How to Connect an Electret Microphone to a Tube Amplifier

In ordinary telephones, instead of carbon microphones, they now begin to use electret microphones, designed in a housing that imitates the body of a carbon microphone, which allows replacement without any problems in any phone.

Once the author had to repair a “TELTA” telephone set, which had an MKE-395-2 “Eltis” electret microphone installed. The appearance of the microphone is shown in Fig. 1.

The frequency characteristics of such a microphone, compared to a carbon microphone, are much better, the volume is comparable to a carbon one, although the volume was expected to be higher.

Rice. 1. Appearance of the microphone.

Amplifier circuit options

In another article, the same author proposed a ready-made preamplifier for a microphone.
This is an AGC (Automatic Gain Control) circuit. This is what this circuit looks like in the original (without a frequency correction circuit): Thanks to the use of a field-effect transistor (KP303Zh) in feedback, this circuit works as a compressor and equalizes the volume of the voice, changing the gain within certain limits.

The circuit is fully working, I personally tested it on a prototype and did not cause any problems. This design is very convenient, for example, for microphones in conference rooms and meeting rooms. But it can also be used as a preamplifier for a microphone when connected to a computer.

Therefore, the AGC had to be abandoned and the circuit was cut down to a conventional non-inverting amplifier with a constant gain. This scheme also copes with its responsibilities perfectly.

Questions

What is a microphone's polar pattern? A microphone's polar pattern is a way of representing the directionality of a microphone. In other words, polar patterns describe the directions in which the microphone is most sensitive, rental sensitive, and all points in between. Polar charts can be qualitative (with specific names) or quantitative (with polar response charts).

For a more in-depth look at microphone polar patterns, read my article, The Ultimate Guide to Microphone Polar Patterns.

What happens if the speaker is connected with reverse polarity? Connecting a speaker with reverse polarity will not damage the speaker, but will likely have a negative impact on playback. Transitional information can be lost when the speaker moves inward rather than outward. When two or more speakers are connected in reverse polarity, major phase compensation problems arise.

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.

In-phase vs. Out of phase

Now let's discuss phase relationships with microphones. Phase is a little more complicated than polarity.

Phase refers to the amount of time a wave has gone through its cycle. Since sound and microphone signals are waves, phase is important. Two identical waves/in-phase signals are added, while two identical waves/in-phase signals cancel each other.

Phase usually applies to signals of the same frequency. If these waves are in phase, they line up along their entire shape. If they are out of phase, there will always be a difference between the two cycles at any given time.

However, audible sound and sound have a frequency range of 20 Hz. - 20000 Hz. For audio or sound to be truly 100% in phase is almost impossible unless we dub the audio digitally or use synthesizers.

By their nature, sound and acoustics have a very complex relationship, and even a pair of microphones placed very close to each other will not be 100% in phase. In doing so, we can get close to being in phase, and that is good enough for us (outside the ears, it is rare, if ever, to hear a sound that is perfectly in phase.

There are two main reasons why sound at two or more points is almost never in phase by nature:

Direct distance from sound source: A sound wave will have different phases of its waveform at different distances from the sound source.
Sound consists of many different frequencies: even if two microphones are located at the same distance from the sound source, the probability of matching all frequencies is almost zero. This is especially true for higher and more directional frequencies.

There are many other ways in which the sound will be different at two different microphone placements. This includes reflections/reverberation, other sound sources, and microphone direction.

So phase only really matters when there are two or more signals. Thus, microphone placement is critical to phase coherence when using more than one microphone.

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.

The most interesting videos on Youtube

How to make a microphone preamp with your own hands

Using a suitable circuit, assembling the device will not be difficult.

Tools and materials

Tools used in the work:

soldering iron;
wire cutters;
scissors;
tweezers;
glue gun

In addition, you will need:

radio components - LED, 2 resistors 200-600 Ohm, 2 capacitors with a capacity of up to 10 μF, transistor KT315 (can be replaced with KT3102 or BC847);
wires;
plug and connectors from an old tape recorder, DVD player or other equipment;
switch from a toy car;
solder and rosin;
bread board.

Step-by-step instruction

They start by choosing a scheme. For a beginner, the simplest option is suitable, based on a transistor cascade with a common emitter. It amplifies the microphone signal by 2 times.

Sequencing:

The breadboard is washed with solvent.
Solder connectors to it - power supply, microphone input and output.
The radio components are soldered onto the board according to the diagram.
A case is made from a plastic box by drilling holes for connectors with a “wood” drill.
The board is glued into the case or screwed with screws if there are threaded stands.
Glue the housing cover.
Connect the preamplifier to the computer sound card or speaker, then check the voltage at the microphone input. It should be 5 V.

If the voltage is different, take another plug and connect it to the connector. Next, check the voltage again between the long terminal and each of the 2 short ones. The multimeter probes are positioned so as not to short-circuit the leads.

How to solder a wire to a headphone speaker

When the wiring breaks near the headphone speaker, it is best to solder it back to the headphone and renew the soldering from the factory. This is how the wires are soldered inside my ancient Aiwa earbuds.

Pay attention to the knot that is tied - it does not allow thin wires to come off when jerking. Before soldering the earphone, be sure to tie the same knot at a certain distance, slightly greater than the distance to the narrow hole.

The question arises - what determines the polarity of connecting the wires to the speaker. The answer is simple - the headphones are connected in the same way so that they work in phase. When the polarity of the headphones is reversed, the sound is smeared and becomes quieter than common-mode sound. If you have a broken wire in one speaker, for proper connection you need to look at the wires and soldering in the other earphone.

What to do if the wire breaks inside the earphone

The most unpleasant thing is when the wire breaks inside the earphone. This means that the speaker winding wire has broken , which is shown in the figure.

The speaker winding is usually glued to the diaphragm and breaks off at the point where the wires are attached to it. Over time, they simply break due to micro-movements.

It is extremely difficult to restore such a break, but it is possible under a microscope. When repairing headphones, you need to be extremely careful not to tear the diaphragm or damage the geometry of the winding. Particular attention should be paid to microdust trapped between the magnet and the winding. Magnetic debris can be easily removed with chewing gum or plasticine.

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.

Receiving +48 V for phantom power

In mixing consoles, phantom power voltage is usually obtained using a separate transformer or DC/DC converter. An example circuit using a DC/DC converter can be found at https://www.epanorama.net/counter.php?url=https://www.paia.com/phantsch.gif (circuit of one microphone preamplifier from PAiA Electronics).

It will be interesting➡ What is current density?

If you use a battery, you might find it helpful to know that many microphones that require phantom power work just fine with voltages less than 48V. Try 9V and then increase it until the mic starts working. It's much easier than using a DC/DC converter. However, it must be remembered that the sound of a microphone powered from a lower voltage may be very different, and this should be taken into account. Five 9V batteries will provide 45V power, which should be enough for any microphone.

If you use batteries, short them with a capacitor to limit their noise in the audio path. To do this, you can use 10 µF and 0.1 µF capacitors in parallel with batteries. The batteries can also be used with a 100 Ohm resistor and a 100 µF 63V capacitor.

Effect on dynamic microphone

Connecting a dynamic microphone with a two-wire shielded cable to the input of a mixing console with phantom power turned on will not cause any physical damage. So there should be no problems with the most popular microphones (if they are wired correctly). Modern balanced dynamic microphones are designed in such a way that their moving parts are not sensitive to the positive potential received from phantom power, and they work great.

The effect of phantom power on a connected dynamic microphone.

Many older dynamic microphones have a center tap grounded to the microphone body and cable shield. This can cause the phantom power to short circuit to ground and burn out the winding. It's easy to check if this is true in your microphone. Using an ohmmeter, the contact between the signal pins (2 and 3) and ground (pin 1, or the microphone body) is checked. If the circuit is not open, do not use this microphone with phantom power.

Do not attempt to connect a microphone with an unbalanced output to the input of a mixing console with phantom power. This may cause equipment damage.

Effect of phantom power on other audio equipment

Phantom power at 48V is a fairly high voltage compared to what conventional audio equipment typically operates with. You must be very careful not to turn on phantom power on inputs that are connected to equipment that is not designed for this purpose.

Otherwise, it may damage the equipment. This is especially true for consumer-grade equipment connected to the remote control via a special adapter/converter. For a safe connection, a transformer isolation is used between the signal source and the remote control input.

Connecting professional microphones to computers

Typical computer audio interfaces provide only 5V power. Often this power is called phantom power, but it should be understood that it has nothing to do with professional audio equipment. Professional microphones typically require 48V power, and many will work with 12 to 15 volts, but a consumer sound card won't be able to provide even that.

Depending on your budget and technical savvy, you can either switch to using consumer microphones or make your own external phantom power supply. You can use either an external voltage source or the power supply built into the computer. As a rule, every computer power supply has a +12V output, so all that remains is to connect it in the correct way.

Additional material: What is a transformer substation.

T-powering and AB powering

T-powering typically has 12V supplied to the balanced pair through 180ohm resistors. Due to the potential difference on the microphone capsule, when a dynamic microphone is connected, current will begin to flow through its coil, which will negatively affect the sound, and after some time will lead to damage to the microphone. Thus, microphones specially designed for power supply using T-powering technology can be connected to this circuit. Dynamic and ribbon microphones will be damaged when connected, and condenser microphones will most likely not work properly.

Microphones using T-powering are, from a circuit design point of view, a capacitor and therefore prevent DC current from flowing. The advantage of T-powering technology is that the shield of the microphone cable does not have to be connected at both ends. This feature avoids the appearance of an earth loop.

T-powering and AB powering.

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.

Advantages and disadvantages

Advantages of electret microphones:

Low price of components and production. When compared with other models, electret models are 20-30% cheaper.
Wide range of applications. They are installed in smartphones, personal computers, and wiretapping gadgets.
High sound quality. The devices are used to measure sound.
Ability to use different connection types. They support XLR, 3.5mm, etc.
Good sensitivity and durability of the membrane.
Resistant to damage and water.

Flaws:

The need to install amplifiers.
Requirement for an additional power source.

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.

What is the difference between microphone polarity and phase?

Often polarity and phase are misunderstood or understood to mean the same thing. Let's see how they differ.

Phase refers to the time shift of a signal shape relative to another signal or another point in time. We measure this shift in a 360° cycle.

Polarity refers to the signal's "right side up" or "up and down" orientation. We measure polarity as positive (right side up) or negative (upside down).

Let's look at a photograph of two sine waves to illustrate these points:

In terms of polarity, we see that the blue and pink sine waves have opposite polarities. They are inverted images of each other. When one reaches its peak, the other finds itself in its trough.

Let's assume that the sine wave has positive polarity and the pink sine wave has negative polarity.

In terms of phase, we can see that these two waveforms are the same, but have been shifted by half a full cycle. Using the 360° measurement, we can see that these two waveforms are 180° out of phase.

It is important to note that reverse polarity does not mean a 180° phase shift; they both produce the same visual and auditory result.

Again, polarity refers to the change in positive and negative amplitude, and phase refers to the change in waveform in a 360° cycle.

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).

What is

In particular, we are talking about additional power, which is usually called phantom power. Whatever the linguistic constructions, this is a device that will immediately add as much as 48 V energy to a suffering device. According to an established tradition, all new and unusual devices are bought on AliExpress and delivered to the customer by mail. The latter only has to understand what he has in his hands and why it is needed.

What is phantom power for a microphone?

Here is a phantom type device, and this is the device that is such a purchase. The device powers a condenser studio microphone, which works much like a capacitor itself. Only instead of a movable capacitor plate there is a microphone membrane. The intensity of the work and the amplitude of the displacement are determined by the strength of the sound that the microphone is currently processing. The operating voltage changes accordingly, and we get the desired effect of improving the performance of the sound recording device.

It should be noted that the scheme is quite original, but it works. In any case, the cost of phantom power is not prohibitive; if you are not satisfied with its capabilities, the financial costs will not be critical. Be that as it may, the new 48 V power supply must be connected somewhere and somehow, and also secured for safety. Moreover, without it, condenser microphones simply will not function. Why 48 V? Because this indicator is supported by most manufacturers of microphones and sound cards, this is already a certain tradition. In fact, a condenser microphone is capable of operating over a wide voltage range.

It will be interesting➡ How a three-phase rectifier works

The device itself, that is, phantom power, should be secured in a convenient place so that it does not interfere and at the same time is easily accessible. All necessary cables are connected to the fixed device, including the wire for connecting the microphone. A dedicated button allows you to turn phantom power on and off as needed. Phantom power is an inexpensive and effective way to improve the performance of your computer's audio recording system as much as possible. The device is popular among consumers because it is safe to use. Unless in the event of a short circuit in the cable, especially in the absence of the grounding required in such cases, the capsule may be damaged, which can be easily replaced. According to the majority of users, it is worth ordering the device from Chinese retailers. Especially if there is a need to work with high-quality sound without buying expensive professional equipment.

Microphone device.

There is only one type of microphone connection, known as phantom power. The specification for phantom power is given in DIN45596. Initially, the power supply was standardized at 48 volts (P48) through 6.8 kOhm resistors. The meaning of the denominations is not as critical as their consistency. It should be within 0.4% for good signal quality. Currently, phantom power is standardized at 24 (P24) and 12 (P12) volts, but it is used much less frequently than 48 volt power. Systems using lower supply voltages use lower value resistors.

Most condenser microphones can operate with a wide range of phantom power voltages. Power supply 48 volts (+10%...-20%) is supported by default by all manufacturers of mixing consoles. There is equipment that uses lower voltage phantom power. Most often this voltage is 15 volts through a 680 ohm resistor (similar, for example, is used in portable sound systems). Some wireless systems can use even lower supply voltages, from 5 to 9 volts.

Studio microphone.

Phantom power is now the most common method of powering microphones due to its safety when connecting a dynamic or ribbon microphone to an input with phantom power enabled. The only danger is that if the microphone cable is shorted, or if you are using an older microphone design (with a grounded terminal), current will flow through the coil and damage the capsule. This is a good reason to regularly check cables for short circuits, and microphones for the presence of a grounded terminal (so as not to accidentally connect it to a live input).

The name “phantom power” comes from the field of telecommunications, where a phantom line represents the transmission of a telegraph signal using ground, while speech is transmitted over a balanced pair.

Phantom power types P48, P24 and P12

There is often confusion about the different but actually similar types of phantom power. DIN 45596 specifies that phantom power can be achieved at one of three standard voltages: 12, 24 and 48 volts. More often than not, the way the microphone is powered can vary depending on the voltage supplied. There is usually no indication that the microphone is receiving power, but a voltage of 48 volts will certainly work.

Creating a clean and stable 48 volt voltage is difficult and expensive, especially when only 9 volt Krona batteries are available. Partly because of this, most modern microphones are capable of operating with voltages ranging from 9-54 volts.

Phantom power for electret microphones

Please note that this is only the simplest way to “spandorize” an electret microphone to the remote control. This scheme works, but has its drawbacks, such as high sensitivity to phantom power noise, unbalanced connection (prone to interference) and high output impedance (long cables cannot be used). This circuit can be used to test the capsule of an electret microphone when connected to a mixing console using a short cable. Also, when using this circuit, the noise of transient processes (for example, when turning on or off phantom power, when connecting to a mixing console, as well as disconnecting from it) is at a very high level. Another disadvantage of this circuit is that it does not symmetrically load the phantom power supply circuit. This may affect the performance of some mixing consoles, especially older models (in some mixing consoles the input transformer may short out and burn out, in this case pins 1 and 3 are shorted through a 47 Ohm resistor).

In practice, this circuit works when used with modern mixing consoles, but it is not recommended for actual recording or any other application. It is much better to use a balanced circuit; it is much more complicated, but much better.

Symmetrical connection diagram for an electret microphone

The output of this circuit (Fig. 20) is symmetrical and has an output impedance of 2 kOhm, making it possible to use it with a microphone cable up to several meters long. The 10uF capacitors that are included at the output of the Hot and Cold pins must be high quality film capacitors. Their rating can be reduced to 2.2 µF if the input impedance of the preamplifier is 10 kOhm or more. If for some reason you use electrolytes instead of film capacitors, then you should select capacitors designed for voltages greater than 50V. In addition, they need to include 100nF film capacitors in parallel. Capacitors connected in parallel with the zener diode should be tantalum, but if desired, 10nF film capacitors can be used in conjunction with them

The connected cable must be two-core shielded. The screen is soldered to the zener diode and not soldered to the capsule. The pinout is standard for an XLR connector.

Improved electret microphone connection to phantom power

BC479 can be used as bipolar PNP transistors. Ideally, they should be matched as closely as possible to minimize noise and gain consistency. Keep in mind that the voltage between collector and emitter can reach 36V. Capacitors of 1 µF should be high quality film capacitors. The circuit can be improved by adding 22pF capacitors in parallel with the 100kΩ resistors. To minimize self-noise, 2.2kΩ resistors must be carefully selected.

Improved circuit for connecting an electret microphone to phantom power.

External phantom power supply

The +48V power supply is grounded to signal ground (pin 1). The +48V voltage can be obtained using a transformer and rectifier, using batteries (5 pieces of 9V each, a total of 45V, which should be enough), or using a DC/DC converter powered by a battery. Between the signal wires and ground there should be two 12V zener diodes connected back to back to prevent a 48V pulse through the capacitors to the input of the mixing console. Resistors with a nominal value of 6.8 kOhm should be used with high precision (1%) to reduce noise levels.

DIY external phantom power supply.

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.

How to find out if the connector is faulty

Insert working headphones into the jack and turn on the music. If music does not play in working headphones, your connector is broken. Also, if you hear a hissing sound when the plug moves, this means that the connector will soon completely fail.

Nowadays, the pinout of headphone wires with a microphone shown in the first picture below is mostly used everywhere, but there is also another one, which is mainly used on old phones and in phones from some manufacturers. They differ in that the microphone and ground contacts are swapped.

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.

Device and parameters

The microphone is made from:

Electret capsule.
Jack 3.5 plugs for connecting to a computer.
Housing that protects the wires.
Clamps for fixing and connecting parts.
Thin wire 1-1.5 m long.
Power supply.

Microphones are characterized by the following parameters:

Sensitivity.
Frequency range
The difference between the highest and lowest frequency.
The magnitude of electrical resistance.
Characteristics of directionality.

Microphone amplifier ready circuit

But I was constantly undermined by the fact that almost all the op-amps I have are dual ones, and I don’t like it when half of the op-amp is hanging in the air. Somehow this is not kosher...

Therefore, without thinking twice, I moved on to my favorite circuit - a headphone amplifier circuit. It is essentially the same non-inverting amplifier, but supplemented with a cleverly included repeater.

The reason for the transition is not only the desire to use both operational amplifiers in the chip package.

Firstly, I have long wanted to try this scheme with unipolar power supply.
Secondly, this circuit is capable of delivering twice the current at the same output voltage. This ensures that there are no signal drops or distortions on the way from the preamplifier to the recording device. The cable can be 5 or 10 meters.

Therefore, all that remained was to simply add an input circuit with a microphone to it and change the values of the capacitors to suit our task. This is what the final diagram ultimately looks like.

How to find out if the connector is faulty

A simple DIY electret microphone amplifier

This microphone preamplifier may be needed if you are not satisfied with the volume level of your microphone when recording sound on a computer.

Often, in order to enhance the sound from a microphone lavalier, software amplification is done in some sound recording program or video editor, but at the same time the sound itself becomes noisy and distorted.

To avoid this, you need to amplify the sound with a preamplifier, to do this, let's solder an amplifier for an electret microphone with your own hands, this is done very simply.

What parts do we need for a microphone amplifier:

Transistor BC547 or KT3102;
Two resistors - 1 kOhm;
One resistor from 150 Ohm to 1 kOhm, selected later by ear;
Ceramic capacitor from 100 to 300 pF;
Electrolytic capacitor 47 uF (from 6.3 V and above, but preferably no more than 25 V);
Electret microphone (lapel microphone).