Preamplifier with tone block. Scheme, description

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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING
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Preamplifier with tone block. Encyclopedia of radio electronics and electrical engineering

Free technical library

Encyclopedia of radio electronics and electrical engineering / Transistor power amplifiers

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The author of the tone block proposed in this article is sure that the use of stream repeaters and op-amps with field-effect transistors in the design makes it possible to obtain very high characteristics. He also notes an interesting effect - the influence of the option of switching on the op-amp (inverting or non-inverting) on the subjective perception of the phonogram. I wonder if those who experiment with such devices will notice it.

The preamplifier (one of the channels is shown in Fig. 1) is used as a built-in one in conjunction with the UMZCH, the sensitivity of which is 0,7 V.

(click to enlarge)

Its main parameters

Rated input voltage, V......0,7
Rated output voltage, V......0,7
Tone control range, dB at a frequency of 40 Hz......±12
at a frequency of 14 kHz ...... ± 12
Harmonic coefficient (up to 20 kHz) for Vin nom,%, no more than......0,05
Signal-to-noise ratio (unweighted), dB, not worse......85

The increased linearity of field-effect transistors is well known; in this they can only be inferior to lamps, and then - not always. Therefore, the buffer stages of the amplifier (source followers) are assembled on field-effect transistors. According to the same criterion, the KR574UD1 op-amp was selected - with field-effect transistors at the input. To increase the maximum level of input signals and improve the parameters in general, the resistors in the repeaters are replaced by current generators (field-effect transistors VT2, VT4).

Experiments have established that correctors with a non-linear frequency response, containing several stages of voltage amplification, are more prone to any kind of self-excitation, not to mention a kind of "multiplication" of distortion in such amplifiers. Therefore, in this device, only one stage was used on the op-amp DA1, and its inverting inclusion was deliberately used, as it is more stable and provides less distortion.

When checking cascades with non-inverting and inverting inclusion of the OS, approximately the same (according to measuring instruments) values of the harmonic coefficient were obtained. A subjective assessment of the quality of sound reproduction gave an interesting result: almost all listeners noted a clear advantage in the natural sounding of an amplifier with an inverting op amp *. Some of the listeners suggested that different types of op amps work! Whoever does not believe what has been said can try and make sure for himself ... Of course, if your UMZCH allows you to hear the difference in the operation of these cascades by ear. It is auditory control, and not measuring instruments, that makes it possible to select the most successful circuit solutions for practical designs.

To eliminate the distortions created by the integrated pn-p transistor in the output stage DA1, the installation and selection of the resistor R31 allows. To select this resistor, it is desirable to perform a simple operation of "testing" the op-amp before installing it on the circuit board. For this purpose, you need to assemble a section of the circuit in the figure with resistors R13, R14, R31 and DA1 (connect a load resistor with a resistance of 6 ... 1 kOhm to pin 2 of DA4). You will also need an audio frequency generator up to 200 kHz and an oscilloscope. A signal from the GZCH is applied to the left tap of the resistor R13 in the figure and the gain KU DA1 \u3d XNUMX is set with this resistor.

By increasing the GZCH signal to the limit level at the DA1 output, the frequency of the input signal is also increased, bringing it up to 100 ... 200 kHz. By selecting the resistor R31, a symmetrical limitation of the output signal of the op-amp is achieved (the oscilloscope cable must be connected to the op-amp through a resistor with a resistance of 0,5 ... 1 kOhm).

This simple process allows you to choose the best copies from a certain number of op-amps, since unusable ones often fall into the hands of a radio amateur. Microcircuits work very well when using other methods of transferring the output stage of the op-amp to class A mode. The described method allows you to reduce the harmonic coefficient at a load with a resistance of 10 kOhm by 10 times!

The necessary amplification of the DA1 stage is achieved by adjusting the tuning resistor R13 (the smaller the input signal at the VT1 gate, the more the DA1 gain is increased). Switch SA1 is designed for stepwise attenuation of the signal by approximately 20 dB. It is very convenient, especially when working with sources of different voltage levels, for example, from a tape recorder - 0,25 ... 0,7 V, from a CD player - 2 ... 4 V.

The tone control is passive, bridge type. Such regulators operate with less distortion than active ones with the formation of a frequency response in the amplifier's feedback circuit, although with a competent circuit design, good op-amps also provide low distortion. In order for the source followers to also work without noticeable distortion, some "subtleties" must be observed.

First, you need to use field-effect transistors with the greatest possible steepness and a large initial drain current, best of all are instances with a maximum cutoff voltage. In addition, by selecting resistors R9 and R24, it is desirable to set voltages close to zero at the sources VT1 and VT3.

Since the amplifier board is located inside the UMZCH case, the power supply of which has bipolar voltages (35V), then the power comes from this block. experiments safe for stabilizers and power supply.Many radio amateurs use such simple voltage stabilizers, but often without limiting resistors in the collector circuits.But in vain!The limiting resistor in emergency situations can prevent the failure of semiconductor devices.Besides, why heat VT5 crystals or VT6 if, by introducing resistors R25 and R28, most of the voltage and power drop can be transferred to them.

The harmonic coefficient mainly depends on the instance of the used op-amp (in this case, op-amps with Kg = 0,1% are used), when choosing a resistor R31, its value decreases ten times. It is also possible to reduce distortion by 1,5 ... 2 times by increasing the values of resistors R16 - R21 by two times (R17 and R22 - 47 kOhm), the capacitances of capacitors C12 - C15 in this case are also reduced by half. If resistors R17 and R22 are each 100 kOhm, distortion DA1 will decrease by 3...4 times. More stringent requirements are imposed on the installation of such amplifiers with low distortion: the conductors are short or shielded; you may need to shield the tone block.

To maintain the "softness" of the low-frequency components, the value of the resistor R29 is increased to 470 kOhm. With some deterioration in the parameters of the amplifier of the op-amp of the KR574 series, it is permissible to replace it with a less high-speed K544UD2. Field-effect transistors VT1 - VT4 - KP302, KP303, KP307 with any letter index, but taking into account the recommendations shown above, and observing the necessary polarity of the supply voltages.

It is very convenient to use assemblies of two field-effect transistors KR504NTZ, KR504NT4, you can also use KP103 with the letter indices K, L, M, but for the reliability of the unit, it is better to reduce the supply voltage to ± 10 V, observing the necessary supply polarity. Resistors R25 and R28 - wire with a power of at least 5 watts.

The printed circuit board is made of double-sided foil fiberglass and is designed for the use of MLT-0,25 resistors, modern electrolytic capacitors of the K50-35 type, or others. 2,a shows the arrangement of elements, in fig. 2b - drawing of the printed circuit board. The foil on the side of the parts is used as an electrostatic shield and a common wire (connections to the foil are shown with the corresponding sign).

Preamplifier with tone block

The attenuator details are located on the SB1 pushbutton switch, which is used as P2K. The bridge tone control was mounted in a similar way. Each of the capacitors C5 and C11 consists of two capacitors of 2,2 microfarads each.

Stabilizers VT5 and VT6 are made on separate boards by surface mounting and are common for both preamplifier channels.

It should be emphasized that the use of a low-speed op-amp, for example, KR544UD1 instead of KR574UD1, will lead to an increase in Kr at high frequencies by more than 10 times. On the contrary, the use of high-quality imported op amps will provide higher parameters.

Resistors R2 and R5 with the input capacitance of the first stage form a low-pass filter, which reduces the likelihood of interference from powerful radio stations or other RF interference entering the UMZCH input. For better suppression of interference, which is extremely undesirable for an amplifier based on bipolar transistors, it is additionally possible to introduce a capacitor Cf with a capacity of 10 ... 100 pF.

The first copy of the amplifier assembled according to this scheme has been in operation for more than five years, and many people evaluate the quality of its work higher than other amplifiers, including tubes.

When collecting and comparing various options for both UMZCH and preliminary blocks, one should not forget that, in general, the quality of the sound reproduction complex, first of all, depends on the elements used in the design: it is impossible, for example, to achieve good sound at low frequencies if the volume of acoustic speakers is small systems. Just like a small, simple CD player can't sound good compared to a desktop PC.

* Much depends on the quality of the used op amps: the structure of their input stages and transistors, methods of frequency correction and broadband.

Author: A.Zyzyuk, Lutsk, Volyn region, Ukraine

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