Crown Audio 800CSL service manual Theory, Voltage Amplification, Voltage Translators, Error Amp

Power Base-2 & 800CSL Amplifier Service Manual

Theory

tivity is achieved by controlling the ratio of feedback to	Voltage Translators
input resistance. The Sensitivity Switch sets the input	A voltage divider network splits the Error Signal (ES)
impedance to this stage and varies the gain such that	into positive and negative drive signals for the bal-
the overall amplifier gain is 26 dB, or is adjusted	anced voltage translator stage. These offset reference
appropriately for 0.775V or 1.4V input to attain rated	voltages drive the input to the Voltage Translator
output. Note that earlier models (before main modules	transistors. A nested NFb loop from the output of the
built with D 7911-7 board) did not support the 1.4V	amplifier mixes with the inverted signal riding on the
sensitivity.	offset references. This negative feedback fixes gain at
	the offset reference points (and the output of the Error
Error Amp	Amp) at a factor of -25.2 with respect to the amplifier
The inverted output from the VGS is fed to the non-	output. The Voltage Translators are arranged in a
inverting input of the Error Amp op-amp stage through	common base configuration for non-inverting voltage
an AC coupling capacitor and input resistor. Amplifier	gain with equal gain. They shift the audio from the
output is fed back via the negative feedback (NFb)	±15V reference to VCC reference. Their outputs drive
loop resistor. The ratio of feedback resistor to input	their respective LVA.
resistor fixes gain from the Error Amp input to the
output of the amplifier at 26 dB. Diodes prevent	Also tied into the Voltage Translator inputs are ODEP
overdriving the Error Amp. Because the Error Amp	limiting transistors and control/protection transistors.
amplifies the difference between input and output	The ODEP transistors steal drive as dictated by the
signals, any difference in the two waveforms will	ODEP circuitry (discussed later). The control/protec-
produce a near open loop gain condition which in turn	tion transistors act as switches to totally shunt audio to
results in high peak output voltage. The output of the	ground during the turn-on delay, or during a DC/LF or
Error Amp, called the Error Signal (ES) drives the	Fault protective action.
Voltage Translators.

VOLTAGE AMPLIFICATION							Last Voltage Amplifiers (LVAs)
							The Voltage Translator stage channels the signal to
The Voltage Translator stage separates the output of							the Last Voltage Amplifiers (LVA's) in a balanced
the Error Amp into balanced positive and negative							configuration. The +LVA and -LVA, with their push-pull
drive voltages for the Last Voltage Amplifiers (LVAs),							effect through the Bias Servo, drive the fully comple-
translating the signal from ground referenced ±15V to							mentary output stage. The LVAs are configured as
±Vcc reference. LVAs provide the main voltage ampli-							common emitter amplifiers. This configuration pro-
fication and drive the High Side output stages. Gain							vides sufficient voltage gain and inverts the audio. The
from Voltage Translator input to amplifier output is a							polarity inversion is necessary to avoid an overall
factor of 25.2.							polarity inversion from input jack to output jack, and it
	BGS	VGS	Error	+15V						Voltage	+VCC
										Translators
Audio	+	+	Amp	Divider							NPN Outputs (+HS)
			+
								Q100	Q121	Q101	Q105
Inputs				Voltage						Q102
	-	-	-					Q103			PNP Outputs (-HS)
									Q122		Q110
						- +		Mute			-VCC
				-15V	ODEP
								NFb Loop			LVA's
		Figure 1. Typical Amplifier Front End and Voltage Amplification Stages.

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