Assembling a tube preamplifier. Track-cascade ULF with direct coupling
HIGH QUALITY PRE-AMPLIFIER CIRCUIT
At the turn of 2004 and 2005, there was a natural desire 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!!!)
EL2125 price in retail sales 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 into a more serious preamplifier, 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 from the passage path sound signal P2K switches (very unreliable in operation), 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 from the circuit).
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, similarly 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
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 structurally designed as 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 the 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 dissipation. 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 that are partially assembled on integrated circuits ah, 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 signal amplifier audio frequency. [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.
The word "preamplifier" is used differently by different manufacturers, marketers, and users. This is one of the most widely interpreted terms when discussing audio equipment; if you ask for "pre-amp", you may also ask for "furniture". No one will know exactly what you want. Let's figure out what a preamplifier is?
Why do I need a preamp and do I need one?
A preamplifier is a “head amplifier,” and as the name suggests, it prepares the signal coming from the source or microphones for further amplification. There are a number of reasons to buy:
Whether or not a preamp is needed.
When you plug your DAC or microphone directly into an amplifier, what does it sound like?
- Is this signal enough?
- Is it balanced?
- Clean?
If this is not the case, then you probably need to buy a preamp.
By the way, a good separate preamplifier produces less interference, interference and other noise than, for example, a full one 
amplifier. Whenever a signal is amplified, the goal is to maintain the signal-to-noise ratio as 
possible in better quality. This makes a lot of sense because interference and interference from the preamp can cause a non-linear sound when the signal is amplified. To avoid introducing additional noise from the preamp, it should be placed in a separate unit and as close to the signal source as possible, like this.
A preamp is part of an amplifier. This means that the preamplifier will allow you to connect a variety of sources, such as a CD tuner or a DAC. 
The preamp allows you to change the volume and possibly change the HF and LF parameters.
By the way, 90% of preamps have a phono stage, which is what you need to connect your turntable.
Finally, one of the reasons to buy a preamp is to switch multiple signals.
All combined systems require pre-amplification.
There's also a multi-channel preamp that combines the signals for you and creates a single output signal for the amplifier. The multi-channel preamp also allows you to adjust the equalizer and power of each signal depending on your needs.
An amplifier can be divided into two main parts - a preamplifier and a power amplifier.
Amplifier 
One way to obtain higher sound quality was to separate the two sections of the amplifier. By separating the preamp and power amplifier, you could design a dedicated power supply to drive finer-signal electronics without interference from noisy power amplifier circuits. In some cases, even the power supply is split in another case to reduce noise in the preamp.
Preamplifiers can also be "passive". They do not require power since the components (mostly 
switch and volume control) are operated directly from your sources (). In theory this is the best way, but in practice they have quite a few disadvantages, but a passive preamplifier is a relatively rare type.
When we talk about a preamplifier, we usually mean a preamplifier in a separate unit. Such a preamplifier is housed in a separate housing, and it has many controls for controlling the power amplifier to control the acoustics and switch.
The preamp can also be built in as an instrument, pedal, rack unit, mixer, sound card, or a variety of other forms; and the preamplifier can also act as the input stage of each head amplifier. 
Not every preamp can effectively drive a power amplifier. Others may be designed to increase the signal level to drive the input.
Some preamps have gain control, while others have a fixed gain amount. In any case, they typically have a volume knob that simply passively turns the overall signal level at the very end of the preamp circuit. Also, the preamp may have a tone that can include something like an equalizer control. Some people want a lot of tonal Shift and EQ control, other people want absolute control.
Find your preamp!
Tell us about your sound system, audio-video equipment, construction, configuration, etc.on .
Send by email: [email protected] text, photos, marked diagrams on, if you don’t know where to start, how to write, then write, we will help you, we will send you a list of ready-made questions for an interview.
Don't be afraid of me and join me
Modern digital sources sound (CD players, DACs, etc.) have a very low noise level. Much lower than vinyl or magnetic tape. Because of this, the noise requirements of the subsequent amplification path today have become much higher than in the era of analogue audio. In light of these requirements, the preamplifier described below was designed with the primary goal of achieving high-quality sound at ultra-low noise levels without the use of exotic or expensive components.
In most stages the author used his favorite operational amplifiers NE5532, but in some nodes they are used LM4562, since recently they have become more accessible and allow one to obtain much less distortion when operating with a low-impedance load.
What kind of music lover (and even more so an audiophile) is without vinyl? It is for them that the preamplifier is equipped with two background correctors for different types of pickups. In addition, the design has tone control, visual level indicator And balanced outputs, which today has become almost a standard for high quality audio equipment.
The block diagram of the preamplifier is shown in the figure:
Click to enlarge
All modules are assembled on separate printed circuit boards, which simplifies their placement in the housing and facilitates switching.
This part of the series of articles describes the circuit of the amplifier itself with volume, balance and tone controls, as well as the organization of a symmetrical output.
Schematic diagram of the pre-amplifier module:
Click to enlarge
All resistances (not only resistors, but also the resistance of active components, for example the base resistance of a transistor) generate noises, the level of which depends on the resistance value and temperature. Since it is quite difficult to influence the temperature in the listening room, the only way to reduce the noise of the resistances is to reduce the value of the resistance itself. This implies the main feature of the presented scheme - the use low resistance resistors along the entire path of the sound signal.
If for constant resistors the choice of low-resistance ratings does not pose a problem, then for variable resistors (for volume, balance and tone controls) the nominal range is significantly limited. Typically in these circuits you can see variable resistors of 47 kOhm, 22 kOhm, or at best 10 kOhm. In this design, Douglas Self used 1kOhm variable resistors - this is perhaps the minimum value available among variable resistors.
By the way, here are the characteristics that we managed to achieve:
(Measurements were carried out at a supply voltage of 17V, with tone controls disabled, using balanced inputs and outputs)
| Harmonic distortion + noise (input signal 0.2V, output signal - 1V) | 0.0015% (1 kHz, B = 22 Hz to 22 kHz) 0.0028% (20 kHz, B = 22 Hz to 80 kHz) |
| Harmonic distortion + noise (input signal 2V, output signal - 1V) | 0.0003% (1 kHz, B = 22 Hz to 22 kHz) 0.0009% (20 kHz, B = 22 Hz to 80 kHz) |
| Signal-to-noise ratio (at 0.2V input signal) | 96 dB (B = 22 Hz to 22 kHz) 98.7 dBA |
| Reproducible frequency band: | 0.2 Hz to 300 kHz |
| Maximum output signal level (at 0.2V input): 1.3V | |
| Balance adjustment | +3.6 dB to -6.3 dB |
| Bass adjustment | ±8 dB (100 Hz) |
| Treble adjustment | ±8.5 dB (10 kHz) |
| Channel separation (R->L) | -98 dB (1 kHz) -74 dB (20 kHz) |
| Channel separation (L->R) | -102 dB (1 kHz) -80 dB (20 kHz) |
The use of low-impedance resistors also reduces the biasing of op-amps by input currents, which also reduces noise caused by fluctuations in op-amp currents.
To reduce the noise of active components, a parallel connection is used in the circuit cascades. Of course, one could use modern low-noise op-amps like AD797. But this will be much more expensive and more complicated (since one package contains only one op-amp). Please note that this is not about parallel connection microcircuits (when they are soldered on top of each other), but about the parallel connection of amplifier stages. Only in this case the noise of the amplifying elements will be uncorrelated, due to which the overall noise level is reduced by 3 dB when 2 stages are parallelized. When 4 stages are connected in parallel, the noise decreases by 6 dB, i.e. twice.
If 8 cascades are parallelized, the noise will decrease by 9 dB, but for such a gain the costs are unreasonably high.
Due to the use of low-resistance resistors in the tone control, the capacitor values were much larger than usual. But today this is not a problem for modern element base.
Line input and balance control.
To reduce noise and interference, a filter R1C1 and R2C2 is installed directly at the amplifier input. Buffer stages IC1A and IC1B provide approximately 50kΩ input impedance and improve common mode rejection. The amplification stage itself is assembled on LM4562 (IC2A), the gain of which is adjusted by potentiometer P1A. The same potentiometer in the right channel is turned on “out of phase” with the left one, due to which the balance is adjusted. Feedback in the cascade is implemented through two parallel buffers IC3A and IC3b, due to which the cascade gain remains constant regardless of load changes. In addition, this solution reduces noise and provides low output impedance.
A typical implementation of a balance control usually negatively affects the stage and the “virtual” arrangement of instruments, which is why it is quite rare in Hi-End equipment. Douglas Self's solution to this node does not have this drawback.
The noise level of this part of the preamplifier is only -109 dB in the middle position of the balance control, -106 dB at the maximum and -116 dB at the minimum position of the control (in the frequency band 22 Hz to 22 kHz).
Tone control.
Despite the fact that the regulator looks somewhat unusual, nevertheless, the classic Baxandall tone control circuit is used here. As noted above, due to the low denominations variable resistances The capacitor ratings are significantly higher than the “typical” values.
Capacitor C7 (1 μF) determines the lower tone control frequency, and capacitors C8 and C9 have a value of 100 nF and determine the tone control frequency at HF. If desired, the depth of tone control can be increased to ± 10 dB. Due to the IC4 elements, the mutual influence of the low-frequency and high-frequency circuits when controlling timbres is eliminated.
Despite the large dimensions and high cost, the use of polypropylene capacitors.
The tone control noise level is only -113 dB in the middle position of the controls.
Relay RE1 serves to turn off the tone control if it is not needed. In this case, the signal is taken from the output of IC2A and goes directly to the input of IC9B, bypassing the tone control. To avoid clicks during switching, resistor R18 is used. To reduce crosstalk, switching in each channel is carried out by a separate relay. In this case, the relay contact groups can be parallelized, which will reduce the contact resistance and further increase the reliability of this part of the circuit.
Active volume control.
The volume control was also implemented according to the idea of Peter Baxandall, which firstly made it possible to obtain ultra-low noise level(especially at low volumes), and secondly, to obtain a logarithmic control characteristic when using potentiometers with a linear dependence of resistance on the angle of rotation. The maximum gain is +16 dB, with the 0 dB point occurring at the middle position of the potentiometer.
Four amplifiers connected in parallel, as noted above, serve to reduce the noise level by 6 dB. The self-noise level of such a regulator is -101 dB at maximum gain and -109 dB at 0 dB gain. In practice, the volume control is usually set to -20 dB, then the noise level will be -115 dB, which is significantly below the hearing threshold.
So that you can evaluate the quality of each cascade, their own noise levels are given. The resulting noise level of a given preamplifier, as you might guess, will vary somewhat depending on the position of the potentiometers.
Symmetrical output implemented using a phase inverter on the op-amp IC9A and has double the signal amplitude compared to an asymmetrical one. However, this is normal for professional audio equipment.
Design and setup.
Placement of amplifier elements on the board:
Click to enlarge
During assembly, the resistors are soldered first, and then the remaining components.
Jumper JP1 is designed to select the optimal ground connection for the vinyl corrector (there are similar jumpers on MC / MD boards). Don't forget to connect them. The connection location is selected experimentally after assembling the structure in the housing.
Photo of the assembled board:
Click to enlarge
This block of settings does not require.
Frequency characteristics of the amplifier and tone control:
Click to enlarge
List of elements:
Resistors:
(1% accuracy; metal-film; 0.25W)
R1,R2,R39,R40 = 100Ohm
R3-R6,R41-R44,R78,R79 = 100kOhm
R7-R12,R16,R17,R21-R24,R33,R34,
R45-R50,R54,R55,R59-R62,R71,R72 = 1kOhm
R13,R51 = 470Ohm
R14,R15,R52,R53 = 430Ohm
R18,R35,R36,R56,R73,R74 = 22kOhm
R19,R20,R57,R58 = 20Ohm
R25-R28,R63-R66 = 3.3kOhm
R29-R32,R67-R70 = 10Ohm
R37,R38,R75,R76 = 47Ohm
R77 = 120Ohm
P1,P2,P3,P4 = 1kOhm, 10%, 1W, stereo potentiometer, linear, for example Vishay Spectrol cermet type 14920F0GJSX13102KA. or, Vishay Spectrol conductive plastic type 148DXG56S102SP.
Capacitors:
C1,C2,C10-C14,C26,C27,C35-C39 = 100pF 630V, 1%, polystyrene, axial
C3,C4,C28,C29 = 47µF 35V, 20%, non-polar, 8mm diameter, 3.5mm pin spacing, e.g. Multicomp p/n NP35V476M8X11.5
C5,C6,C30,C31 = 470pF 630V, 1%, polystyrene, axial
C7,C32 = 1µF 250V, 5%, polypropylene, pin spacing 15mm
C8,C9,C33,C34 = 100nF 250V, 5%, polypropylene, lead spacing 10mm
C15,C16,C40,C41 = 220µF 35V, 20%, non-polar, 13mm diameter, 5mm pin spacing, e.g. Multicomp p/n NP35V227M13X20
C17-C25,C42-C50 = 100nF 100V, 10%, pin spacing 7.5mm
C51 = 470nF 100V, 10%, pin spacing 7.5mm
C52,C53 = 100µF 25V, 20%, diameter 6.3mm, pin spacing 2.5mm
Chips:
IC1,IC3,IC5-IC10,IC12,IC14-IC18 = NE5532, for example ON Semiconductor type NE5532ANG
IC2,IC4,IC11,IC13 = LM4562, for example National Semiconductor type LM4562NA/NOPB
Miscellaneous:
K1-K4 = 4-pin connector, pitch 0.1’’ (2.54mm)
K5,K6,K7 = 2-pin connector, pitch 0.1’’ (2.54mm)
JP1 = 2-pin jumper, pitch 0.1’’ (2.54mm)
K8 = 3-pin screw block, 5mm pitch
RE1,RE2 = relay, 12V/960Ohm, 230VAC/3A, DPDT, TE Connectivity/Axicom type V23105-A5003-A201
To be continued...
The article was prepared based on materials from the magazine “Elector” (Germany)
Happy creativity!
Editor-in-Chief of RadioGazeta
As is known, the rated output voltage modern sources audio frequency signal (3Ch) does not exceed 0.5 V, while the nominal input voltage of most 3Ch power amplifiers (UMZCH) is usually 0.7..1 V. To increase the signal voltage to a level that ensures normal operation of the UMZCH, and 3Ch preamplifiers are also used to match the output impedances of signal sources with its input impedance. 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 the Muscovite V. Orlov (he took the AU-X1 amplifier circuit of the Japanese company "Sansui" as a basis). Nominal input "and output voltage amplifier 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, the tone control limits (at frequencies of 100 and 10000 Hz) are from -10 to +6 dB. The device is designed to work with UMZCH, the input impedance of which is at least 5 kOhm.
Amplifier (Fig. 1 shows circuit diagram 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 regulated by a variable resistor R7.1, in the region higher frequencies- 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.
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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 it is necessary to select according to the initial drain current, which at a voltage Uc = 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 described one, and of many other similar transistor devices, is relatively big number elements and, as a consequence, quite 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).
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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 virtual absence of “step” distortion even without a negative feedback(OOS). 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 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).
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All parts of the amplifier, with the exception of the tone control elements, are mounted on 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 acceptable reverse voltage not less than 50 V. For connection to the amplification path, a detachable connector MRN14-1 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 last years 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.
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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 with a signal source resistance of 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, harmonic distortion in the frequency range 20..20000 Hz no more than 0.02%, signal-to-noise ratio (unweighted) 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. Bridged tone control included in this case into 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.
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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, switchable low-pass (LPF) and high-pass (HPF) filters with a large frequency response slope outside the transparency band are sometimes used to suppress interference. However, in this case too desired result This 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.