The so-called floating active speaker refers to an active speaker made by directly using commercial power as a high voltage on the circuit and using a reference point on the potential to isolate external devices.
The speaker uses a small transformer for filament power supply, the high voltage directly uses the city power, and the input end uses a small input transformer as the 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 nonlinear distortion. However, if the transformer is wound well, or using appropriate negative feedback can be solved. In this type of speaker, the author introduces two circuits, one is a pure vacuum tube; the other is a mixture of a transistor and a vacuum tube.
1. Pure vacuum tube suspended active speaker
The circuit adopts push-pull type, the power tube is 6P1 × 2, and the pre-phase inverter adopts 6N11, or 6N3.
(1) The circuit of the whole machine is as follows:
(2) A few notes:
①Input transformer 1: 3, using an iron core with a cross-sectional area of ​​1.25 square centimeters, the primary winding is 2,000 turns, and the secondary winding is 6,000 turns. When winding, the secondary winding is in the coil, and the primary winding is outside. The voltage between primary and secondary should be greater than 500V.
If it is unconditionally wound, it can be replaced with a small transistor power supply. The primary uses 12V and the secondary uses 220V. Due to the influence of the input ratio, a 10K resistor can be connected in series to adjust the impedance. If the quality of the transformer core is good, the effect is justified.
The small input transformer can not be omitted in the circuit, this is because the machine directly uses the commercial power, in order to prevent the ground terminal of the signal input terminal (DVD output, computer sound card output) from connecting with the zero line, causing a trip; or to prevent The plugs of this machine are connected in reverse, and the fire is exposed to the public, causing the device to burn, so the input signal must be isolated using 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. It is used in series with 4 windings. The secondary is wound in the middle, the secondary 8 is wound 92 turns, and the 4 ohm is tapped at 66 turns. , Wire diameter 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. The full-wave rectified high-voltage winding is used as the primary, and the filament windings are used in series as the secondary. When the output is 8 ohms, the primary secondary voltage ratio is about 35.5; When the output is 4 ohms, the primary secondary voltage ratio is about 50: 1.
â‘¢ The power transformer is used for the filament, and the power supply can be satisfied if the filament is 6.3V greater than 1.6A. If the filament has no center tap, one end can be grounded.
â‘£ It is better not to use magnetically conductive material bottom plate for the whole machine. In the past, iron base plates were commonly used in radios. It is easy for input transformers to be affected by power transformer magnetic leakage, that is, power leakage through the iron bottom to the output transformer, causing hum. This kind of hum cannot be solved by improving filtering. Do not install the input transformer directly on the circuit. It is best to install it on the back cover of the speaker. Use the printed circuit board to install the small input transformer and the signal tap together, and connect it to the circuit with the audio isolation cable. This can effectively prevent the input transformer from causing feedback or introducing power supply magnetic field interference.
⑤Because the commercial power is directly used as 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 should be connected to the common ground wire.
2. Hybrid suspended active speaker
The circuit uses 6P1 × 2 push-pull output, and the transistor is used for pre-amplification.
(1) The whole circuit is as follows:
(2) A few notes:
①The turns ratio of the input transformer is the same as that in Figure 1. The core with a cross-sectional area of ​​1.25 square centimeters is used. The primary winding is 2,000 turns and the secondary winding is 6,000 turns. That's it. When winding, the secondary winding is in the coil, and the primary winding is outside. The 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. The primary uses 12V and the secondary uses 220V. Due to the influence of the input ratio, the circuit can be changed to a RC coupling input to the transformer. The transistor load is 600 ohms and the coupling capacitance is 100μ.
â‘¡The output transformer is the same as Figure 1.
③In addition to supplying the filament, the power transformer must also have an 8V winding, which is supplied to the transistor pre-amplifier through the bridge rectifier filter. If you wind it yourself, you can use an iron core with a cross-sectional area of ​​11 square centimeters. The primary is wrapped with a 0.2mm enameled wire for 2200 turns, and the secondary is wrapped with a 0.8mm enameled wire for 68 turns (no-load voltage 6.8V), with a wire diameter of 0.33 The enameled wire is wound around 80 turns (with no load 8V) for the transistor circuit.
â‘£ The common ground terminal of the amplifying part of the transistor is not communicated with the ground of the tube power amplifier. The two circuits are separated by an input transformer. To isolate the mains.
⑤The tube push-pull input transformer has no center tap to prevent the unbalance caused by the bad winding of the transformer. The phase reversal is performed by the gate leakage resistance at the input secondary to form a loop.
â‘¥The two transistors are all made of silicon tubes, the withstand voltage is above 50V, and the magnification is 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 adjusted, it is marked by working in class A, that is, the collector current remains basically unchanged regardless of the signal input amplitude.
For pre-amplification, low-power output integrated blocks can also be used.
The above speakers are mono, for two, that is, dual channels. Or increase the filament transformer and put 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 curtain grid. The negative feedback of the large loop is not used. The open loop gain is high. During the production, pay attention to the circuit arrangement to prevent hum.
The woofer uses an 8-inch paper cone, and the treble uses a 3-inch small top ball. 30μ electrolytic connection between tweeter and bass. Do not make the speaker fully enclosed. Open holes in the back cover to facilitate heat dissipation.
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