Kurzweil K2000 Service Manual page 101

SECTION 5
AUDPWRJ.DOC
12122/92
5.1.3.5 MUTING CIRCUITS
During power-up and power-down, muting circuits on all audio outputs are asserted to prevent
transient noises and damage to speakers and human ears. The muting circuit description provided
here applies to all other outputs as well.
Using the Qll circuitry as an example (I-I), this JI09 N-channel JFET is either ON or OFF. When
OFF, it is not conducting, thus any audio present passes through R163 and out Jll. When ON this
JFET shorts the output line to ground, thus muting the circuit. To control this function take
note -UNMUTE arriving in the upper left corner of the schematic (A-I), being delivered from the
collector of Q23 (H-7) on schematic page 2. This signal, when high, cuts off Q11, and all other
JFETs so connected and allows the passage of audio. When low, the JFETS are ON and the mutes are
asserted.
5.1.3.6 AUDIO SWITCHING MATRIX
The scanner microprocessor has direct, individual control over the 6 analog switches and pairwise
control of the 4 variable gain amplifiers. Five of the analog switches (ASW 1, ASW 3 - ASW 6)
are in audio signal paths while the sixth is in a gain control signal path (AsW_2).- -
Note: ASW 1 and ASW 2 audio switching controls are drawn on schematic page 4.
Refer to fIgure 5.1.3.1 below for a simplified detail of the Audio Switching Matrix.
"" ...
---~------l
fA"'" .
~
~l
....... _..J...l!t.. _ _
-+ ___
...oj.
(
...... ,
(:'.::rl-----------..;...---''----'
Figure 5.1.3.1 - Audio switching matrix
In all of the audio path switching cases, the switch is connected to the summing junction of a n
inverting op-amp with the input resistor at the input of the switch. Signal current through the
input resistor either enters the summing junction when the switch is on or is diverted to ground
when the switch is off. This makes each switch a current-mode switch which allows it to block
large voltages when off and avoid large voltage swings when on. This minimizes distortion of the
signal and crosstalk. It also allows the switch to handle signal voltages greater than its own
power voltage. A simple detail of the Analog Switch follows in figure 5.1.3.2.
'NPVT
CONTR~L
ll"'AlO~
SWITCH
15K
1--- ---,
IS'K :
:NO
~
IN:
I
.NC
•• _ _ _ _
,~I-J
,
,--- ---'
>---..
OVTPVT
•
Figure 5.1.3.2 - Analog switch connection for audio signals
Note that the IN and NC terminals are connected when the control voltage is low (switch off). IN
and NO are connected when the control voltage is high (switch on).
5.1.3.7 MIX OUTPUT AMPLIFIERS
The two sections of U1 (C-6 to C-7) do the final mixing of audio signals from the switching
matrix and Effects Unit. Each sums 5 signals from various sources into one. A low noise
amplifier is used here because the "noise gain" of the circuit is 5 even though the effective
gain for anyone signal is unity. The 100pF capacitors across the feed back resistors compensate
for stray capacitance at the large summing junction and also the finite load capacitance
presented by the cable to the volume slider. The signal level going into the volume slider is 6
volts peak if one of the mixed sources should reach peaks near 6 volts simultaneously.
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