On the interaction of UMZCH with the load. Scheme, description

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On the interaction of UMZCH with the load. Encyclopedia of radio electronics and electrical engineering

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Encyclopedia of radio electronics and electrical engineering / Transistor power amplifiers

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We often hear that the standard specifications do not fully reflect the quality of the UMZCH. Is it so? Maybe it's just that the measurements of the parameters of the amplifiers are carried out incompletely or incorrectly?

Well-known researchers in the field of electroacoustics Matti Otala and Mamoru Sekiya already in 1987 in their article published in the journal "Funkschau" No. 23 answered this question in many ways, and here we present it in an abbreviated form. However, hardware developers have not yet made, as it seems to us, the proper conclusions from this publication.

If you believe advertising and branded information materials, the state of affairs with UMZCH now looks simply brilliant - their technical characteristics are so high. Output power, expressed as a three- or even four-digit number, harmonic coefficient - hundredths and thousandths of a percent. The parameters that were not so long ago a sign of extremely expensive models have now become common for mass equipment. All this does not explain, however, the empirical fact that amplifiers with the same specifications sound different. And what's more: many UMZCH models are distinguished by audible signal distortion, despite excellent passport data. Such facts give rise to bewilderment among users, and also serve as a breeding ground for all kinds of paradoxical conclusions on the verge of mysticism and parapsychology.

In fact, the reason for such discrepancies lies in the generally accepted measurement technique, using a "standard" load - usually a purely active one with a nominal value of 8 ohms - which does not correspond at all to real operating conditions. On fig. 1a shows impedance versus frequency graphs for two common loudspeakers - a professional studio monitor "Yamaha NS1000M" (blue curve) and a home speaker "Infinity 4,5" (red curve).

On the interaction of UMZCH with the load

It is useful to recall that according to the IEC standard, the loudspeaker impedance should not be lower than 80% of the nominal value, i.e. 6,4 ohms at a nominal value of 8 ohms. Numerous measurements carried out in recent years in Japan and the USA. showed that for 60% of the loudspeakers, the impedance can drop at certain frequencies from 8 to 5 ohms, and for 25% - up to 4 ohms; the impedance of some household models (nominal - 4 ohms) can even drop to 2 ... 2,5 ohms. It follows that the current supplied by the amplifier to the load can be many times higher than the nominal one, which means that the technical characteristics of the amplifiers, if they claim to be objective, should be determined at the same low load values.

An equally important difference between a real load and a bench load is its complex nature, which is expressed in the phase-frequency characteristic of loudspeakers. However, there are no industry standards or generally recognized experience data regarding acceptable phase distortion. The results obtained during the above tests have shown that the phase shift of a conventional loudspeaker reaches ±50°, in more severe cases it reaches ±60° and even ±80°. The corresponding PFCs are shown in fig. 1b. It becomes clear that reliable information about the quality of the amplifier can only be obtained by testing it with a complex load.

A special passive load, created by the authors to simulate a real one, consists of powerful non-inductive resistors, high-quality metal-film capacitors and coils without magnetic circuits. Switches designed for high currents allow you to create various configurations. The inherent non-linearity of the load, measured as total Kg, does not exceed 0,03% up to a power of 250 W, and in this case this value can be neglected. According to the IEC 268-3 standard, the amplifiers were warmed up for 30 minutes before testing with a signal at the input with a frequency of 1 kHz and an amplitude equal to 1/8 of the nominal one. During the tests, the level of the test signal gradually increased from zero to the level at which Kg of the amplifier reached 1%, and all data were entered into a computer.

In total, five amplifiers of the "hundred-watt" category were tested: "Harman-Kardon PM665", "Luxman L-510X", "Sansui AU-D907X", "Yamaha A-1000" and "Yamaha M-50". The measurement results are shown in random order in Figs. 2a - 2e in the form of three-dimensional diagrams. Ideally, the diagram should have the shape of a parallelepiped - this means that the output voltage of the amplifier at K, = 1% does not depend on the active and reactive components of the load. In other words, the load does not affect the distortion of the amplifier. It is clearly seen that only one amplifier from the group turned out to be sufficiently close to the ideal (Fig. 2, a), while the others demonstrate a noticeable dependence of the output voltage (and distortion) on the magnitude and phase shift of the load, and in Fig. 2d, this dependence is so strong that it raises doubts whether this amplifier is even capable of working with a real AS.

On the interaction of UMZCH with the load

Naturally, such measurements can also be carried out at other frequencies. A typical example for a low-frequency signal, in which the shortcomings of the power supply and protective devices are noticeably affected, is shown in fig. 2.e. It is easy to calculate that such a 100-watt amplifier is capable of delivering to a load at a frequency of 50 Hz. where a deep phase shift and strong impedance drops are usually observed, no more than 5 W (!) at K, = 1% (a typical normalized value of Kr is 0.01%). In other words, the operation of the amplifier in the nominal mode is accompanied by significant non-linear distortions. Thus, technical data in manuals and catalogs on the output power and harmonic distortion of amplifiers should be evaluated very carefully.

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