OTL is the abbreviation of English Output Transformer Less Amplifier, which is a power amplifier without output transformer.
One. The characteristics of OTL tube power amplifier circuit The output load of ordinary tube power amplifier is a dynamic coil speaker, and its impedance is very low, only 4 ~ 16Ω. In general, the internal resistance of the power amplifier tube is relatively high. In the ordinary push-pull power amplifier, the load impedance of the screen to the screen is generally 5 to 10kΩ, so it cannot directly drive the low impedance speaker, and the output transformer must be used for impedance conversion. Since the output transformer is an inductive element, the impedance of the inductance coil is also different when the frequency of the signal passing through the transformer is different. In order to extend the low-frequency response, the inductance of the coil should be large enough, and the number of turns will be more, so the distributed capacitance between each layer will also increase accordingly, limiting the high-frequency expansion, and will also cause nonlinear distortion and phase distortion.
In order to eliminate these adverse effects, various types of tube OTL no-output transformer power amplifiers came into being. Many new power tubes suitable for OTL power amplifiers have also been continuously designed and manufactured abroad. The sound quality of the tube OTL power amplifier is clear and transparent, with high fidelity and wide frequency response. The frequency extension range of high and low frequency bands can generally reach 10HZ ～ 100kHz, and its technical performance such as phase distortion, nonlinear distortion, transient response, etc. There is a significant improvement.
Two The form of the OTL power amplifier circuit of the electron tube Figure 1 (a) ~ Figure 1 (f) is the basic circuit of OTL no output power amplifier. Figure 1 (a) and Figure 1 (b) are the two power supply modes of OTL power amplifier, that is, positive and negative dual power supply and single power supply. In the positive and negative dual power supply OTL amplifier, the center is the ground potential. This can ensure the symmetry of the push-pull circuit, so the output capacitor can be omitted, making the frequency response characteristics of the power amplifier better. In order to make the two push-pull tubes have the same working voltage, the single-supply OTL circuit must make the working voltage at the center point equal to half of the power supply voltage. At the same time, the capacity of its output capacitor C1 must be large enough to not affect the output impedance and low frequency response requirements.
Figure 1 (c) and Figure 1 (d) for the OTL power amplifier tube grid bias method. Since the cathode of the upper tube is not grounded, the push signal of the upper tube is added between the grid and the cathode, while the push signal of the lower tube can be added between the grid and the ground. As for its bias mode, the upper side tube can be taken out after dividing the voltage to the ground through the center point, and the bias voltage of the lower side tube must be provided by a special negative voltage power supply.
Figure 1 (e) and Figure 1 (f) for the application of OTL inverter circuit. Figure 1 (e) is the use of screen-inverted split-phase inverter circuit to excite the OTL power amplifier. As long as the resistance value of the screen load resistance RL of the inverter tube and the cathode load resistance RK are equal, the excitation voltage output by the inverter can always be balanced.
Figure 1 (f) shows the use of a common cathode differential inverter circuit. Because the resistance of the common cathode resistance RK is large, it has a deep negative feedback effect, so the circuit is stable and reliable. At the same time, as long as the load resistances of the upper and lower tubes of the differential amplifier are equal, the screens of the two tubes can always output a pair of push signal voltages with opposite phases and equal amplitudes.
3. Selection of the OTL power amplifier circuit For the output stage of the OTL power amplifier of the tube, not all power tubes can be applied. Only power tubes that meet the following conditions must be selected to achieve good results.
1. Low internal resistance characteristics The internal resistance of the screen of a general power tube is about 10kΩ, which is not suitable for OTL amplifiers. The OTL power amplifier must use a power tube with an internal resistance of 200 to 800Ω. These low internal resistance power tubes are 6AS7, 6N5P, 6C33C-B, 6080, 6336 and so on.
2. Low screen voltage, high current characteristics The screen voltage of general power tubes is about 400V, and the high screen voltage tube can reach 800 ~ 1000V, and OTL amplifiers must choose low screen voltage, large current between 150 ~ 250V The characteristics of the power tube come to serve. The low internal resistance power tubes listed above all have the working characteristics of low screen voltage and large current. In addition, there are 6C19, 6KD6, 421A, 6146 and other power tubes. These tubes have the characteristics of low screen voltage and high current, but the internal resistance of the screen is slightly higher, and multiple tubes should be connected in parallel to be suitable for OTL power amplifiers.
3. Adopt new type OTL power amplifier dedicated power electron tube. This kind of electron tube not only has low internal resistance, but also has low screen voltage and high current characteristics, such as 6HB5, 6LF6, 17KV6, 26LW6, 30KD6, 40KG6, etc. In order to reduce the power consumption of the tube filament, many of the power tubes used in OTL power amplifiers have the filament voltage increased to 20-40V, so that they can be used in series.
4. Several typical OTL power amplifier circuits
1. New triode power tube OTL power amplifier Figure 2 is a 6C33 CB dual triode OTL power amplifier circuit diagram. This circuit uses a new type of low internal resistance, high power dual triode 6C33C-B as an OTL power amplifier. Each channel uses a pair of 6C33C-B for power amplification. When an 8Ω load is output, the output power of each channel can reach 40W.
The input stage of this OTL power amplifier adopts high amplification factor dual triode tube 12AX7 to form the previous differential and inverting circuit. This circuit has the characteristics of high input impedance and large dynamic range. In order to broaden the frequency response and reduce phase distortion, a direct coupling method is used between the input stage and the driving stage. In order to improve the gain of the front stage, a voltage of -22 V is added to the cathode of the differential input tube 12AX7, and a 1.1mA constant current diode is connected in series to make the front stage work more stable and reliable.
The booster stage is served by the medium amplification factor dual triode tube 12BH7. The characteristics of the tube are similar to those of the bipolar transistors such as l2AU7, 12JD8, and 5687. In order to increase the screen current and improve the output capacity of the driving stage, two triodes are used in parallel, and the screen voltage of each tube is as high as 265V, forming a common cathode driving amplifier circuit. In order to improve the electrical performance of the driving level, reduce distortion, and broaden the frequency response, a deep current negative feedback is added to the cathodes of the two tubes.
Each channel of the OTL power amplifier output stage uses a pair of new dual triode power tubes 6C33C-B. A pair of push signals with equal amplitude and opposite phase in the previous stage are coupled to the power amplifier tube through two 0.47 F capacitors.
This OTL power amplifier stage adopts the form of positive and negative dual power supply, and its working voltage is ± 182V. The highest negative pressure value between the grid and the cathode of the power amplifier tube 6C33C-B is -60V, the grid negative pressure of the upper tube is supplied by a separate negative voltage power supply, and the grid negative pressure of the lower tube is supplied by another set of negative voltages.
In order to improve the electrical performance of the OTL power amplifier, an appropriate voltage negative feedback is added between the OTL midpoint output terminal and the input terminal through a 1.8kΩ resistor, so that the overall mechanical and electrical performance is stable and reliable. The frequency response of this machine is 10Hz ~ 200kHz (± 0.1dB).
In terms of the power supply of OTL power amplifier, the positive and negative high voltage of the power amplifier stage is obtained from the 135V / 1.3A winding of the power transformer through the positive and reverse rectifier filtering of the diode to obtain a high voltage of ± 182V. The high voltage of the input stage and the driving stage is output by the power transformer 300V / 0.1A winding through diode bridge rectification and filtering and output + 395V high voltage, and the voltage of + 265V and + 140V is obtained after the decoupling resistor is stepped down, which is supplied to 12AX7 and 12BH7 . The grid negative voltage power supply is divided into two groups. The two independent windings of the power transformer, 60V / 50mA, are rectified and filtered, and are respectively supplied to the grid of the OTL power amplifier tube as the grid negative bias, and are carried out by two 20kΩ variable potentiometers Adjust. The filament power supply is divided into 3 groups, and each channel of the front stage is 2 groups. Amplifier tube 6C33C-B filament has two usages, when used in series is 12.6V / 3.3A, when used in parallel is 6.3V / 6.6A, this machine is used in series.
2. Ordinary triode OTL power amplifier Figure 3 is a 6KD6 pentode OTL power amplifier circuit diagram. It is an OTL power amplifier that changes the ordinary beam radiating quadrupole or pentode power electronic tube to the triode connection method, and utilizes the characteristic that the electron tube curtain grid can output a large current under the same grid voltage. It turned out that due to the relatively large internal resistance of the screen, the working current was limited, but after changing to the triode connection method, the voltage of the curtain grid and the voltage of the screen were at the same potential, the internal resistance of the screen was greatly reduced, which strengthened the screen. The ability of larger current, so it can output larger power under low impedance load.
For the OTL power amplifier with the common power tube changed to the triode connection method, not all power tubes can be used, and it is necessary to select a beam tetrode or pentode power tube with a large screen voltage range, such as 6KD6, 6L6, 6P3P , 6146, etc. At the same time, the power amplifier stage must also use multiple power tubes in parallel. In the case of 8Ω low-impedance load, 6 power tubes in parallel per channel can meet the requirements of low-impedance load, and the output power is only about 30W.
The input stage of this OTL power amplifier is served by the high amplification factor electron tube 6J2, which can greatly improve the input audio signal, and the single-stage voltage gain can reach more than 30dB. The amplified signal voltage is transmitted to the inverting stage by direct coupling. The phase reversal stage is served by the high-screen voltage double transistor 6SN7, and the screen voltage value is 340V. The screen is composed of a screen-inverted split-inverting circuit, and the load resistances of the screen electrode and the cathode are both 33kΩ. In this way, a pair of push signal voltages of equal amplitude and opposite phase can be obtained at the output.
The OTL power amplifier stage adopts SEPP parallel push-pull circuit. 6KD6, 6L6, 6P3P and other power amplifier tubes with large screen voltage range can be selected and changed to triode connection method. The output mode of 6 power amplifier tubes in parallel is adopted to make the output impedance reach 8 ~ 16Ω.
Amplifier power supply is in the form of positive and negative dual power supply, the value is ± 230V. The negative voltage of the grid of the power amplifier tube should be determined according to the characteristics of different power tubes. The upper side tube and the lower side tube are obtained through their respective voltage division networks and through the adjustment of the potentiometer.
Connector 1.5Mm Pitch,Zh Connector,Zh Terminal,Pitch Connector
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