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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING Thermal distortion in HiFi amplifiers. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering /Transistor power amplifiers And only here we begin to realize why vacuum tube amplifiers have much less distortion. After all, the cathode of an electron lamp or its other important details are much more massive than the "legs" of semiconductor devices. Therefore, the possible thermal time constants in lamps are much larger, and thermal phenomena can, in principle, be neglected. A normal electronic lamp with an indirect glow almost does not respond to a "brisk" sound-frequency signal. Only by coincidence of many circumstances can any slow thermal effect appear. We also note as an additional argument that the magnitude of the signals in a tube amplifier is noticeably larger than in semiconductor circuits. In addition, vacuum tube stages almost always operate under conditions close to power matching. The same circumstance explains why class "A" solid-state power amplifiers have, on average, the best quality when listening. Such amplifying stages are mostly used in power matching conditions, so there will be less thermal distortion. The question arises: what can be done to reduce thermal distortion in an already finished amplifier? Of course, specific recipes can only be given for a specific scheme. Naturally, it is recommended first of all to carefully study this amplifier - whether there are definitely weak points in it from a thermal point of view. Ignoring other aspects, we can say that in a low-power amplifier (preamplifier) the most thermally favorable stages are those operating with a low signal level (in relation to the supply voltage), or, which is almost the same, stages with a large supply voltage. The resulting thermal interference signal is relatively small. Therefore, it is necessary to strive to use as much supply voltage as possible in the pre-cascades. The classical rules for designing amplifiers recommend the opposite. It is not at all excluded that when using a well-designed amplifier with a different (for example, lower) supply voltage, thermal interference that was previously minimal will appear or significantly increase at the new operating point. It is very important to set up the stages for proper power matching. At rest, the voltage across this transistor should be approximately equal to the voltage across the collector (or emitter) load resistor. It is possible that in order to comply with this condition, you will have to thoroughly "tinker" with the amplifier. Where possible, symmetrical cascades should be used, always maintaining the principle of correct power matching. If many symmetrical stages follow each other in the amplifier, it is necessary to strive to create obstacles in the path of the interference signal, i.e. it is necessary to use cascades with a large coefficient of attenuation of the common mode signal. Unfortunately, it is impossible to prevent the appearance of an interference signal (even with the help of correct power matching), you can only prevent its further propagation. In particular, this can be done using a large emitter resistance (compared to the collector), for example, an emitter current generator. Typically, the inclusion of each new stage in the amplifier leads to an increase in thermal distortion. Therefore, there is no guarantee that a pre-amplifier or final amplifier, assembled according to a complex “own invention” circuit, will give a better result than the original simple amplifier. It may be thought to somehow use thermal compensation; however, due to the fact that the time constants are unknown, we find ourselves here "in a rather swampy place." However, it makes sense to experiment with different types of semiconductors, using them as compensating elements. In the case of devices on integrated circuits (operational amplifiers, final stages on ICs), they proceed in the same way as before: if possible, all known sources of interference are excluded, and by listening, a decision is made whether this sample (product) is suitable or not. And so far there are no other methods, except for listening, that would lead to the same goal. I would like to make the following considerations, which can serve as a starting point for further reflection, but should not be taken literally. Before proceeding with the alterations, it is necessary to carefully study the circuit of this particular amplifier and evaluate its capabilities. It is usually easy to ascertain that most popular preamplifier and power amplifier circuits have a stage(s) that are thermally poorly built. The improvements undertaken should not only take into account the principles of operation of this circuit, but also the type of semiconductor devices used. For example, in a poorly designed stage, the thermal cut-off frequency will be higher and the resulting distortion will be less when using small size transistors, such as SM. The use of different types of parts is probably one of the main reasons that amplifiers calculated by the same method, having the same circuits and almost the same design, still give different results when listening. The trouble is that many firms manufacture semiconductors of any type, not necessarily using the same technology. Moreover, the technology sometimes changes while the semiconductor brand remains the same, and even the package, at first glance, is the same. Some modifications of the amplifiers undertaken by the author and a series of measurements have already given the first encouraging results. After the amplifier has been modified according to the above rules, transients of "unexplained" origin are reduced (with a high probability) to a favorable low level, so that they are either not registered or are difficult to detect. Calculations show that we can expect a decrease in this kind of transient distortion by about an order of magnitude. You can eliminate or greatly reduce the strange change in sound quality after a strong input signal is stopped or greatly reduced. The sometimes occurring "mysterious" cross distortions of some musical melodies do not appear (at least, they are not distinguishable or almost indistinguishable by ear). Once again, it must be emphasized that we are not talking about those distortions in the amplifier that are controlled by traditional measurement methods. On the contrary, we are talking about those cases where these methods fail (do not detect noticeable distortion), and the sound quality is still unsatisfactory. Of course, with the help of ingenious (for example, differential) methods, one could still measure the thermal changes in the parameters of the amplifier. However, here, again, the problem arises of estimating measurement errors that arise for similar reasons in the measuring instruments used. And anyway, the corrected "by ear" amplifier will have a higher category. Naturally, redesigning and experimenting is not a job for beginners. Caution, accuracy and imagination are needed. There may be other side effects (eg, ultra-excitation, etc.) that worsen more. which improve the situation. The author does not try to hide the fact that the purpose of this article, first of all, is to awaken the thought of readers, to make them look at the "evergreen" problem of distortions from a new angle. Literature
Translation by A. Belsky; Publication: N. Bolshakov, rf.atnn.ru
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