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再转贴书上一些保护电路
先贴保护电路的保护说明曲线图
1.仅仅限流保护，保护曲线就是A－B段
2.单斜线保护电路，保护曲线就是图C－D段，其中右面的电路加了电容延时，避免电源的HF干扰。
3.改进的斜线保护，保护曲线是C－D－DD段，
左面的特征就是加上稳压管，
右面的保护曲线更复杂，不同的文献中有的描述是具有双重或三重斜线，甚至半椭圆形
4. 3斜线保护，保护曲线是E－F－G（45度虚线那段），
5.用在hi-end的保护电路Mid-Floyd
Turning to the circuit, CCB and CCE show two locations for compensation capacitors
where parasitic HF oscillation caused by positive feedback at RF, and other
instabilities respectively, may be slugged − in this or any of the preceding limiter
circuits. Notice the voltage sensing is not from the rails, but from 0v (a cleaner
place that gives the inverse information) through the diode and resistor array, to
give the dashed section down to G again, at high voltages. Note also the relatively
elaborate control of the limiter’s ‘attack and release’ response with the 47μF, 1.5nF
and 220pF capacitors.
6.dimensions
US maker Crown (Amcron) had patented their own scheme as early as 1967. Here,
the V-I limit turns back into a current-only limiter at low bass frequencies. This
would be like the load line towards E in Figure 5.20 turning hard left before it
reaches point E. But only at low frequencies. So a V-I load limit-line’s slope changes
may be frequency conscious, as well as varying with temperature, and five spacial
dimensions would be needed to ‘see’ what is going on dynamically.
The next circuit illustrates a comparatively recent development used in designs by
UK professional makers, after Tim Isaac. In Figure 5.25, improved protection fitting
is attained by splitting the voltage sensing, with R6 diverting around the zener
clamp. The zener-clamped voltage is then controlled by a second small signal transistor,
TR.2. This turns on hard when the output swings more than a volt into the
opposite polarity, effectively shunting the zener with R7. Other than providing enhanced
short circuit protection near zero volt output, this circuit provides up to five
slopes, including the ‘anti-flyback’ simple current limit line.
保护电路的缺点
Although the V-I limiter in all its forms is still in widespread and continuing use in
amplifiers with BJT outputs today, it is rare in the best sounding, high performance
amplifiers. It is easy to disconnect a V-I limiter and many listeners have noted just
how much load-line limiter circuits degrade sonics even when they are not overtly
at work. Causes include:
(i) Current clipping, an occurrence with particular speakers (Figure 5.9) and
program, which is not visible with a conventional voltage-sensing scope
connection.
(ii) Negative resistance conditions. If not destructive, these result in RF
oscillation(s) that may be local and not readily noticeable at the output.
(iii) Related to these, flyback pulses caused (with the 3 and more particularly, 2 slope
limiters) by instantaneous shutdown of current into an inductive load after it has
caused voltage spiking. The flyback pulse is heard as a popping or rasping sound.
(iv) V-I limiting is an urgent, brute force form of protection. The side can often be
‘bitten out of’ a few cycles of a bass sinewave. This takes the ‘edge’, ‘slam’
and ‘thrust’ out of music that has it. Otherwise it changes the tonal and
harmonic structure. More subtle means of protection would not interfere with
instantaneous values of the waveform so abruptly or crudely. See below.
(v) Non-linear positive feedback.
(vi) Supply noise injection. As V-I limiting circuitry is required to act within a few
milliseconds to Vce changes, to be of use, this means that anti-musical halfwave
supply rail noise is inevitably injected into the signal path, through the
limiter transistor, which can act as a common emitter amplifier stage.
The sonic effect of a V-I limiter may be ameliorated by delaying its action. This is
risking transistor longevity if pushed too far without simultaneous derating, much
beyond 10mS, say. In more sophisticated V-I load line limiter circuits, the load
limit-line may be continuously adjusted against the lowest large signal frequency.
Anything that backs off unnecessary V-I limiting will help recover power delivery
and maximum sound level. This was the secret of one of Panasonic’s ‘Ramsa’ PA
amplifiers sounding more powerful than a certain US east-coast PA amplifier that
had twice the rated continuous power. |
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楼主
发表于 2007-6-14 08:42
