Simple LM358 Mic Preamplifier

This is a simple LM358 microphone preamplifier schematic diagram. The pre-amp circuit is very easy to build and.. it's a low cost project... The variable resistor R5 is to adjust the LM358 op-amp gain. The LM358 has dual op-amp circuit modules, you may use a single LM358 to build two channels mic preamplifier.

Parts List:
R1, R3, R4 = 10K
R2 = 1K
R5 = 100K-1M Potensiometer
C1 = 0.1uF
C2 = 4.7uF/16V
IC1 = LM358 dual op-amp single power supply
Mic = Electret Microphone

50 Watt Amplifier

This is a handy, easy to build general purpose 50 watt amp. The amp has an input for a radio, TV, stereo or other line level device. It also has a phono input for a record player, guitar, microphone or other un-amplified source. With the addition of a low pass filter at the input, it makes a great amp for a small subwoofer.

Parts

R1(1)200 Ohm 1/4 W Resistor

R2(1)200K 1/4 W Resistor

R3(1)30K 1/4 W Resistor

R5(1)1K 1/4 W Resistor

R6(1)5K 1/4 W Resistor

R7,R10(2)1 Meg (5%) 1/2 W Resistor

R8,R9(2)0.4 Ohm 5 W Resistor

R11(1)10K Pot

R12,R13(2)51K 1/4 W Resistor

R14(1)47K 1/4 W Resistor

C1(1)100uF 35V Electrolytic Capacitor

C2(1)0.011uF Capacitor

C3(1)3750pF Capacitor

C4,C6(2)1000pF Capacitor

C5,C7,C8(3)0.001uF Capacitor

C9(1)50pF Capacitor

C10(1)0.3uF Capacitor

C11,C12(2)10,000uF 50V Electrolytic Capacitor

U1,U2(2)741 Op Amp

U3(1)ICL8063 Audio Amp Transister Driver thingy

Q1(1)2N3055 NPN Power Transistor

Q2(1)2N3791 PNP Power Transistor

BR1(1)250 V 6 Amp Bridge Rectifier

T1(1)50V Center Tapped 5 Amp Transformer

S1(1)SPST 3 Amp Switch

S2(1)DPDT Switch

F1(1)2 Amp Fuse

SPKR1(1)8 Ohm 50W Speaker

MISC(1)Case, Knobs, Line Cord, Binding Posts Or Phono Plugs (For Input And Output), Heatsinks For Q1 And Q2

1. I know I skipped R4. That is not a problem :-)


2. Distortion is less than 0.1% up to 100HZ and increases to about 1% at 20kHz.


3. I haven't been able to find anyone who sells a suitable T1. You can always use two 24V 5A units in series. If you are building two amps (for stereo), then I would suggest using an old microwave transformer and rewinding it. Follow the instructions in the 12V To 120V Inverter, execpt wind 26 turns, twist a loop (center tap) and wind 26 more turns. That should work out to around 50 volts. You may need to add or remove turns depending on your transformer.


4. Q1 and Q2 will require heatsinks.


5. You may have trouble finding U3 because it is discontinued. Please don't email me about sources...I can't find it either. See if any of the sources in Where To Get Parts has it. A possible source was sent in by JBWilliams

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easy and reliable amp

Headphone monitor amp

This simple amplifier is ideal for adding a headphone jack to equipment that lacks this feature. The Headphone Buffer circuit board is small enough (1.2" X 1.4") to squeeze into even the smallest spaces and power requirements are so low that existing supplies in retrofitted gear can be used. It's a useful addition to many PAiA products such as the Submixer or Submix-Master. Adding a headphone output turns a Stack-In-a-Box it into an incredible practice amp. 

The key component is the 5532 Dual OpAmp. While ordinarily this part is chosen for it's low noise characteristics, it is also capable of delivering nearly 350mW of output power per side, more than enough to drive headphones. The circuit can operate from bipolar voltages from +/-5V to +/-18V and it is not absolutely necessary that the + and - supply voltages be the same magnitude. The superior supply voltage rejection of the IC allows operation with unregulated supplies.

In the typical mono application such as adding headphones to a FatMan or Theremax, the Left and Right inputs of the amp are both connected to the Volume control supplied with the kit. For stereo applications a dual ganged pot should be used and this modification is covered in the instructions.

The Headphone Buffer amp kit includes circuit board, all electronic components, 1/4" Stereo Jack, Volume control w/knob, wire and assembly instructions.

Two Transistors Audio Amplifier

Mini-MosFet Audio Amplifier

Notes
This project was a sort of challenge: designing an audio amplifier capable of delivering a decent output power with a minimum parts count, without sacrificing quality.
The Power Amplifier section employs only three transistors and a handful of resistors and capacitors in a shunt feedback configuration but can deliver more than 18W into 8 Ohm with <0.08% THD @ 1KHz at the onset of clipping (0.04% @ 1W - 1KHz and 0.02% @ 1W - 10KHz) and up to 30W into a 4 Ohm load.
To obtain such a performance and to ensure overall stability of this very simple circuitry, a suitable regulated dc power supply is mandatory. This is not a snag because it also helps in keeping noise and hum of the preamp to very low levels and guarantees a predictable output power into different load impedances. Finally, as the amplifier requires only a single rail supply, a very good dc voltage regulator capable of supplying more than 2 Amps @ 40V can be implemented with a few parts also.

Build a good Audio Buffer Amplifier

General Description:
This circuit will amplify voice audio in the range of 50 to 10,000 cycles with little distortion, and have the ability to drive a low impedance load to 16 ohms.  The circuit will run from 6 to 15 vdc and give about 20 dB of gain.
Concept:
To build an inexpensive audio amplifier with little parts count that is very reliable and easy to build and implement. Parts are available from Radio Shack.
Construction:
Build the circuit on a breadboard and use a socket so when the lightning hits, well you know......
If the audio input is biased above ground (audio floats on a voltage) a coupling cap will need installed in the "Audio In" lead to the pot.  The pot can be any value from 5 K to 100 K.  The 100 pF helps eliminate RF bombardment, making it suitable for higher RF environments. Don't forget the 100 uF cap or the circuit will oscillate.
Many op-amp circuits don't drive low impedance loads well, this circuit will handle a load impedance to about 16 ohms but doesn't need to be loaded down to that impedance.  The output cap is a 10 uF non polarized electrolytic for impedance's to 600 ohms.  The output capacitance should be raised to 100 uF for impedance's to 100 ohms and to 1000 uf for impedance's below 100 ohms (speaker.)
Operating bandwidth is from 50 cycles to 10 kc.  Over 10 kc and the unit suffers from poor slew rate, causing distortion, but for NBFM this bandwidth is acceptable.  Actually, for this purpose slew rate limitations work to our advantage as it helps make the amplifier less RF susceptible.  Increased audio amplification can also be had with the addition of a feedback loop.  Consult the RS parts substitution manual for examples.

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2W Audio Amplifier using BD139 and BD140 transistors

A 2 Watt audio amplifier made from discrete components.

Notes

This was one of the earliest circuits that I ever designed and built, in Spring 1982. At that time I had only an analogue meter and a calculator to work with. Although not perfect, this amplifier does have a wide frequency response, low harmonic distortion about 3%, and is capable of driving an 8 ohm speaker to output levels of around 5 watts with slightly higher distortion. Any power supply in the range 12 to 18 Volts DC may be used. 

Circuit
The amplifier operates in Class AB mode; the single 470R preset resistor, PR1 controls the quiescent current flowing through the BD139/140 complimentary output transistors. Adjustment here, is a trade-off between low distortion and low quiescent current. Typically, under quiescent conditions, current is about 15 mA rising to 150 mA with a 50 mV input signal. The frequency response is shown below and is flat from 20Hz to 100kHz:

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FET Audio Mixer

This simple circuit mixes two or more channels into one channel (eg. stereo into mono). The circuit can mix as many or as few channels as you like and consumes very little power. The mixer is shown with two inputs, but you can add as many as you want by just duplicating the "sections" which are clearly visible on the schematic.

Parts
R1,R3            2               10k pot
R2,R4            2               100k
R5                 1               6.8k
C!,C2,C3        3              0.1uF capacitor
Q1                1               2N3819 junction fet
MISC            1               wire,shielded(metal)case,phono or other plug for ouput


Notes

1. As many or as few channels as are required can be added to the mixer. Do this by just duplicating the input "sections" which are shown on the schematic.
2. A shielded case is probably needed to reduce hum and help stop oscillations.
3. The circuit can be powered by a single 9 volt battery.

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Digital Volume Control

This digital volume control has no pot to wear out and introduces almost no noise in the circuit. Instead, the volume is controlled by pressing UP and DOWN buttons. This simple circuit would be a great touch to any home audio project.

Parts
C1                1           0.1uF ceramic disc capacitor
U1                1           DS1669 digital pot ic
S!,S2           2           momentary push button switch
MICS           1           board,wire,socket for u1


Notes

1. S1 turns the volume up, S2 turns it down.
2. The input signal should not fall below -0.2 volts.
3. Using a dual polariity power supply (+-5V works fine) will cure most clipping problems. You will have to check the data sheet for the correct pins to connect your voltages.

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Simple Microphone Preamplifier

This simple microphone preamplifier circuit can use between your microphone and stereo amplifier. This amplifier microphone suitable for use with normal home stereo amplifier line/CD/aux/tape inputs. This microphone preamplifier can take both dynamic and electret microphone inputs (preamplifier provides power foe electret microphone elements).

This circuit idea is to keep the design as simple as possible to be easy to build. That was my goal when I needed a simple external microphone preamplifier for my mixer. The performance of the circuit is nothing superior but can be used with many not so serious projects. 

Parts

Capacitor

C1,C2,C3,C4         10uF

C5                          100uF

Resistors

R1                          4.7k

R2                          220k

R3                          2.2k

R4                          120ohm

LEDs

led                           1 red

Transistor           

Q1                           BC547B 

Mics

dynamic mic,input pin,output pin            

Microphone Preamplifier Circuit Features

• Brief description of operation: Simple microphone preamplifier
• Circuit protection: No special protection circuits used
• Circuit complexity: Very simple one transistor circuit
• Circuit performance: Amplification 35 dB, flat frequency response from 20 Hz to 20 kHz, quite poor distortion performance figures, a little bit noisy
• Availability of components: Uses common and easily available components
• Design testing: I have built few microphone preamplifiers based on this circuit and theu have worked without problems.
• Applications: Interface dynamic or electret microphone to a line level audio input in HIFI amplifier or computer soundcard.
• Power supply: 9V battery, takes less than 10 mA current
• Estimated component cost: Electronics components than $10
• Safety considerations: No special electrical safety considerations.

The circuit is a simple one transistor amplifier with amplification of about 30-40 dB (depends on transitor, temperature and voltage). The dynamic mic input is just a simple one transistor amplifier circuit with nothing special in it. LED D1 is in the circuitto show that the circuit operates. The voltage drop caused by LED (around 1.8V for RED led) has been taten in account when designing the amplifier circuit built around Q1. Resistor R4 and capacitor C5 make a filter to filter out possible noise from battery or other power source which is used to feed this circuit. Capacitors C1, C2 and C3 are used to block the DC bias on Q1 base to flow out of microphone input to microphone (the polarity of all capactors is straigh line = + and curved line = -).

Electret microphone input has a resistor R1 fo feeding current through electret microphone capsule when it is connected to the electret microphone input. Electret microphone needs some current (about 1 mA) flowing through it to operate, because there is a small amplifier circuit inside the microphone capsule. This circuit is suitable for all typical cheap electret capsules which available from any electronic component shop. Because electret microphones have higher signal level output, it is quite easy to overdrive the amplifier when you shout to electret microphone.

The circuit is bet to build to a small metal box like in the picture above. Put the 9V battery inside the case too. Battery power and metal box keep external noise and interference sources away. I used standard 6.3 mm jack for dynamic microphone and 3.5 mm mono jack for electret micrphone both installed to from, panel of the metal box. The LED and power switches are also installed to front panel.


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