DIY microphone amplifier with transistors. A simple DIY microphone amplifier for a computer
Microphone preamp, also known as a pre-amplifier or amplifier for a microphone, is a type of amplifier whose purpose is to amplify a weak signal to a linear level (about 0.5-1.5 volts), that is, to an acceptable value at which conventional audio power amplifiers operate .
The input source of acoustic signals for a preamplifier is usually vinyl record pickups, microphones, and pickups of various musical instruments. Below are three circuits of microphone amplifiers on transistors, as well as a variant of a microphone amplifier on the 4558 chip. All of them can be easily assembled with your own hands.
Circuit of a simple microphone preamplifier using one transistor
This microphone preamplifier circuit works with both dynamic and electret microphones.
Dynamic microphones are similar in design to loudspeakers. The acoustic wave affects the membrane and the acoustic coil attached to it. When the membrane oscillates, an electric current is generated in a coil exposed to the magnetic field of a permanent magnet.
The operation of electret microphones is based on the ability of certain types of materials with increased dielectric constant (electrets) to change the surface charge under the influence of an acoustic wave. This type of microphone differs from dynamic microphones in its high input impedance.
When using an electret microphone, to bias the voltage on the microphone, it is necessary to set the resistance R1
single transistor microphone amplifier
Since this microphone amplifier circuit is for a dynamic microphone, when using an electrodynamic microphone, its resistance should be in the range from 200 to 600 Ohms. In this case, C1 must be set to 10 microfarads. If it is an electrolytic capacitor, then its positive terminal must be connected towards the transistor.
Power is supplied from the crown battery or from a stabilized power source. Although it is better to use a battery to eliminate noise. can be replaced with a domestic one. Electrolytic capacitors for a voltage of 16 volts. To prevent interference, connect the preamplifier to the signal source and to the amplifier input using a shielded wire. If further powerful sound amplification is needed, then you can assemble an amplifier on a microcircuit.
Microphone preamplifier with 2 transistors
The structure of any preamplifier greatly affects its noise characteristics. If we take into account the fact that the high-quality radio components used in the preamplifier circuit still lead to distortion (noise) to one degree or another, then it is obvious that the only way to get a more or less high-quality microphone amplifier is to reduce the number of radio components in the circuit. An example is the following two-stage preliminary circuit.
With this option, the number of decoupling capacitors is minimized, since the transistors are connected in a circuit with a common emitter. There is also a direct connection between the cascades. To stabilize the operating mode of the circuit when the external temperature and supply voltage change, a direct current feedback loop has been added to the circuit.
Preamplifier for electret microphone with three transistors
This is another option. The peculiarity of this microphone amplifier circuit is that power is supplied to the preamplifier circuit through the same conductor (phantom power) through which the input signal travels.
This microphone preamplifier is designed to work together with, for example, MKE-3. The supply voltage to the microphone goes through resistance R1. The audio signal from the microphone output is supplied to the VT1 base through capacitor C1. , consisting of resistances R2, R3, creates the necessary bias at the base of VT1 (approximately 0.6 V). The amplified signal from resistor R5, acting as a load, goes to the base of VT2 which is part of the emitter follower on VT2 and VT3.
Near the output connector, two additional elements are installed: load resistor R6, through which power is supplied, and separating capacitor SZ, which separates the output audio signal from the supply voltage.
Pre-microphone amplifier based on 4558 chip
The 4558 operational amplifier is manufactured by ROHM. It is characterized as a low power and low noise amplifier. This microcircuit is used in a microphone amplifier, audio amplifiers, active filters, and voltage-controlled generators. The 4558 chip has internal phase compensation, increased input voltage threshold, high gain and low noise. This op amp also has short circuit protection.
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microphone preamplifier for 4558
This is a good option for building a microphone preamp on a chip. The microphone preamplifier circuit is characterized by high amplification quality, simplicity and does not require much wiring. This dynamic microphone amplifier also works well with electret microphones.
If the microphone's sound is very weak and there is distortion, then this problem can be eliminated using a preamplifier. This is a device that can amplify a weak signal to the required volume level. And the sound wave immediately gets amplified into the computer and without extraneous sounds. You don’t have to buy an amplifier in a store, but you can make it yourself.
How to make a microphone amplifier with your own hands
To make a microphone preamplifier that will take energy not from batteries or not pull long wires from another power source, but so that it is recharged directly from the sound card, you need to make a circuit with a phantom power source. That is, a circuit where the transmission of the information signal and the power supply of the device occur together via a common wire.
This option is the most optimal, because a regular battery often runs out, and using the battery also requires recharging it from time to time. Using the power supply is also not entirely convenient, because there are wires that can interfere with movement and third-party interference. These factors lead to inconvenience in using the device.
Important! The operation of the microphone is based on the property of some materials with increased dielectric permeability to change their charge when exposed to a sound wave. And to amplify the microphone signal, you need to set the resistance in the range from 200 to 600 Ohms, and the capacitance of the capacitor should be up to 10 microfarads.
For this purpose you must have:
- resistors;
- capacitors;
- transistor;
- plug and sockets for connecting the device;
- wires;
- frame;
- microphone;
- additional tools - wire cutters, soldering iron, scissors, tweezers, glue gun.
Amplifier circuit
There are many ways to assemble an amplifier, but this circuit is distinguished by its simplicity and it is based on a classic transistor stage, where a common emitter is installed. Also, to assemble it you do not need to purchase expensive parts. It will take only one hour of free time to make it. The circuit consumes 9 mA of current in operation, and 3 mA at rest.
It has two capacitors and two resistors, one plug, a transistor and an electret microphone. The amplifier board turns out to be very small in size, which can be attached to the plug; if it is slightly larger in size, then you need to take some kind of plastic part to make the case.
The principle of its operation is such that power is supplied to the elements through resistors R1 and R2, in order to prevent feedback in the frequencies of the supplied signal, capacitor C1 is used, and a resistor is needed to eliminate extraneous clicks when connecting a microphone to work. The signal comes from the resistor and goes to the transistor to amplify it. Thanks to this circuit, the signal from a dynamic microphone can be doubled.
Microphone amplifier: step by step
We take a resistor, it will perform the function of biasing the voltage. We take a transistor model KT 315 and can replace KT 3102 or BC847. To make a circuit, we can take a homemade breadboard. Before use, rinse it thoroughly with any solvent. You need to solder the connectors through which the power is supplied; we also use this method to connect the microphone input and output connectors. We take the connectors and solder them to our board. They can be taken from an old DVD player or tape recorder. The switch can be taken from an old toy car. Solder all the parts to the board.
To make a housing for a microphone amplifier, we take a plastic box. We make holes in it for the connectors and for the switch. We glue the board to the box and cover it with the top of the plastic box.
If assembled correctly, the circuit does not need to be further configured and the microphone can be immediately connected to work. This microphone amplifier greatly improves the sound quality and there is no extraneous noise. The circuit also works well with an electret microphone.
Important! Before connecting a microphone to the device, you should check its contacts, and also that the power at the microphone input is at least 5 volts.
If there is no such voltage, then we take another plug and attach it to the connector and measure with a voltmeter the voltage that exists between the large tap and the other two taps, which are shorter. When measuring voltage, you need to be careful not to short the plug terminals to each other.
To check, take a dynamic microphone, connect it, connect the amplifier output with a wire to the computer or speakers, or to the device you need, and turn on the power. If an LED was used during assembly, then its glow indicates that the amplifier is working. But the electrode itself is not required in the circuit.
Hello! In this article I want to tell you about a microphone preamp.
From the very title of the article it is clear that we will strengthen something. First, let's look at one example. You connected a dynamic microphone to your computer and decided to record your voice. But apart from very quiet speech, filled with a lot of noise and interference, you heard nothing. And all because 1.5 V appears at the input of the computer’s audio card. This very one and a half volts presses the coil inside the microphone, and when you speak, they prevent it from moving. This means that this voltage needs to be somehow removed and the signal strengthened. For this we will make a pre-amplifier. That is, the sound from the microphone will enter the computer already amplified and without noise.
So, let's get started.
To do this you need the following components:
Resistors – 4.7 kOhm – 2 pcs., 470 kOhm, 100 kOhm.
Capacitors – 4.7 µF, 10 µF, 100 µF.
Transistor– KT315.
Light-emitting diode – not necessary.
Tools:
Soldering iron, wire cutters, tweezers, scissors, glue gun, etc..
Let's start manufacturing.
1.
First, let's look at the diagram and details.
Resistor R5 put for electret microphone and acts as a voltage bias. We don't use it. The KT315 transistor can be replaced with KT3102, BC847. KT3102 has a higher gain, so it is preferable to install it. LED is optional. If it is not needed, replace it with a diode. I found a piece of a homemade breadboard at my place. I will make a diagram on it.
2. Now, according to the diagram, solder all the components.
3. Next, we solder the power connectors, microphone input and output, and power switch. 6.3 mm jack connector. I took a 3.5 mm jack from an old DVD player. - from a tape recorder. A connector for a battery from a non-working crown, a switch from a toy car. Solder everything to the board.
There is no LED in the photo; it appeared later.
4. Now let's take care of the body. I found some kind of plastic box without a bottom. She just fit all the details. We drill holes in it for connectors, an LED, and cut out a rectangular hole for a switch.
5. Now we assemble everything into the case. We glue the crown and board with double-sided tape, and the connectors with hot-melt adhesive.
The bottom was made of durable black cardboard.
6. Check. I had the cheapest BBK karaoke microphone. I connected it. Next, using a jack-jack wire, we connect the amplifier output to the computer, speakers, or whatever you need. Turn on the power. The LED lit up. The preamp is working.
If your computer microphone is “hard of hearing” and you have to literally shout out to your interlocutor, do not rush to write it off as scrap: maybe a simple amplifier will help. Owners of laptops and netbooks will immediately snort at me: “No, it won’t work - extra wires!” Calm down, they won't be there. We organize phantom power.
The circuit is more than simple; it takes longer to look for parts than to solder. You can remake an existing microphone, you can make it from scratch, or you can use it for some other crafts.
Travel notes:
If you measure the voltage at the microphone input of a PC/laptop in any convenient way, you will get something like a green number (my Studebaker produces 3.2 volts, variations are possible on other computers). This voltage is used to power electret microphones, and the circuit design, when power is supplied through the same wire as the signal, is called phantom power.
When connecting the circuit, the voltage drops to 0.9 volts. At the base of the transistor - 0.6 - 0.7 volts assigned to it for opening.
Almost all sites where this scheme is available recommend KT3102. On my own behalf I will add that it is preferable in an iron case. But if it is not there, then any silicon low-power transistor will do, for example, BC547, S9014. In very cramped circumstances, you can take KT315.
This option is on S9014 I got together with a friend in the fall of 2013 to capture the “corridor air” in order to know who was rowdy at night and who to snort at later. At that time, we had just appeared soldering irons with an “eternal” tip, and such miniaturization of crafts was simply a breakthrough after the 25-watt EPSN with a 6 mm rod.
I assembled it in a new way, using the miniaturization skill “I soldered so much in two years.” Above is another option on a smaller capsule. First I soldered the transistor, then C1, then “electrolyte” and two resistors.
I extended the leads and doused the structure with hot glue.
And wrapped it in self-adhesive aluminum foil for shielding. In order for the foil to come into contact with the capsule, you need to wrap it, like you wrap a collar: there is no conductivity on the adhesive side.
If you remake a factory product, then most likely there will not be a place next to the microphone. No problem! The amplifier can be soldered on a small scarf or the same “canopy” and placed somewhere to the side, if the case allows it. In the same way, isolate it from the external environment (not necessarily with hot glue - electrical tape, “heat shrink”, paper, in the end) and shield it, if possible, hooking the screen to the minus of the “circuit”.
The microphone amplifier designs discussed in the thematic selection use only inexpensive and accessible radio components, as well as good technical characteristics.
Thanks to the combination of just such bipolar transistors, there is no need for a transition capacitance between both stages, and the stable operation of the amplifier in terms of direct current is guaranteed, even when the supply voltage fluctuates or when replacing transistors with new ones.
This design does not require selection of elements, since transistors with a transmitted current coefficient above 50 are used. This means that in this design you can use, without selection, transistors like KT3102 or KT3107 with any letter indices. A good result can also be obtained by using foreign analogues VS307A, VS307B, VS308A, VS308V as the first one. The circuit provides a gain of at least 150-200 in the frequency range 50 Hz - 20 kHz.
The use of bipolar transistors of the same type of conductivity made it possible to simplify the procedure for their selection, since direct contact between the cascades stabilizes the functioning of all three transistors in terms of direct current.
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The peculiarity of such a circuit is that it is possible to adjust the frequency characteristics of the second transistor stage due to the presence of frequency-dependent negative feedback. To implement it, a parallel connection to resistance R7 is made of a chain of capacitor C4 and resistor R5. The reactance of capacitance C4 at low frequencies is quite high, and therefore R5 does not affect the amplifier stage. At high frequencies, C5 is connected in parallel with R7. And the gain increases as a result of a decrease in the resistance of the emitter circuit.
Another feature of this design is that the signal to its output follows through the emitter follower on the last transistor. This combination reduces the output impedance and also reduces the influence of the length of the connecting cable on the quality of the amplifier as a whole.
The proposed circuit solution makes it possible to use fewer radio components and increase the gain to 1000, due to the presence of negative feedback in terms of voltage in the middle stage. This perfectly stabilizes the gain and also increases the increase in the input impedance of the circuit. If necessary, the gain is reduced due to an increase in resistance R3. For example, using R3 = 1 kΩ, the gain ( K u) dropped to 100.
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Taking into account the dependence of the operating modes of transistors for direct current on the performance of the first and second transistor. For normal operation of the device, the constant voltage at the emitter junction of the last transistor should be about 1.4 V. This control voltage is adjusted by supporting resistor R1.
Microphone DEMSH-1A is an electromagnetic, differential and noise-proof microphone used for radio communications. The DEMSH-1A microphone capsule is a symmetrical electromagnetic system with a diaphragm, open on both sides. Therefore, provided that the microphone is located close and asymmetrically relative to the sound source, it produces a high level of output signal while significantly reducing various noises present at the transmission site.
To pre-amplify the sound frequency of the microphone and set the frequency response, as well as match the output impedance of the microphone with subsequent stages, this circuit is used:
All stages of the microphone amplifier are assembled according to a direct coupling circuit. This reduced the number of electrolytic capacitors and added some reliability to the design. Voltage amplification is carried out by two transistors VT 1 and VT2. The third has an emitter follower, which is used to achieve low output impedance. To thermally stabilize the operating modes of the amplifier transistors, the bias voltage to the base of the first of them is supplied from the emitter resistance of the second through R4. Let us assume that, under the influence of some negative factors, the current of transistor VT1 also increases, this will lead to a decrease in the voltage level at its collector and at the base of VT2. This will reduce the collector current of VT2 and the voltage drop across the emitter resistance R6, which will lead to a decrease in the voltage at the base of VT1 and a decrease in its collector current. Thus, the stabilization of the operating modes of the microphone amplifier is set. Capacitance C1 is the supply voltage filter capacitor, C2 is the separating capacitor. Through capacitor C3, the negative feedback signal voltage, removed from R6, is supplied in antiphase to the base of VT1. This guarantees a rollover in the frequency response in the high-frequency region and eliminates excitation at high frequencies. Capacity C4, like C2, is a separation container. The amplifier is adjusted for direct current by changing the value of resistor R4. The amplifier operates in class A mode. The value of resistor R4 should be such that as the input signal from the low-frequency generator increases, the amplitude of the positive and negative half-waves of the sine wave is limited simultaneously.