01-11-2011, 03:40 PM
dari http://sound.westhost.com/articles/class-g.htm
A similar (at least in some respects) arrangement is called Class-H, and it can be difficult to decide exactly into which camp some amplifiers fall. Class-H is often described as using a "bootstrap" capacitor that lifts the rails as needed, but cannot maintain them at the full voltage for more than a few cycles. After a short period, the capacitor discharges, and the high voltage supply collapses. Originally, there were used for car audio, and allowed much more power than can normally be expected (about 18W for a BTL (bridge-tied-load) amp operating from 13.8V DC). Being far cheaper than a switchmode power supply, this is a convenient way to get extra power for very little expense. A number of specialised ICs were/are produced for just this purpose.
Because the difference is rather blurred, you may see Class-G amps described as Class-H and vice versa. My preferred terminology is that only amps that use a bootstrap circuit are "real" Class-H. Likewise, if the supply is switched or boosted using a separate fixed high voltage supply, then Class-G is the most appropriate description of their topology.
Hitachi is usually credited with the first Class-G amplifier, but from the descriptions I've seen, it actually appears to have been Class-H. The peak power of 400W into 8 ohms was not available continuously, but only for relatively brief periods. This implies that the high voltage rail was produced by bootstrapping a capacitor, rather than a switched rail design. By my definition above, that makes it Class-H, although Hitachi described it at the time (1978) as Class-G.
A similar (at least in some respects) arrangement is called Class-H, and it can be difficult to decide exactly into which camp some amplifiers fall. Class-H is often described as using a "bootstrap" capacitor that lifts the rails as needed, but cannot maintain them at the full voltage for more than a few cycles. After a short period, the capacitor discharges, and the high voltage supply collapses. Originally, there were used for car audio, and allowed much more power than can normally be expected (about 18W for a BTL (bridge-tied-load) amp operating from 13.8V DC). Being far cheaper than a switchmode power supply, this is a convenient way to get extra power for very little expense. A number of specialised ICs were/are produced for just this purpose.
Because the difference is rather blurred, you may see Class-G amps described as Class-H and vice versa. My preferred terminology is that only amps that use a bootstrap circuit are "real" Class-H. Likewise, if the supply is switched or boosted using a separate fixed high voltage supply, then Class-G is the most appropriate description of their topology.
Hitachi is usually credited with the first Class-G amplifier, but from the descriptions I've seen, it actually appears to have been Class-H. The peak power of 400W into 8 ohms was not available continuously, but only for relatively brief periods. This implies that the high voltage rail was produced by bootstrapping a capacitor, rather than a switched rail design. By my definition above, that makes it Class-H, although Hitachi described it at the time (1978) as Class-G.