The circuit of a homemade pre-amplifier (preamp) with a tone block is made on the LM4558 microcircuit. An important part of an audio amplifier is the pre-amplifier. It is desirable that it can not only amplify the signal, but also regulate its frequency response.

The figure on the right shows a diagram of a simple stereo pre-amplifier with volume control separately in each channel and a common tone control for both channels for low, mid and high frequencies.

Schematic diagram

The circuit is based on a dual operational amplifier type LM4558. And it is intended primarily for working with simple automobile amplifiers built on microcircuits - integrated bridge UMZCH. Therefore, the “automobile” supply voltage is unipolar 12V.

But this does not limit the scope of application of this scheme only to automotive vehicles.

Rice. 1. Schematic diagram of a homemade pre-amplifier with a tone block based on LM4558.

The supply voltage can be up to 30V. And you can even switch to bipolar power supply. To do this, you need to remove the divider on resistors R1, R2 and C2. And connect pins 3 and 5 of the microcircuit to the common power supply negative. In this case, disconnect pin 4 from the minus power supply and apply a negative supply voltage to it.

Volume adjustment is carried out by variable resistors R7 and R15, as already mentioned, separately in each channel. On the operational amplifiers of the A1 chip, two active tone controls are made, in which both amplification of the audio signal and frequency correction using three-band tone controls occur.

The adjustment circuits are included in the OOS circuits of the operational amplifiers. Volume control for high frequencies - with a double variable resistor R8, for mid frequencies - R9, for low frequencies - R10.

Details

The LM4558 microcircuit can be replaced by any IC - two op-amps for general use, or you can make a circuit using two ICs, one op-amp in each.

Most audio lovers are quite categorical and are not ready to compromise when choosing equipment, rightly believing that the perceived sound must be clear, strong and impressive. How to achieve this?

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Perhaps the main role in resolving this issue will be played by the choice of amplifier.
Function
The amplifier is responsible for the quality and power of sound reproduction. At the same time, when purchasing, you should pay attention to the following designations, which mark the introduction of high technologies in the production of audio equipment:

• Hi-fi. Provides maximum purity and accuracy of sound, freeing it from extraneous noise and distortion.
• Hi-end. The choice of a perfectionist who is willing to pay a lot for the pleasure of discerning the smallest nuances of his favorite musical compositions. Hand-assembled equipment is often included in this category.

Specifications you should pay attention to:

• Input and output power. The rated output power is of decisive importance, because edge values ​​are often unreliable.
• Frequency range. Varies from 20 to 20000 Hz.
• Nonlinear distortion factor. Everything is simple here - the less the better. The ideal value, according to experts, is 0.1%.
• Signal to noise ratio. Modern technology assumes a value of this indicator over 100 dB, which minimizes extraneous noise when listening.
• Dumping factor. Reflects the output impedance of the amplifier in its relation to the nominal load impedance. In other words, a sufficient damping factor (more than 100) reduces the occurrence of unnecessary vibrations of equipment, etc.

It should be remembered: the manufacture of high-quality amplifiers is a labor-intensive and high-tech process; accordingly, too low a price with decent characteristics should alert you.

Classification

To understand the variety of market offers, it is necessary to distinguish the product according to various criteria. Amplifiers can be classified:

• By power. Preliminary is a kind of intermediate link between the sound source and the final power amplifier. The power amplifier, in turn, is responsible for the strength and volume of the output signal. Together they form a complete amplifier.

Important: the primary conversion and signal processing takes place in the preamplifiers.

• Based on the element base, there are tube, transistor and integrated minds. The latter arose with the goal of combining the advantages and minimizing the disadvantages of the first two, for example, the sound quality of tube amplifiers and the compactness of transistor amplifiers.
• Based on their operating mode, amplifiers are divided into classes. The main classes are A, B, AB. If Class A amplifiers use a lot of power, but produce high-quality sound, Class B amplifiers are exactly the opposite, Class AB seems to be the optimal choice, representing a compromise between signal quality and fairly high efficiency. There are also classes C, D, H and G, which arose with the use of digital technologies. There are also single-cycle and push-pull operating modes of the output stage.
• Depending on the number of channels, amplifiers can be single-, double- and multi-channel. The latter are actively used in home theaters to create volumetric and realistic sound. Most often there are two-channel ones for right and left audio systems, respectively.

Attention: studying the technical components of the purchase is, of course, necessary, but often the decisive factor is simply listening to the equipment according to the principle of whether it sounds or not.

Application

The choice of amplifier is largely justified by the purposes for which it is purchased. We list the main areas of use of audio amplifiers:

1. As part of a home audio system. Obviously, the best choice is a tube two-channel single-cycle in class A, and the optimal choice can be a three-channel class AB, where one channel is designated for a subwoofer, with a Hi-fi function.
2. For car audio system. The most popular are four-channel AB or D class amplifiers, depending on the financial capabilities of the buyer. Cars also require a crossover function for smooth frequency control, allowing frequencies in the high or low range to be cut as needed.
3. In concert equipment. The quality and capabilities of professional equipment are reasonably subject to higher demands due to the large propagation space of sound signals, as well as the high need for intensity and duration of use. Thus, it is recommended to purchase an amplifier of at least class D, capable of operating almost at the limit of its power (70-80% of the declared one), preferably in a housing made of high-tech materials that protects from negative weather conditions and mechanical influences.
4. In studio equipment. All of the above is also true for studio equipment. We can add about the largest frequency reproduction range - from 10 Hz to 100 kHz in comparison with that from 20 Hz to 20 kHz in a household amplifier. Also noteworthy is the ability to separately adjust the volume on different channels.

Thus, in order to enjoy clear and high-quality sound for a long time, it is advisable to study in advance all the variety of offers and select the audio equipment option that best suits your needs.

Microphone preamp, also known as a pre-amplifier or amplifier for a microphone, is a type of amplifier whose purpose is to amplify a weak signal to a linear level (about 0.5-1.5 volts), that is, to an acceptable value at which conventional audio power amplifiers operate .

The input source of acoustic signals for a preamplifier is usually vinyl record pickups, microphones, and pickups of various musical instruments. Below are three circuits of microphone amplifiers on transistors, as well as a variant of a microphone amplifier on the 4558 chip. All of them can be easily assembled with your own hands.

Circuit of a simple microphone preamplifier using one transistor

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

Dynamic microphones are similar in design to loudspeakers. The acoustic wave affects the membrane and the acoustic coil attached to it. When the membrane oscillates, an electric current is generated in a coil exposed to the magnetic field of a permanent magnet.

The operation of electret microphones is based on the ability of certain types of materials with increased dielectric constant (electrets) to change the surface charge under the influence of an acoustic wave. This type of microphone differs from dynamic microphones in its high input impedance.

When using an electret microphone, to bias the voltage on the microphone, it is necessary to set the resistance R1

single transistor microphone amplifier

Since this microphone amplifier circuit is for a dynamic microphone, when using an electrodynamic microphone, its resistance should be in the range from 200 to 600 Ohms. In this case, C1 must be set to 10 microfarads. If it is an electrolytic capacitor, then its positive terminal must be connected towards the transistor.

Power is supplied from the crown battery or from a stabilized power source. Although it is better to use a battery to eliminate noise. can be replaced with a domestic one. Electrolytic capacitors for a voltage of 16 volts. To prevent interference, connect the preamplifier to the signal source and to the amplifier input using a shielded wire. If further powerful sound amplification is needed, then you can assemble an amplifier on a microcircuit.

Microphone preamplifier with 2 transistors

The structure of any preamplifier greatly affects its noise characteristics. If we take into account the fact that the high-quality radio components used in the preamplifier circuit still lead to distortion (noise) to one degree or another, then it is obvious that the only way to get a more or less high-quality microphone amplifier is to reduce the number of radio components in the circuit. An example is the following two-stage preliminary circuit.

With this option, the number of decoupling capacitors is minimized, since the transistors are connected in a circuit with a common emitter. There is also a direct connection between the cascades. To stabilize the operating mode of the circuit when the external temperature and supply voltage change, a direct current feedback loop has been added to the circuit.

Preamplifier for electret microphone with three transistors

This is another option. The peculiarity of this microphone amplifier circuit is that power is supplied to the preamplifier circuit through the same conductor (phantom power) through which the input signal travels.

This microphone preamplifier is designed to work together with, for example, MKE-3. The supply voltage to the microphone goes through resistance R1. The audio signal from the microphone output is supplied to the VT1 base through capacitor C1. , consisting of resistances R2, R3, creates the necessary bias at the base of VT1 (approximately 0.6 V). The amplified signal from resistor R5, acting as a load, goes to the base of VT2 which is part of the emitter follower on VT2 and VT3.

Near the output connector, two additional elements are installed: load resistor R6, through which power is supplied, and separating capacitor SZ, which separates the output audio signal from the supply voltage.

Pre-microphone amplifier based on 4558 chip

The 4558 operational amplifier is manufactured by ROHM. It is characterized as a low power and low noise amplifier. This microcircuit is used in a microphone amplifier, audio amplifiers, active filters, and voltage-controlled generators. The 4558 chip has internal phase compensation, increased input voltage threshold, high gain and low noise. This op amp also has short circuit protection.

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microphone preamplifier for 4558

This is a good option for building a microphone preamp on a chip. The microphone preamplifier circuit is characterized by high amplification quality, simplicity and does not require much wiring. This dynamic microphone amplifier also works well with electret microphones.

With error-free assembly, the circuit does not require configuration and starts working immediately. The highest current consumption is 9 mA, and at rest the current consumption is around 3 mA.

For beginners Microphone amplifier. (017)

Let's look at the design of a high-quality microphone amplifier. The amplifier is assembled on the BA4558 operational amplifier (different manufacturers produce microcircuits with different letter designations, the essence does not change). This microphone amplifier is designed to monitor the sound environment indoors and outdoors, as an addition to a video surveillance, security and safety system. The circuit is characterized by high sensitivity, low noise level of the operational amplifier, provides high-quality sound on monitors, recording devices, headphones, has low current consumption (measurement showed a current of about 2 mA), is operational when the supply voltage is reduced to 4.5 volts. When repeating the circuit in order to minimize the dimensions of the device, you can replace the microphone capsule with another one with minimal dimensions (about 3mm), not use a socket, using this type of microcircuit in the CMD version, replace the electrolytic capacitors with non-polar multilayer ones. Acoustic range - up to 7 meters, length of the connecting line from the amplifier to the signal consumer (headphones, monitor, recording device) - up to 300 meters. Powered by a DC source of 5 - 12 volts. If a studio (dynamic) microphone is used instead of an electret microphone, the electret microphone power resistor R1 must be excluded from the circuit. Considering that the microcircuit contains two identical amplifiers, the second amplifier (pins 5,6,7) can be used for the second channel or used as a pre-amplifier for the first channel. If instead of a dynamic microphone you connect a coil to the amplifier input,wound on a ferrite rod and containing about 3000 turns of thin (0.08 - 0.12 mm) copper wire such as PEL, PEV, then the amplifier will turn into a radio receiver of ultra-low range radio waves emitted by the speakers of telephones, televisions, and telephone wires. If it is impossible to connect to the amplifier with a wire line, you can modify the amplifier with a radio extender assembled on one transistor VT1, which is a low-power HF generator operating in the broadcast range 88 - 108 MHz. The tuning capacitor C6 allows you to change the frequency of the generator, tuning to frequencies free from broadcasting. You can also change the frequency by stretching/compressing the turns of the frameless coil L1, wound with copper wire with a diameter of 0.4 - 0.7 mm on a mandrel of 3 - 4 mm (for example, on the shank of a drill) and containing 6 turns. If the circuit is modified with a high-frequency amplifier using any RF transistor (for example, KT361, shown in blue in the diagram and not included in the kit), then the communication range can reach 1 km, but this may run counter to existing legislation. The antenna is a piece of copper mounting wire 50 - 80 cm. The antenna can be made from a rigid copper wire with a cross section of 0.7 - 1 mm, rolled into a spiral.
Configure the radio microphone to a radio receiver in the 88 - 108 MHz range that is turned on on a free frequency and located nearby.

Chip BA4558 is a low noise dual operational amplifier. The scope of application of this microcircuit is quite wide - from household audio systems to radio transmitters (bugs) and professional radio microphones for karaoke. The microcircuit is very often used as a headphone amplifier.

IL4558, BA4558, F4558, LM4558, H4558, L4558, C4558 - they are all complete analogues, and sometimes the letter(s) are missing at the beginning, there is only the inscription 4558.

In our version, we will consider a microphone amplifier from a DVD player, implemented on the BA4558 chip.

The upper frequency range is up to 3 MHz, the maximum permissible case temperature is up to 70 degrees Celsius. There is built-in protection against overvoltage and short circuit; the microcircuit itself is produced in an 8-pin package. The upper limit of the supply voltage is 25 Volts, it is not recommended to exceed the input voltage rating above 15 Volts, power dissipation is 550-570 MW.

Often, this microcircuit is used to build pre-circuits and filters in home amplification systems. An excellent option as a microphone amplifier, with only a few electronic components in the harness. When using this microcircuit as a microphone amplifier, you can use both electret and dynamic microphones; with the use of an electronic microphone, the overall sensitivity will be very high, it is for this reason that the microcircuit has found wide use in homemade radio bugs and remote microphones.

Simple microphone amplifier circuit

The microphone amplifier on the BA4558 chip works even with a wide range of components used and does not require any additional configuration, and the quiescent current is only 2-3 mA, the maximum current consumption does not exceed 9 mA.