For the first time, the requirements for audio amplifiers were established by the German DIN 45500 standard in the middle of the 60s. Then the recommendations of the International Electrotechnical Commission IEC 268-3, IEC-581-6 were approved. Methods for measuring and testing the means and conditions for measuring the main parameters of amplifiers are established by GOST 23849-90 “Household radio-electronic equipment. Methods for measuring electrical parameters of audio frequency amplifiers ”, GOST 24388-88 (ST SEV 1079-78)“ Household audio frequency signal amplifiers. General technical requirements ", GOST 36033 -91" Amplifiers for measuring direct current and direct current voltage. General technical requirements and test methods ", GOST 12090-80" Frequencies for acoustic measurements. Preferred Rows ".
The main operations carried out when measuring the main parameters of the amplifier are as follows:
By the amplitude characteristic, we determine the dynamic range of the amplifier
D = 20 lg U in max / U in min; (1.94)
To determine the nominal power of the amplifier, we use the amplitude characteristic of the amplifier (Figure 1.94) and the device connection diagram shown in Figure 1.95. Inflection of the amplitude characteristic (point a) usually corresponds to the rated power, as well as the harmonic distortion of the output voltage, which is indicated in the technical specifications.
R number. = U 2 input max. 1000 / R n. (1.95)
where U inmax. 1000 - maximum voltage in current a;
R n - load resistance.
We draw a horizontal line at a level of - 3 dB, which meets the generally accepted tolerance for unevenness frequency response... And we determine the bandwidth of the P amplifier.
4. Definition sensitivity amplifier.
The parameter "sensitivity" is usually understood to mean the voltage of the low-frequency signal, which must be applied to the input of the amplifier in order to obtain the nominal initial power at the load. The input sensitivity is determined at a frequency of 1000 Hz. The volume (gain) control is then set to the maximum volume position and the position of the other controls corresponding to the nominal conditions.
5 We define volume control limits measured under the above conditions. First, the voltage at the output of the amplifier is measured. The position of the volume control slider is changed within the limits of smooth adjustment until the voltage at the amplifier input changes abruptly. Then the output voltage is measured again.
The measurement results are determined by the value of the volume control limits D g, calculated in decibels by the formula
D g = 20 lq (U out max / U out min), (1.96)
where U out max is the voltage at the amplifier output when the volume control is in the maximum volume position;
U out min - voltage at the amplifier output when the volume control is in the minimum volume position.
6. We define tone control limits- at the lowest and highest frequencies. Tone control limits (see Figure 1.95) is determined at the frequencies specified in the technical conditions at an input voltage, the value of which is equal to 0.3 of the nominal voltage value.
Frequency characteristics are recorded in such amplifiers at least three times. First, both tone controls are set to positions that correspond to the greatest blockage of the extreme low and high frequencies. The resulting characteristic can have the form of a curve, indicated in Figure 1.97, number 1. Then the knob of both tone controls is returned to the other extreme position, which corresponds to the maximum lower and higher frequencies, and the measurement is done at the input voltage, which is ten times (by 20 dB) less than nominal. This characteristic can look like curve 2 in Figure 1.97. After that, the knobs of both regulators are set to the middle position and a third measurement is made. If a characteristic is obtained or is close to curve 3, then this measurement ends. If it differs significantly from this curve, then by trial such positions of the knobs of the regulators are found, at which the characteristic comes out the most straightforward, and the corresponding estimates are made on the knobs of the regulators.
Figure 1.97 - Frequency response of the timbre
From the graph, Figure 1.97, it can be seen that for an amplifier with such characteristics, the tone control limit at low frequencies is f n = 70 Hz, and at the highest one, equal to f in = 7500 kHz. The tone control is carried out within the range of +5 dB to - 10 dB.
The value of the limits of tone control (rise and fall) D at frequencies F n and F b is determined in decibels by the formula
D t = 20 lq (U out / U 1000), (1.97)
where U out is the output voltage, respectively, at the frequencies F h and F b at the given positions of the tone controls (rise and fall); U 1000 - output voltage at a frequency of 1000 Hz, with P out = P nom.
7. Harmonic coefficient are measured using special devices - nonlinear distortion meters or spectrum analyzers (Figure 1.95) Measurements are made at the frequencies specified by the technical specifications. On the scale of the meter of harmonic distortion, you can directly determine the harmonic distortion.
8. In determining intermodulation distortion coefficient it is necessary to use two measuring generators to set the frequencies at which the measurements are made. Depending on the frequency range of the amplifier, the values of these frequencies are indicated in the regulatory and technical documentation. For example, for low-frequency amplifiers with a range of 40 Hz ... 16 kHz, in accordance with GOST 23849-87, these frequencies are 250 Hz and 8 kHz, respectively.
The measurement circuit (Figure 1.98) consists of generators, a harmonic combiner, a spectrum analyzer and the amplifier under test.
Figure 1.98 - Connection diagram of instruments for measuring intermodulation distortion
At the output of the first generator, a voltage is set, the value of which is 0.8 of the nominal voltage value; and at the output of the second - 0.2 · U nom. With the help of the volume control in the load, the power is set equal to the nominal one. The spectrum analyzer measures the output voltage at the following frequency combinations: (F 2 + F 1), (F 2 - F l), (F 2 + 2F 1), (F 2 -2F1) ....
The measurement result is the value of the inter-modulation distortion coefficient, calculated by the formula
K g = V 2 + 2 / U F 2 * 100,%. (1.98)
The above method for performing individual operations is recommended by GOST 23849-87 "Household radio-electronic equipment, methods for measuring the electrical parameters of audio amplifiers".
The purpose of the calibration of measuring amplifiers is to determine their suitability in accordance with the specified metrological characteristics. Calibration of measuring instruments, including measuring amplifiers, is carried out on the basis of the current regulatory and technical documentation, the state standard of Ukraine. The fundamental documents in matters of calibration and testing of measuring amplifiers are DSTU 3989-2000. Metrology. Calibrating using vimiruvalnoy technology. The main provisions, organization, procedure for carrying out and formalizing the results. Calibration is carried out periodically by the bodies of state or departmental metrological services. DSTU 2708: 2006. Metrology. Verification of the imaging technology. Organizatsiya and the order of carrying out. DSTU 3406: 2006. Metrology. State-of-the-art technology. The main provisions, organization, procedure for carrying out and viewing the results.
Before proceeding with the calibration, it is necessary to familiarize yourself with the technical documentation for this amplifier and the procedure for its calibration. After that, model and auxiliary measuring instruments are selected and the question of matching the input and output parameters of these means and the amplifier under test is decided. Calibration is carried out using more accurate reference measuring instruments. The minimum permissible error ratio of the sample and verified means is 1: 3. When choosing an exemplary measuring instrument, not only its accuracy in general is taken into account, but also the degree of reliability of determining the errors of the exemplary and calibrated measuring instrument is assessed. Voltmeters, attenuators, nonlinear distortion meters, spectrum analyzers, frequency and transient characteristics meters, and measuring generators are used as measuring instruments in the calibration of measuring amplifiers. In addition, for the calibration of amplifiers, an installation of the K2-41 type is produced, used in the frequency range 20 Hz ... 200 kHz, which allows you to set the voltage ratio from 10 to 10 6 with a relative measurement error of 0.3%.
Calibration of amplifiers consists of external inspection, testing (performance check), determination metrological characteristics and parameters. The main operations for determining metrological characteristics and parameters are the following: F(its value is indicated in the standard or technical description of the device; for low-frequency amplifiers - 1 kHz); unevenness of the frequency response relative to frequency F; output voltage harmonic coefficient; the noise voltage of the amplifier brought to the input. The error in setting the gain is determined by the substitution method using an exemplary attenuator or setting K2-41 by directly reading the error on the indicator scale. The technique for carrying out other operations is similar to the methods discussed above. electrical measurements when testing amplifiers.
Measurement methods. The frequency is measured by comparing it with the frequency of the frequency-setting process, taken as a unit (the frequency-setting process can be reference, exemplary or working, depending on the measure that reproduces it). This type of measurement is one of the important tasks of measuring technology. In electronics, radio engineering, automation and other related industries, signals of a wide variety of frequencies are used - from fractions of a hertz to thousands of GHz.
A distinction is made between analog and digital methods for measuring frequency. Analog method is an indirect measurement method based on comparing the measured frequency with the frequency of another source (usually a reference one) using an oscilloscope, heterodyne and resonance method.
For comparison, it is necessary to have an exemplary generator, the accuracy of which is at least 5 times higher than the accuracy of the controlled source, and a device for comparison of frequencies. An oscilloscope is often used as such a device.
To measure frequencies that are multiples of a known frequency, apply method of Lissajous figures. The voltage of the known frequency freb of the reference source is applied to one input of the oscilloscope (for example, input X) , and the voltage of the measured frequency fmeas - to the second one (for example, input Y). The frequency of the reference generator is tuned until a stable image of the simplest interference figure is obtained on the screen: a straight line, a circle or an ellipse. The appearance of one of these figures indicates the equality of frequencies (ratio fmeas: frev = 1: 1). When the frequencies are not equal to each other, but multiples, more complex shapes are observed on the oscilloscope screen.
The frequency ratio is determined in the following way. Two straight lines are mentally drawn through the image of the figure: horizontal and vertical. Number ratio T intersections of a horizontal line with a figure to a number NS of intersections of a vertical line with a figure is equal to the ratio of the frequency fed to the input of channel Y to the frequency fed to the input of channel X:
Rice. 3.1
If the compared frequencies are multiples, but their ratio is large, apply circular sweep method with brightness modulation. The voltage of the reference frequency frev is applied simultaneously to both inputs of the oscilloscope with a phase shift of 90 °, achieved using a phase shifter. The gain of both channels is adjusted so that the beam traces a circle on the screen. The voltage of the measured frequency is supplied to the brightness control channel. The frequency of the reference source is reconstructed until a fixed image of a dashed circle is obtained on the screen (Fig. 3. 1). The number of bright arcs or dark gaps between them uniquely determines the ratio N = fmeas / frev (7: 1 in Fig. 3. 1).
If the ratio of frequencies fmeas and frev slightly differs from an integer, i.e. fmeas = Nfrev Fp (frequency Fp is relatively small), then the figure rotates, and the direction of rotation shows the sign of the frequency divergence (it is easiest to determine experimentally, fixing the direction of rotation when the established ratios f 'meas> Nfo6p and f' meas> Nfo6p). The degree of discrepancy (and the resulting frequency measurement error) can be determined as follows: count the number d arcs running through a certain radial line on the screen in a fixed amount of time. Then the divergence is Fp = d / t.
Digital method(discrete counting method) occupies a dominant position in modern measuring technology. It has many advantages: a very wide frequency range that can be measured with a single instrument (for example, from 10 Hz to 32 GHz); high measurement accuracy; obtaining a reading in digital form; the ability to process measurement results using a computer, etc.
Rice. 3.2
The problem of measuring the frequency by the digital method is the opposite of the problem of measuring the period. If, when measuring the period, the time interval t x = Tx filled with timestamps T 0 , then when measuring the frequency, the reference time interval T 0 is filled with pulses with a period T x = 1 / f x . For this, the signal under investigation is converted into a periodic sequence of short pulses, the moments of occurrence of which correspond to the moments of transition of a sinusoidal signal through a zero level with a derivative of the same sign. Thus, the pulse repetition period is equal to the period of the signal under study. From two adjacent pulses of the reference frequency, which are separated by a time interval T 0 , a strobe pulse is formed - a time gate with a duration t = T 0 . The number of impulses entering the gate NS = t / T x . Obviously, the desired frequency will be determined from the relation fx = n / t.
Measurements are indirect. To get direct readings, in frequency meters. Built according to a scheme with rigid logic (without a microprocessor), the duration of temporary gates is set t = with, where p = 0; ± 1; ± 2; ... ... ... (on the panel of the device, the gate duration switch is indicated by the inscription MEASUREMENT TIME). When p = 0 (t = 1c) fx = n Hz;
if t== 1ms then fx == NS kHz.
Digital frequency counter. Modern digital frequency meters are multifunctional devices. They measure the frequency of a sinusoidal and pulse signals, signal repetition period, pulse duration, time intervals given by two pulses from one or different sources, frequency variation, ratio of two frequencies; count the number of pulses received at the input, etc. 3.3 structural scheme refers to the frequency measurement mode. The work of the circuit is as follows.
A periodic signal, the frequency of which must be measured, enters the input of the device (usually denoted by the letter A). After amplification or attenuation in the input unit, the signal is fed to the shaper, where it is converted into a periodic sequence of pulses with a repetition rate f x . These pulses are fed to input 1 of the time selector and pass through it to the counter, if the input 2 the selector has a strobe pulse. The strobe is generated from the voltage of a high-frequency crystal oscillator. Since the period of its output signal is small, a frequency divider is provided in the circuit to obtain the required duration of the strobe pulse (on the front panel of the device it is designated as the PERIOD MULTIPLIER). The divider is a set of decades, each of which reduces the pulse repetition rate by 10 times. Division ratio q depends on the number of included decades. From a periodic sequence of pulses formed at the output of the divider, the automation unit (time gate circuit) forms a strobe pulse (time gate) with a duration t == T 0 , input 2 a time selector that determines the duration of the count.
Rice. 3.3
Consider the process frequency ratio measurements Fx1 / Fx2 (Fx1> Fx2). The higher frequency Fx1 is fed to the input of the frequency meter, and the lower frequency Fx2 is fed through an additional shaper to the automation unit (in this case, the crystal oscillator and the divider are turned off). Pulses with a period of Tx1 during a period of Tx2 pass through the time selector and are counted. Number of pulses m = Tx2 / Tx1 = Fx1 / Fx2 . To increase the measurement accuracy, the frequency Fx2 supplied through a divider (only the crystal oscillator is turned off).
Frequency measurement errors are similar to those considered in the analysis of time intervals measurements.
Before testing your speakers, speakers, or headphones, make sure your amplifier (either stationary, or built into powered speakers, or sound card computer) has fairly good technical characteristics (parameters). Those. how straightforward and broad is it Frequency response, can he give out everything frequency with the same level, without blockage at low frequencies (which is often the fault of low-quality amplifiers).
At the same time, you can determine whether it develops the manufacturer's declared maximum power(Pmax) and what output impedance(Rout) has.
Frequency response test method
To measure the amplitude-frequency characteristic ( Frequency response) into one of the channels (left or right) instead of the speaker as an amplifier load, connect conductors with a resistance of 5-10 ohms. In parallel with the resistor, connect an AC voltmeter (digital in this case is more convenient than a pointer), and, by sending a signal from the computer sound frequency generator(22Kb.) At a frequency of 1000 hertz, use the volume control to set the output voltage, for example, 1 volt (1000 millivolts), then, without changing the signal level, reduce the frequency of the generator (in the range of 1000-100 hertz with the "-100" button, in the range of 100-20 hertz button "-10") starting from 1000Hz. and up to 20Hz. inclusive (in this case, the tone controls on the amplifier must be in the middle position or disabled, i.e. its frequency response must be rectilinear (horizontal).
The voltage at the amplifier output MUST NOT vary by more than ± 2 decibels (or 1.25 times), but the less the better (in our case, it should be between 0.8-1.25 volts, or 800 -1250 millivolts). Ideally, all frequencies are output at the same level.
Well, if the voltage drop at low frequencies is 2 or more times, which corresponds to 6 decibels or more (i.e., the voltage drops to 0.5 volts or less), then your speakers will never sound in all their glory. TO besides, at non-linear characteristic amplifier you will not be able to accurately determine resonant frequency speakers. An example of such a non-linear frequency response is shown in the figure on the left (see the blue curve).
The second channel of the amplifier is checked in the same way. In the event of a significant drop in the signal at low frequencies, it is advisable to change the amplifier to a better one.
Measuring the output impedance of an amplifier
The damping factor and intermodulation distortion depend on the value of the output impedance, and it also directly affects the overall quality of the system. The output impedance of the power amplifier should be within 1 / 10-1 / 1000 of the load resistance and modern amplifiers has a value of the order of 0.01-0.1 ohms.
To measure it as the load of the amplifier, connect with conductors with a resistance of 4 or 8 ohms of the corresponding power. In parallel with the amplifier output, connect an AC voltmeter (digital in this case is more convenient than a pointer), and, by sending a signal from the computer sound frequency generator(22Kb.) At a frequency of 1000 hertz, use the volume control to set the output voltage in the range from 1 to 5 volts.
First, you need to measure the output voltage of the amplifier at idle (no load). Then do the same by loading it on a resistor. All quantities, including Rload, must be measured as accurately as possible. Output impedance is calculated by the formula
Rout = [(Uхх / Uload) -1] × Rload or
Rout = [(Uхх-Uload) / Uload] × Rload. example: [(5-4.9) / 4.9] × 8 = 0.163 ohm.
Thus, you can determine the output impedance both on the second channel and at any frequency.
Maximum power measurement
Some users want to know how much power their amplifiers actually deliver to the load, not trusting the characteristics declared by manufacturers. This can be done, but you will need:
- powerful pull-up resistor
- sound frequency generator
- ac voltmeter
- oscilloscope.
The most difficult thing is to buy or make a powerful load resistor yourself and find an oscilloscope. As a last resort, as an oscilloscope, you can use a computer or laptop with the "Virtual Oscilloscope" program from (volume 0.3 Mb.). Detailed description its work and the adapter circuit (voltage divider for matching the input of the computer sound card with the source of the voltage under investigation) are available in the program help. The resistor can be made from a spiral of an ancient iron, an electric stove, or a fan heater.
Connect to one of the channels (left or right) instead of the speaker as the amplifier load with conductors, with a resistance corresponding to the calculated load resistance of your amplifier. It is indicated in the instructions for the equipment and is usually 8 or 4. The power of the resistor must be sufficient so that it does not burn out during operation, i.e. not less than the estimated output power of the amplifier (if the amplifier is declared for 100 watts per channel, the resistor power must be 100 watts or more).
In parallel with the resistor, connect an AC voltmeter (preferably a pointer, it shows the effective voltage value), as well as an oscilloscope and, having sent a signal from the computer sound frequency generator(22Kb.) At a frequency of 1000 hertz, use the volume control to set the output voltage, for example, 1 volt (1000 millivolts). Observe the waveform on an oscilloscope, then, without changing the frequency, increase the signal amplitude.
The sinusoid will increase in 
height, without distorting its shape, but at some point its clipping will occur, it seems to rest against the "ceiling and floor", instead of rounded, its upper and / or lower parts will become horizontal, as in the picture on the right, i.e. signal amplitude limiting will begin. Reduce the amplitude so that the signal is on the verge of clipping (still retaining its rounded shape). The voltage shown at this moment on the voltmeter is equal to Umax. Calculate the maximum power of the amplifier using the formula P = U² / R.
For example, Umax = 21v. R = 4om. Pmax = 21² / 4 = 110 watts. If R = 8 ohm, then Pmax = 55 watts.
In the same way, you can check the maximum output power at the lower frequency of the amplifier's frequency response (20 hertz), or at the lower frequency of the frequency range specified for your speakers, for example 40, 45 or 50 hertz. The limitation of the sinusoid in amplitude, ideally, should occur strictly symmetrically, on both half-waves of the signal.
Measure the power in the second channel of the amplifier in the same way.
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Copyright © Poluboyartsev A.V.
In the descriptions of low-frequency amplifiers published in the journal Radio and other radio engineering literature, it is customary to indicate their nominal power, total harmonic distortion, sensitivity and frequency response. By these four main parameters, one can already judge the quality of the amplifier and its suitability for certain purposes.
What are these parameters of the LF amplifier? With a further increase in power, the distortion increases significantly. The power at which the distortion reaches 10% is considered to be the maximum (P max).
Nonlinear distortion. In the process of amplifying any, even purely sinusoidal signal due to the nonlinearity of the characteristics of transistors, vacuum tubes, transformers and a number of other elements of equipment in amplified signal harmonics appear - oscillations, the frequencies of which are 2, 3 or more times higher than the fundamental frequency. This is the non-linear or harmonic distortion, which grows as the power delivered by the amplifier to the load increases. They are estimated by the harmonic distortion factor.
Harmonic distortion (Kg) measured at sinusoidal input signal constant frequency, expressed as a percentage of the total voltage of all harmonics U g to the output voltage U out
Permissible K g is established by the relevant standards (GOST). For example, for low-frequency amplifiers of radio receivers, radio tape recorders, radio tape recorders, electrophones, it can be 5-7%, for household tape recorders - 5%. The higher the class of radio equipment, the less it should be K g.
Sensitivity. The term sensitivity is usually understood to mean the voltage of the low-frequency signal in millivolts, which must be applied to the input of the amplifier in order to obtain the rated output power at the load. The sensitivity of most amplifiers for reproducing a gram recording is 100-200 mV, and the sensitivity of recording amplifiers for household tape recorders, measured from the microphone input, is 1-2 mV.
The frequency response of the amplifier is the dependence of the output signal voltage on the frequency at a constant input voltage U in. For a number of reasons, bass amplifiers amplify signals of different frequencies differently. Usually, the lowest (f n) and highest (f in) amplifiers are amplified worst of all, therefore the frequency characteristics of the amplifiers are uneven and have slopes or blockages at the edges. The extreme frequencies at which) a decrease in gain of 30% (-ZdB) is observed are considered the boundaries of the amplified frequency band, they are indicated in the amplifier's passport data. The frequency response or band of amplified frequencies of low-frequency amplifiers of network radio receivers can be from 100 to 10,000 Hz, and amplifiers of small-sized transistor receivers - from 200 to 3500 Hz. The higher the class of the amplifier, the wider the band of amplified frequencies.
In addition to these parameters, there are some others, but they are secondary or arising from the main ones.
But the radio amateur installed, tested and adjusted the amplifier. How to measure its main parameters in order to compare them with the given ones?
Measurements of the parameters of low-frequency amplifiers are usually performed using special high-precision measuring equipment. However, in an amateur setting, this can be done using simple measuring instruments, for example, those described in our magazine in 1971 and 1972. under the heading Laboratory of the radio amateur. You will need a LF generator, a transistor AC millivoltmeter and rectifiers to power these devices separately. The amplifier under test usually has its own power supply. You also need an equivalent load R e - a wire-wound resistor, the resistance of which is equal to the resistance of the voice coil of the loudspeaker used in the amplifier, or a special device described in the article Universal equivalent load, published in Radio No. 12 of 1973.
The set of instruments of the Radio Amateurs Laboratory does not have a nonlinear distortion meter (INI), therefore, measurements of this parameter of the amplifier will have to be carried out according to a simplified method, using additionally any low-frequency electronic oscilloscope, for example LO-70. In this case, measurements begin with taking the amplitude characteristic of the amplifier - the dependence of the output voltage U out of the amplified signal on the input voltage U in, measured at a frequency of 1000 Hz (1 kHz) with a constant load R n = R e.
So, we proceed to taking the amplitude characteristics of the amplifier. The connection diagram of the measuring instruments with the amplifier under test is shown in Fig. 1, a (power circuits are not shown). A signal with a frequency of 1000 Hz from the output of the LF generator (LFO) is fed to the input of the LF amplifier (ULF) with a shielded two-core cable. We ground the cable sheath and one of its cores at the amplifier input. Connect the millivoltmeter to the Generator Output Control jacks. We smoothly increase the amplitude of the generator signal to a voltage of 0.3 V. In this case, the actual signal voltage at the amplifier input will be 30 mV, since it is removed from the generator attenuator, which attenuates the signal by a factor of 10 (1: 10). Having measured the input voltage U in, we switch the millivoltmeter to the measurement limit of 10 V and measure the output voltage U out at the equivalent load R e (Fig. 1, b). Suppose the voltage U out is 1.2 V. We make a table (Table 1) and write down the measurement results in it: U in = 30 mV, U out = 1.2 V. Next, we increase the input voltage in steps of 10 mV, and write the measurement results to the table. And so on until the proportionality of the increase in the output voltage U out is violated. In this case, on the screen of the oscilloscope, there should be a noticeable cutting off of the tops of the sinusoid (Fig. 1, c). Clipping occurs due to symmetrical limiting of the output signal amplitude and is accompanied by an increase in distortion up to about 10%. This means that the amplifier reaches its maximum power (P max). Then we slightly reduce U in until the distortions of the sinusoid disappear (see Fig. 1, b) and we believe that now the amplifier is delivering the nominal power P nom. Output voltages corresponding to P max and P nom, for example 4.1 and 3.6V, are highlighted in the table.
Now, using the data in Table. 1, we build, the amplitude characteristic of the amplifier (Fig. 2). To do this, along the horizontal axis to the right of zero, we mark the input voltages U in in millivolts, and along the vertical axis up - the output voltages U out in volts. We mark all the measured values of U out on the graph with crosses and draw a smooth line through them. This line is straight up to point a, and then noticeably deviates downward, which indicates a violation of the direct relationship U out / U in and a sharp increase in distortion.
Knowing the voltage U out and the resistance of the equivalent load R e, it is possible to calculate the output power P out of the amplifier for various voltages U out.
The output power P out is calculated according to the formula arising from Ohm's law:
For example, with P n = 6.5 Ohm and Uout = 1.0 V
at U out corresponding to 1.8 V, Pout ≈ 0.5 W, etc. In Fig. 2 parallel to the U out axis, the second vertical axis is drawn, on which the calculated output powers P out are marked.
The inflection of the amplitude characteristic usually corresponds to the nominal power P nom of the amplifier, in our example 2 W (maximum power P max ≈ 2.5 W). If the inflection in the characteristic is not clearly pronounced, it is refined using the oscilloscope by repeated measurements. Then take the arithmetic mean value U out, at which the distortions of the sinusoid on the oscilloscope screen become indistinguishable by eye.
The numerical value of the harmonic distortion factor Kg can be measured using a suppression filter tuned to the fundamental frequency of 1 kHz. The filter is connected between the output of the LF amplifier and the millivoltmeter (Fig. 3). First, measure U out at the first position of the switch B. Let's assume that it is equal to 3.6 V (3600 mV). Then, setting the switch to the second position to turn on the filter, measure the harmonic voltage U g. Let's say it is equal to 72 mV. After that, the harmonic distortion is calculated according to the previously given formula:
Now, using the amplitude characteristic, we determine the sensitivity of the amplifier. Since U in at P nom is equal to 90 mV, therefore, the nominal sensitivity of the amplifier is also equal to 90 mV,
The connection diagram of instruments with an amplifier for measuring the frequency response remains the same (see Fig. 1). The original frequency of the input signal is the same - 1000 Hz. Using the generator Amplitude knob, set the voltage U in, equal to 20 mV, which is further maintained constant at all frequencies (this voltage, which is almost five times less than the nominal sensitivity of the amplifier, was chosen for the convenience of reading the measurement results on the dial gauge of the avometer). Then, by switching the voltmeter to the output of the amplifier, we measure the voltage at the equivalent load R e. The measurement results are written in table. 2 in two lines: in the first - the frequency f of the input signal, in the second - the output voltage U out. In the heading of the table we write the name of the amplifier, the resistance of the equivalent load R e, the input voltage U in, at which we make the measurements (in this example 20 mV).
Having recorded the measurement results at a frequency of 1000 Hz, we switch the LF generator to the frequency, let it go, it is equal to 72 mV. After that, the harmonic distortion is calculated according to the previously given formula:
Now, using the amplitude characteristic, we determine the sensitivity of the amplifier. Since U in at P nom is equal to 90 mV, therefore, the nominal sensitivity of the amplifier is also equal to 90 mV.
The frequency response of the amplifier is measured at an output power that is significantly less than the nominal power, which eliminates any overload of the amplifier. The frequency characteristics of amplifiers in industrial receivers, for example, are measured at an output power of 50 or even 5 mW.
If the amplifier is relatively simple and does not have any tone controls, then the volume control is set to maximum and its position is not changed during the frequency response. In the presence of a loudness-compensated volume control, the frequency response is removed at the maximum, minimum and several, at the request of the designer, intermediate positions of the volume control.
The connection diagram of instruments with an amplifier for measuring the frequency response remains the same (see Fig. 1). The original frequency of the input signal is the same - 1000 Hz. Using the generator Amplitude knob, set the voltage U in, equal to 20 mV, which is further maintained constant at all frequencies (this voltage, which is almost five times less than the nominal sensitivity of the amplifier, was chosen for the convenience of reading the measurement results on the dial gauge of the avometer). Then, by switching the voltmeter to the output of the amplifier, we measure the voltage at the equivalent load R e. The measurement results are written in table. 2 in two lines: in the first - the frequency f of the input signal, in the second - the output voltage U out. In the heading of the table we write the name of the amplifier, the resistance of the equivalent load R e, the input voltage U in, at which we make the measurements (in this example, 20 mV).
Having recorded the measurement results at a frequency of 1000 Hz, we switch the LF generator to a frequency of 500 Hz. We check the input voltage of 20 mV with a voltmeter, then measure the output voltage of the amplifier as accurately as possible at the equivalent load R e. Further, in the same way, we make measurements at frequencies of 250, 150, 100, 75, 50 Hz and write down the measurement results in a table (amateur amplifiers at a frequency of 25 Hz are usually not checked). After that, we carry out a repeated control measurement at a frequency of 1000 Hz to check the stability of the amplifier and measuring instruments.
Then we make measurements at higher frequencies. After a control frequency of 1000 Hz, we send signals with frequencies of 2.5 to the amplifier input; 5; 7.5; ten; 15 kHz (measurements at a frequency of 20 kHz are made only when checking amplifiers top class). We write the measurement results into a table and use them to calculate the ratios of the output voltages U in to the voltage of the control frequency U1000. The resulting relations are written in the corresponding row of the table.
For example. At frequencies of 50 Hz and 15 kHz, the output voltage is U out = 300 mV. Hence the relationship
At frequencies of 100 Hz and 10 kHz, we have a relationship
Now, having all the preliminary data, we proceed to plotting the frequency response of the amplifier (Fig. 4). Usually, a special logarithmic paper is used for this purpose (the auditory perception of sounds of different frequencies and loudness obeys a logarithmic law). However, to build the frequency response, you can use any paper in the box or graph paper. It is marked as shown in Fig. 4. First, along the horizontal ordinate, we plot the frequency values. In fig. 4, the top row of numbers corresponds to the fixed frequencies of the LF generator of the Radio Amateurs Laboratory. The lower row of numbers, highlighted in color, corresponds to the frequencies recommended by GOST when taking characteristics using industrial measuring equipment.
Then, along the vertical axis, having previously made 8-10 equally spaced marks on it, is the ratio U f / U 1000 in decibels. Since the measured drop or slope of the frequency response does not exceed 6 dB, we draw the zero line at the 6th mark and on the left we put the numbers 0, -1, -2 ... -6 dB. We also draw a line of the control frequency of 1000 Hz. Further, using the data in Table. 2, sequentially put marks on measuring frequencies 50 Hz to 15 kHz. Since the characteristic has recessions at the edges, we postpone the marks in decibels down from the zero line. For example, at a frequency of 50 Hz there was a drop of 6 dB, therefore, we set the mark at a level of -6 dB. For a frequency of 75 Hz, we place the mark at a level of –3 dB, etc. A smooth line drawn through these marks will be the frequency response. A -3 dB horizontal line, which is generally accepted for flatness tolerance, crosses this response at 75 Hz and approximately 12 kHz. Therefore, the amplified frequency bandwidth, or the passband of the amplifier under test, is 75-12000 Hz with a flatness of 3 dB.
High-quality bass amplifiers, in addition to volume controls, usually have two separate tone controls - for low and high frequencies. Frequency characteristics of such amplifiers are removed at least three times. First, both tone controls are set to the positions corresponding to the greatest blockage of the extreme low and high frequencies. The resulting characteristic can have the form of a curve indicated in Fig. 5 with number 1. Then the knobs of both tone controls are turned to the other extreme position, corresponding to the maximum rise of the lower and higher frequencies, and measurements are made at an input voltage that is ten times (20 dB) less than the nominal one. This characteristic can take the form of curve 2 (Fig. 5).
After that, the knobs of both regulators are set to the middle positions and a third measurement is made. If the obtained characteristic corresponds to or is close to curve 3, then the measurements are finished at this. If it differs significantly from this curve, then by means of tests such positions of the knobs of the regulators are found, at which the characteristic is obtained the most straightforward, and the corresponding marks are made on the knobs of the regulators.
From the graph in Fig. it is clearly seen that for a bass amplifier having such characteristics, the tone control limit is lowest frequency 63 Hz (according to GOST) is +6 and -6 dB, and at the highest, equal to 12 kHz, it is approximately from +5 to -10 dB.
In the design activities of many radio amateurs, the audio frequency amplifier (34) occupies one of the first places. The sound quality of a broadcasting receiver, TV, or tape recorder largely depends on the amplifier 34.
Descriptions of amplifiers 34, intended for electrophones, tape recorders, broadcasting receivers, usually indicate their nominal output power, nominal input voltage, harmonic distortion and frequency response parameters. From these basic data, it is already possible to judge the quality of the amplifier and its suitability for certain purposes.
Let us briefly recall what the named parameters of the amplifier 34 are.
The nominal output power P HO m, expressed in watts or milliwatts, is the power released at the load (voice coil of the dynamic head of the loudspeaker, winding of the headphone), at which the nonlinear distortions introduced by the amplifier correspond to those indicated in the description. With a further increase in the output power, these distortions increase significantly *.
In the process of amplification of any signal, due to the nonlinearity of the characteristics of transistors or electronic tubes, oscillations with a frequency of 2, 3, 4 or more times higher than the fundamental frequency appear in the amplified signal, i.e., the second, third, etc. harmonics of the signal appear. They also distort the amplified signal. Harmonic distortion increases as the output power of the 34 * amplifier increases. Their harmonic coefficient is estimated. The power at which the distortion (harmonic distortion) reaches 10% is usually called the maximum output power of the amplifier 34 (it is denoted Pmax).
The harmonic coefficient Kg, measured with a sinusoidal input signal, can be expressed as the percentage of the total voltage of all harmonics U r to the output voltage and output:
frequency response number in the operating range, crosses the frequency response at frequencies of 75 to 11 000 Hz. Consequently, the operating frequency range of the amplifier extends from 75 Hz to 10 Hz.
Many amplifiers 34, in addition to the volume control, are equipped with two (less often - three or more) tone controls - for the lowest and highest sound frequencies. The frequency response of such amplifiers is removed at least three times, and with the input voltage reduced by about 20 dB (10 times) compared to * the nominal (to avoid overload when the gain rises at the edges of the operating range). First, both tone controls of such an amplifier 34 are set to positions corresponding to the rolloff of the frequency response at the edges of the range. The resulting frequency response can have the form of curve 1 (Fig. 107). Then both tone controls are moved to other extreme positions (raising the frequency response at the edges of the range). The frequency response of the amplifier in this case may have the form of curve 2. After that, the tone controls are set to the middle positions and the frequency response is removed again. If it is close to curve 3, then the measurements are finished at this, and if it differs significantly from it, then by means of tests such positions of the tone controls are found, at which the frequency response is obtained as the most even and parallel to the frequency axis in the widest possible band, and on the knobs of the controls they make corresponding marks.
From the graphs in fig. 107 it is clearly seen that for this amplifier 34, the range of tone control at the lowest frequency of 63 Hz is + 6 ... -6 dB, and at the highest, equal to 11,000 Hz, approximately + 5 ... -10 dB. So with the help of simple laboratory instruments, using the described methodology, it is possible to measure the basic parameters of almost any amplifier 34 with sufficient accuracy for a radio amateur.
