Circuit board with microphone pre-amplifier. Microphone amplifiers: circuit
Hello! In this article I want to tell you about a microphone preamp.
From the very title of the article it is clear that we will strengthen something. First, let's look at one example. You connected a dynamic microphone to your computer and decided to record your voice. But apart from very quiet speech, filled with a lot of noise and interference, you heard nothing. And all because 1.5 V appears at the input of the computer’s audio card. This very one and a half volts presses the coil inside the microphone, and when you speak, they prevent it from moving. This means that this voltage needs to be somehow removed and the signal strengthened. For this we will make a pre-amplifier. That is, the sound from the microphone will enter the computer already amplified and without noise.
So, let's get started.
To do this you need the following components:
Resistors – 4.7 kOhm – 2 pcs., 470 kOhm, 100 kOhm.
Capacitors – 4.7 µF, 10 µF, 100 µF.
Transistor– KT315.
Light-emitting diode – not necessary.
Tools:
Soldering iron, wire cutters, tweezers, scissors, glue gun, etc..
Let's start manufacturing.
1.
First, let's look at the diagram and details.
Resistor R5 put for electret microphone and acts as a voltage bias. We don't use it. The KT315 transistor can be replaced with KT3102, BC847. KT3102 has a higher gain, so it is preferable to install it. LED is optional. If it is not needed, replace it with a diode. I found a piece of a homemade breadboard at my place. I will make a diagram on it.
2. Now, according to the diagram, solder all the components.
3. Next, we solder the power connectors, microphone input and output, and power switch. 6.3 mm jack connector. I took a 3.5 mm jack from an old DVD player. - from a tape recorder. A connector for a battery from a non-working crown, a switch from a toy car. Solder everything to the board.
There is no LED in the photo; it appeared later.
4. Now let's take care of the body. I found some kind of plastic box without a bottom. She just fit all the details. We drill holes in it for connectors, an LED, and cut out a rectangular hole for a switch.
5. Now we assemble everything into the case. We glue the crown and board with double-sided tape, and the connectors with hot-melt adhesive.
The bottom was made of durable black cardboard.
6. Check. I had the cheapest BBK karaoke microphone. I connected it. Next, using a jack-jack wire, we connect the amplifier output to the computer, speakers, or whatever you need. Turn on the power. The LED lit up. The preamp is working.
A simple DIY microphone amplifier for a computer
This article focuses on the design of a simple microphone amplifier that can be used to amplify the signal of an electret or dynamic microphone.
With a minimum number of parts, such an amplifier allows you to improve the signal-to-noise ratio and increase the microphone signal gain compared to the amplifier of the built-in audio card. https://site/
I'm about to record my first video lesson. Already made it. But the very first attempt to record a voice stumbled over incredibly high noise and insufficient gain of the microphone amplifier of the built-in audio card.
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By turning off the Microphone Boost mode, it was possible to reduce the noise, but the gain level became so low that it became impossible to record anything.
I had already decided to buy a separate audio card, but I discovered that a good audio card is very expensive, and a budget one for $10, although it has a lower noise level, also has a microphone amplifier with a not very high gain.
So, I set about making a simple microphone amplifier.
The first experiments with prototypes of microphone amplifiers showed that the noise level can be reduced and the gain increased.
One can only wonder how computer hardware developers manage to produce such “pearls”, while just a few cheap parts solve the problem of noise and amplification.
Construction and details.
When choosing an amplifier circuit, I focused mainly on ease of operation and the minimum number of parts spent on construction. The goal was not to produce a super-duper amplifier with record performance.
After prototyping several circuits on Sovdepov microcircuits, I settled on the K538UN3A (KR538UN3A) chip. https://site/
The reasons are as follows:
Why DL123A (CR-P2)? Due to the toxic filling, the cases of these elements are made of stainless steel and carefully sealed, which prevents the destruction of the case and damage to the amplifier circuit. The latter often happens when using salt and alkaline (alkaline) elements. (The GP alkaline elements damaged my beloved Maglite).
Technical parameters of K538UN3A.
Below I publish technical data taken from a paper reference book on analog microcircuits, since I did not find detailed information about this microcircuit on the Internet.
The microcircuit is an ultra-low-noise broadband signal amplifier with a frequency of up to 3 MHz. The amplifier's noise characteristics are optimized for operation with low-impedance signal generators. The gain is fixed by the internal divider, but it can be adjusted externally. The amplifier is intended for use as a playback pre-amplifier in high-end equipment, as well as an amplifier for low-impedance sensors. Housing 2101.8-1 (DIP8) or 301.8-2.
Electrical parameters.
Rated supply voltage – +6V.
Current consumption at Up = 6V, T = -45… +70C, no more than – 5mA.
Voltage amplification factor with internal feedback at Up = 6V, f = 1 MHz, Uin. = 1mV, Rn = 10kOhm, T = +25C:
not less than 200,
no more than 300,
typical value is 250.
Voltage amplification factor without internal feedback at Up = 6V, f = 1 MHz, Uin = 1 mV, Rn = 10 kOhm, T = +25 C, typical value – 3000.
Normalized voltage of self-noise at Up = 6V, f = 1 MHz, Uin = 1 mV, Rg = 500 Ohm, Rn. = 10kOhm, T = +25C, no more than – 5nV/√Hz, typical value – 2.1nV/√Hz.
Maximum output voltage Up = 6V, Rn = 2kOhm, Kg = ≤ 10%, T = -45C, not less than 0.5V, typical value – 1V.
Upper cutoff frequency at Up = 6V, Rn = 2 kOhm, Ku = 100, T = +25C, typical value – 3 MHz.
Input impedance – 10 kOhm.
Limit operating data.
The maximum supply voltage is 7.5V.
The maximum input voltage is 200mV.
Minimum load resistance (short-term) – 0 Ohm.
Ambient temperature, long-term exposure: –45… +70С, short-term exposure: –60… +125С.
Pin assignment of the K538UN3A microcircuit.
Housing 2101.8-1.
- Nutrition.
- Not used.
- Correction.
- Entrance.
- Gain adjustment pin.
- Connecting the OS DC filter.
- General.
- Exit.
Housing 301.8-2.
A somewhat outdated version of the microcircuit.
Typical circuit diagram for connecting a microcircuit.
- C2 – power filter.
- C5 – separating.
- C6 – corrective.
- C8 – DC filter.
- R4 – adjustment of operating system for alternating current.
The presented microphone amplifier circuit can amplify the signal of both an electret and a dynamic microphone.
The value of resistor R4 determines the gain of the DA1 chip.
The maximum gain is achieved at R4 = 0.
To quickly adjust and limit the input signal level during overload, potentiometer R3 is used.
Resistor R2, diode VD2 and LED HL1 represent a voltage divider on which 2.2V is generated to power the electret microphone. Resistor R1 is the load of the electret microphone. The HL1 LED also functions as a power indicator.
The circuit can be significantly simplified if you rely only on the use of a dynamic microphone. You just need to keep in mind that when using a passive dynamic microphone with low sensitivity, you may need to increase the gain, which will lead to a slight increase in the noise level of the microphone amplifier.
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 R3 motor.
- LED anode HL1.
- Frame.
- Power supply +6V.
- Exit.
- Frame.
An example of a printed circuit board layout for a dynamic microphone amplifier.
- Entrance.
- Frame.
- Power supply +6V.
- Exit.
- Frame.
I myself made a printed circuit board based on the dimensions of the controls and housing at my disposal.
Frame.
It would be good to choose a metal case to house the structure. If a plastic case is used, then it is advisable to place the entire structure in the screen. The screen can be made from the tin of a condensed milk can. These cans are still tin plated and they solder just fine (they don't even need to be tinned). Both tasty and healthy... for the homemade person. The housing of the signal level control must be connected to the shield of the entire amplifier.
The picture shows a duralumin housing and a printed circuit board assembly. The board has two independent amplifiers with separate power management. To be able to record a stereo signal using two arbitrary microphones, the amplifier of each channel is equipped with a separate input jack.
Control elements are installed directly on the printed circuit board. Gain adjustment is carried out once by selecting fixed resistors when setting up the amplifier.
Microphone amplifier assembly. The microphone amplifier is connected to the computer with a shielded cable, at the end of which there is a 3.5mm Jack connector.
Comparative tests.
During the comparative test, the controls were set to a position that would provide the same level of the recorded signal, both with and without a microphone amplifier.
Green - noise level.
Raspberry is a type of noise.
The graph shows the noise level of the microphone amplifier of the built-in audio card in the “Microphone Boost” mode.
Recording level is 1.0.
The noise level is about -80dB.
In order to obtain a minimum noise level, I set the maximum signal level with resistor R3. This made it possible to use the audio card's line-in amplifier with a low gain level.
This graph shows the noise level of a homemade microphone amplifier.
Recording level 0.05.
The noise level is about -110dB.
Audio cart drivers usually do not allow you to set the recording level with such high precision.
You can set the recording level with an accuracy of a fraction of a percent using the free portable audio editor Audacity, a link to which is in the “Additional Materials”.
The recording or broadcasting of sound itself can be done using any other programs.
How to properly connect a dynamic microphone to a cable.
Having a stereo microphone from an old reel-to-reel tape recorder, I wanted to record stereo sound. But it was not there…
The sensitivity of dynamic microphones is inferior to that of electret microphones, which places increased demands on the former for shielding from interference and interference. However, these requirements are often ignored by the manufacturer. This was exactly the case with my microphones. They were connected to the cable in different ways, but each was incorrect in its own way.
- Frame.
- Coil output.
- Coil output.
The figure shows that the left microphone had no housing connected at all, while the right one had one of the coil terminals connected to the housing. Both of these connections were made incorrectly, especially considering that a shielded twisted pair cable was used.
The picture shows how to properly connect a dynamic microphone to a microphone amplifier with an asymmetrical input.
And this is connecting a microphone to a microphone amplifier with a balanced input.
The cheapest dynamic microphones are connected using a single-wire shielded cable. The figure shows a diagram of such a connection.
If you hear interference in the form of background with a frequency of 50 Hz, then it is better to connect the microphone using shielded twisted pair cable.
The dotted line in the diagrams shows the metal body of the microphone, which should be connected to the braided shielding cable. The coil terminals must be connected to a twisted pair. Not all budget dynamic microphones allow you to do this painlessly. Often one of the coil wires is already connected to the metal body of the microphone.
Do not try to solder the coil wire to another contact yourself. The coil is wound with wire 0.05 mm or thinner. For comparison, the thickness of a human hair is 0.03-0.04mm. Any careless touching of the coil leads will inevitably lead to a break. In addition, the coil terminals are additionally coated with glue, which also complicates the task.
Hooray! It's working!
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A five-second stereo recording made using two dynamic microphones and a homemade microphone amplifier. (You need to click on the picture).
The value of the resistor in the feedback circuit R4 = 50 Ohms.
The microphone amplifier signal level is maximum.
Recording level at the linear input of the audio card = 0.2.
Details Created 10/21/2014 07:27
The fundamental component without which not a single modern electronic device could exist is the transistor. To understand how this semiconductor device works, let's assemble a simple amplifier using a single transistor.
Since the goal was to get acquainted with the operation of the transistor, and not to assemble a final device for use in everyday life, I did not choose and specifically buy a specific transistor, but took the one that was at hand - P307V. I downloaded from the Internet the so-called datasheet for P307 from which I learned that this type of transistor has an n-p-n structure, low-frequency, low-power and is suitable for use in amplifiers.
As you know from the school physics curriculum, a transistor is, figuratively speaking, a layer cake consisting of three layers of semiconductor material. A semiconductor is a material that is characterized by a strong dependence of its conductivity on the concentration of impurities and other factors. The most common semiconductor is silicon.
Depending on the impurity introduced into the semiconductor, it becomes p-type or n-type. Transistors can have an n-p-n or p-n-p structure. The central layer of the semiconductor is called the base, and the two outer layers are the emitter and collector. In the diagrams they are designated as follows:
The principle of operation of the transistor comes down to the fact that small currents supplied to the base can be controlled by large currents flowing between the emitter and collector.
N-p-n transistors are controlled (activated) by a positive voltage that is applied to the base of the transistor relative to the emitter.
PNP transistors are driven by a negative voltage that is created at the base relative to the emitter.
Electronics engineers have one catchphrase: “No one dies as quietly and unnoticed as a transistor.” If too much current is applied to the terminals of the transistor, it will immediately fail. The permissible currents for different transistors can be found in the datasheet; for low-power transistors it is usually no more than 20 mA.
You can check the transistor using a conventional multimeter. We turn the multimeter into resistance measurement mode in the range of thousands of Ohms, connect the red probe to the base, and the common black probe, alternately to the emitter, then to the collector, the device should show resistance, in my case about 300 Ohms. Next, we connect the common probe to the base, and the red probe alternately to the emitter, then to the collector; the device should not show resistance, as if it were a dielectric. If it still shows resistance in both directions, then the pn junction is broken. That is, from the base to the emitter and from the base to the collector, current must flow in only one direction. When checking a transistor, the base-emitter and base-collector transitions can be compared to two diodes connected to each other. Transistors of pnp structures are tested in the same way, but the directions of conduction will be opposite.
In addition to the transistor, a microphone, a speaker, a variable resistor and a power source were needed.
I happened to have this speaker on hand, but you can take any one, even regular earbuds
variable resistor at 20 kOhm, fixed resistors at 10 kOhm and 300 Ohm
power source - two 3.7v batteries connected in series, giving a total of 7.4v
It is very convenient to do all manipulations with electronic components on a breadboard that does not require soldering. To include a part in the circuit, you just need to stick it into the holes on the board. The cheapest way to order a development board is on Aliexpress; I bought this development board complete with a USB power adapter and a set of jumpers
To begin with, I decided to check the operation of the transistor in switch mode. The resistor for protecting against excess current on the LED is 200 ohms, although the power supply is not powerful enough to damage the LED. Thus, the emitter-collector circuit is assembled, but the LED does not light. In order for current to flow, you need to apply a small positive resistance to the base. To do this, I took two conductors, one connected to the plus, and the second to the base, and closed them with my finger so that they did not touch each other. That is, I used the resistance of a small area of the skin of my finger. The resistance of the finger is quite large and the current has decreased significantly, but even this small current at the base of the transistor was enough to slightly open the emitter-collector junction and the LED began to glow.
To make a microphone amplifier from a simple electronic switch using a single transistor, you need to connect a speaker instead of an LED, and a resistor and a microphone to the base.
Here I encountered two difficulties: firstly, I did not know what resistance the required current would have on the base. It is on this so-called “bias current based on the transistor” that the gain, that is, the volume in the speaker, will depend. So I decided to take variable resistance. Through selection, it turned out that the amplifier worked with a resistance in the range from 11 kOhm to 33 kOhm; beyond these limits nothing was heard in the speakers. The highest volume was achieved at approximately 14 kOhm. This value depends on the input signal, in this case the microphone used.
This amplifier will work if the speaker is connected to the gap between the emitter and the minus and between the plus and the collector.
Although this amplifier was made only for the purpose of familiarizing itself with the operation of the transistor, it is quite functional and can be used. Sounds in front of the microphone are clearly audible through the speaker.
If your computer microphone is “hard of hearing” and you have to literally shout out to your interlocutor, do not rush to write it off as scrap: maybe a simple amplifier will help. Owners of laptops and netbooks will immediately snort at me: “No, it won’t work - extra wires!” Calm down, they won't be there. We organize phantom power.
The circuit is more than simple; it takes longer to look for parts than to solder. You can remake an existing microphone, you can make it from scratch, or you can use it for some other crafts.
Travel notes:
If you measure the voltage at the microphone input of a PC/laptop in any convenient way, you will get something like a green number (my Studebaker produces 3.2 volts, variations are possible on other computers). This voltage is used to power electret microphones, and the circuit design, when power is supplied through the same wire as the signal, is called phantom power.
When connecting the circuit, the voltage drops to 0.9 volts. At the base of the transistor - 0.6 - 0.7 volts are assigned to it for opening.
Almost all sites where this scheme is available recommend KT3102. On my own behalf I will add that it is preferable in an iron case. But if it is not there, then any silicon low-power transistor will do, for example, BC547, S9014. In very cramped circumstances, you can take KT315.
This option is on S9014 I got together with a friend in the fall of 2013 to capture the “corridor air” in order to know who was rowdy at night and who to snort at later. At that time, we had just appeared soldering irons with an “eternal” tip, and such miniaturization of crafts was simply a breakthrough after the 25-watt EPSN with a 6 mm rod.
I assembled it in a new way, using the miniaturization skill “I soldered so many things in two years.” Above is another option on a smaller capsule. First I soldered the transistor, then C1, then “electrolyte” and two resistors.
I extended the leads and doused the structure with hot glue.
And wrapped it in self-adhesive aluminum foil for shielding. In order for the foil to come into contact with the capsule, you need to wrap it, like you wrap a collar: there is no conductivity on the adhesive side.
If you remake a factory product, then most likely there will not be a place next to the microphone. No problem! The amplifier can be soldered on a small scarf or the same “canopy” and placed somewhere to the side, if the case allows it. In the same way, isolate it from the external environment (not necessarily with hot glue - electrical tape, “heat shrink”, paper, in the end) and shield it, if possible, hooking the screen to the minus of the “circuit”.
This microphone amplifier was made because the noise and lack of sensitivity of store-bought headsets and computer microphones was extremely annoying, and I couldn’t afford to buy high-quality ones for $50+.
The proposed circuit showed really high sensitivity, a powerful output signal, low noise level and a pleasant frequency response.
Schematic of a homemade microphone amplifier using an op-amp
The basis of the circuit is the NE5532 operational amplifier. Of course, you can put the best one, but this one meets these requirements 100%. This circuit uses both halves of the amplifier in a single housing, so the output signal will be very strong (you can even feed it to headphones). The device must be connected to the LINE-IN input because the typical microphone input is too sensitive and the recording will be overloaded.
In the photo, the top layer is a seal with double-sided adhesive tape. Electret microphone, standard. If you need to use dynamic - . The microcircuit was in the bins and the only thing I had to buy was . But even if you buy absolutely everything, the total cost will be close to a ridiculous 1 dollar.
All electronics were built into a ready-made plastic case (although metal is also welcome). The board is glued to the base with hot glue. The microphone is glued to the body with the same glue as the 9 V battery connector (so that the battery does not dangle).
Gluing a microphone to the body is generally not a good idea; it is better to do something like this through a soft rubber band - it will filter vibrations.
After assembly, the board was coated with a clear varnish to protect the copper from corrosion. The microphone usually works suspended on a stand. The cable for the microphone is 5 meters, naturally it is a good quality shielded cable.
Microphone tests and conclusions
The microphone is used for recording audio books and dubbing translated films. If necessary, it can be used as a karaoke microphone or even a small amplifier - the output signal is so strong that it can drive 32 Ohm headphones.
Lower power will not work - this is the limit for this microcircuit, which operates from 9 to 30 V according to the datasheet.
The noise parameter can be further improved by using a special low noise operational amplifier (OPA type).
Perhaps for some the microphone will not seem too light and comfortable. But you can do it your way by reducing the size of the board and case. The battery lasts a very long time, I recently recorded an audiobook for 10 hours and no problems.
