Model Overview

No.26 LINE STAGE

Production: 1995-1998

Design: Reference line-level preamplifier with fully balanced circuitry, microprocessor control, and dual-mono construction.

Key Features:

6 line-level inputs (balanced & single-ended)
Fully balanced signal path
Microprocessor-controlled relay switching
Dual-mono power supplies
No phono stage included

No.28 LINE + PHONO

Production: 1996-1999

Design: Reference preamplifier with integrated phono stage, sharing the No.26's line stage design with added phono circuitry.

Key Features:

4 line inputs + 2 phono inputs (MM/MC)
Discrete phono stage with RIAA equalization
Selectable loading for MC cartridges
Same control system as No.26
Additional power supply for phono stage

Shared Architecture: Both models share identical line stage circuitry, control systems, power supply topology (excluding phono supply), and construction. The No.28 adds a sophisticated discrete phono stage with its own regulated power supply.

Technical Specifications

Parameter	No.26 Specification	No.28 Specification	Notes
Frequency Response	10Hz-100kHz ±0.1dB	10Hz-100kHz ±0.1dB RIAA: ±0.25dB	Both: 20Hz-20kHz ±0.05dB
Total Harmonic Distortion	<0.005% @ 2V out	<0.005% (line) <0.01% (phono)	20Hz-20kHz, typical
Signal-to-Noise Ratio	>110dB (A-weighted)	>110dB (line) >90dB (phono MM)	Reference: 2V output
Input Sensitivity	250mV for 2V out	250mV (line) 0.5mV (phono MC)	Gain: 18dB (line), 60dB (phono)
Output Capability	20V RMS balanced	20V RMS balanced	Into 600Ω load
Channel Separation	>100dB @ 1kHz	>100dB @ 1kHz	Dual-mono design

Common Failure Modes

Power Supply Capacitors

Failure Rate: 85%

Main filter caps develop high ESR. Regulator output caps leak. Digital supply caps fail causing microprocessor issues.

Control Microprocessor

Failure Rate: 70%

EEPROM corruption loses settings. Clock crystal failure. Reset circuit issues causing lockup.

Signal Relays

Failure Rate: 65%

Gold contacts oxidize causing intermittent connections. Coils weaken over time. Arcing damage from switching.

Volume Control System

Failure Rate: 60%

Relay-based attenuator failures. Resistor ladder drift. Position sensor degradation.

No.28 Specific Issues

1

Phono Stage Capacitors

RIAA equalization capacitors drift causing frequency response errors. Coupling caps leak DC.

2

Input JFETs

2SK170/2SJ74 pairs degrade over time. Noise increases. Idss matching drifts causing imbalance.

3

Loading Selector

MM/MC loading switch contacts oxidize. Resistor networks drift from specified values.

Power Supply Restoration

Shared Power Supply Components

Component	Location	Original Value	Modern Replacement	Critical Specs
Main Filter Caps	C101, C102	4,700µF 35V	Nichicon KG Gold Tune	Low ESR, audio grade
Analog Regulators	C201-C208	1,000µF 25V	Elna Silmic II	Low leakage, 105°C
Digital Supply	C301-C304	470µF 16V	Panasonic FC	Low ESR, long life
Display Supply	C401, C402	220µF 50V	Nichicon UPW	105°C, low impedance

No.28 Additional Power Supply

Component	Function	Original Value	Replacement	Notes
Phono Filter Caps	Main phono supply	2,200µF 50V (x2)	Nichicon KZ	Critical for low noise
Phono Regulators	±24V regulation	1,000µF 35V (x4)	Elna Silmic II	Audio grade required
RIAA Supply	Equalization stage	470µF 63V (x4)	Nichicon FG	Low leakage critical

Audio Stage Components

Line Stage (Both Models)

Input Stage

JFET differential pairs
2SK170/2SJ74 matched
Idss: 6-12mA range
Modern: LSK170/LSJ74

Voltage Gain

Discrete bipolar design
2SC2240/2SA970
Cascode configuration
Modern: KSC1845/KSA992

Output Buffer

Complementary emitter followers
MJE15032/MJE15033
High current capability
Still available

Current Sources

JFET based
2N5564 or similar
Low noise design
Modern: LSK489

No.28 Phono Stage Specifics

Stage	Components	Critical Values	Replacement Notes
Input	2SK170 BL grade (x4) Matched for noise	Idss: 8-12mA matched Vgs: -0.5 to -1.5V	Use LSK170 grade B Match within 5%
RIAA Network	Polystyrene capacitors Metal film resistors	Capacitors: ±1% Resistors: ±0.5%	Use polypropylene if polystyrene unavailable
Gain Stage	2SC2240/2SA970 Cascode configuration	High hFE: 400-800 Low noise: < 2nV/√Hz	KSC1845/KSA992 Match pairs
Loading Network	Metal film resistors Rotary switches	MM: 47kΩ MC: 100Ω, 470Ω, etc.	Use low-noise metal film Clean switch contacts

Control System Restoration

Microcontroller

Type: 80C51 family
Clock: 12MHz crystal
EEPROM: External 24C series
Memory: 2K-8K bytes

Volume Control

Type: Relay-based R-2R
Relays: 20-30 signal relays
Resolution: 0.5dB steps
Range: -90dB to +18dB

Input Selection

Relays: Gold-contact signal type
Control: Microprocessor driven
Isolation: Optical or relay

Display System

Type: Vacuum fluorescent
Supply: -27V regulated
Driver: HV driver IC
Interface: Serial or parallel

Common Control System Issues

1

EEPROM Corruption

Settings lost after power cycle. Requires EEPROM replacement or microcontroller reprogramming.

2

Volume Relay Failure

Relay contacts oxidize causing intermittent operation. Requires complete relay replacement.

3

Display Issues

Vacuum fluorescent display dims or loses segments. Requires -27V supply check or display replacement.

Calibration Procedures

Required Test Equipment

True RMS multimeter (4.5 digit minimum)
Audio analyzer or low distortion generator
Oscilloscope (50MHz minimum)
Dummy loads: 10kΩ, 600Ω
Inverse RIAA network (No.28)
DC power supply for board testing

Calibration Steps for Both Models

Step	Procedure	Target Value	Notes
1. Power Supply	Verify all regulator outputs	±15.00V ±0.1V +5.00V ±0.05V -27.0V ±0.5V	Ripple < 10mV p-p
2. DC Offset	Adjust for minimum offset	< 5mV all outputs	Both balanced & SE
3. Volume Calibration	Verify attenuation accuracy	±0.5dB steps	All volume settings
4. Channel Balance	Check all inputs/outputs	< 0.5dB mismatch	Multiple volume levels

No.28 Additional Calibration

1

RIAA Equalization

Use inverse RIAA network to verify frequency response within ±0.25dB 20Hz-20kHz.

2

Phono Gain

Verify gain: 40dB (MM), 60dB (MC) with appropriate loading resistors.

3

Loading Selection

Test all loading positions for proper resistance values and clean switching.

Troubleshooting Guide

Symptom	No.26 Likely Causes	No.28 Likely Causes	Diagnostic Steps
No power, no display	Main fuse blown Transformer failure Power switch failure	Same as No.26 Phono supply short Additional fuse	Check primary fuse Measure transformer secondary Test power switch continuity
Display works, no audio	Mute relay failure Input selector issue Volume control failure	Same as No.26 Phono/line selector Input overload protection	Test mute relay operation Check input selector relays Verify volume control signals
Intermittent operation	Loose connections Failing relay contacts Microprocessor reset	Same as No.26 Phono stage connectors Loading switch contacts	Check all connectors Test relay contact resistance Monitor reset circuit
Excessive noise/hum	Power supply caps Grounding issues Input stage transistors	Same as No.26 Phono supply ripple Phono input JFETs	Measure supply ripple Check ground continuity Test input devices
Phono stage issues (No.28)	N/A	RIAA cap drift JFET degradation Loading switch oxidation	Measure RIAA response Test JFET parameters Clean loading switch

Replacement Parts Guide

Critical Replacement Components

Component Type	No.26 Part Numbers	No.28 Part Numbers	Modern Equivalents
Microcontroller	80C51 or similar	Same as No.26	AT89C51 or reprogram original
Signal Relays	Omron G6K or similar	Same as No.26	G6K-2P-DC5 (5V coil)
Volume Control Relays	Signal relay type	Same as No.26	Takamisawa RY-S or equivalent
Display	VFD, custom part	Same as No.26	Must match original Check Noritake or Futaba
Phono JFETs (No.28)	N/A	2SK170/2SJ74	LSK170/LSJ74 (Linear Systems)
RIAA Caps (No.28)	N/A	Polystyrene ±1%	WIMA FKP2 or polystyrene

Important Note: Many components in these preamplifiers are custom or selected for specific parameters. When replacing semiconductors, always match original specifications carefully. For the microprocessor, programming files may be required from Madrigal/Levinson or must be extracted from a working unit.

Experts on Mark Levinson No.26 & No.28 Preamplifier repairs

Experts Reference Stereo Preamplifiers Technical Service