output 1W Power of , Then the maximum output voltage Vpp=4V, connect MIC Amplification and power amplifier , Adjusting potentiometer , This can avoid the trouble of matching input and output impedance . The test results are as follows :

stay 20HZ~50KHZ All frequencies are distorted , from 4*0.707=2.828 Available , The measured frequency attenuation is less than 3dB.
3, Sine wave generating circuit
Simulation diagram :

In practice, double potentiometer is used to control the frequency , Control amplitude with potentiometer .

principle : Sine wave oscillation circuit with venturi Bridge .f=1/(2Π*R1R4C3C1), Starting diode not conducting , Magnification is 1+30/10=4>3, After diode conduction , Magnification is 1+(30//47)/10=2.83<3, Magnification at 3 Left right oscillation , Form sine wave .
Simulation effect :

At about 10ms Start vibration after ,15ms Reach a stable amplitude around .
Actual effect :

Output can be achieved by adjusting two potentiometers Vpp=100mv,f=1kHZ Sine wave signal of .
4, Square wave generating circuit
Simulation diagram :

principle : Adopt zero crossing comparator , Input as sine wave , Output as square wave .
simulation result :
Due to the problem of simulation software , The amplitude after simulation reaches several thousand volts , Subject to actual effect .
Actual effect :

The frequency of sine wave and square wave is jointly adjusted by a double potentiometer , The frequency of the two is the same .
5, Additive circuit
Simulation diagram :

principle : Using in-phase adder , All resistors adopt 1K, Realize mixing only , Do not enlarge .
simulation result :

1V Sine wave and 1V Square wave mixing of , The result is correct .
Actual effect :
Connect the sine wave generation circuit and square wave generation circuit to the adder , The following results are obtained :

It is consistent with the simulation results .
6,Line-in Circuit
Simulation diagram :

principle :C4 Play the role of filtering ,R2 Used to adjust the output amplitude .
simulation result :

input 2Vp Sine wave of ,Av=200.05/(200.05+1)=0.5, In the waveform diagram 1Vp Signal of , The result is correct .
7, Audio adjustment circuit
Simulation diagram :

principle : In the figure 1KH The signal of can pass , And adjust R6,R7,1KHZ The signal of does not change . input 125HZ Signal of , adjust R6 Amplitude variation can be achieved ; input 8KHZ Signal of , adjust R7 Amplitude variation can be achieved .
simulation result :
①1KHZ

It can be seen from the above results , Basically meet the requirements , stay 1KHZ Gain at 1.
Actual effect :
input 100mv RMS sine wave signal

②125HZ

According to the above simulation , from -12dB~+13dB Gain of .
Actual effect :
input 100mv RMS sine wave signal

③8KHZ

It can be seen from the above , The adjustment range is -20dB~+13dB.
Actual effect :
input 100mv RMS sine wave signal

It can be seen from the above actual results :
1KHZ Output meets requirements ,125HZ Only -10.911.48dB Adjustable ;8KHZ only -8.528.26dB Adjustable .

<> Chapter IV : summary

1, Simulate first , Rewelded circuit .
2, Modular thinking , Reserved interface .
3, Practice brings true knowledge , Cannot rely entirely on Simulation .
4, Circuit implementation mode , In the early stage, the hole plate can be used , It is convenient to modify and debug the circuit , In the later stage, you can make boards or plates quickly .

<> Chapter V : appendix

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