Old but golden. Scheme, description

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Old but golden. The tube amp renaissance. Encyclopedia of radio electronics and electrical engineering

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Amplifier circuitry has already gone through a spiral in its development, and now we are witnessing a "tube renaissance". In accordance with the laws of dialectics, which we were so stubbornly drummed into, the "transistor renaissance" should come next. The very fact of this is inevitable, because the lamps, for all their beauty, are already very inconvenient. Even at home. But transistor amplifiers have accumulated their drawbacks ...

The reason for the "transistor" sound was explained back in the mid-70s - deep feedback. It gives rise to two problems at once. The first is transient intermodulation distortion (TIM) in the amplifier itself, caused by signal delay in the feedback loop. There is only one way to deal with this - by increasing the speed and amplification of the original amplifier (without feedback), which is fraught with serious complication of the circuit. The result is difficult to predict: whether it will be, or not.

The second problem is that deep feedback greatly reduces the output impedance of the amplifier. And this for most loudspeakers is fraught with the occurrence of those very intermodulation distortions right in the dynamic heads. The reason is that when the coil moves in the gap of the magnetic system, its inductance changes significantly, so the head impedance also changes. With a low output impedance of the amplifier, this leads to additional changes in the current through the coil, which gives rise to unpleasant overtones that are mistaken for amplifier distortion. This can also explain the paradoxical fact that with an arbitrary choice of speakers and amplifiers, one set "sounds" and the other "does not sound".

The Secret of Tube Sound = High Amplifier Output Impedance + Shallow Feedback

However, similar results can be achieved with transistor amplifiers. All the circuits below are united by one thing - unconventional and now forgotten "asymmetric" and "wrong" circuitry. However, is it as bad as it is made out to be? For example, a phase inverter with a transformer is a real Hi-End! (Fig. 1) A phase inverter with a divided load (Fig. 2) is borrowed from lamp circuitry ...

Rice. 1 (click to enlarge)

Rice. 2 (click to enlarge)

Rice. 3 (click to enlarge)

These schemes are now undeservedly forgotten. But in vain. Based on them, using a modern element base, you can create simple amplifiers with a very high sound quality. In any case, what I happened to collect and listen to sounded decent - soft and "tasty". The depth of feedback in all circuits is small, there are local OOS, and the output impedance is significant. There is also no general OOS for direct current. However, the above schemes work in the class B, so they have "switching" distortions. To eliminate them, it is necessary to work the output stage in a "pure" class A. And such a scheme also appeared. The author of the scheme is JLLinsley Hood. The first mentions in domestic sources date back to the second half of the 70s.

Rice. 4 (click to enlarge)

Here you can also see a phase inverter with a divided load and a voltage boost circuit, as in circuits 2 and 3. The amplifier is non-inverting and has a very wide frequency response band, therefore, if the installation is unsuccessful, self-excitation may occur due to parasitic feedback. In this case, the RC circuit at the output of the amplifier can correct the situation. The main disadvantage of class amplifiers A, limiting the scope of their application - a large quiescent current. However, there is another way to eliminate switching distortion - the use of germanium transistors. Their advantage is small distortions in the mode B. (Someday I will write a saga dedicated to germanium.) Another issue is that these transistors are not easy to find now, and the choice is limited. When repeating the following designs, you need to remember that the heat resistance of germanium transistors is low, so you do not need to save on radiators for the output stage.

Old but golden. The tube amp renaissance
Fig. 5

In this diagram, there is an interesting symbiosis of germanium transistors with a field one. The sound quality, despite the more than modest characteristics, is very good. In order to refresh the impressions of a quarter of a century ago, I was not too lazy to assemble the design on a mock-up, slightly modernizing it to match the modern denominations of parts. The MP37 transistor can be replaced with a silicon KT315, since when setting up, you still have to select the resistance of the resistor R1. When working with an 8 ohm load, the power will increase to about 3,5 W, the capacitance of the capacitor C3 will have to be increased to 1000 microfarads. And to work with a load of 4 ohms, you will have to reduce the supply voltage to 15 volts so as not to exceed the maximum power dissipation of the output stage transistors. Since there is no general DC CNF, thermal stability is only sufficient for home use.

The next two schemes have an interesting feature. The AC output stage transistors are connected in a common-emitter circuit, so they require a small excitation voltage. No traditional voltage boost is required. However, for direct current, they are connected in a common collector circuit, so a floating power supply that is not connected to ground is used to power the output stage. Therefore, a separate power supply must be used for the output stage of each channel. In the case of using pulse voltage converters, this is not a problem. The power supply of the prestages can be shared. The AC and DC FOS circuits are separated, which, in combination with the quiescent current stabilization circuit, guarantees high thermal stability at a shallow AC FOS depth. For MF / HF channels - an excellent scheme.

Rice. 6 (click to enlarge)

Rice. 7 (click to enlarge)

Author: A. Shikhatov; Publication: bluesmobile.com/shikhman

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Vladimir
It is much more profitable to combine "business with pleasure" - a hybrid amplifier. At the input - SRPP on a double triode, at the output a follower in two transistors. Lamps amplify voltage well, transistors do an excellent job with current. It turns out that there are only three or four active elements, the simplest circuit, small dimensions of the device and excellent sound.

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