Crown SST-SBSC Specifications download pdf (Page 24)

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Reference Manual

CE 4000 The amp with an Attitude!CE 4000 The amp with an Attitude!

Principles of Operation

Audio Signal Path

For the sake of simplicity, only channel one of the audio signal

path is described.

Signal is presented to the CE 4000 through one of three con-

nectors when using the standard input module. Each channel

is outfitted with a balanced XLR / phone jack, and a barrier

strip. These connectors are wired in parallel, which allows

daisy chaining when needed. The signal is then converted

from balanced to unbalanced in the Balanced Input Stage

where it also receives RFI protection. Signal then flows into the

Variable Gain Stage where the front panel level controls are

allowed to affect the gain.

Following this stage, the signal goes through a gain stage that

allows for the various positions of the sensitivity switch. The

signal is then put under the control of a full-time compressor

circuit comprised of a symmetrical window detector, a buffer

amplifier, and the gating op amp which uses several small

components to set the compressor’s attack and decay char-

acteristics. The actual compressing is accomplished by an

opto-isolator that affects the gain in the signal path.

The signal then is passed through a series of switchable filters

that allow the signal to be low-pass and high-pass filtered at

various frequencies. The switches are located at the back

panel. The low-pass filter is a fourth-order Linkwitz-Riley type

(24-dB/octave rolloff) and the high-pass filter is a third-order

Butterworth type (18-dB/octave rolloff).

Following the switchable filters, the signal enters a 32 kHz 7

Order Gaussian Low-Pass Filter. This filter prevents the modu-

lator stage and the output filter (both described below) from

receiving signals that are too high. Without the 32 kHz filter,

the modulator would be unable to process signals that are too

high and the output filter would not yield the proper frequency

response behavior. The Gaussian filter type is unique in that it

has minimal ringing and excellent phase response so even a

high-order filter such as this one does not adversely affect the

sonic excellence of the product.

The signal next enters the main amplifier error amp where it is

mixed with a small portion of the output voltage and current in

such a way as to control the amplifier’s overall output perfor-

mance. Warning: Details of closed loop amplifier design are

beyond the scope of this description and if discussed, would

surely put most readers to sleep!

Following the error amp is the modulator stage where the au-

dio signal is compared to an extremely accurate 250 kHz tri-

angle waveform. Comparators output a Pulse Width Modulated

(PWM) string of pulses at 250 kHz that vary in width depend-

ing on the level of the input signal. These strings of pulses, one

for the positive side and one for the negative side, are con-

nected to the output stage via optocouplers.

The signals from the optos are then passed to gate drivers that

amplify the pulses to the level required to drive output devices.

The driven ou

tput devices are now able to produce PWM pulses that have

an output voltage from the negative high-voltage rail (-Vcc) to

the positive high-voltage rail (+Vcc). This output voltage is

always the same (2 * Vcc) but the width of the pulses is still

dependent on the level of the input signal. The positive and

negative output PWM pulses then pass through inductors and

are summed together. Summing the output signals through

inductors reconstructs the audio signal, amplified to the de-

sired level. There is a small amount of ripple on the output that

is at double the switching frequency (500 kHz).

The amplified audio signal is then passed through an output

filter that removes the residual ripple voltage. The power is

delivered to the load cables through the output connector

panel which consists of one of several options.

Protection for the output devices is performed by a very pre-

cise pulse-by-pulse current limiter circuit that operates each

time the output devices switch. The current limiting is “flat”

meaning that, regardless of the output voltage, the output cur-

rent always limits at a certain value.

The turn-on delay circuitry functions to keep the modulators

turned off (which keeps the outputs from switching) until all

supplies are up and stable.

Thermal probes monitor Heatsink temperatures and power

transformer temperature. As the temperatures rise, the probes

send a proportional voltage to the fan control circuit and the

Thermal Limit Control (TLC) circuit. The fan normally runs at

very low speed when the amplifier is idling or when it is being

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Crown SST-SBSC Specifications Page 24

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