Three-pole headset and line amplifier, line amplifier
Keywords: HIFI electronic tube headphone amplifier production, electronic pipe ear
High-fidelity stereo headphones are getting better and better. Its sound is not affected by the acoustic conditions of the room, nor does it affect the work or rest of the people, more helpful to listen to the melody of music, rhythm, and touching atmosphere. In addition, the cost of the earphone listening system is low, the sound quality is good, and it is increasingly favored by enthusiasts. However, the effect of listening to music is not satisfactory with general amplifier, the effect is not ideal, and the better way is to customize a headphone amplifier for headphones, and now it has become a consensus of music with headphones. Original location
This paper introduces the low-impedance line amplifier and 32Ω headphone amplifier assembled with 3 electronic tube. It uses differential push-off and transformer output. The circuit is simple and easy to make, and it can be used in one machine, it is worthy of interest.
I. Circuit and principle
Figure 1 is a schematic of the electromechanical schematic (only the left channel, the power supply is shared by left and right channels). Most of the whole machine only uses 3 MT tubes. A 12AT7 / ECC81 double triode is left, the right channel input stage, one 12au7 / ECC82 double triode as a channel of differential push output level. Since this circuit itself has inverted effects, the primary inverting circuit can be omitted to simplify the circuit structure.
The input is provided with two sets of input terminals A, B, which can be switched to switch. In order to improve the characteristics of the whole machine, the input stage adds negative feedback from the output stage. Feedback amount is compared to the LODB. The tube can also be used 12AU7, but the gain is slightly low, and the circuit is selected to ensure gain and the necessary negative refueling.
The output stage is pushed with 12au7, as described above, it has a inverted effect, so it does not need to enter the inverted signal and there is only one signal input. This circuit seems less easy to understand, in fact it is the differential amplifier we are familiar with. In the transistor circuit, the differential amplifier is the use of Siki, and the use of differential amplifiers in the electronic pipe circuit is relatively rare. Here below explains.
Figure 2 is the basic structure of the electronic pipe difference amplifier.
This circuit structure is identical to the transistor differential amplifier. It has two signal input terminals, two signal output terminals, and a cathode share a resistor. The screen is electrically active. The application is quite flexible. It can be added to both input terminals, or only add signals on one input terminal. The output signal can be obtained from two output terminals or from an output terminal.
The following is a common way.
(1) When two inputs are added, the same amplitude signal is added. The same phase, the same amplitude output signal is obtained on both output terminals. If the push-pull output is made, the two signals cancel each other, which is advantageous to suppress the output of the input noise.
(2) When the two inputs add inverted epitonal signal, the output is obtained by the same inverted output signals as the same amplitude.
(3) When only the signal is only added in one input, the output signals of the same amplitude are the same, the phase opposite.
The unit output stage uses a single-ended input signal (another input), the two-terminal output signals (the screen is used to use an output transformer instead of two load resistances). It has the inverted effect of the (3) point, and there is a (1) point in the inhibition effect, and the circuit is simple.
In order to improve the symmetry and its characteristics of the differential amplification circuit, Figure 1 outputting two triode cathodes using a constant current source circuit, using an LM317 regulator IC. As the headphone amplifier, the rated output power is 100MW is enough, this The machine uses 12AU7 differentials to push the pull, the screen is electrified by 180V, the screen current is about 7.5mA, the gate bias is about one 6V, and the primary impedance of the output transformer is used. At the time of 10kΩ, the maximum output power of about 150rt is about 150ROW. At this time, the screen is power consumption of 180V × 7.5mA = 1.35W. It is quite safe for less than 12AU7 maximum screen loss.
Finally, when the unit is used as a headphone amplifier, the 18Ω terminal is used when using a 16Ω terminal, and the actual load impedance of the output stage is 20 kq. When the line amplifier is used, the 16 Q terminal is also used. However, in order to avoid a large-scale change of the load impedance of the local output stage due to the post-matching of the post-level amplifier input impedance, the 16 Ω terminal should be connected in parallel to the ground. In addition, the gain of this unit is about 3.2 times, and when the line amplifier is used, the unit seems to be too large, and it is preferable to access the attenuation network shown in FIG. 3 at the output. Select the appropriate amount of attenuation; allow the system to listen properly, the corner stroke of the volume potentiometer of this unit is about half, so that the unit works in the best state. As for how to achieve corresponding conversion (with appropriate conversion switches, etc.) between the headphone amplifier, the line amplifier, and the mating attenuation network, please create according to its own actual needs, and will not be described here.
II. Components and production
Most components of this unit are generally general, and the requirements have been indicated in Figure 1, and the resistors that have not been indicated are 1/2 W. The resistance (50 Q, 82 Q) and the resistance (240 T2, 100 T2, 2k Ω) used by the filament regulator IC should be used in the resistance of 1% error. The capacitance in the high-frequency phase compensation network (200PF ', 30kΩ) can be used in general mica capacitors.
The output transformer uses the Fe-10 LO of the Japanese Patynt Motor Production, and its shape and terminal block are shown in Figure 4. The main features are as follows:
(1) Output power: 10W (50 Hz)
(2) Frequency response: 20Hz ~ 50KHz (1 LDB, input 4V, 2Rn = ZP)
(3) One side inductor: L 0 0 h (1 mW), up to 50 hours (50 Hz)
(4) One side allows DC current: Double Tube 90mA (unbalanced current 4mA)?
(5) Power loss: 0.41dB (16 Ω)
(6) One-time DC resistance: 410Ω output stage cathode constant streaming adopts the adjustable triple voltage regulator IC to be configured as constant current circuitry, using the LM31 7T can be mounted on a small heat sink, and pay attention to the insulation between the chassis to be reliable. To make the regulator IC as constant current devices, only the specified constant current value can be reached at the L = M317T output terminal. This resistance can be obtained from 1.25 (V) / I (a). The output stage of the unit is 7.5mA per tube, and the two tubes are 15mA ie O.015A, so R = 1.25V / O.015A = 83 Q, 82 Ω is selected in Figure 1. When the LM317T is working normally, it is required to enter the pressure drop between the outputs greater than 3V. The local output stage cathode is about 6V, and the output terminal is 1.2 5V, and the input terminal voltage can be 4.25V, which can be Meet the normal work requirements. The 50Ω resistance between the LM 3L 7T input terminal and 12au7 cathode is used to detect the cathode current. The filament is powered by LM3 50T regulating (also using LM317T), and a small heat sink should be added. No matter whether the LM 3L 7 is still LM350, do not make mistakes.
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