Preliminary ultrasound on microcircuits of the K174 series

A two-channel pre-amplifier of audio frequencies with thinly compensated electronic volume and tone control for low and high frequencies can be built on specialized low-noise K174 series microcircuits. A simple device, which will be discussed in this article, is designed to modernize stationary industrial equipment of the 2nd ... 4th complexity group of previous years of production or for installation in similar home-made devices.

The use of electronic adjustments makes it possible to eliminate the passage of a signal through variable resistors, thereby eliminating their inherent noise, rustles and distortions that occur when the current-collecting contact is not in good enough contact with the resistive layer. In addition, the control circuits become insensitive to background pickups of the mains frequency of 50 Hz, which makes it possible to place variable resistors in any convenient place of the apparatus.

(click to enlarge)

The balance adjustment unit and the volume control with loudness are built on a DA1 chip of the K174UN12 type (import analogue - TCA730). The AC voltage of the audio frequency through the input resistive voltage dividers and separating capacitors C2, C3 is fed to the inputs of the DA1 microcircuit (pin 11 is the input of the first channel, pin 14 is the input of the second channel). The voltage transfer coefficient (-60...+18 dB) for both channels depends on the control voltage at pin 13 DA1. Its adjustment is made by the resistor R20 "Volume". The channel balance is regulated by a variable resistor R22. The output voltages change by ±8dB when the balance is adjusted.

Features of human hearing are such that when the volume level is reduced, the sensitivity to the low-frequency and high-frequency parts of the sound range decreases. Therefore, so that the frequency perception of the phonogram does not change when the volume is reduced, it is necessary to raise the low-frequency and high-frequency components of the sound. This is what is called tone compensation. Its depth depends on the value of the control voltage at pin 4 of DA1.

The DA1 chip is powered by a unipolar voltage of +15 V through an RC filter R27-C16-C13. A feature of this microcircuit is that when the supply voltage drops below 2 V, both of its channels are closed, which prevents the appearance of a distorted, gradually fading sound after the device is turned off.

The electronic tone control is built on a DA2 chip of the K174UN10A type, (import analogue - TCA740). The range of tone control for bass and treble for each channel is at least ±15 dB. The transmission coefficient at a frequency of 1 kHz with a change in the control voltage at pins 4 and 12 in the range of 1 ... 10 V changes by no more than ± 2 dB. The timbre at low frequencies is regulated by a variable resistor R40, at high frequencies - R42. The output voltage of the first and second channels is taken from pins 3 and 5 of DA2, respectively. The input impedance of the connected load must be at least 22 kOhm, which in most cases is easily feasible.

The audio frequency preamplifier is powered by a stabilized voltage of +15±1,5 V and consumes no more than 80 mA.

Details. Fixed resistors can be of types C2-8, C1-4, C2-14, ML T. It should be borne in mind that resistors of the same type but of lower power have a higher intrinsic noise voltage. Variable resistors - any type, for example, SPZ-30, SPZ-33, SPZ-4. Resistor R22 "Balance" - necessarily with a linear characteristic (group "A"). Volume control resistor R20 - group "B" or "A". Tone control resistors R40, R42 - group "A".

Oxide capacitors can be taken types K50-24, K50-35 or their imported counterparts. Blocking capacitors C13, C29, C31 - ceramic, types K10-17, KM-5, KM-6. The rest are K73-9, K73-2, K73-17. Capacitors C7, C8, C14, C15, C21, C22, if possible, it is better to take non-polar (film).

The K174UN10A chip can be replaced with the K174UN10B, but the latter has a slightly higher level of noise and harmonics.

We will consider the establishment of a properly assembled amplifier using the example of its installation in a stationary tape recorder. Both inputs of the amplifier are temporarily connected together and connected to the output of the playback amplifier or noise reduction system. In the absence of a tape recorder circuit, you need to find a volume control resistor and connect the amplifier inputs to its engine. If the device has balance and tone control circuits, then they must be excluded from the signal flow circuit.

Next, a tape with a recorded signal with a frequency of 1000 Hz and a level of 0 dB is inserted into the tape recorder. The volume, balance and tone controls of the tuned amplifier are set to the middle position. If necessary, by selecting resistors R3, R4, a voltage of 11 ... 14 mV is set at the inputs 1, 100 of DA200. The voltage at the outputs of the amplifier is measured. Since the technology for manufacturing microcircuits is not ideal, and the attached components have some variation, it is likely that the output voltages of the channels will differ slightly from one another. You can equalize them by additional selection of the resistance of one of the resistors R3 or R4.

Installing trimmers instead of fixed resistors R1, R3 and R2, R4 is undesirable, since reliability is reduced, the noise level may increase significantly. The inputs are now connected to their channels. Resistor R20 is set to the lower position according to the diagram. Resistor R21 is selected so that when the volume is close to zero, a slight movement of the slider of the resistor R20 upwards would lead to an increase in volume. Resistors R23, R24, R41, R43 set the voltage of 9 ... 10 V at the corresponding points, resistor R5 sets the desired loudness value. With a decrease in its resistance, loudness increases. If instead of this resistor we install a variable resistance of 2,2 kOhm, then it will be possible to adjust the depth of loudness.

The device can be mounted on a 120x50 mm printed circuit board made of double-sided foil fiberglass. The foil on the part installation side is used as a common wire. This reduces the requirements for the wiring of signal and power circuits, which is essential for those who have little experience in designing and manufacturing high-quality sound-reproducing equipment. If one-sided foil material is used, then the foil layer serves as a common wire, and all other connections are made by wire jumpers.

Depending on the circumstances, it is not at all necessary to assemble the considered amplifier completely. If, for example, you only need a tone control, then you can assemble the corresponding node on the K174UN10 chip with the details of its frame.

Electronic adjustment of operating modes opens the way for further development of the design. For example, instead of a resistor, you can make a pseudo-touch button volume and tone control.

Sources of

1. A.V. Nefedov, A.I. Aksenov. Elements of circuits of household radio equipment. Microcircuits. Directory. - M.: Radio and communication, 1995, pp. 113-118.
2. D.I. Ataev, V.A. Bolotnikov. Functional units of high-quality sound reproduction amplifiers. - M.: Radio and communication, 1989, S.121-138.

Author: A.Butov, s.Kurba; Publication: cxem.net

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