Do-it-yourself microphone preamplifier: diagram, how to make

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Microphone preamplifier , 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.

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.
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, capacitor C1 must be set to 10 μF. 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. The BC547 bipolar transistor can be replaced with the domestic KT3102. 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 you need further powerful sound amplification, you can build an amplifier using the TDA2030 chip.

Benefits of External Preamps

Higher sound quality, which is especially noticeable at high volumes. Built-in preamplifiers can satisfy the sound quality at a volume level of 40-50 dB, but as the level increases, you may notice extraneous noise. This is due to the fact that more expensive and high-quality preamps (which are rarely used as integrated ones) have more complex amplification circuits, providing transparent sound at any volume level.
A large amount of gain, something that built-in preamps cannot boast of, offering no more than 60 dB of gain. This is not enough for many dynamic microphones.
Noise, or rather the absence of it, in external preamplifiers when recording very quiet sound sources. Values above -127 dBu can be considered good equivalent output noise (EIN) values for built-in preamps.
Character of sound. As mentioned above, an external microphone preamplifier can not only pleasantly color the sound, but also give it a certain character, which cannot be said about the built-in preamps, which provide a “sterile” sound. Tube and transistor representatives of preamplifiers of the 60-70s of the last century are a striking example of this.
Advanced functionality. External preamps usually have additional features such as phase change, low cut, gain switching, etc.

Circuit of a simple microphone preamplifier using one transistor

This microphone preamplifier circuit works with both dynamic and electret microphones.

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

What types of preamplifiers are there?

1. By form factor: built-in and external

, but this was mentioned above.

2. According to the scheme: tube, transistor and modeling

. Tube circuits are traditionally considered “warmer” and “live” in terms of sound compared to transistor ones. But they usually cost more. To notice that special tube sound, the preamp must be in the higher price range. If you want to save money, choose a transistor preamplifier; at an average price, you are unlikely to feel the difference with a tube one.

You can also find a hybrid microphone

a preamplifier that combines tube and transistor components.

Modeling Preamps

are based on transistor circuits, but using digital conversion they simulate the sound characteristics of tube preamplifiers.

3. By the nature of the influence on the signal: coloring and transparent

. Color preamps, as the name suggests, add character to the sound. Typically, they are used by sound engineers in an attempt to add uniqueness to their mixes. Transparent preamps are designed to produce clear sound without any impurities. Typically, these are transistor transformerless preamplifiers.

4. By the presence of a transformer: transformer and transformerless

. The name speaks for itself - either a transformer is present in the family or it is not. Transformer preamps sound fatter, but still color the sound.

Low-noise ULF for microphone on K548UN1A

Figure 1 shows an example of a ULF based on a specialized microcircuit - IC K548UN1A, containing 2 low-noise op-amps. The op-amp and ULF created on the basis of these op-amps (IC K548UN1A) are designed for a unipolar supply voltage of 9V - ZOV. In the above ULF circuit, the first op-amp is included in a version that ensures the minimum noise level of the op-amp.

Rice. 1. ULF circuit on the K548UN1A op-amp and microphone connection options: a - ULF on the K548UN1A op-amp, b - connection of a dynamic microphone, c - connection of an electret microphone, d - connection of a remote microphone.

Elements for the circuit in Figure 1:

R1 =240-510, R2=2.4k, R3=24k-51k (gain adjustment),
R4=3k-10k, R5=1k-3k, R6=240k, R7=20k-100k (gain adjustment), R8=10; R9=820-1.6k (for 9V);
C1 =0.2-0.47, C2=10µF-50µF, C3=0.1, C4=4.7µF-50µF,
C5=4.7uF-50uF, C6=10uF-50uF, C7=10uF-50uF, C8=0.1-0.47, C9=100uF-500uF;
Op-amps 1 and 2 - IS K548UN1A op-amp (B), two op-amps in one IC package;
T1, T2 - KT315, KT361 or KT3102, KT3107 or similar;
D1 - zener diode, for example, KS133, you can use an LED in normal switching, for example, AL307;
M - MD64, MD200 (b), IEC-3 or similar (c),
T - TM-2A.

The output transistors of this ULF circuit operate without an initial bias (with Irest = 0). There is virtually no step distortion due to deep negative feedback covering the second op-amp of the chip and the output transistors. If it is necessary to change the mode of the output transistors (Iquiescent = 0), the circuit must be adjusted accordingly: include a resistor or diodes in the circuit between the bases T1 and T2, two 3-5k resistors from the bases of the transistors to the common wire and the power wire.

By the way, outdated germanium transistors work well in ULF in push-pull output stages without an initial bias. This allows the use of op-amps with a relatively low slew rate of the output voltage with this output stage structure without the risk of distortion associated with zero quiescent current. To eliminate the danger of excitation of the amplifier at high frequencies, a capacitor SZ is used, connected next to the op-amp, and the R8C8 chain at the ULF output (quite often RC at the amplifier output can be eliminated).

Electret microphones

Recently, electret condenser microphones have been used in household tape recorders. Electret microphones have the widest frequency range - 30...20000 Hz.

Microphones of this type produce an electrical signal twice as large as conventional carbon ones.

The industry produces electret microphones MKE-82 and MKE-01 in size similar to carbon microphones MK-59 and the like, which can be installed in ordinary telephone handsets instead of carbon ones without any modification of the telephone set.

This type of microphone is much cheaper than conventional condenser microphones, and is therefore more accessible to radio amateurs.

The domestic industry produces a wide range of electret microphones, among them MKE-2 unidirectional for reel-to-reel tape recorders of class 1 and for integration into radio-electronic equipment - MKE-3, MKE-332 and MKE-333.

For radio amateurs, the MKE-3 condenser electret microphone, which has a microminiature design, is of greatest interest.

The microphone is used as a built-in device in domestic tape recorders, radios and tape recorders, such as Sigma-VEF-260, Tom-303, Romantic-306, etc.

The MKE-3 microphone is manufactured in a plastic case with a flange for mounting on the front panel of the radio device from the inside. The microphone is omnidirectional and has a circle pattern.

The microphone does not allow shocks or strong shaking. In table 2 shows the main technical parameters of some brands of miniature condenser electret microphones.

Table 2.

Microphone type	MKE-3	MKE-332	MKE-333	MKE-84
Nominal operating frequency range, Hz	50…16000	50… 15000	50… 15000	300…3400
Free field sensitivity at a frequency of 1000 Hz, µV/Pa	no more than 3	at least 3	at least 3	A - 6...12 V - 10...20
Uneven frequency response of sensitivity in the range 50… 16000 Hz, dB, not less	10	—	—	—
Total electrical resistance module at 1000 Hz, Ohm, no more	250	600 ±120	600 ± 120	—
Equivalent sound pressure level due to the microphone’s own noise, dB, no more	25	—	—	—
Average difference in sensitivity levels “front - rear”, dB	—	no, less than 12	no more than 3	—
Operating conditions: temperature, C relative air humidity, no more	5…30 85% at 20″С	-10…+50 95±3% at 25″С	10…+5095±3%at 25″С	0…+4593%at 25″С
Supply voltage, V	—	1,5…9	1,5…9	1,3…4,5
Weight, g	8	1	1	8
Overall dimensions (diameter x length), mm	14×22	10.5 x 6.5	10.5 x 6.5	22,4×9,7

In Fig. Figure 5 shows the connection diagram for the MKE-3 type electret microphone, which is common in amateur radio designs.

Rice. 5. Schematic diagram of connecting a microphone of the MKE-3 type at the input of a transistor ultrasonic sounder.

Rice. 6. Photo and internal circuit diagram of the MKE-3 microphone, location of colored conductors.

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 circuit of a two-stage transistor pre-amplifier

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.

Active microphone amplifier

I have been struggling with my sound card for a long time, namely its “microphone” input. My previous attempts to make a proper microphone amplifier were Amplifier 1.

The whole problem was that my “microphone volume” slider was set to maximum. By lowering its level, I got rid of one problem and got another: the amplification of the previous ULFs was not sufficient.

Therefore, I decided to make an amplifier for a microphone with sufficient gain, low-voltage power supply and low current consumption. During the calculations, a good circuit was obtained, with easily accessible parts - it is based on an LM358 op-amp. And now I share it with you:

Stereo version of active microphone amplifier

For anyone who wants to do it, here's a little reminder about wiring the audio plug:

Now I will briefly describe its work. Power supply: lithium battery from 3 to 4.2 V. Current up to 1 mA. Gain is selected using the formula:

Ku = -(R2/R1)

The diagrams show a gain of 100 times (100k/1k). The minus in the formula is due to the fact that the amplifier inverts the output signal. For me this is not critical, and it does not affect the sound much. For those who are interested in how I calculated it, selected the denominations, and ran it in the simulator, here is the video:

Next is the printed circuit board. I don’t like drilling, so I did everything on planar parts. The sign was made in the Diptrace program:

Since most site users use SprintLayout, I transferred the printed one to LAY6 format. I warn you right away - the signets must be “mirrored”.

By the way, this video was already recorded with this amplifier, so you can evaluate the quality of the recording. And the fully assembled amplifier looks like this:

And this is the calculated frequency response and phase response of the amplifier, when adding a capacitor in parallel R8R2R14 - 510 pF. You can click on it and enlarge it:

If desired, the circuit can be further miniaturized if you want to install it inside a small case. All files—signets, diagrams—are in the archive. Author of the material BFG5000.

MU Forum

Discuss the article Active microphone amplifier

Problem

Most cheap microphones don't have a default sensitivity enough to be heard clearly. You have to scream, but you can’t do that on a regular basis; yelling is a tiresome and harmful activity.

Having carefully studied the issue, I came to the conclusion that the manufacturers are to blame for the situation, overly simplifying the design of the device. Having given his hard-earned 100-500 rubles, the buyer essentially receives a module (capsule) of an electret microphone without any electronic “piping”.

Electret microphone and standard 3.5 mm jack. This design does not allow the microphone to be sensitive, but you can record sound

All sorts of flexible legs and clothespins are optional tinsel. Formally, such microphones work, but their sensitivity and recording quality are low (noise is heard). There's nothing stopping you from adding a few electronic components to the circuit to improve the microphone's ability to pick up quiet sounds.

A typical representative of electret microphones

Here and further we will talk about electret microphones, as the most affordable on the market. And, partly, capacitors. Not dynamic!

I'm also not considering purchasing a separate sound card. This was already in the article “How to set up a microphone, record and process sound - instructions for beginners.”

Dynamic microphones already have a built-in amplifier

Amplifier circuits are quite simple, so people who know how to use a soldering iron remake microphones and enjoy life.

By the way, even cheap buttonholes for 100 rubles include good electret modules. For example, I have a Genius clip-on microphone from ten years ago, it works great. After modifications, of course.

In addition to low sensitivity, you can hear a low hiss on the recordings. It can be suppressed by filters in an audio editor, but when the interference is too strong, removing noise will distort the useful part of the recording and the voice will sound dull, as if from a barrel.

Noise (in 99% of cases this is interference from electromagnetic fields) appears at several stages of sound delivery:

In the electret capsule of the microphone.
In the microphone preamp, if available.
When transmitting a signal via a connecting cable that is not shielded from interference.
In the sound card amplifier.

The most painful place is the computer's sound card. Replacing with a better one and/or moving it outside the computer case can get rid of the noise, but not everyone has the money for such an upgrade.

Most often, the user is left alone with a cheap microphone plugged into a loud hissing sound card soldered to the computer motherboard. You can try to make the sound louder programmatically.

Improved amplifiers for sensitive microphones

The use of low-speed op-amps in ULF output stages and the operation of silicon transistors in power amplifiers in a mode without an initial bias (quiescent current is zero - mode B) can, as noted above, lead to transient distortions of the “step” type. In this case, to eliminate these distortions, it is advisable to change the structure of the output stage so that the output transistors operate with a small initial current (AB mode).

Figure 4 shows an example of such a modernization of the above amplifier circuit with a differential input (Figure 3).

Fig.4. ULF circuit using an op-amp with a differential input and a low-distortion output stage.

Elements for the circuit in Figure 4:

R1=R2=20k (equal to or slightly higher than the maximum source resistance in the operating frequency range),
RЗ=R4=1m-2m; R5=2k-10k, R6=1k-Zk,
R7=47k-300k (gain adjustment, K=1+R7/R6),
R8=10, R10=10k-20k, R11=10k-20k;
C1 =0.1-0.22, C2=0.1-0.22, SZ=4.7uF-20uF, C4=0.1;
OU - K140UD8, KR1407UD2, KR140UD12, KR140UD20, KR1401UD2B or other op amps in standard configuration and preferably with internal correction;
T1, T2 - KT3102, KT3107 or KT315, KT361, or similar;
D2, D3 - KD523 or similar;
M - MD64, MD200, IEC-3 or similar (c),
T - TM-2A.

Figure 5 shows an example of ULF on transistors. In the first stages, transistors operate in microcurrent mode, which minimizes ULF noise. The circuit is in many ways similar to the circuit in Figure 2. To increase the share of the useful low-level signal against the background of inevitable interference, a bandpass filter is included in the ULF circuit, which ensures the selection of frequencies in the 300 Hz -3.5 kHz band.

Op-amp microphone amplifier with differential input

Figure 3 shows an example of a ULF on an op-amp with a differential input . A properly assembled and tuned ULF provides significant suppression of common-mode interference (60 dB or more). This ensures that the useful signal is isolated with a significant level of common-mode interference.

It should be recalled that common-mode interference is interference arriving in equal phases at both inputs of the ULF op-amp, for example, interference induced on both signal wires from a microphone. To ensure correct operation of the differential cascade, it is necessary to precisely fulfill the condition: R1 = R2, R3 = R4.

Fig.3. ULF circuit on an op-amp with a differential input and options for connecting microphones: a - ULF with a differential input, b - connecting a dynamic microphone, c - connecting an electret microphone, d - connecting a remote microphone.

Elements for the circuit in Figure 3:

R1=R2=20k (equal to or slightly higher than the maximum source resistance in the operating frequency range),
RЗ=R4=1m-2m; R5=2k-10k, R6=1k-Zk,
R7=47k-300k (gain adjustment, K=1+R7/R6), R8=10, R9=1.2k-2.4k;
C1=0.1-0.22, C2=0.1-0.22, SZ=4.7uF-20uF, C4=0.1;
Op-amp - KR1407UD2, KR140UD20, KR1401UD2B, K140UD8 or other op-amps in a standard connection, preferably with internal correction;
T1, T2 - KT3102, KT3107 or KT315, KT361, or similar;
D1 - zener diode, for example, KS133, you can use an LED in normal switching, for example, AL307;
M - MD64, MD200 (b), IEC-3 or similar (c),
T - TM-2A.

It is advisable to select resistors using an ohmmeter among 1% resistors with good temperature stability. To ensure the necessary balance, it is recommended that one of the four resistors (for example, R2 or R4) be made variable. This can be a high-precision variable resistor trimmer with an internal gearbox.

To minimize noise, the input resistance of the ULF (the values of resistors R1 and R2) must match the resistance of the microphone or a sensor replacing it. The ULF output transistors operate without an initial bias (from 1 rest = 0). There is virtually no step distortion due to deep negative feedback between the second op amp and the output transistors. If necessary, the transistor connection circuit can be changed.

Setting up the differential cascade: apply a 50 Hz sinusoidal signal to both inputs of the differential channel simultaneously, selecting the value of RЗ or R4 to ensure a zero signal level of 50 Hz at the output of op amp 1. For tuning, a 50 Hz signal is used, because A power supply with a frequency of 50 Hz makes the maximum contribution to the total value of the interference voltage. Good resistors and careful tuning can achieve common-mode noise suppression of 60dB-80dB or more.

To increase the stability of the ULF operation, it is advisable to bypass the power supply pins of the op-amp with capacitors and turn on an RC integer at the amplifier output (as in the amplifier circuit in Figure 1). For this purpose, you can use KM6 capacitors.

To connect the microphone, a twisted pair of wires in the screen is used. The screen is connected to the ULF (only at one point!!) as close as possible to the op-amp input.

Guitar preamp - circuit diagram

We solder the guitar pre-amp on a separate board, later placed in an interference shield. A photo of the preamp board is shown below. It is based on two operational amplifiers with a tone and gain control unit.

This is a simple but proven circuit design that provides excellent tonality throughout the entire range. The design is ideal for those guitarists who would like to get great sound. The tone controls have enough range to cover almost anything, from violin to bass guitar.

The preamplifier uses dual op-amp for amplification. The transistor is connected according to the emitter follower circuit and has a low output impedance, after the master volume control. As shown in the diagram, there is a typical guitar input from which you can get a very fat overdrive and then adjust it to the appropriate level

Please note that when using the TL072 op amp, noise with a lot of high frequencies is possible. We highly recommend using the OPA2134 op amp from Texas Instruments for truly the quietest guitar amp you've ever heard!

Elementary base

Modern element base makes it possible to create high-quality ULFs based on low-noise operational amplifiers (op-amps), for example, K548UN1, K548UN2, K548UNZ, KR140UD12, KR140UD20, etc.

However, despite the wide range of specialized microcircuits and op-amps, and their high parameters, ULF transistors have not lost their importance at present. The use of modern, low-noise transistors, especially in the first stage, makes it possible to create amplifiers with optimal parameters and complexity: low-noise, compact, economical, designed for low-voltage power supply. Therefore, transistor ULFs often turn out to be a good alternative to integrated circuit amplifiers.

To minimize the noise level in amplifiers, especially in the first stages, it is advisable to use high-quality elements. Such elements include low-noise bipolar transistors with high gain, for example, KT3102, KT3107. However, depending on the purpose of the ULF, field-effect transistors are also used.

The parameters of other elements are also of great importance. In low-noise cascades of electronic circuits, oxide capacitors K53-1, K53-14, K50-35, etc. are used, non-polar ones - KM6, MBM, etc., resistors - no worse than traditional 5% MLT-0.25 and ML T- 0.125, the best option for resistors is wirewound, non-inductive resistors.

The input impedance of the ULF must match the impedance of the signal source - a microphone or a sensor replacing it. Typically, they try to make the input impedance of the ULF equal (or slightly greater) to the resistance of the signal source-converter at fundamental frequencies.

To minimize electrical interference, it is advisable to use shielded wires of a minimum length to connect the microphone to the ULF. It is recommended to mount the IEC-3 electret microphone directly on the board of the first stage of the microphone amplifier.

If it is necessary to significantly distance the microphone from the ULF, you should use an amplifier with a differential input, and the connection should be made using a twisted pair of wires in the screen. The screen is connected to the circuit at one point of the common wire as close as possible to the first op-amp. This ensures that the level of electrical noise induced in the wires is minimized.

Scheme

The circuit is extremely simple, containing only two resistors, two capacitors, a transistor and an electret microphone capsule. The transistor can be used in almost any low-power npn structure, for example, KT3102, BC547, BC337. An electret microphone can be obtained, for example, from a broken headset or handset, or you can buy it at a radio parts store. The sensitivity of the microphone will greatly depend on this element, so it is advisable to take several and check which one is best suited. The advantage of this circuit is that it uses phantom power. Those. The sound signal is transmitted through the same wires as the power supply. If you take a voltmeter and measure the voltage at the microphone input of your computer, it will be about 3-4 volts. When connecting the microphone circuit, this voltage should drop to a level of 0.6-0.7 volts, thus, an external power source will not be needed and there will be no extra wires at the workplace.

We make the amplifier ourselves

I warn you right away: it is not advisable to power homemade microphone preamplifiers from the power supply - you will have to install a separate circuit to filter the power from interference. The batteries will last a long time and there will be no problems with power supply.

Ready-made microphone module on the MAX9812 chip

The easiest option is to buy a microphone module for Arduino on a MAX9812 chip (70 rubles), a cable (30 rubles), a 3.5 mm plug (15 rubles) and a CR-2032 coin-cell battery (from 30 rubles). The components will cost 150 rubles.

The scarf can be turned into a full-fledged microphone with minimal soldering skills or by asking those who know how to solder it.

The plug plugs into the line input, the batteries will last a long time.

Gain 3-5 times with phantom power

This is enough for communicating on Skype; you no longer have to swallow the microphone.

If there are normal radio parts stores in your city, you should take a closer look at it, because all the components are standard. I don’t have a single normal radio parts store in Essentuki, I couldn’t even find a capacitor of a suitable value, I had to order it online. The transistor does not have to be BC547, there are many analogues, they are easy to Google.

Connects to the microphone input of a computer or video camera. That is, this option is portable; video recording can be improved if the camera supports connecting external microphones.

The modification is cheap and effective, but requires a shielded cable, otherwise the hiss is too noticeable, because the microphone input is still there.

Gain 3-5 times with battery power

An analogue of a module for Arduino, a transistor is used instead of a microchip.

Connects to line input, noise is minimal. Simple, but only suitable for inherently sensitive microphones, because... the gain is too low.

10-1000 times amplification, battery powered

In my research I stopped at a diagram that I spotted somewhere in a thread on the RadioKot forum. I redrew it in Qucs-S to test and make sure the values were correct.

P1 and P2 are the plus and ground of the electret microphone, respectively, P3 and P4 are connected to the line input of the computer.

In reality, the circuit turned out to be very sensitive, I could hear the breathing of a parrot in a cage two meters away from me, I had to add a 10 kOhm resistor R6 to muffle the signal from the microphone capsule. Also, the signal amplitude at the output of the amplifier may be too large, so it can also be limited by a resistor by placing it in front of pin P3.

It runs on two AA batteries, I don’t know how long they will last, they haven’t run out in a week.

Additional Information

This version of a homemade preamplifier does not require configuration. Connect the microphone and start recording.

Current consumption (mA):

during work – 9;
in standby mode – 3.

The preamp is supplied with phantom power, i.e. its source is the sound card itself. In such options, the supply wire simultaneously transmits the signal from the microphone. Alternative nutrition methods have disadvantages:

batteries have to be changed periodically;
connecting to an outlet via a power supply, additional wires appear.

This version of a homemade preamplifier does not produce noise interference and improves sound quality. Works with both dynamic and electret microphones.

Additional functions

When buying a microphone preamplifier, first of all, of course, you need to listen and choose with your ears

But then it's worth plugging in your head and paying attention to some factors and features that may be useful to you. The number of inputs means the number of microphones that can be recorded at the same time

The bigger, the better.

The number of inputs refers to the number of microphones that can be recorded simultaneously. The bigger, the better.

Low pass filter.
Its task is to cut low frequencies, which helps cut off various low-frequency interference.
Phantom power
, which is necessary for the operation of condenser microphones. By the way, almost all models have it, but make sure of it, just in case.
Built-in limiter (compressor)
, which is used to compress dynamic range. The limiter also helps to avoid unnecessary overloads. To be honest, this is not a particularly necessary feature, and sometimes it is not worth paying extra for it.
Level indicators
– an optional feature for every preamplifier, but certainly useful. It's worth paying extra for it.
Phase switch
will help cope with phase problems when you record several microphones at the same time.
Equalizer
adjusts the frequencies of the sound signal before sending it to the sound card. Before purchasing, evaluate: do you really need this option?

What to choose?

If you are a beginner, then you should not choose an external microphone preamplifier. It's better to buy a more expensive audio interface with a good built-in preamp and one or two condenser microphones. An external preamp can be the next step in your improvement and experience.

The work of external preamplifiers is especially noticeable when using microphones with weak output (dynamic). Condenser microphones have a stronger output signal, so you won't notice much of a difference between built-in and external preamps.

The most popular preamplifiers are models from Focusrite manufacturers. Behringer and Art. Choose, experiment and good luck in your creativity!

Elementary base

Modern element base makes it possible to create high-quality ULFs based on low-noise operational amplifiers (op-amps), for example, K548UN1, K548UN2, K548UNZ, KR140UD12, KR140UD20, etc.

Preamplifier for electret microphone with three transistors

This is another microphone amplifier option for an electret microphone. 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 an electret microphone, 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. A voltage divider 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.

Printed circuit boards.

The images of printed circuit boards show a view from the side of the elements. The tracks are visible through the board.

The picture shows an example of the PCB layout of a universal microphone amplifier.

Entrance.
The top end of potentiometer R3 according to the diagram.
Potentiometer motor R3.
LED anode HL1.
Frame.
Power supply +6V.
Exit.
Frame.

An example of a printed circuit board layout for a dynamic microphone amplifier.

I myself made a printed circuit board based on the dimensions of the controls and housing at my disposal.

Link to printed circuit board drawings at the end of the article.

Low-noise microphone ULF using transistors

Figure 2 shows an example of a ULF circuit using transistors. In the first stages, the transistors operate in microcurrent mode, which minimizes internal ULF noise. Here it is advisable to use transistors with a high gain but low reverse current.

This could be, for example, 159NT1V (Ik0=20nA) or KT3102 (Ik0=50nA), or similar.

Rice. 2. ULF circuit on transistors and options for connecting microphones: a ULF on transistors, b - connection of a dynamic microphone, c - connection of an electret microphone, d - connection of a remote microphone.

Elements for the circuit in Figure 2:

R1=43k-51k, R2=510k (adjustment, Ukt2=1.2V-1.8V),
R3=5.6k-6.8k (volume control), R4=3k, R5=750,
R6=150k, R7=150k, R8=33k; R9=820-1.2k, R10=200-330,
R11=100k (adjustment, Uet5=Uet6=1.5V),
R12=1 k (adjustment of the quiescent current T5 and T6, 1-2 mA);
C1=10uF-50uF, C2=0.15uF-1uF, C3=1800,
C4=10µF-20µF, C5=1µF, C6=10µF-50µF, C7=100µF-500µF;
T1, T2, T3 -159NT1 V, KT3102E or similar,
T4, T5 - KT315 or similar, but MP38A is also possible,
T6 - KT361 or similar, but MP42B is also possible;
M - MD64, MD200 (b), IEC-3 or similar (c),
T - TM-2A.

The use of such transistors allows not only to ensure stable operation of the transistors at low collector currents, but also to achieve good amplification characteristics with a low noise level.

Output transistors can be used either silicon (KT315 and KT361, KT3102 and KT3107, etc.) or germanium (MP38A and MP42B, etc.). Setting up the circuit comes down to setting resistor R2 and resistor RЗ the corresponding voltages on the transistors: 1.5V on the collector T2 and 1.5V on the emitters T5 and T6.

Preamplifier for electret microphone with three transistors

Ready amplifiers

I won’t consider expensive options, sorry. It is assumed that the budget is extremely limited.

Speaker/headphone amplifiers will not work. They are not sensitive enough, do not supply phantom power to the microphone, and the output power is too high even for a line input.

On Aliexpress, you need to search for devices using the queries “microphone preamplifier” and “microphone preamplifier”. The cheapest options cost one and a half to two thousand rubles. Designed for karaoke, but if you don’t turn it up to full volume, you can connect it to the line input.

For three thousand rubles you can find a full-fledged preamplifier, to which a musical instrument is also connected. For example, a guitar with a pickup.

To connect a cheap computer microphone, you will need a 3.5 mm jack > 6.3 mm jack adapter. The computer must have a line input.

And don’t forget about such a miracle as the BM 800 condenser microphone, which has conquered the vocal cords of YouTubers reviewing goods from Chinese shops:

BM 800 computer microphone Condenser 3.5 mm Wired

Let me clarify: I do not recommend it for purchase. It is not entirely clear under what conditions it works normally; the reviews are too contradictory. But sometimes VM 800 can be found for 300-500 rubles, which is not much more expensive than primitive electret ones, but with a preamplifier. But it connects to the microphone input, which means hello, sound card interference.

Power supply for electret microphone

For some reason, there is very little information on the Internet about how to properly turn on electret microphones. Usually the standard version is used, in which the voltage is supplied through a current-limiting resistor, and then a capacitor is installed to cut off the direct voltage.

However, in most circuits, not a word is said about the selection of this resistor and simply indicates a specific value. Although in general this is not entirely true. The value of this resistor should not be chosen out of thin air, but selected for each specific microphone capsule.

But how to choose it?

Fortunately, a very interesting article was found, in which the author carried out a number of measurements and made a very useful, from a practical point of view, conclusion.

In addition to the optimal operating mode of the microphone, this feature is also convenient because as a bonus we get a bias for the operational amplifier when powered from a unipolar source. This means that you can remove the extra capacitor and two resistors from the circuit.

The most common problems with microphones and headphones

The modern microphone dates back to the 20th century, but it's getting smaller and smaller every year, thanks to advances in science and technology, as well as silicon semiconductors, which are ideal for creating multi-element circuits such as microphones and headphones.

There are two types of microphones: condenser microphones, in which capacitors try to absorb all noise, and coil microphones, they are cheaper, but practically not inferior to their condenser brother. First, it’s worth understanding what a modern microphone consists of:

A special thin wire through which electrical charges will flow freely to the amplifier or microphone.
Sensitive membrane.
Electromagnetic coil.

When any sound waves hit a very sensitive membrane, which, by the way, is located in a protected iron casing, a pulse is created in the coil, after which the sound card or amplifier catches the sent signal, decodes it into an acceptable form and we hear the desired sound. Previously, this sound recording process took a lot of time, but modern technologies have reduced it several times.

Microphone preamplifier. Selection of schemes

A microphone preamplifier, also known as a preamplifier or microphone amplifier, 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.

Amplifier circuit option for a dynamic microphone

The circuit is distinguished by its super-simplicity and mega-repeatability; the circuit contains two resistors (R1, 2), two capacitors (C2, 3), a 3.5 plug (J1), one electret microphone and a transistor. Capacitor C3 works as a microphone filter. Capacity C2 should not be neglected, that is, it should not be set more or less than the nominal value indicated in the diagram, otherwise this will entail a lot of interference. We install the domestic KT3102 . To reduce the size of the device, I used an SMD transistor marked “1Ks”. If you don’t know how to solder at all, go to the forum.

When replacing T1 there were no significant changes in quality. All other parts are also in SMD cases, including capacitor C3. The entire board turned out to be quite small, although it can be made even smaller using the LUT printed circuit board manufacturing technology. But I made do with a simple half-millimeter permanent marker. I etched the board in ferric chloride in 5 minutes. The result is a microphone amplifier board that is attached to a 3.5 plug.

All this fits well inside the plug casing. If you do this too, I advise you to make the board as small as possible, since for me it deformed the casing and changed its shape. It is advisable to wash the board with solvent or acetone. The result was a useful device with good sensitivity:

Before connecting the microphone to the computer, check all the contacts and whether there is +5v power at the microphone input (and there should be), in order to avoid comments like: “I assembled it exactly as in the diagram, but it doesn’t work!” This can be done this way: connect a new plug to the microphone connector and measure the voltage with a voltmeter between ground (large tap) and two short solder taps. Just in case, try not to short-circuit the plug leads together when you measure the voltage. I don’t know what will happen then and I don’t want to check. My microphone amplifier has been working for 3 months now, and I am completely satisfied with the quality and sensitivity. Collect and post on the forum about your results, questions, and maybe even about modifications to the case, circuits and methods of their manufacture. BFG5000 was with you , good luck!

Discuss the article ELECTRIC MICROPHONE AMPLIFIER

When studying the connection diagrams for electret microphones [1], their uniformity is deeply surprising. The connection point between the microphone and the load resistor is connected to the amplifier itself through a coupling capacitor (Fig. 1) in 100% of the studied circuits.

Rice. 1

There may be other connection schemes, but the author has not seen them. At the same time, anyone who has been closely and long associated with audio reproduction will probably not strongly object to the fact that any capacitor in the path of the audio signal is an undesirable component. This is especially true for electrolytic capacitors, which are inevitably used in the case of a sufficiently low input resistance of the amplifier stage.

An approximate study of the operating modes of electret microphones [2] showed that, firstly, they are current sources and, secondly, the maximum amplitude of their output signal is observed when the voltage drops across the microphone and the load resistor are the same.

Let's consider one of the well-known [3] circuits of a microphone amplifier with an AGC system made using an op-amp (Fig. 2).

Rice. 2

The circuit consists of a non-inverting amplifier itself based on op-amp DA1, the non-inverting input of which receives an artificial midpoint from the divider R3R4, as well as the input signal through the isolation capacitor C2; controlled divider of the OOS signal (resistor R5, capacitor C1 and channel resistance of the field-effect transistor with PN junction VT1); output amplified signal detector (capacitors C3, C4 and diodes VD1, VD2). The detected output signal of negative polarity controls the conductivity of the VT1 channel, increasing it, thereby reducing the gain of the op-amp.

Considering the presence of a constant component of the divider formed by the electret microphone and its load resistor, we can conclude that the components C2R3R4 are completely unnecessary. The role of R4 is perfectly performed by the microphone itself, and R3 is its load resistor. Capacitor C2 is generally superfluous, as a class.

The result was the diagram shown in Fig. 3.

Rice. 3

The R3C1 RC filter provides additional filtering of the electret microphone supply voltage. In principle, it is optional (optional), but actually quite useful. The value of resistor R1 is selected such that at the point of its connection with the microphone there is approximately half the supply voltage. Resistors R4R6 linearize the transfer function of the controlled resistor on the field-effect transistor VT1.

Instead of resistor R5, a double T-shaped filter (on the right) can be included in the OOS circuit, raising the frequency band corresponding to the voice range. Its frequency response is shown on a Bode plotter from Multisim measuring instruments (below)

Naturally, any theoretical rants can be taken into account only if they are confirmed by practice. Therefore, the diagram shown in Fig. 3, was studied on a prototype.

Available micropower op-amps based on TLC271 and TL081 MOS transistors were used. The results were identical. In principle, any “sound” op amp can be used as an op amp (which absolutely does NOT include the LM358/324 and their clones.). The electret microphone used for these experiments was type J60. Repeating the experiments with other microphones was considered inappropriate in terms of time. Signal diagrams from the op-amp output were recorded with a RIGOL DS1052E digital oscilloscope. The “test phrase” spoken into the microphone at approximately the same volume was: “One-two-three-four-five, the bunny went out for a walk.” Of course, for the purity of the experiment, it would be desirable to use a recording played back through a speaker, but what happened is what happened.

First, a circuit without AGC was investigated. The detector and field-effect transistor were not connected, and a 10 kOhm resistor was connected from the lower terminal of capacitor C2 to the common negative bus. Thus, the gain was 11. The output signal for fast (10 ms/div) and slow (100 ms/div) sweeps at a distance of 20 cm from the mouth to the microphone are shown, respectively, in Fig. 4.

What was surprising was the signal range (peak-to-peak), which was more than 2 V. This means that the signal from the microphone was about 200 mV.

Next, instead of a 10 kOhm resistor, a field-effect transistor KP303Zh was connected with an initial drain current of 0.85 mA and a cut-off voltage of 0.7 V. Its gate was connected to the negative bus, which ensured the minimum resistance of its channel and, accordingly, maximum gain. The output signal of such a circuit is shown in Fig. 5.

As you can see, the signal from the microphone is excessively amplified, right up to clipping, which indicates the applicability of a field-effect transistor with such a small initial drain current with an OOS resistor resistance of about 100 kOhm.

Next, the complete circuit was studied, with all shown in Fig. 3 components. The output signals when pronouncing the “test phrase” from a distance of 20 and 60 cm, respectively (with slow scan) are shown in Fig. 6, and from a distance of 60 (with fast scan) - in Fig. 7.

As can be seen from these diagrams, the signal swing was about 4 V with a satisfactory shape, which is quite sufficient for ordinary applications. Unfortunately, the initial “surge” of amplitude (while the AGC system had not yet activated) could not be registered. The gopher was not visible, but he was present by ear.

Finally, two more field-effect transistors with a higher initial drain current and cutoff voltage were studied (respectively, another KP303Zh with an initial drain current of 1.2 mA and a cutoff voltage of 0.9 V, as well as KP303V with an initial drain current of 2.6 mA and cut-off voltage 1.2 V). The output signal with the first of them at a distance to the microphone of 20 cm (with slow scan) is shown in Fig. 8, and the output signals with the second at a distance to the microphone of 10 cm and 40 cm (with slow scan) are shown in Fig. 9.

In the first case, the signal swing was almost 5 V, and in the second - almost 7 V!

From these experimentally obtained data it is clear that for practical purposes it is desirable to use field-effect transistors with the lowest possible cutoff voltage. The initial drain current does not significantly affect the stabilized amplitude of the output signal for a given resistance of the OOS resistor.

Finally, the microphone “muting” mode was tested by short-circuiting the inverting and non-inverting inputs of the op-amp. There were no audible “clicks” with this method of muting.

For a “snack” - a circuit similar in function, made with transistors (maybe someone will like it): Fig. 10. True, it was not mocked up “live”, it was only simulated in Multisim. It showed almost the same results as the op-amp circuit.

Field effect transistor Q1 with resistor R1 is a model of an electret microphone. The value of the load resistor R2 selects half the supply voltage at the point where it connects to the microphone. The value of resistor R4 selects the equality of the collector currents Q2 and Q3. Field-effect transistor Q4 with resistor R5 is a parametric current generator for the differential stage on transistors Q2 and Q3. A similar role is played by transistor Q7 with resistor R9. for transistor Q6. In principle, these current generators can be replaced with ordinary resistors, but with them the amplifier parameters are by definition better. Finally, the variable resistor in the OOS circuit on transistor Q5 and the output signal detector are the same as in the op-amp circuit.

Conclusions:

Another amplifier for an electret microphone is presented to the court, which does not pretend to be exclusive, but is somewhat simpler than the known ones. Due to the elimination of one coupling capacitor in the audio signal path, it is of higher quality by definition.
Considering the fairly high gain provided by this amplifier, the op-amps for it, in order to provide sufficient bandwidth, must have a cutoff frequency of at least 5.10 MHz.
This amplifier without an AGC system can be used for highly sensitive amplification of signals from an electret microphone.

Buy or make it yourself?

A DIY microphone preamplifier has three main advantages over those models that can be bought in the appropriate store:

Price.
Perfect adaptation for a specific task.
Sound quality.

Price

So, the price of a finished product sold in a store, in addition to the cost of components, includes a fee for the brand, compensation for advertising expenses and the profit that everyone receives: the manufacturer, wholesaler and retailer, plus transportation costs. So it turns out that in a purchased amplifier, the housing alone will cost more than the entire microphone amplifier made by hand.

In addition, there are a number of consumer qualities that almost all manufacturers necessarily follow in order to achieve a certain versatility for possible applications of microphone preamplifiers. After all, the developers are faced with the task of achieving maximum compatibility with all possible microphones and the equipment with which it will have to work.

This leads to the fact that the microphone amplifier circuit acquires significant redundancy in the form of various operating modes, protection, controls and indicators. And the more parts there are in the device, the greater the impact they have on the sound quality, and not for the better.

Adaptation for a specific task

But in a home recording studio, a microphone amplifier usually works with one specific microphone, in a stationary environment, and always performs the same task. This means that we simply don’t need most of the universal capabilities of a purchased preamp. But we can focus on the maximum quality of exactly what we need, ideally adapting our own design to a specific task.

Sound quality

What is the difference between a good microphone amplifier for vocal recording and a regular one? First of all, the fact that a good preamplifier does not introduce its own artifacts and distortions into the sound, and at the same time creates the most optimal matching for the microphone to obtain the highest possible quality of conversion of sound into an electrical signal.

It is difficult to hear this audibly during a routine check. To evaluate the quality of a microphone amplifier, you need to work with it in real conditions, applying a variety of processing to vocals already recorded with it. All the shortcomings are especially pronounced at high compression levels and attempts to place vocals in a dense mix.

The sound quality of modern microphone preamplifiers, especially branded ones, as a rule, does not cause any particular complaints. But the natural desire of manufacturers to make the product as cheap as possible leads to the fact that formally all the characteristics correspond to the declared ones, but the components can be inexpensive, purely for marketing expediency.

Moreover, it is not always possible to check what a finished preamplifier is made of before you buy it.

So until you buy a preamp and work with it properly, you will not appreciate its quality. But you can quite easily make changes to your own design if you don’t like something.

And further.

For what?

Optimal signal level without distortion is an important point in the recording process, when the ideal ratio between clean signal and noise is achieved. The purpose of a preamplifier is to amplify a weak signal to linear level, that is, to the level of the recording equipment signal. The fact is that the microphone signal is quite weak, so you need to “finish off” it with additional decibels (around 30-60 dB). Guitars also require amplification of their signal, but not as significant as a microphone (about 20-30 dB).

That is, amplification of the microphone and instruments during the recording process is necessary. If you are using an interface or mixing console, then you can get by with the internal preamp circuitry that is found on all such devices. Of course, the quality of the built-in preamplifiers varies between models and significantly depends on the price that is asked for a particular audio interface. That is, expensive audio cards have significantly better preamplifiers. But for the first steps in sound recording, for home studios, the quality of the built-in preamplifiers is quite sufficient.

Does it make sense then to buy a separate mic preamp? In pursuit of a high-quality signal - yes. Since microphone preamplifiers, as a separate device, most often (but not always) provide higher quality sound than those integrated into an audio interface, mixing console, etc. In addition, an external microphone preamplifier colors the sound, giving it some individuality and character, but this mainly applies to top-end models.

How to connect a microphone correctly

To achieve high-quality sound, you need to know how to connect a microphone to a computer. Not everyone understands what kind of multi-colored inputs are located on the back panel of a desktop computer. It’s easier with laptops: there are always explanatory icons near the connectors; on desktop PCs this is a luxury.

(There may be connectors for connecting additional speakers, which is great for creating a home theater; we don’t need them.)

There are three main connectors: speaker output (headphones), microphone and line inputs, each assigned a specific color.

Why you need a microphone input and speaker/headphone output is clear from the name. And with linear (Line in) the situation is more interesting. It is also designed for recording sound, but is simpler.

Microphone input	Line input
The voltage supplied is 2 - 5 Volts, the exact value depends on the sound card model	No output voltage
A preamplifier is installed, amplifying the signal approximately 30-50 times	No amplifier

A device connected to the microphone jack is supplied with voltage (so-called “phantom power”), and the return signal passes through an amplifier. This is where the noise in the recording arises: firstly, the supplied power has its own frequency, and secondly, the electronic components of the sound card catch and make louder all the noise and signals from the microphone and surrounding devices.

The linear input does not have phantom power, and there is no amplifier as such. A powerful signal is needed, but when digitizing the signal, a minimum of extraneous noise is mixed in. For example, you can take an old cassette player and connect its headphone output to Line in - this way you can digitize audio cassettes.

Electret and condenser microphones cannot simply be plugged into a line input. More precisely, the electret will work, but without power it, being a very weak current generator, will produce a too quiet sound, almost inaudible.

What to do? Why know this nerd? And to the fact that there are two types of amplifiers that can increase the sound volume, connected either to the microphone or to the line inputs. And you need to understand which option is right for you.

Built into the microphone, powered by voltage running along the microphone cable. They amplify the signal up to 10 times (I can’t say exactly in decibels), and are highly vulnerable to interference.
With external power supply from batteries or a separate unit. They can amplify the signal 10-1000 times and connect to the linear input. The noise does not disappear anywhere, but relative to the useful signal it is a hundred times quieter, therefore, by connecting even a cheap hundred-ruble lavalier through an amplifier, you can get high-quality sound.

That is, ideally, the microphone should be connected through an amplifier to the line input and everything will be OK.

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.

Source: setafi.com