Philips FW-C50/37, Service Manual

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• Super class G - operation
The power amplifiers operate as so-called super class G - amplifiers:
The supply pins 12 (Vcc) are not just connected to one fixed DC-supply as in conventional amplifiers.
Dependent on the output power there are three different DC-voltages supplied to the power amplifiers:
+C (+18V) for low output power
+B (+25V) for medium output power
+A (+36V for high output power
Principle / benefit of Super Class G
Display Super Class G Output Benefits
• advantages: - best efficiency
- less power drawn from the mains transformer than by conventional operating amplifiers reduces
transformer heating.
- reduced power dissipation at the amplifier ICs results in
• less junction temperature and better reliability
• possibility of higher output power with smaller cooling fin
• smaller size
• Functional description of the super class G - circiuitry used in Power4-module
The DC-level on the amplifier output pins is normally Vcc/2.
With low sC is supplying the amplifiers via decoupling diode 6312. The DC-level on the output pins is
therefore approx. 8,6V and approx. 8V on the base of 7315.
When the output signal increases, also DC-level on base of 7315 increases via diodes 6305, 6306, 6307 and
6308. At a certain output power 7315 becomes slightly conductive and enables low base current for 7304 which
becomes conductive too and pulls gate of FET 7303 up to a more positive level. Thus FET 7303 begins to switch
through and connects the higher DC B1 slowly to the power stages.
This does not end up in a hard switching but in a smooth regulating because Vcc is coupled back to the emitter
of 7315 via Zener diode 6310. As soon as Vcc increases also the level on emitter 7315 is increased by a 3,9V
lower level than Vcc. When the output power is increased fB1 would not be high enough to enable
undistorted output signal. The more the output level increases the more increases the DC-level on base of 7315
which causes the transistor more and more conducting until the summary of the voltage drop on 3340+E/B
7304+3342 becomes approx. 1,4V. Now the necessary VBE for a darlington-type transistor is obtained, 7305
begins to switch through and connects the again higher DC A slowly to the power stages. 7305
regA, same as described before for +B.
7322 and 7316 switch the ripple capacitor 2355, dependent on the output power.
With low output power the DC-level on base 7322 is approx. 8V. Via Zener diode 6310 and resistor 3333 the
emitter is pulled to Vcc (+C at low levels). 7322 is switched through and in turn 7316. The ripple capacitor 2325
is connected to ground and functions as in normal amplifiers. Hum is suppressed and good S/N-ratio is
guaranteed even during silent music passages.
When the supply voltage has to be switched to a higher level the DC-level of the ripple capacitor has to increase
in the same relation, otherwise the reference voltages inside the IC would not fit to the actual Vcc. Because of
the different delays this relation cannot be obtained and a continuously connected capacitor to the ripple input
would cause distortion. For that reason the ripple capacitor 2325 is disconnected as soon as the output power
exceeds a certain value. When the output signal increases, also DC-level on base of 7322 increases via diodes
6305, 6306, 6307 and 6308. 7322 blocks and in turn 7316. The ripple capacitor 2325 is disconnected from
ground. The circuitry is designed so that 2325 is disconnected just before 7303 begins to B through (see
above).
• For Center/Surround-amplifier the function of the Super Class G circuit is similar. Instead of +B1 there +B2 is
connected.
• For the /37-versions with two channel-application the so called MATRIX SURROUND is added. The 2
surround-speakers are added in a way, that in case of STEREO a high signal can be measured (up to 10W per
speaker at 6 Ohm). In MONO only a few 100mW are available. Result: The widening of the STEREO base is
increased without any additional electronic or amplifier.
• In all four channel versions a pre-amplifier out for SURROUND is available to add a wireless speaker system