QSC III Series Technical & Service Manual page 20

resistors R25 and R26 are used to stabilize the gain of Q1 and Q2, and to minimize any tendency to draw more idle
current as they heat up. The final collector current from Q1 and Q2 is about 500 mA, and goes to the bases of the
lower voltage output transistors Q5, Q6, Q7 and Q11, Q12, Q13.
These parts handle the first 50% of the output voltage, but then approach saturation. In order to deliver the
remaining 50% of the output swing, we need to transfer to the high-voltage drivers Q3 and Q4, which drive the full-
voltage output transistors Q8, Q9, Q10 and Q14, Q15, Q16. This is done by putting Q3 and Q4 in series with Q1
and Q2, and using 3.9 V Zeners Z15 and Z16 for turn-on threshold.
To explain this fully, we will look at just the positive half of the circuit, keeping in mind that the same thing will
happen on the negative half-cycle. Remembering that the speaker voltage is superimposed on the main power
supply rails (see section 1), you will note that as the positive driver Q1 is driven harder, its collector current,
amplified by its bank of outputs, will pull the positive half-voltage power supply closer to ground. Since the entire
supply is coupled together as a unit, this means all voltages move negative together. The speaker is connected to
the common midpoint, so this results in a negative speaker output. As we approach the point of 50% output, the
positive half-voltage supply will come within a few volts of ground, This carries the emitter of Q3within a few volts of
ground, until its base voltage drops below the voltage established by the 3.9V zener Z15. At this time, D12 becomes
forward biased and begins to drive Q3. Q3 takes over the current coming from Q1, thus cutting off the low-voltage
outputs and starting to drive the high-voltage outputs. These then pull the full voltage supply the rest of the way to
ground, resulting in 100% output to the speakers. Since the half-voltage supply is carried below ground during this
period, high-current diode D11 protects the low-voltage outputs from reverse polarity.
This reduces waste heat considerably, compared to the normal single-stage circuit. A power amp with a single level
DC supply has a lot of voltage across the power transistors for all levels up to full o0utput. This means that more
power is wasted at moderate power levels than at full output. By having a lower level DC supply for lower levels of
output voltages (up to 50%), we greatly reduce waste voltage and power for lower level signals.
Of course, the big problem is to switch over smoothly between high an low power stages. To assist this, especially
at high frequencies, we have a speed-up network, R23 and C17, which makes Q3 switch sooner at very high
frequencies, and coil, L1, between the low and high voltage filter capacitors to slightly "cushion" the transition.
One problem with the original scheme is that the zener voltage (about 3.9V) is added to the overall saturation loss,
which reduces peak output voltage by this amount. A second-generation circuit uses a small transistor after the
zener to isolate the loading effect of Q3 and Q4. This permits full signal swing, which improves dynamic headroom
somewhat.
5. Complementary Output Transistors
The emitters of the positive and negative output transistors are connected to their respective power supply rails
through load-sharing emitter resistors. The parallel bases of each bank are bypassed with 22 ohm resistors, R15,
R16, R17 and R18, which assure positive shut-off of the outputs. The 500 mA current from the collectors of the
driver transistors are connected to the output transistor bases and amplified to about 25 A peak.
Here you will see another unique feature of the QSC circuit. Note that the collectors of all output transistors meet at
a common ground. (This is possible because of the special power supply arrangement, as described in Section 1.)
This means we can simply screw the cases of all four banks to a grounded heat sink, with no need for the usual
insulating mica. This saves money and improves reliability through better cooling.
As mentioned in Sections 1 and 4, the speaker output is taken from the midpoint of the power supply capacitors.
This means that the audio output voltage is superimposed on top of the DC supply voltages, which must be kept in
mind when checking these voltages.
6. Protection and Signal Readout Circuits
A small, independent –24V power supply is tapped off the low voltage transformer secondary through diode D4 and
(in later versions) dropping resistor R32, and filtered by E5. A high current relay is put in series with the speaker
output, which is switched by small transistors Q17 and Q18. Note that the relay connects the load to ground when
not activated. This not only helps protect the load from DC fault, but enables the "bridged mono auto back-up"
feature (see Section 7).
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