Electronic tubes - transistor hybrid output stage

Abstract

An electronic tube-transistor hybrid output stage is characterized by the fact that both a tube and a transistor system simultaneously perform the amplification of a signal.

Description

Application:

[0001] Low-frequency amplifiers are used in various electronic designs as instrument amplifiers and significantly influence the sound of a musical instrument. They are also used in hi-fi and PA systems for music and speech reproduction. State of the art:

[0002] Although the transistor, as a semiconductor, has increasingly replaced the vacuum tube as an electronic component since the 1960s, there are areas in amplifier design that are very sensitive to the disadvantages of the transistor compared to the tube. Human hearing is crucial to this sensitivity. Disadvantages of the semiconductor, such as knee voltage or overload behavior, cannot be completely compensated for despite modern circuit designs. And so, tube amplifier design remains a crucial foundation of modern manufacturing.

[0003] The basic amplifier circuits, whether tube or transistor, have remained unchanged since the beginning of amplifier development. The two most important types are single-ended and push-pull amplifiers.

[0004] Single-ended amplifiers generate the voltage swing required to operate an electronic transducer (loudspeaker) using an active electronic power component (tube or transistor). Ideally, the base of the alternating output voltage is at half the amplifier's operating voltage. The active component amplifies the alternating voltage peaks above and below this half-operating voltage, the virtual base.

[0005] Push-pull amplifiers, on the other hand, have two active power components, one of which processes the positive AC portion and the other the negative AC portion of the signal to be amplified. These components are then combined into a single AC signal. This is exemplified by the tube amplifier within the iron core of an output transformer. Transistor amplifiers typically decouple the overall signal to suppress the DC component by using a low-impedance capacitor.

[0006] Regardless of the advantages and disadvantages of tubes versus transistors, the described amplifier concepts produce different sound spectra. Different doesn't mean better or worse, but rather that each covers a specific area of ​​application. For example, musical instruments vary in their reproduction of their sound spectrum. One could say that certain amplifiers are suitable for certain instruments or even musical genres. Technical innovation:

[0007] Hybrid amplifiers combining transistors and tubes are numerous on the market. However, these are exclusively hybrid versions of various preamplifier and power amplifier stages. The characteristic tube sound is primarily determined by a tube power amplifier stage in combination with an output transformer. Key differences between transistor and tube amplifiers lie in their overdrive characteristics and the sonic properties of the output transformer.

[0008] Preamplifiers that are not overdriven and are operated almost exclusively as Class A amplifiers play no role in the overall sound. In this case, tubes and transistors behave almost identically. However, the situation is different in push-pull operation. Crossover distortion and the transformer required in tube power amplifiers create significant differences between the two types. When preamplifiers and power amplifiers from various systems are combined, the technical term is "hybrid amplifier." This term is misleading, however, as the sonic character depends solely on the factors described. For example, a tube power amplifier in push-pull operation with driving transistor preamplifiers can sound quite like a fully equipped tube amplifier in the hi-fi realm. Conversely, an amplifier equipped exclusively with preamp tubes will never achieve the full-scale sound of a tube amplifier. The term "hybrid" is commonly used for both variants, and is therefore contradictory.True hybrid technology, however, only exists in a real combination of tubes and transistors within a power amplifier.

[0009] In studies aimed at increasing the efficiency of various tube amplifiers, the control and amplification methods are examined analogously to those of a transistor, using both negative (NPN) and positive (PNP) signals. A tube with a positive charge equalization characteristic, unlike a PNP transistor, does not exist. What a push-pull transistor output stage accomplishes with a transistor pair driven by a symmetrical voltage, a tube output stage must achieve with a transformer. This transformer consists of two antiphase primary windings, each transmitting a positive and a negative half-wave to the secondary winding without any potential difference.

[0010] Apart from the winding-related problems that arise inductively as well as capacitively within such a transformer, such components are usually manufactured individually for specific output tubes at considerable expense.

[0011] Simple mains transformers, with two secondary windings (e.g. 2 × 6 V) and two primary windings (2 × 115 V), previously rather excluded as transformers in a tube circuit, are intended to replace an output transformer in this application. Basic features of the solution:

[0012] The idea is based on the possibilities of using a transformer (here as a transformer) to transfer both high- and low-impedance alternating signals, according to the induction requirements of tube and transistor output stages, simultaneously with their magnetic fluxes, into an output winding of the transformer. Solved tasks:

[0013] The basic circuit is based on the principle of a single-ended output stage. Here, it is implemented using a P- or ECL... tube as an example. Both cathodes of the tube are connected to ground potential. The negative grid bias of the output tube creates an adjustable auxiliary voltage (RV1). The control grid of the preamplifier is connected to a high-impedance leakage resistor. This ensures that the electrons also generate a sufficiently negative bias on this grid.

[0014] Up to this point, it's a classic single-ended Class A tube amplifier. As such, a 100V output transformer can be used, but it lacks the air gap in the iron core to compensate for the bias caused by the DC component. However, if a 100V transformer is used in push-pull configuration via a center tap, the air gap is no longer strictly necessary. This reduces the individual inductance of the connected tubes, which significantly limits bass response. Sufficient impedance on the tube side is achieved by connecting both primary windings (2 × 115V) of a mains transformer in series; in this case, 3.5VA to 7VA. To prevent DC current in the loudspeaker, the transistor stage is galvanically isolated one-to-one via the two 6V secondary windings.The presented circuit based on a MOSFET output transistor (Q1) extends the tube circuit to a true hybrid push-pull output stage. Advantages: 1. No air gap is required in the iron core. Compensation of the DC components of both systems within the transformer is achieved. 2. The total inductance of the transformer's primary winding is available to the tube circuit, which improves bass reproduction.

[0015] A split in the secondary winding of transformer T2 prevents the permanent DC current from the transistor output stage from passing through the loudspeaker. Here, a low-impedance connection is established between the windings. The quiescent currents of the output stages must be adjusted according to the turns ratio of this transformer. Class B operation of the tube output stage (approx. 5 mA) requires a quiescent current of approximately 50 mA for the transistor output stage. This can be adjusted using trimmers RV1 and RV2. The quiescent current can be measured and thus determined at resistors R5 and R7.

[0016] Separating the input signal of both stages via an additional transformer T1, and thus enabling phase inversion of the signal, allows the use of both N-type and P-type transistors in the output stage. This can be switched using jumpers JP3 and JP4. Simultaneously, the 12 V supply voltage is reversed. This can be switched using jumpers JP1 and JP2. A miniature 0.35 VA mains transformer is used as the input signal transformer T1 in the circuit diagram.

Claims

[1] Electronic tube-transistor hybrid output stage characterized by , that both a tube and a transistor system can simultaneously amplify a signal. [2] Electronic tube-transistor hybrid output stage according to claim 1, characterized by , that, according to a push-pull circuit, the tube system amplifies one half-wave and the transistor system amplifies the wave of the signal that follows by 180°. [3] Electronic tube-transistor hybrid output stage according to claim 2, characterized by , that the tube system drives a high-impedance primary winding of an output transformer. [4] Electronic tube-transistor hybrid output stage according to claim 2, characterized by , that the transistor system drives a low-resistance primary winding of an output transformer. [5] Electronic tube-transistor hybrid output stage according to claim 4, characterized by, that due to multiple possible 180° phase shifts of the signal within the transformers, either N- or P-conducting transistors are used. [6] Electronic tube-transistor hybrid output stage according to claims 3 and 4, characterized by , that the two primary windings of the output transformer, which are phase-shifted by 180°, excite a further low-impedance secondary winding for coupling out the signal.

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