Pre-amplifiers AF. High quality preamplifier

Circuit design and application

Tube bass amplifier

An audio amplifier usually consists of a preamplifier and a power amplifier (PA). The preamplifier is designed to increase the voltage and bring it to the value required for the operation of the final power amplifier; it often includes volume controls, tone controls or an equalizer; sometimes it can be constructed as a separate device. The power amplifier must supply the specified power of electrical oscillations to the load (consumer) circuit. Its load can be sound emitters: acoustic systems (speakers), headphones (headphones); radio broadcast network or radio transmitter modulator. A low-frequency amplifier is an integral part of all sound reproducing, recording and radio broadcasting equipment.

Power amplifier as a separate unit

Technics preamplifier

Classification

Signal half-wave cutoff angles in various modes

By type of input signal processing and design of the amplifier output stage:

• class “A” - analog signal processing, linear mode of operation of the amplification element
• class “AB” - analog signal processing, operating mode with a large cut-off angle (>90°)
• class “B” - analog signal processing, operating mode with a cutoff angle of 90°
• class “C” - analog signal processing, operating mode with a small cutoff angle (<90°)
• class “D” - digital signal processing, pulse-width modulation is used, the amplification element operates in key mode
• class “T” - digital signal processing, pulse-width modulation is used with changing the frequency and duty cycle of pulses

IC for use in power amplifiers

By type of application in the amplifier design of active elements:

• tube- on electronic, vacuum tubes. They formed the basis of the entire ULF fleet until the 70s. In the 60s, tube amplifiers of very high power (up to tens of kilowatts) were produced. They had significant dimensions and weight, low efficiency. and high heat generation. Currently, low power tube amplifiers (a few watts) are used only as part of high fidelity circuits.
• transistor- on bipolar or field-effect transistors. This design of the final amplifier stage is quite popular due to its simplicity and the ability to achieve high output power, although recently it has been actively replaced by integrated ones even in powerful amplifiers.
• integral- on integrated circuits (ICs). There are microcircuits that contain both preamplifiers and final power amplifiers on the same chip, built according to different circuits and operating in different classes. Among the advantages are the minimum number of elements and, accordingly, small dimensions.
• hybrid- some of the cascades are assembled on semiconductor elements, and some on electronic tubes. Sometimes hybrid amplifiers are also called amplifiers, which are partly assembled on integrated circuits, and partly on transistors or vacuum tubes.

Transformer matching with load

By type of matching of the amplifier output stage with the load:

• transformer- this matching circuit is mainly used in tube amplifiers. This is due to the need to match the high output resistance of the lamp with the low load resistance. High-end transistor amplifiers also have transformer matching to the load.
• transformerless- the most common matching circuit for transistor and integrated amplifiers, because the transistor stage has a low output resistance, which is well suited to low-resistance loads.

Links

Wikimedia Foundation. 2010.

See what a “Low Frequency Amplifier” is in other dictionaries:

low frequency amplifier- ULF Amplifier designed to amplify audio frequency signals; in a radio receiver, the ULF is turned on after the detector. [L.M. Nevdyaev. Telecommunication technologies. English-Russian explanatory dictionary reference book. Edited by Yu.M. Gornostaeva... ...

low frequency amplifier- žemadažnis stiprintuvas statusas T sritis automatika atitikmenys: engl. low frequency amplifier vok. Niederfrequenzverstärker, m rus. low frequency amplifier, m pranc. amplificateur à basse fréquence, m … Automatikos terminų žodynas

audio amplifier- NDP. low frequency amplifier Electronic amplifier for audio frequency signals. [GOST 24375 80] Inadmissible, not recommended low frequency amplifier Topics radio communications General terms radio transmitters ... Technical Translator's Guide

audio amplifier- 360 audio amplifier; UZCH (Low frequency amplifier) ​​Amplifier for electrical audio signals Source: PR 45.02 97: Industry standardization system. Principles for the development of regulatory documents 360. Sound amplifier... ... Dictionary-reference book of terms of normative and technical documentation

It is proposed to rename this page to Audio Amplifier. Explanation of the reasons and discussion on the Wikipedia page: To rename / November 3, 2012. Perhaps its current name does not correspond to the norms of the modern Russian language ... Wikipedia

An electronic amplifier is an amplifier of electrical signals, the amplification elements of which use the phenomenon of electrical conductivity in gases, vacuum and semiconductors. An electronic amplifier can be an independent... ... Wikipedia

amplifier- 3.1.1 amplifier: An amplifier for audio frequency signals in a block-type detachable design or included in a single-case equipment.

HIGH QUALITY PRE-AMPLIFIER CIRCUIT

At the turn of 2004 and 2005, a natural desire arises to build amplifiers on a modern element base, taking advantage of the advanced achievements of global electronic technology.
I bring to your attention a high-quality preamplifier based on the EL2125.
The basic materials are FREE and DIYers are free to use them to replicate them in their own designs.
WHY EL2125?
An excellent chip, according to its characteristics it almost ranks 2nd in the top ten op amps according to model reviews in 2004.
This is, of course, not the AD8099 (first place in the world, award from Intel “Innovation of 2004”), but the EL2125 has already appeared on the CIS market and it is quite possible to get it, especially for those who live in capitals and large cities.
JUDGE FOR YOURSELF HOW GOOD THE EL2125'S CHARACTERISTICS ARE:

Ability to operate on loads up to - 500 Ohm
Operating frequency range up to - 180 MHz
Supply voltage - ±4.5 ... ±16.5 V.
Nonlinear distortion coefficient - less than 0.001%
Output slew rate - 190 V/µs
Noise level - 0.86 nV/vHz (better than AD8099!!!)

The EL2125 retail price is usually $3 each, not very cheap, but worth it.
Most often, EL2125 is found in SO-8 type housing (prepare micro-tips for soldering irons).
I should note that I would add “amazing musicality” to the list of characteristics. This indicator cannot be measured with instruments and expressed in numbers; it is felt only by ear.

1. As an amplifier for phones with a wide range of impedances:

2. As a high-quality preamplifier for power amplifiers with bipolar power supply (ranging from ± 22 to ± 35 V.) and sensitivity 20 ... 26 dB:

This op-amp involuntarily suggests itself as a more serious pre-amplifier, created on the basis of the Solntsev amplifier and described on the Soldering Iron website:
The amplifier uses dual variable resistors R11 and R17 of any type of group B, R1 and R21 of any type of group B or A. A 100 kOhm variable resistor (tapped from the middle) can be used as a loud-compensated volume control (R21). Transistors can be replaced with KT3107I, KT313B, KT361V,K (VT1, VT4) and KT312V, KT315V (others). Replacing the K574UD1 op amp with other types of op amp is not recommended. If the DC component is at a significant level (in rare cases) at point A, it is necessary to install a capacitor with a capacity of 2.2 - 5 μF.

The described preamplifier is connected to an AF power amplifier with an input impedance of at least 10 kOhm. With a significant increase in Kg, this control unit can also be loaded onto an UMZCH with Rin up to 2 kOhm (which is extremely undesirable), in such cases (if the Rin of your UMZCH is less than 10 kOhm), you just need to once again power up the output stage (a copy of the circuit section VT1-VT2- VT3-VT4-R4-R5-R6-R7, connect to output DA2), connect resistors R23 and R24 in the same way as resistors R2 and R3, although in this case the noise level may increase. And if Rin of your UMZCH is greater than or equal to 100 kOhm, then it is recommended to use K574UD1A(B) as an operational amplifier DA2, this will reduce the level of distortion and noise.

Possible changes in the scheme (improving):
- To exclude P2K switches (very unreliable in operation) from the audio signal path, it is recommended to exclude switch SA1 from the circuit (together with resistors R8, R9), and move switch SA2 to the last stage by short-circuiting resistor R23 (resistors R13, R14 are excluded in this case from the diagram).

Preamp circuit:

It would also not be useless to use this op-amp in a universal pre-amplifier that can also serve as a headphone amplifier. The circuit diagram is shown below:

Emitter followers VT1-VT2 unload the output of the op-amp, and then follows a circuit with local feedback, which further reduces non-linear distortions. Resistors R19 and R20 set the quiescent current of the window stage of the preamplifier, similar to power amplifiers, within 7-12 mA. In this regard, the last stage must be installed on a small heat sink

The page was prepared based on materials from the site http://yooree.narod.ru and http://cxem.net

As is known, the rated output voltage of modern audio frequency signal sources (3Ch) does not exceed 0.5 V, while the rated input voltage of most 3Ch power amplifiers (UMZCH) is usually 0.7..1 V. To increase the signal voltage to level that ensures normal operation of the UMZCH, as well as to match the output impedances of the signal sources with its input impedance, 3CH pre-amplifiers are used. As a rule, it is in this part of the sound reproduction path that volume, timbre and stereo balance are adjusted. The main requirements for preamplifiers are low nonlinear signal distortion (harmonic distortion - no more than a few hundredths of a percent) and a low relative level of noise and interference (not higher than -66..-70 dB), as well as sufficient overload capacity. All these requirements are largely met by the pre-amplifier of Muscovite V. Orlov (he took the AU-X1 amplifier circuit of the Japanese company "Sansui" as a basis). The nominal input and output voltages of the amplifier are 0.25 and 1 V, respectively, the harmonic coefficient in the frequency range 20..20000 Hz at the rated output voltage does not exceed 0.05%, and the signal-to-noise ratio is 66 dB. The input impedance of the amplifier is 150 kOhm, tone control limits (at frequencies of 100 and 10000 Hz) from -10 to +6 dB. The device is designed to work with UMZCH, the input impedance of which is at least 5 kOhm.

The amplifier (Fig. 1 shows a schematic diagram of one of its channels) consists of a source follower on transistor VT1, a so-called bridge passive tone control (elements R6-R11.1, C2-C8) and a three-stage symmetrical signal voltage amplifier. The volume control - variable resistor R1.1 - is included at the amplifier input, which reduces the likelihood of its overload. The timbre in the region of lower frequencies of the audio range is controlled by a variable resistor R7.1, in the region of higher frequencies - by a variable resistor R11.1 (resistors R7.2 and R11.2 are used in another channel of the amplifier). The transfer coefficient of a symmetrical amplifier is determined by the ratio of the resistances of resistors R18, R17 and, with the values ​​​​indicated in the diagram, is approximately 16. The operating mode of the final stage transistors (VT6, VT7) is determined by the voltage drop created by the collector currents of transistors VT4, VT5 on diodes VD1 connected in the forward direction - VD3. Trimmer resistor R15 serves to balance the amplifier. The amplifier can be powered either from the source that powers the UMZCH, or from any unstabilized rectifier with output voltages of +18..22 and -18..22 V.

A possible version of the printed circuit board for one channel of the device is shown in Fig. 2.

It is made of foil fiberglass laminate with a thickness of 1.5 mm and is designed for the installation of resistors MLT and SP4-1 (R15), capacitors MBM (C1, C4, C8, C11), BM-2 (C3, C5-C7) and K50-6 , K50-16 (rest). Capacitors MBM and BM-2 are mounted vertically on the board (one of their terminals is extended to the locally required length using tinned wire with a diameter of 0.5..0.6 mm). Double variable resistor R1 of any type of group B, resistors R7 and R11 - group B. Transistors KP303D can be replaced with KP303G, KP303E, transistor KP103M - with KP103L, transistors KT315V and KT361V - with transistors of these series with index G. Field-effect transistors must be selected according to the initial drain current, which at voltage Uс=8 V should not go beyond 5.5..6.5 mA. D104 diodes are completely interchangeable with diodes of the D220, D223, etc. series. The adjustment comes down to setting the trimmer resistor R15 to zero voltage at the output and selecting the resistor R18 until an output voltage equal to 1 V is obtained at an input voltage of 250 mV with a frequency of 1000 Hz (the sliders of resistors R7, R11 are in the middle position, and resistor R1 is in the upper position in the circuit ).

A significant drawback of the one described, and many other similar devices using transistors, is the relatively large number of elements and, as a consequence of this, the rather large dimensions of the circuit board. Pre-amplifiers based on operational amplifiers (op-amps) are much more compact.

An example is a device developed by Muscovite Yu. Solntsev based on the general-purpose OS K574UD1A (Fig. 3).

His studies showed that the harmonic coefficient of this op-amp strongly depends on the load: it is quite acceptable when its resistance is more than 100 kOhm, it increases to 0.1% when the load resistance decreases to 10 kOhm. To obtain sufficiently small nonlinear distortions, the author added to the specified op-amp a so-called parallel amplifier, characterized by the practical absence of “step” distortion even without negative feedback (NFB). With OOS, the harmonic coefficient does not exceed 0.03% in the entire audio frequency range with a load resistance of more than 500 Ohms. The remaining parameters of the pre-amplifier are as follows: nominal input and output voltages 250 mV, signal-to-noise ratio at least 80 dB, overload capacity 15..20 dB. As can be seen from the diagram, the device consists of a linear amplifier with a horizontal frequency response using op-amp DA1 and transistors VT1-VT4 ("parallel" amplifier) ​​and a passive bridge tone control (elements R12-R14, R17-R19, C6-C9). If necessary, this regulator can be excluded from the path using relay K1 (the signal in this case is removed from the voltage divider R10R11). The amplifier's transmission coefficient is determined by the ratio of the resistance of resistor R3 to the total resistance of resistors R2, R4. The bridge regulator has no special features. At lower frequencies, the timbre is adjusted with a variable resistor R18.1, at higher frequencies with a resistor R13.1. Resistors R12, R14 prevent monotonous rise and fall of the frequency response outside the nominal frequency range of the amplifier. For normal operation of the tone control, the load resistance must be at least 50 kOhm. When working with a signal source whose output voltage contains a constant component, it is necessary to turn on a separating capacitor at the amplifier input (shown in the diagram with dashed lines).

All parts of the amplifier, with the exception of the tone control elements, are mounted on a printed circuit board made of foil fiberglass (Fig. 4 shows part of it for one channel). The board is designed for mounting resistors MLT, SP4-1 (R4), capacitors K53-1a, K53-18 (C1, C4), KM-6B (C2, C3, C5, C6) and MBM (others). Double variable resistors R13 and R18 - any type of group B. The tone control elements are mounted directly on their terminals and connected to the board with shielded wires. Instead of those indicated in the diagram, transistors KT3107I, KT313B, KT361K (VT1, VT4) and KT312V, KT315V (VT2, VT3) can be used in the amplifier. Relay K1 - brand RES60 (passport RS4.569.436) or any other with suitable dimensions and operating current and voltage. Diode VD1 - any with a permissible reverse voltage of at least 50 V. For connection to the amplification path, a detachable connector MPH14-1 is used (its plug is installed on the board). To power the amplifier, a bipolar power supply is required, capable of delivering a current of about 30 mA to the load at a ripple voltage of no more than 10 mV (otherwise, if the installation is unsuccessful, a noticeable background may appear). Adjusting the amplifier comes down to setting the required transmission ratio with and without a connected tone control. In the first case, the desired result is achieved by changing the resistance of the tuning resistor R4 (and, if necessary, by selecting resistor R2), in the second, by selecting resistor R11. The amplifier is designed to work with UMZCH, described in the article by Yu. Solntsev “High-quality power amplifier” (Radio, 1984, No. 5, pp. 29-34). The volume control (double variable resistor of group B with a resistance of 100 kOhm) is switched on in this case between its input and the output of the pre-amplifier. The same resistor, but group A, is used as a stereo balance regulator (one of its outer terminals and the engine output in each channel is connected to the volume control slider, and the other outer terminal is connected to the UMZCH input).

In recent years, the industry has mastered the production of integrated circuits (ICs KM551UD, KM551UD2), specially designed for operation in the input stages of audio frequency paths of household radio equipment (preamplifiers-correctors of electric players, amplifiers for recording and playback of tape recorders, microphone amplifiers, etc. devices). They are distinguished by a reduced level of self-noise, low harmonic distortion, and good overload capacity.

Figure 5 shows the circuit of a pre-amplifier based on the KM551UD2 IC (proposed by Muscovite A. Shadrov). This IC is a dual op-amp with a supply voltage from +5 to +16.5 V. An IC with index A differs from a device with index B in half the input common-mode voltage (4 V) and the normalized noise voltage referred to the input (no more than 1 µV when the signal source resistance is 600 Ohms; for KM551UD2B it is not standardized). The nominal input and output voltages of this amplifier are the same as those of the device according to the circuit in Fig. 1, the harmonic coefficient in the frequency range 20..20000 Hz is not more than 0.02%, the signal-to-noise ratio (unweighted) is 90 dB, Adjustment range volume and timbre (at frequencies 60 and 16000 Hz) respectively 60 and +10 dB, transition attenuation between channels in the frequency range 100..10000 Hz is not less than 50 dB. The input and output impedances of the amplifier are 220 and 3 kOhm, respectively. The bridge tone control is included in this case in the OOS circuit, covering the op-amp DA1.1 (hereinafter, the pin numbers of the second op-amp of the microcircuit are indicated in parentheses). At the input there is a fine-compensated volume control on a variable resistor R2.1 with a tap from a conductive element. Loudness compensation (raising low-frequency components at low volume levels) can be turned off using switch SA1.1. Stable operation of the KM551UD2 IC (its frequency response has three bends) is ensured by capacitor C7 and circuit R5C5, the values ​​of which are selected for the transfer coefficient Ki = 10 (the rate of rise of the output voltage with such amplification reaches 3..4 V/μs). Capacitors C12, C13 prevent the amplifier from interconnecting with other devices in the path when powered from a common source. The variable resistor R12.1 (in another channel R12.2) regulates the stereo balance.

All parts of the amplifier, except for variable resistors R2, R7, R11 and switch SA1, are mounted on a printed circuit board made of foil fiberglass. It is designed for the installation of MLT resistors, capacitors MBM (C1, C10), BM-2 (C3-C5, C11), KM (C6, C7, C12, C13) and K50-6, K50-16 (others). Capacitors MBM and BM-2 are mounted vertically. Any dual variable resistors of group A are suitable for regulating volume and stereo balance; resistors of group B are suitable for regulating tone. The amplifier does not require adjustment. The frequency response of bridge tone controls, as is known, has fixed inflection frequencies, therefore, in essence, only the slope of the frequency response sections to the left and right of these frequencies is smoothly adjusted, and its maximum value does not exceed 5..6 dB per octave. To obtain the required limits of tone control at higher and lower frequencies of the audio range, the inflection frequencies must be selected in the mid-frequency region. Such a regulator is ineffective if it is necessary to suppress low- or high-frequency interference in the signal spectrum. For example, with a corner frequency of 2 kHz, the tone control can reduce the level of interference at a frequency of 16 kHz by 15 dB, only at the same time attenuating the spectrum components of 8 and 4 kHz by 10 and 5 dB, respectively. It is clear that in such a case this is not a way out, therefore, to suppress interference at the edges of the spectrum, sometimes switchable low-pass (LPF) and high-pass (HPF) filters with a large slope of the frequency response slope outside the transparency band are used. However, even in this case, the desired result is not always achieved, since these filters usually have fixed cutoff frequencies. It's a different matter if the filters are made tunable in frequency. Then, by smoothly shifting the boundaries of the transmitted frequency range in the desired direction, it will be possible to “remove” the interference beyond its limits without affecting the shape of the frequency response within the range. By the way, it is advisable to make such filters non-switchable: they will help combat infra-low-frequency interference from the mechanism of an insufficiently advanced electric player.

Good afternoon.

I would like to continue the story about a tube preamp for a hybrid amplifier.

The scheme is very simple. We didn't invent anything. The basis chosen last time is a resistive cascade. There is nothing unusual about it.

Active filters on transistors VT1 and VT2 were added to the circuit. They provide additional nutritional cleansing. Since the main filtration will be performed by an external source, the filter circuits were simplified - they were made single-stage.

We plan to power the filament from an external stabilized source. Using powerful filtering of all voltages will ensure that there is no background.

With the prototype board, everything is as usual: we draw, print, translate, etch, drill and clean it with fine sandpaper... After that, put a respirator on your face, a can of black heat-resistant paint in your hands... paint the board black. This way it will not be visible in the body of the assembled amplifier.

Set the board aside and let it dry. It's time to shake out the boxes and pick up the parts. Some of the components are new, others are soldered from early prototypes (well, good, almost new components shouldn’t go to waste?!).

Note: It is more convenient to solder, starting with the lowest profile components and moving to higher ones. Those. First we solder diodes, zener diodes, then resistors, a socket for a lamp, capacitors, etc... We, of course, broke this sequence and soldered as necessary :)

Capacitors installed. This project uses domestic K73-16. Good capacitors. We carried out a series of measurements of their nonlinearity spectra in different modes. The results were encouraging. We will definitely write about this someday.

Note: When soldering a lamp socket, you must insert a lamp into it. If this is not done, then after assembly there may be problems with installing the lamp. In some (the most “severe” cases) you can even damage the lamp base.

The scheme is simple and the likelihood of error is minimal. But you need to check. Connect the amplifier to the power source and turn on:

10 seconds - normal flight... 20... 30... everything is fine: nothing exploded or started smoking. The glow glows quietly, the test power supply protections do not operate. You can exhale with relief and check the modes: all deviations are within acceptable limits for an unheated lamp.

After a 10-minute warm-up, all parameters were established and reached the calculated values. The operating point is set.

Since everything is good, we can continue. We connect a test signal source to the input. At the output there is a resistor simulating the input resistance of a power amplifier. We turn on and measure all the main parameters of the cascade.

Everything is within normal limits. The distortion and gain coincided with what was obtained in the previous article. There is no background.

So our tube preamplifier is ready. It's time to move on to creating a powerful transistor output buffer for it. It can be used with the same success in a purely tube design. To do this, you will need to make a powerful tube output for it.

Perhaps it makes sense to make a universal tube preamplifier (maybe in the form of a designer) for use in tube and hybrid designs?