Crown POWER BASE 1 Service Manual download pdf (Page 9)

Power Base-3 & 1400CSL Amplifier Service Manual

Theory

output of the amplifier at 26 dB. Diodes 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.

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.

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

transistors. A nested NFb loop from the output 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 voltage

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 ODEP

limiting transistors and control/protection transistors.

The ODEP transistors steal drive as dictated by the

ODEP circuitry (discussed later). The control/protec-

tion transistors act as switches to totally shunt audio to

ground during the turn-on delay, or during a DC/LF or

Fault protective action.

Last Voltage Amplifiers (LVAs)

The Voltage Translator stage channels the signal to

the Last Voltage Amplifiers (LVA's) in a balanced

configuration. The +LVA and -LVA, with their push-pull

effect through the Bias Servo, drive the fully comple-

mentary 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

polarity inversion from input jack to output jack, and it

allows the NFb loop to control Error Amp gain by

feeding back to its non-inverting input (with its polarity

opposite to the output of the VGS). With the added

voltage swing provided by the LVAs, the signal then

gains current amplification through the Darlington

emitter-follower output stage.

GROUNDED BRIDGE TOPOLOGY

Figure 2 is a simplified example of the grounded

bridge output topology. It consists of four quadrants

of three deep Darlington (composite) emitter-follower

stages per channel: one NPN and one PNP on the

High Side of the bridge (driving the load), and one

NPN and one PNP on the Low Side of the bridge

(controlling the ground reference for the rails). The

output stages are biased to operate class AB+B for

BGS VGS Error

Amp

Audio

Inputs

Voltage Divider

NFb Loop

ODEP

Mute

+15V

-15V

+VCC

-VCC

NPN Outputs (+HS)

PNP Outputs (-HS)

Q100

Q103

Q121

Q122

Q101

Q102

Q105

Q110

Voltage

Translators

LVA's

Figure 1. Typical Amplifier Front End and Voltage Amplification Stages.

1 2 3 4 5 6 7 8 9 10 11 12 13 14 ... 26 27

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