Power Amplifier Stage#

The power amplifier converts the low-level preamp signal into high-current drive for speakers. The MRP-F250 uses a Class AB topology with discrete transistors for each of its four channels.

Architecture#

Each channel is identical in design. The schematic shows channels 1-4 with components numbered in the 100s, 200s, 300s, and 400s respectively.

Stage-by-Stage Analysis#

Input Stage (Q151)#

Transistor: 2SC3326 (NPN)

The input stage is a common-emitter amplifier that:

• Accepts the signal from the preamp output buffer
• Provides initial voltage gain
• Sets the input impedance

Key Components:

• R151: 100K input resistor
• C151: 10µF coupling capacitor
• D151: DAN217 protection diode

Mute Stage (Q152)#

Transistor: 2SC4207 (NPN)

The mute transistor shunts the signal to ground when protection circuits activate:

• When muted: Q152 conducts, shorting the signal path
• When unmuted: Q152 is off, signal passes through

Voltage Amplifier Stage (Q153-Q158)#

This multi-transistor stage provides the bulk of the voltage gain.

Bias Network:

• R154, R155, R156: Set operating points
• C152-C154: Compensation capacitors (22pF, 47pF)

Driver Stage (Q159, Q160)#

Transistors:

• Q159: 2SC2235 (NPN) - Drives positive output transistor
• Q160: 2SA965 (PNP) - Drives negative output transistor

The driver stage:

• Provides current gain to drive the output transistors
• Operates in Class AB with slight overlap
• Powered from ±25V rails

Output Stage (Q161, Q162)#

Transistors:

• Q161: 2SC5100 (NPN) - Positive half-cycle
• Q162: 2SA1908 (PNP) - Negative half-cycle

The output stage:

• Complementary emitter-follower configuration
• Powered from ±23V rails
• Current sensing via emitter resistors (R168, R169 = 2.2Ω)

Output Current Detection:

• Q163 (2SC2412K) monitors output current
• Feeds back to protection circuit

Transistor Inventory (Per Channel)#

Note: Channels 2, 3, 4 use identical transistors with 2xx, 3xx, 4xx numbering.

Normal Operating Voltages#

From service manual page 12, with 14.4V supply, Remote ON, no signal:

Common Power Amp Failures#

Output Transistor Failure (Q161/Q162)#

Symptoms:

• Blown fuses
• DC on speaker output
• Smoke from output stage area

Common Causes:

• Speaker short circuit
• Operating into too-low impedance
• Thermal overload

Test: Check E-C and B-C junctions with diode mode

Driver Transistor Failure (Q159/Q160)#

Symptoms:

• Distorted output
• One half of waveform missing
• Can cause output transistor failure

Test: Check all junctions, compare to known-good channel

DC Offset at Output#

Symptoms:

• Speaker cone pushes in or out at idle
• Excessive heat in output stage
• Protection circuit may trigger

Causes:

• Mismatched or failed transistor in differential pair
• Failed bias components
• DC contamination from preamp

Test: Measure DC voltage at speaker output (should be <50mV)