Measurement technique amplifier, tools, goals

Any more or less complex electronic device requires a functional check and adjustment

The audio power amplifier is no exception and measurements of the characteristics of this device are not only a development tool, but also an indicator of the quality and level of the product.

Earlier measurements of audio amplifiers were made in factories and required a lot of special equipment and specially trained personnel. There was no question of any more or less accurate measurements at home or semi-professional conditions. However, the emergence of personal computers, the development of sound interfaces and the creation of specialized software systems have radically changed the situation:

Today, every person with a PC and desire can measure their audio device with a fairly high level of accuracy, and if you add a sound card of an above average level here, then the accuracy of such measurements could be envied by almost any manufacturer of transistor or integrated audio power amplifiers 30 years ago

On the other hand, in the modern world ruled by marketers, it is not very difficult to get practically any measurement figures, because in different conditions, an electronic device behaves differently and, as a rule, very "beautiful" or necessary numbers are indicated without specifying the conditions. in which the measurement was carried out

This section, on the kit-amp.com website, was created for the purpose of measuring modules, ready-made amplifiers and other audio devices that we have on sale. We do not pretend to be the ultimate truth, but only show the figures of the characteristics we received in certain conditions

Measurement conditions:

1. Load resistance of all amplifiers for acoustics 4 Ohm. This is the most difficult mode for an amplifier and with an increase in resistance to 6-8 Ohm, it will ALWAYS have a positive effect on characteristics except for power

2. Measurements of amplifiers, including maximum power, are carried out using a resistive load. We are well aware that the dynamic head of a speaker system or headphone is a complex load. At the same time, the heads themselves are quite different from each other and measurements on a particular head can be objective only in relation to it. We are convinced that in order to obtain reference and comparative information, just measurements of all amplifiers at the same load (albeit less complicated) will be more objective.

3. Power supply (single and double polarity), with voltage up to 30V, of all tested devices was carried out through a powerful, linear stabilized power supply. Power supply above 30V is carried out through a power supply unit based on a CRC filter, with a total capacitance of smoothing capacitors of 48,000 uF. The choice fell on a stabilized power supply and a power supply based on a CRC filter. the organization of power in different projects is quite different and a high-quality power supply allows you to see the potential of the device.

Measuring bench and tools:

1. Personal computer

2. Sound interface E-MU 1616m (one of the best sound cards in spite of its obscenely advanced age).

3. Powerful 15A, two-polar, transformer, stabilized power supply 0-30V or transformer, power supply unit based on a CRC filter 20μF smoothing capacitors per arm when powered over 30V.

4. Resistive load in the form of an assembly of 5 kW heating elements, with a resistance of 3.8 Ohm at the ends of the cables (amplifiers for acoustics)

5. Resistive load in the form of an assembly of 1W resistors, with a resistance of 68 Ohm at the ends of the cables (headphone amplifiers). The choice fell on 64 Ohm, tk. this impedance is "average" and may indicate the overall picture for owners of both high impedance and low impedance headphones

6. Oscilloscope

7. Multimeter

Software package:

RightMark Audio Analyzer 6.4.5 PRO

The advantages of this program:

A) Batch testing

B) Informative and concise output of the information received

D) The ability to convey information to a user who does not want to understand the characteristics and their values using such categories as "Excellent", "Average", "Bad"

E) Program operation mode for measurements at 24-bit, 48 kHz. This mode allows you to take measurements at frequencies up to 24 kHz, which is much higher than the audibility of high frequencies for any person. In addition, the internal filter of the audio interface is tuned to 20 kHz. On the other hand, the frequency response drop after 20 kHz is not very large and is only 1 dB per first octave. What happens in ultrasound is by and large not interesting to us

Batch testing includes the main characteristics of an audio device and consists of:

(as an example, the graphs of the E-MU 1616m audio interface will be given to itself)

1. Frequency response

The frequency response of an amplifier shows the dependence of the gain on the frequency of the signal applied to the amplifier input. This is one of the most important parameters, since if the frequency response is uneven, that is, not straightforward, then this signals that the amplifier amplifies signals of different frequencies in different ways, thereby introducing frequency distortion

2. Noise level

Signal-to-noise ratio is a parameter of an ADC, DAC, mixer, microphone, pre-amplifier, or final amplifier, for example, an amplifier of active speakers. It shows how much noise the audio device makes (usually from 60 to 135.5 dB) when there is no signal. The higher the S / N value, the clearer the sound and the greater the dynamic range provided by the system. For musical purposes, it is desirable that this parameter be at least 75 dB, and for systems with high-quality sound, at least 90 dB

3. Dynamic range

This is the ratio of the maximum input voltage of the amplifier to the minimum input voltage that can be amplified with allowable distortion and noise level. Typically, the dynamic range of an amplifier is indicated in decibels.

4. Harmonic distortion

Signal distortions are caused by the nonlinearity of the input and output characteristics of the amplifying elements and are inherent in any power amplifiers. If a sinusoidal signal is applied to the input of the amplifier, then in the spectrum of the output signal, in addition to the fundamental, additional harmonics will be found, the frequency of which is a multiple of the frequency of the useful signal. Such harmonics are parasitic and their power is usually low. However, their summation with the useful signal leads to a significant distortion of its shape, and as a result, distorted sound

Total Harmonic Distortion measures the audible harmonic distortion in the output signal and is defined as the ratio of the total power of the parasitic signals to the power of the wanted harmonic signal. Typically, measurements are taken at 1 kHz

When measuring, attention is paid to the spectral distribution and the nature of the distortions. The audibility of parasitic harmonics depends on the relative level in relation to the test signal, on the order of the harmonics, on the type (even / odd), as well as on the volume at which the test fragment is listened to

Typical THD for a Hi-Fi amplifier is 0.1%. However, it has already been noted more than once: an amplifier with THD 0.001% may sound worse than another one with THD 0.1%. The fact is that with such small values of this parameter, it is difficult to trace the distortion in the form of the output signal or to feel it by ear. Therefore, the difference between 0.1% and 0.001% will not be heard.

5. Intermodulation distortion

The nonlinearity of the characteristics of the amplifying elements leads to the occurrence of nonlinear distortions. Most amplifier manufacturers measure and state only the harmonic distortion (THD). Measurements are made using a harmonic signal. With such testing, higher harmonics appear at the output of the amplifying circuit, the frequency of which is a multiple of the frequency of the fundamental tone. However, as already mentioned, the musical signal is far from harmonic. Moreover, any musical instrument reproduces not only the main tone, but "overtones", which are a prime example of harmonic distortion. It is known that the presence of "overtones" in a musical signal does not spoil, but enrich the sound. Therefore, it is very important to indicate not the harmonic distortion factor, but the entire spectrum of the output signal, from which it is possible to determine the type (even or odd) of parasitic harmonics and their level relative to the useful signal. From the point of view of psychoacoustics, for example, the presence in the output signal of even harmonics perceptible by the level is perceived by ear better than the presence of small odd harmonics.

The greatest harm to a music signal is brought about by Inter Modulation Distortion, which occurs when a multi-tone signal is applied to the input of a nonlinear system. In this case, parasitic signals appear at the output with frequencies that are the sum or difference of the frequencies of the input signals, as well as the sum or difference of the frequencies of the signals caused by harmonic distortion and through the feedback returned to the input of the amplifier. Such distortion does not correspond to the fundamental tones of the music signal and introduces background noise into it.

It should be noted that there are no uniform standards for measuring intermodulation distortion, and the measurement results significantly depend on the levels of input signals and their frequencies. More often than not, IMD is not specified simply because it is not known how to measure it. Nevertheless, this parameter is the most promising for assessing the nonlinear properties of a power amplifier.

6. Interpenetration of stereo channels

This parameter determines the degree of signal penetration from one channel to another. A high level of crosstalk leads to a slight deterioration in the clarity of the stereo image. However, a sensitive listener will immediately feel that the sound does not give an idea of the relative position and size of musical instruments, i.e. absence or indistinctness of the sound 3D picture

7. Intermodulation distortion (variable frequency)

Measurement of intermodulation distortion at frequencies of 5/10/15 kHz

8. Maximum power amplifier

not included in batch testing and measured manually

The maximum power is calculated by applying a sinusoidal signal with a frequency of 1 kHz to the amplifier input and raising the output signal power to the limits of the sinusoid peaks, followed by a return to the sinusoid without limitations. In this case, the rms voltage is measured and substituted into the formula for calculating the power:

P = U ^ 2 / R

where

U voltage at the amplifier output

R - load resistance

Summary:

However, it should be remembered that all people differ in varying degrees from each other, including in audio preferences and excellent numbers in the characteristics of an audio device are not always the key to the success of this device, and especially among experienced and sophisticated listeners. The very same developer of the RMAA program - Maxim Lyadov ALWAYS focuses on the fact that the device needs to be listened to and no measurements and voting can even closely replace listening

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