Kurzweil K2000 - K2000 Audio;Power Board Theory - Part 8

SECTION 5
A1JDPWR3. DOC 12/22/'
The return signal from the volume slider varies from 0% to 100% of the signal driven to the
slider by Ul. The final mix output amplifier for the Right Channel is U33-1 (G-6) which simply
inverts the slider's signal at unity gain. If no plug is inserted into the Right mix output jack
(JI0, I-6), the Right output signal is also sent to the Left output amplifier to be mixed with
the Left signal from the volume slider. This provides automatic mono output through the Left
output jack when no Right channel cable is used. Note the Right signal must be mixed without
inversion into the Left output amplifier to keep phasing correct in the mono signal. The 600 ohm
source impedance and muting arrangement for the mix outputs is the same as for the separate
outputs described elsewhere.
5.1.3.8 HEADPHONE AMPLIFIERS
To
drive the headphones adequately, a non-inverting gain of 2 amplifier with a power booster
stage is used. Using the Left channel as an example, op-amp U5-1 (F-6) provides the voltage gain
and Q2-Q3 provide current gain to drive low-impedance headphones. R91, between the bases and
emitters, allows the op-amp to drive the load directly for currents less than 6mA peaks and
eliminates crossover distortion. R160 (H-7) protects the transistors from user overload caused
by a short-circuited cable or driving a too great load.
It is possible for the headphone amplifier to clip at high volume slider settings. This occurs
at about 10 volts peak for typical headphone loads and would normally be extremely loud. This
clipping will not affect any other audio outputs.
Muting of the headphones is accomplished by Q4 and Q7 (F-6) by shorting the op-amp's (U5)
outputs. If the Audio/Power Board is operated with a signal but the mute is activated for an
extended time (such as on the test bench), U5 will become quite warm, but will not be damaged.
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