Tweet
Originally posted by vachira
View Post
-----------------
น่าทำดีเหมือนกัน....เหมาะสำหรับหัดทำ
The MOSFET can be any power MOSFET, so long as the voltage and current ratings are adequate. MOSFETs have the ?soft? overload characteristics of vacuum tubes and are preferred in this type of application over bipolars. The gate resistor helps to stabilize the MOSFET. The VDS spec should be at least twice the idle voltage. If the MOSFET idles at 1/2 the total supply voltage, then VDS should be at least the value of the total supply or higher. Rs is a power resistor. Starting with an idle current Id of about 100mA and -V = -12V, then RS = 12/.1 = 120 ohms. The resistor?s power rating should be much greater than 12 * 0.1 = 1.8W (at least 3.6W to be safe). Also make sure that the MOSFET is heatsinked to dissipate a similar amount of power.
When a headphone plug is inserted or removed from the jack, the possibility arises that the amplifier outputs will be shorted, if only briefly. Without current limiting, such a short could burn out opamps and/or output stage transistors. Rather than resort to complex current sensing schemes, figure 14 shows two common limiting mechanisms that protect against short-circuit damage: current limiting resistors and incandescent bulbs. Current limiting resistors set the minimum load that the amplifier can see ? typically 100 ohms, 1/2W. Output resistors will reduce the output power and increase the amplifier?s output resistance, but most headphones will be unaffected. Another option is to locate the current limiting resistors inside the feedback loop (figure 12) so that the effective output impedance of the amplifier is minimized from the feedback. See Headphone FAQs for more information about the impact of amplifier output impedance on headphone sound.
In place of a current limiting resistor, an incandescent lamp has the advantage of very low resistance when the filament is cold. Lamp filaments have a positive temperature coefficient. As increasing current heats the filament, the resistance also goes up, thereby reducing the output current. Choose lamps with voltage and current characteristics similar to that of the output stage. Incandescent lamps were once popularly deployed to protect loudspeakers from overdrive. The idea resurfaced as output limiting for headphone amplifiers in Ben Duncan?s PHONES-01 headphone amplifier project.
เครดิต
http://headwize.com/?page_id=147
Comment