Sansui AU-70 Stereo Control Amplifier
Operating Instructions & Service Manual
This amplifier boasts a maximum output of 50 watts (25W-25W) and features an exceptionally low distortion factor, not exceeding 0.15% at 20W. It is a high-quality amplifier equipped with numerous advanced features, delivered with Sansui's utmost confidence. This manual details the necessary steps for operating and maintaining your new AU-70. Please read all instructions carefully and retain them for future reference.
Features
- 7189A P.P. Output: Produces 40W without distortion, with a maximum of 0.95% at 30 CPS and 0.15% at 1,000 CPS. Utilizes class AB, P.P. fixed bias for high-fidelity reproduction. Incorporates a multiplex feedback circuit (26 dB negative feedback) for low distortion across the frequency range. Features a damping factor of 18 for clear bass reproduction.
- NFB Used in All Circuits: Employs a three-stage amplifier circuit in the intermediate/control amplifier and a single negative-feedback loop for all circuits, ensuring freedom from noise and distortion.
- Super-Precision Transistor Preamplifier: Eliminates hum and noise with a four-transistor preamplifier, offering a superior S/N ratio and gain compared to conventional vacuum-tube amplifiers.
- Independent Channel Operation with Tone Defeat: Each channel has independent tone controls (bass and treble) and a tone defeat switch to easily achieve a flat frequency response without affecting overall sound quality.
- Blend Circuit: Allows continuous shift from monaural to stereophonic reproduction, enabling compensation for program sources with strong concert hall effects.
- Three-Dimensional Performance: Features a center-channel output terminal for connecting a monaural amplifier to create a three-dimensional sound effect.
- Advanced Accessory Circuits: Equipped with loudness control, low and high filters, tape monitor, tone defeat, presence, blend, speaker switch, and headphone jack.
- Easy-to-Read Output Level Meters: Three-range output level meters (5, 10, and 25 watts) provide direct readings for both channels and indicate any balance issues.
Specifications
| Parameter | Value |
|---|---|
| MAIN AMPLIFIER: MAXIMUM OUTPUT | 25W-25W (50W total) |
| HARMONIC DISTORTION | Max. 0.15% at 20W for 1,000 Hz; Max. 0.95% at 20W for 30 Hz |
| INTERMODULATION DISTORTION | Max. 0.85% at 20W (50 Hz + 5,500 Hz) |
| FREQUENCY RESPONSE | 10-80,000 Hz at ±1dB |
| OUTPUT IMPEDANCE | 8 and 16 ohms |
| GAIN AND SENSITIVITY (PHONO MAG) | 84 dB (Output 20W at 1.1mV input) |
| GAIN AND SENSITIVITY (PHONO X-TAL) | 54 dB (Output 20W at 36mV input) |
| GAIN AND SENSITIVITY (TAPE) | 86 dB (Output 20W at 0.9mV input) |
| GAIN AND SENSITIVITY (MIC) | 86 dB (Output 20W at 0.9mV input) |
| GAIN AND SENSITIVITY (AUX/TAPE MON) | 51 dB (Output 20W at 50mV input) |
| GAIN AND SENSITIVITY (TUNER) | 51 dB (Output 20W at 50mV input) |
| RESIDUAL NOISE | below 0.25 µW |
| S/N RATIO | 52 dB (at TAPE terminal) |
| TONE CONTROL | Type CR; 50 Hz +11 to -15 dB; 10,000 Hz +12 to -13 dB |
| EQUALIZER (TAPE) | Type NF; NARTB (50µs) |
| EQUALIZER (PHONO) | RIAA |
| ACCESSORY CIRCUITS | Presence Control (Turnover at 150 Hz), Low Filter (50 Hz -12.5 dB), High Filter (10,000 Hz -13.5 dB), Loudness Control, Tone Defeat, Tape Monitor, Headphone Jack |
| TAPE RECORDER PLAYBACK CONNECTION | DIN |
| TERMINAL FOR CENTER-CHANNEL AMPLIFIER | Yes |
| LEVEL METER (FOR OUTPUT) | Three ranges: 5, 10, and 25W |
| BLEND CONTROL | Yes |
| VACUUM TUBES, TRANSISTORS AND DIODES | 9 tubes (7189AX4, 6AN8x2, 12AX7x3) and 9 diodes (OA-91x2, 2SB-381x4, SD-1Bx2, SW-0503x1) |
| POWER SUPPLY | 117V AC, 50 and 60 Hz |
| POWER CONSUMPTION | 150VA |
| DIMENSIONS | Width 15¹¹⁄₁₆", Depth 12⁹⁄₃₂", Height 5¹⁹⁄₃₂" |
| WEIGHT | 30 lbs |
Characteristics
This section presents graphical representations of the amplifier's performance:
- Tone Control Characteristics: Graphs showing the effect of bass and treble controls, including the tone defeat function.
- Frequency Response Characteristics: Graph illustrating the amplifier's frequency response with negative feedback (NF: 26dB) and load resistance (16 Ohms) at different output levels (1W and 20W).
- High and Low Filter Characteristics: Graphs depicting the attenuation provided by the low and high filters.
- Input and Output Characteristics: Graph showing the relationship between input and output levels, indicating linearity and gain.
- Harmonic Distortion: Graph showing harmonic distortion percentage versus output power at 1kHz.
- Inter Modulation Distortion: Graph showing intermodulation distortion percentage versus output power at 5.5kHz + 50Hz.
Connections
Detailed instructions are provided for connecting various audio components:
Speaker Connections
- Stereophonic Reproduction: Connect right speaker (+) to upper R 8/16-ohm terminal, (-) to upper R (C) terminal. Connect left speaker (+) to lower L 8/16-ohm terminal, (-) to lower L (C) terminal.
- Monaural Reproduction: Connect upper and lower 16-ohm speaker terminals to the speaker's (+) terminal, and the upper/lower (C) terminals to the speaker's (-) terminal.
- Three-Dimensional Stereo-Phonic Reproduction: Utilize the center-channel output terminal to connect to a monaural amplifier for a three-dimensional effect. Connect the input terminal of the monaural amplifier to the center-channel output pin-jack using a shielded wire.
External Component Connections
- Record Player: Connect player output to "PHONO-R" and "PHONO-L" terminals. Set the cartridge switch (MAG or X-TAL) according to the cartridge type. Connect the player's power cord to the amplifier's power receptacle.
- Microphone: Connect microphone to "MIC" terminals (R or L). For stereo, use one for each channel.
- Tuner: Connect tuner output to "TUNER" terminals (R and L for stereo, R or L for monaural). For FM-MPX adaptors, connect tuner output to the adaptor's input, and the adaptor's output to the amplifier's "AUX" terminals.
- Tape Recorder:
- Single-Connection (DIN): Connect the DIN connector to the "TAPE-REC" receptacle.
- Pin-Jack:
- Recording: Connect recorder input to "REC" terminals (R/L).
- Playback: Connect recorder output ("LINE") to "TAPE MON" terminals (R/L).
- Playing Tapes (Tape Deck): Connect tape deck output to "TAPE HEAD" terminals (R/L).
- Monitoring (Three-Head Tape Recorder): Connect via "TAPE-REC" (single) or "REC" and "TAPE MON" terminals (pin-jack). Ensure "TAPE MONITOR" switch is ON for monitoring.
Switches and Controls
Description of front panel controls and their functions:
- POWER INDICATOR: Illuminates when the amplifier is powered on.
- HEADPHONE JACK: For connecting headphones.
- SPEAKER (SPEAKER OFF): Disables speaker output when headphones are used.
- PRESENCE: Adjusts turnover frequency to 150 cps for enhanced bass response.
- POWER: Main power switch.
- OUTPUT LEVEL METERS: Indicate output levels for right and left channels, with selectable ranges (5, 10, 25W).
- LOUDNESS: Compensates for bass and treble at low volume levels.
- LOW FILTER: Reduces low-frequency noise (e.g., from record players).
- HIGH FILTER: Reduces high-frequency noise (e.g., record scratch, tuner noise).
- TONE DEFEAT: Bypasses tone control circuits for a flat frequency response.
- TAPE MONITOR: Allows monitoring of recorded material on three-head tape recorders.
- BALANCE: Adjusts volume balance between left and right speakers.
- BLEND: Continuously shifts from stereo to monaural sound.
- VOLUME: Controls overall sound volume.
- SELECTOR (Input Selector): Selects input source (TUNER, PHONO, MIC, TAPE HEAD, AUX).
- MODE (Stereophonic/Monaural Switch): Selects stereo, stereo reverse, or monaural (L+R, Mono R, Mono L) playback modes.
- BASS (L/R) & TREBLE (L/R): Tone controls for left and right channels.
Operations
Guidance on using the amplifier for various audio sources:
Record Playing
Set SELECTOR to PHONO, MODE to STEREO (or STEREO-REV/R/L for monaural). Play record, adjust balance using BALANCE knob while monitoring output level meters. Adjust volume with VOLUME knob. Use BLEND knob as needed. Ensure player power is switched off with the amplifier.
Broadcasts
Set SELECTOR to TUNER (or AUX for FM-MPX adaptor). Set MODE to STEREO/STEREO-REV (or R/L for monaural). Tune in with the tuner. Adjust other controls for optimal performance. Refer to tuner/adaptor manuals for specific instructions.
Use a Microphone
Set SELECTOR to MIC. Set MODE to STEREO (for stereo effect), R (for monaural), or L+R (for mixing). Use BLEND knob for mixing. Only high-impedance microphones are recommended. Separate tone controls for L/R channels offer added versatility.
Record and Play Tapes
Recording: Set SELECTOR to the appropriate source. Set MODE to STEREO, R, L, or L+R. Prepare tape recorder and operate controls. Amplifier controls affect speaker output, not recording levels. Connect tape recorder directly for best results.
Playback: Set SELECTOR to TAPE (for tape deck) or set TAPE MONITOR to ON (for tape recorder). Set tape recorder to play. Adjust other controls for optimal reproduction.
Caution: When TAPE MONITOR is ON, the MODE switch is inoperative. For MODE switch functionality with tape recorders, connect recorder output to AUX and set TAPE MONITOR to OFF.
Tape Monitoring: Use the TAPE MONITOR switch ON for monitoring during recording with a three-head tape recorder. Ensure the switch is OFF otherwise.
Hints on Use
- Speaker Polarity: Incorrect polarity can weaken center-range sounds and bass. Reverse speaker connections if sounds do not mix well, ensuring consistent polarity for both speakers.
- Fuse: If the amplifier does not power on, check the fuse. Replace with a 3-ampere glass tube fuse. Do not use substitutes. If the fuse blows repeatedly, check for short circuits in the power transformer or condenser.
- Heat Generation: The amplifier's top may become hot during extended use, which is normal due to air vents. Avoid placing objects on or in enclosed spaces to ensure proper ventilation.
- Connections: Use adequately thick shielded wires for connections to prevent hum. Keep connecting wires short (under 2 meters) to minimize treble attenuation.
- Hum and Howling: Hum or howling can result from placing the record player near speakers, using unshielded wires, or improper grounding. Ensure proper placement and wiring.
- Connect Leadwires Properly: Loose or touching connections can cause noise or damage. Ensure all connections are secure.
- Level Meter Switch: Select the appropriate meter sensitivity (5, 10, or 25 watts) for accurate needle movement indication.
- AC Outlet: The back panel AC outlets are controlled by the power switch. Do not exceed the total capacity (80VA-30VA + 50VA).
- Placement: Install the amplifier in a well-ventilated area, away from moisture, dust, direct sunlight, and vibrations, with easy access to grounding.
Service Note
Troubleshooting guide for common issues:
| Symptom | Probable Cause | What to Do |
|---|---|---|
| No power when power switch is pushed on | 1. Defective power switch 2. Defective line cord 3. Loose contact/defective plug 4. Blown fuse | Replace. Replace. Recondition or replace. Replace. (If fuse blows again, check transformer/condenser for short-circuit.) |
| Power indicator lit, but unit does not work | 1. Defective tube 2. Abnormal voltage in tubes/parts | Check tubes & diodes. Check voltages, replace if necessary; check AUX input circuit if voltage is OK. |
| PHONO and TAPE do not function | 1. Defective head amplifier (transistor section) 2. Defective selector switch 3. Loose contact/short-circuit at input terminal 4. Defective connected equipment 5. Defective coupling condenser | Replace transistor head amplifier unit. Replace or repair switch. Replace or repair. Replace. Replace C₁ and/or C₅. |
| Weak sound on AUX, PHONO, TAPE | 1. Abnormal voltage in power circuit/parts | Check and repair. |
| Normal voltage, but weak sound on AUX | 1. Defective fixed resistance 2. Short-circuit in output transformer 3. Discharged capacitor 4. Aged tube 5. Defective slide/selector switch | Check specific resistors. Check and repair T₂ and T₃. Check specific capacitors. Check tubes. Replace or repair switches. |
| Normal voltage, but weak sound on PHONO/TAPE | 1. Defective selector switch 2. Abnormal input circuit (loose contact, defective wire) 3. Defective connected equipment 4. Defective coupling condenser 5. Discharged bypass condenser | Replace or repair switch. Replace or repair. Replace. Check C₁, C₂, C₅, C₆, C₇, C₈. Check C₃, C₄, C₉, C₁₀. |
| Distorted sound on AUX | 1. Aged tube 2. Short-circuit in output transformer 3. Defective loudspeaker 4. Defective connected equipment | Check tubes. Check and repair T₂ and T₃. Replace. Replace. |
| Distorted sound on PHONO and TAPE | 1. Defective connected equipment 2. Defective tubular electrolytic capacitor 3. Disconnection at fixed resistance | Replace. Replace C₁ and/or C₂. Check specific resistors. |
| Hum on AUX | 1. Electrolytic capacitor discharged 2. Defective tube 3. Defective connected equipment 4. Hum balancer not properly adjusted 5. Disconnected NF resistor | Replace C₁ and/or C₂. Check tubes. Replace. Check VR₄ and VR₁₅. Check R₅₂ and R₅₃. |
| Hum on PHONO and TAPE | 1. Defective shielded wire/wrong connection 2. Audio system/wire induced from outside 3. Residual hum of connected audio system | Replace/connect correctly. Maintain distance from inductors. Replace or repair. |
| Noise on AUX and TUNER | 1. Fixed resistor badly connected/touched 2. Capacitor nearly short-circuited/touched 3. Primary coil of output transformer nearly disconnected 4. Defective tube | Check specific resistors. Check specific capacitors. Check T₂ and T₃. Check tubes. |
| Noise on PHONO and TAPE | 1. Fixed resistor defective/disconnected 2. Defective capacitor 3. Noise or bad connection of connected equipment | Check specific resistors. Check C₁-C₁₀. Replace or repair. |
Parts List Summary
This document contains a detailed parts list enumerating all components used in the Sansui AU-70 amplifier. It includes resistors (R1-R130), capacitors (C1-C81), variable resistors (VR1-VR15), vacuum tubes (V1-V9), transistors (TR1-TR4), transformers (T1-T3), diodes (SD-1B, SW-0503, OA-91), and various switches (S1-S14). Each entry specifies the part number, nomenclature, and technical details such as resistance, capacitance, type, tolerance, and material where applicable. Due to the extensive nature of this list, it is summarized here. For complete details, please refer to the original document.
Diagrams
This document includes technical diagrams essential for understanding the amplifier's construction and circuitry:
- Schematic Diagram: A detailed circuit diagram illustrating the electronic components and their interconnections, showing the signal flow and power supply paths. It includes symbols for various components like tubes, transistors, resistors, capacitors, and switches.
- Parts Layout Diagram: A visual representation showing the physical placement of components on the circuit boards and chassis, aiding in assembly, repair, and identification of parts.
