Crown Com-Tech 1610, Service Manual

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Com-Tech 810 and 1610 Amplifier Service Manual
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5-2
Rev. 0
fed to its op-amp stage. Because gain after this stage
is fixed at 26 dB (factor of 20), greater amplifier sensi-
tivity is achieved by controlling the ratio of feedback
to input resistance. The Sensitivity Switch sets the in-
put impedance to this stage and varies the gain such
that the overall amplifier gain is 26 dB, or is adjusted
appropriately for 0.775V input to attain rated output.
5.2.3 Error Amp
The inverted output from the VGS is fed to the non-
inverting input of the Error Amp op-amp stage through
an AC coupling capacitor (C107) and input resistor
(R113). Amplifier output is fed back via the negative
feedback (NFb) loop resistor (R114). The ratio of feed-
back resistor to input resistor fixes gain from the Error
Amp input to the output of the amplifier at 26 dB. Di-
odes prevent overdriving the Error Amp. Because the
Error Amp amplifies the difference between input and
output signals, any difference in the two waveforms
will produce a near open loop gain condition, which in
turn results in high peak output voltage. The output of
the Error Amp, called the Error Signal (ES) drives the
Voltage Translators.
5.3 Voltage Amplification
The Voltage Translator stage separates the output of
the Error Amp into balanced positive and negative
drive voltages for the Last Voltage Amplifiers (LVAs),
translating the signal from ground referenced ±15V to
±Vcc reference. LVAs provide the main voltage ampli-
fication and drive the High Side output stages. Gain
from Voltage Translator input to amplifier output is a
factor of 25.2.
5.3.1 Voltage Translators
A voltage divider network splits the Error Signal (ES)
into positive and negative drive signals for the bal-
anced voltage translator stage. These offset reference
voltages drive the input to the Voltage Translator tran-
sistors (Q104/Q105). A nested NFb loop from the out-
put of the amplifier mixes with the inverted signal riding
on the offset references. This negative feedback fixes
gain at the offset reference points (and the output of
the Error Amp) at a factor of -25.2 with respect to the
amplifier output. The Voltage Translators are arranged
in a common base configuration for non-inverting volt-
age gain with equal gain. They shift the audio from the
±15V reference to VCC reference. Their outputs drive
their respective LVA.
Also tied into the Voltage Translator inputs are the lim-
iting transistors (Q102/Q103). These transistors steal
drive as dictated by the
ODEP circuitry (discussed
later), or act as switches totally shunting audio to
ground during the turn-on delay, or during a DC/LF or
Fault protective action.
5.3.2 Last Voltage Amplifiers (LVAs)
The Voltage Translator stage channels the signal to the
Last Voltage Amplifiers (LVAs) in a balanced configu-
ration. The +LVA (Q106) and –LVA (Q109), with their
push-pull effect through the Bias Servo, drive the fully
complementary output stage. The LVAs are configured
as common emitter amplifiers. This configuration pro-
vides sufficient voltage gain and inverts the audio. The
polarity inversion is necessary to avoid an overall po-
larity inversion from input jack to output jack, and it
allows the NFb loop to control Error Amp gain by feed-
ing back to its non-inverting input (with its polarity op-
posite to the output of the VGS). With the added volt-
age swing provided by the LVAs, the signal then gains
Figure 5.1 Typical Amplifier Front End and Voltage Amplification Stages.
+
-
+
-
+
-
Audio
Inputs
BGS VGS Error
Amp
+15V
-15V
NFb Loop
Q102
Q103
Q104
Q105
Q106
Q109
+VCC
-VCC
NPN Outputs (+HS)
PNP Outputs (-HS)
Mute
ODEP
+
Voltage
Translators
LVA's
_
Voltage Divider