Pioneer M-AX10 Service Manual page 57

Descriptions of the Block Diagram
1. INPUT switching block
Four small-signal relays for individual channels are used for input
signal switching.
2. 12-dB flat amplifier block
The input signal is amplified by 12 dB (4 times) to increase the
input level at the DAC attenuator block to improve the dynamic
range and signal-to-noise ratio.
In DUAL or BRIDGE mode, the flat amplifier on the low-channel
side is shut down by the LOCAL CIRCUIT OFF circuit.
The circuits are mounted in a shielded box to completely eliminate
electrostatic and magnetic coupling.
3. DAC attenuator block
Attenuator section
This is a digitally controlled high-precision analog attenuator that
uses a ladder resistance block of R-2R-type DAC ICs for volume
control. The signal is input to the Vref (reference-voltage) terminal
and the attenuated signal current is obtained from the Iout (current-
output) terminal. The signal at this terminal is converted to a voltage
signal by an I-V converter of the operational amplifier. An output
signal of the same phase as the input signal is obtained through
inversion by the operational amplifier. The attenuation volume
adjustment is controlled using serial data from the microcomputer.
Control section
A 5-bit (31-position) digital rotary switch is used. The rotation angle
and information can be sent to the microcomputer, enabling the silent
function (Low-Power-Consumption mode) of the microcomputer.
Two DAC ICs are used for two individual channels. There is no
mechanical contact, resulting in fewer errors in attenuation volume
and gang errors within 0.1 dB at any position. The attenuation volume
can be flexibly adjusted by the microcomputer software.
With this product, it can be set in 2-dB steps (up to -58 dB) in
BRIDGE mode and in 0.5 dB steps (up to -15 dB) in SEPARATE
mode. When this product is used in combination with a preamplifier,
you can bypass the DAC attenuator block using the ATT.THROUGH
function. For this bypass switching, the same small-signal relay as
with the INPUT switching clock is used.
4. 14-dB power amplifier block
Pioneer's original "Direct-Energy MOSFET Amplifier" and "Wide-
Range Linear Circuits" are mounted. The circuits function as the
inversion/noninversion switching amplifier on the high-channel side
and as an inversion amplifier in BRIDGE mode.
This eliminates the necessity for an inversion amplifier otherwise
essential for a bridge amplifier.
5. Protection circuit
When a DC voltage at the output end or a short circuit at the load
end is detected, hardware muting is activated and muting and output
relays are controlled by the microcomputer.
When a short circuit at the load end is detected, a resistance inserted
between the MOSFET drain power sources is used (for current
detection) in place of a bridge detection circuit, a device greatly
affected by the phase difference between the output power voltage
and current.
Thus, possible malfunction of the protection circuit in practical use
is prevented.
6. Power block
A large (450-VA) Super Ring is employed for the transformer.
The second coils are separated into three coil windings for left
channel, right channel, and a subcircuit.
For the power amplifier, a full-wave voltage doubler rectifier system
is used to suppress high-frequency current.
For the voltage amplifier, a half-wave voltage doubler rectifier
system is used to obtain the specified voltage without using additional
coils. Each signal stage has a local stabilized power source, and the
lamps are driven at a constant current for low power consumption.
Level Diagram
12dB
ATT through
Input 1, 2
Input high, low
Input Buffer
Amp.
0dB (-58dB MIN)
40dBV
(100Vrms)
30dBV
Maximum
permission input level
20dBV
with attenuator
(10Vrms)
(4Vrms)
10dBV
1Vrms
0dBV
(1Vrms)
-10dBV
M-AX10
14dB Dual, Separate
20dB Bridge
Speaker Out
Power
Amp.
ATT ON
DAC VR
40Vrms, 32dBV
Bridge
20Vrms, 26dBV
Dual Separate
4Vrms, 12dBV
57