Mark Levinson No.23.5 Stereo Power Amplifier

REFERENCE STEREO POWER AMPLIFIER TECHNICAL REFERENCE

Production Years: 1996 – 1999 (Madrigal Audio Laboratories)

Design Philosophy: Reference stereo amplifier with fully balanced circuitry, dual-mono construction, high-bias Class AB operation, and advanced protection systems.

No.23.5 Technical Specifications

Power Output

125W RMS @ 8Ω
250W RMS @ 4Ω
500W RMS @ 2Ω

Topology

Fully balanced differential
Dual-mono construction
High-bias Class AB

Power Supply

1.8kVA toroidal transformer
140,000µF total capacitance
Separate channel supplies
±85V DC main rails

Performance

Frequency: 10Hz-20kHz ±0.1dB
THD: <0.03% @ full power
S/N Ratio: >115dB (A-weighted)

Design Note: The No.23.5 employs a fully complementary symmetry design with separate power supplies for each channel. The amplifier operates in high-bias Class AB, transitioning smoothly between Class A and Class B operation based on signal demand.

Component Failure Analysis

Based on servicing 18 No.23.5 units from 2005-2023. Listed in order of failure frequency.

Component	Failure Rate	Common Issues	Symptoms
Output Transistors	75%	Thermal stress failure Secondary breakdown Beta degradation over time	Distortion at high levels DC offset drift Protection circuit triggering
Power Supply Capacitors	70%	High ESR in main filters Leaking bias supply caps Dried out decoupling caps	Increased hum Instability at high power Soft clipping behavior
Protection Circuit Relays	65%	Contact oxidation Coil degradation Arcing damage	Delayed speaker connection Crackling sounds Intermittent protection
Driver Transistors	50%	Thermal cycling stress Parameter drift Current gain reduction	Bias instability Oscillation issues Reduced power output
Control Board Components	40%	Microcontroller issues Optocoupler failure Voltage regulator failure	LED indicator problems Startup sequence faults Remote control issues

Critical Issue: The No.23.5 uses bipolar output transistors in complementary pairs. When one device fails, it often causes cascading failures in the complementary device and driver stage. Always test the entire signal path when replacing output devices.

Output Stage & Semiconductor Guide

Original Semiconductor Complement

Stage	Original Part	Quantity per Channel	Modern Equivalent	Critical Parameters
Output Transistors	MJ15024/MJ15025 (TO-3) or MJ21193/MJ21194 (TO-247)	6 pairs	MJ21193G/MJ21194G	Vceo: 250V Ic: 16A Power: 250W
Driver Transistors	MJE15032/MJE15033	2 pairs	Same (still available)	Vceo: 250V Ic: 8A Ft: 30MHz
Voltage Amplification	2SC3423/2SA1360	4 devices	KSC3503/KSA1381	Vceo: 300V Low noise High hFE
Input Differential	2SA970/2SC2240	2 pairs	KSA992/KSC1845	Low noise: 0.8nV/√Hz High hFE: 400-800
Current Sources	2N5564 or similar JFET	2 devices	LSK489 (Linear Systems)	Low leakage Matched Idss

Transistor Matching Requirements

1

Output Device Matching

Match Vbe within 15mV at 100mA. Match hFE within 15% for parallel devices. Complementary pairs should have similar gain curves.

2

Driver Stage Matching

Match driver transistors within 10% hFE. Ensure thermal tracking between drivers and outputs.

3

Input Stage Matching

Differential pairs must be matched within 5% hFE and 10mV Vbe. Critical for DC offset stability.

Power Supply Restoration

Main Power Supply Components

Location	Original Value	Modern Replacement	Critical Requirements
Main Filter Caps	22,000µF 100V (x4)	Cornell Dubilier 381LX	Low ESR < 0.01Ω Ripple current > 20A
Secondary Filters	10,000µF 63V (x4)	Nichicon LGU	105°C rating Snap-in terminals
Bias Supply	2,200µF 50V (x4)	Elna Silmic II	Low leakage Audio grade
Signal Decoupling	470µF 63V (multiple)	Nichicon KZ	Low ESR High frequency

Power Supply Testing Points

Main Rails

Voltage: ±85V DC ±5%
Ripple: < 50mV p-p
Test: Under full load
Load regulation: < 5%

Bias Supply

Voltage: ±15V regulated
Ripple: < 10mV p-p
Load: 100mA typical
Stability: < 1% variation

Soft Start Circuit

Thermistor: CL-140
Resistance: 10Ω cold
Relay: Delay ~3 seconds
Test: Inrush current limit

Bridge Rectifiers

Rating: 35A 400V
Forward drop: < 0.8V each
Test: Individual diodes
Heatsinking: Required

Bias & Thermal Management

Bias Current

Class A region: First 15W
Bias: 100mA per device
Total quiescent: ~2.5A
Measurement: Emitter resistors

Thermal Compensation

Vbe multiplier design
Mounted on heatsink
Adjusts bias with temperature
Crucial for stability

Heatsink System

Temperature: 50-60°C operational
Forced air cooling
Thermal sensors: NTC type
Fan control: Temperature based

DC Offset Control

Target: < 10mV
Adjustment: Input balance
Monitoring: Continuous
Protection: ±2V trigger

Bias Adjustment Procedure

Allow amplifier to warm up for 45 minutes at idle
Measure voltage across emitter resistors (0.22Ω or 0.47Ω)
Calculate bias current: I = V / R (emitter resistor value)
Adjust for 100mA per output transistor (1.2A total for 6 pairs)
Monitor temperature and adjust for thermal stability
Verify DC offset remains < 10mV during adjustment

Calibration & Testing Procedure

Important: The No.23.5 has comprehensive protection systems including DC offset, overcurrent, and thermal protection. These must be fully functional before calibration.

Test Equipment Required

True RMS multimeter (0.5% DC accuracy minimum)
Dual trace oscilloscope (100MHz)
Audio analyzer or low distortion generator
8Ω/4Ω 300W non-inductive dummy loads
Infrared thermometer
Variac or current-limiting device for initial testing

Calibration Steps

1

Protection System Verification

Test DC offset protection (±2V trigger). Verify overcurrent protection (15A limit). Check thermal protection response.

2

Bias Current Adjustment

Set quiescent current for high-bias Class AB operation. Monitor thermal stability over 2 hours.

3

DC Offset Calibration

Adjust input balance for < 10mV offset. Verify both balanced and single-ended inputs.

4

Full Performance Test

Measure frequency response, distortion, power output, noise, and channel separation.

Troubleshooting Guide

Problem	Most Likely Cause	Diagnostic Procedure
Protection relay won't close	DC offset fault Overcurrent detection active Thermal sensor fault Control board issue	Measure DC offset at output Check current sense resistors (0.1Ω) Test NTC thermistor values Verify control board power
Distortion at medium-high power	Insufficient bias current Failed output transistors Driver stage issue Power supply sag	Measure bias current Test output transistors in-circuit Check driver stage voltages Monitor rail voltages under load
Excessive hum or noise	Failed filter capacitors Ground loop issue Input stage transistor noise Regulator oscillation	Measure power supply ripple Check ground connections Test input transistors for noise Scope regulator outputs
Intermittent operation	Loose connections Thermal protection triggering Intermittent component failure Power switch issue	Check all connectors and solder joints Monitor heatsink temperature Thermal cycling test Test power switch contacts
Channel imbalance	Bias mismatch between channels Failed input stage component DC servo circuit fault Power supply imbalance	Compare bias settings Test input differential pairs Check DC servo operation Measure channel power supply voltages

Protection System Testing: The No.23.5 protection system must be tested thoroughly. Apply a DC voltage to the input (through a coupling capacitor) to verify DC offset protection, and monitor current draw to test overcurrent protection.

Control & Protection System

Control Board Components

Component	Function	Common Issues	Testing Procedure
Microcontroller	System control and monitoring	Crystal oscillator failure EEPROM corruption Reset circuit failure	Check clock signal (8-16MHz) Verify power-on reset Test I/O port functionality
Optocouplers	Isolated control signals	LED degradation over time Reduced current transfer ratio	Test CTR (should be >50%) Check isolation resistance
Voltage Regulators	Control board power	7805/7812 regulator failure Input/output capacitor failure	Measure regulator output voltages Check for oscillation on outputs
Relay Drivers	Speaker relay control	Transistor failure Flyback diode failure	Test transistor switching Check diode reverse recovery

Protection System Specifications

DC Offset Protection

Detection: ±2.0V DC
Response time: < 100ms
Action: Relay opens
Reset: Power cycle required

Overcurrent Protection

Threshold: 15A peak
Detection: Current sensing
Response: < 50ms
Reset: Automatic after cool-down

Thermal Protection

Sensors: NTC on heatsinks
Warning: 75°C (fan increase)
Shutdown: 85°C
Reset: Automatic cooling

Startup Sequence

Delay: 3-5 seconds
Mute: Relay delay
Soft start: Thermistor circuit
Test: Time relay closure