The so-called floating active speaker refers to an active speaker that is made by directly using the mains power as a high voltage on the circuit and using a reference point on the potential to isolate the external equipment.
This speaker uses a small transformer for filament power supply, the high voltage directly uses city power, and the input end uses a small input transformer for signal isolation. The advantage of this is that it reduces the overall weight and cost; the disadvantage is that the input transformer is not handled properly, which causes a certain degree of non-linear distortion. But if the transformer is well wound, or if appropriate negative feedback is used, it can be completely solved. In this category of speakers, the author introduces two circuits, one is pure vacuum tube; the other is a combination of transistor and vacuum tube.
1. Pure vacuum tube suspension active speaker
The circuit adopts push-pull type, the power tube is 6P1×2, and the pre-phase inversion adopts 6N11 or 6N3.
(1) The complete circuit is as follows:
(2) A few explanations:
①Input transformer 1:3, using iron core with a cross-sectional area of 1.25 square centimeters, the primary winding is 2000 turns, and the secondary winding is 6000 turns. There is no requirement on the wire diameter, so that it can be wound around the window. When winding, the secondary is wound in the coil and the primary is wound outside. The withstand voltage between primary and secondary should be greater than 500V.
If it is unconditionally wound, it can be replaced by a small transistor power supply, with 12V for the primary and 220V for the secondary. Due to the influence of the input ratio, a 10K resistor can be connected in series to the primary to adjust the impedance. If the transformer core is of good quality, the effect is justified.
The small input transformer cannot be omitted in the circuit. This is because the machine directly uses the city power to prevent the ground terminal of the signal input terminal (DVD output, computer sound card output) from being connected to the neutral line, causing tripping; or to prevent The pin of this machine is reversed and the public area is fired, causing the device to burn out. Therefore, the input signal must be isolated by an input transformer.
②The output transformer adopts an iron core with a cross-sectional area of 9 square centimeters, the primary winding is 3200 turns, the wire diameter is 0.16, and it is divided into 4 windings for use in series. The secondary is wound in the middle, the secondary is 8 ohms and 92 turns, and 4 ohms are tapped at 66 turns. , The wire diameter is 0.7.
If it is unconditionally wound, it is not easy to buy. It can be replaced by the old five-lamp radio power transformer, with full-wave rectified high-voltage winding as the primary, and each filament winding in series as the secondary. When the output is 8 ohms, the primary-to-secondary voltage ratio is about 35.5; When the output is 4 ohms, the primary to secondary voltage ratio is about 50:1.
③The power transformer is for the filament, and it is sufficient to supply the filament with 6.3V greater than 1.6A. If the filament does not have a center tap, one end can be grounded.
④It is better not to use a magnetic base plate for the whole machine. In the past, iron base plates are commonly used in radios. The input transformer is likely to be affected by the leakage of the power transformer, that is, the leakage of the power is transmitted to the output transformer through the iron bottom, causing hum. This kind of humming cannot be solved by improving filtering. The input transformer of this unit should not be directly installed on the circuit, it is best to install it on the back cover of the speaker. Use a printed circuit board to install the small input transformer and the signal tap together, and connect the audio isolation cable to the circuit. This can effectively prevent the input transformer from triggering feedback or introducing power magnetic field interference.
⑤As the mains power is directly used for high voltage, in order to prevent electric shock during debugging, the live wire terminal must be connected to the rectifier input terminal, and the neutral wire is connected to the public ground wire.
2. Hybrid suspended active speaker
The circuit adopts 6P1×2 push-pull output, and transistors are used for pre-amplification.
(1) The complete circuit is as follows:
(2) A few explanations:
①The turns ratio of the input transformer is the same as that shown in Figure 1. It uses an iron core with a cross-sectional area of 1.25 square centimeters. The primary winding is 2000 turns, and the secondary winding is 6000 turns. The primary must be able to flow 30mA, and the secondary has no requirement on the wire diameter to be able to wind around the lower window. That's it. When winding, the secondary is wound in the coil and the primary is wound outside. The withstand voltage between primary and secondary should be greater than 500V.
If it is unconditionally wound, it can be replaced by a small transistor power supply, with 12V for the primary and 220V for the secondary. Due to the influence of the input ratio, the circuit can be changed to a resistance-capacitance coupling input to the transformer. The transistor load is 600 ohms, and the coupling capacitance is 100μ.
②The output transformer is the same as in Figure 1.
③ In addition to supplying the filament, the power transformer also needs an 8V winding, which is supplied to the transistor pre-amplifier through the bridge rectifier filter. For self-winding, an iron core with a cross-sectional area of 11 square centimeters can be used. The primary uses an enameled wire with a wire diameter of 0.2MM to wind 2200 turns, and the secondary uses a wire diameter of 0.8mm enameled wire to wind 68 turns (no-load voltage 6.8V), with a wire diameter of 0.33 The enameled wire is wound 80 times (no load 8V) for use in transistor circuits.
④ The common ground terminal of the transistor amplifying part is not connected to the ground of the tube amplifier. The two circuits are separated by an input transformer. To isolate the mains.
⑤The electronic tube push-pull input transformer has no center tap to prevent unbalance due to poor winding of the transformer. The phase inversion is carried out by forming a loop on the input secondary by the gate leakage resistance.
⑥ The two transistors are all silicon tubes, with a withstand voltage above 50V, and a magnification greater than 60. The power of the excitation tube must be 1W. The collector current of the input signal amplifier tube is adjusted to 0.8mA, and the excitation tube current is about 8-20mA. When adjusting, it is marked to work in Class A, that is, regardless of the signal input amplitude, the collector current remains basically unchanged.
For pre-amplification, low-power output integrated blocks can also be used.
The speakers above are single-channel, make two, that is, dual-channel. Or increase the filament transformer and make the two amplifiers together. After testing the above two circuits can achieve better results. The negative feedback of this machine adopts the negative feedback of the screen grid. The negative feedback of the large loop is not used, and the open-loop gain is high. When making, pay attention to the wiring arrangement to prevent humming.
The woofer uses an 8-inch paper cone and the treble uses a 3-inch small top ball. A 30μ electrolytic connection is used between the tweeter and the bass. Do not make the speaker fully enclosed, and open holes on the back cover to facilitate heat dissipation.