1. Introduction
This manual provides detailed instructions for the assembly, operation, and maintenance of the MOFI-L1.12AU7/12AX7 Tube Pre-Amplifier Printed Circuit Board (PCB). This product is a bare PCB designed for enthusiasts with a high level of electronics knowledge and experience in soldering and testing. It does not include any components, which must be sourced and installed by the user.
Successful completion of this project requires careful attention to detail, adherence to safety guidelines, and a thorough understanding of electronic circuits. Please read this manual completely before beginning assembly.
2. Safety Information
- Always disconnect power before working on the PCB or any connected components.
- Tube amplifier circuits can involve high voltages, which are dangerous and can be lethal. Exercise extreme caution.
- Ensure proper ventilation when soldering to avoid inhaling fumes.
- Wear appropriate personal protective equipment, such as safety glasses, during assembly.
- Verify all component polarities and orientations before soldering. Incorrect installation can lead to damage or malfunction.
- After assembly, perform thorough checks for short circuits before applying power.
- The mounting tab of components like LT1085/LM317T is internally connected to pin 2 and carries live voltages. Ensure it is properly insulated and not shorted to the chassis, ground, or other circuit parts.
3. Product Overview
The MOFI-L1.12AU7/12AX7 Tube Pre-Amplifier PCB is a high-quality circuit board designed for building a tube-based audio pre-amplifier. It supports both 12AU7 and 12AX7 vacuum tubes, offering flexibility for different audio characteristics. The PCB features clearly marked component positions and test points to aid in assembly and debugging.
Package Contents:
- 1 x MOFI-L1.12AU7/12AX7 Tube Pre-Amplifier PCB (bare board)
Note: No electronic components are included with the PCB.
Figure 1: Top view of the MOFI-L1.12AU7/12AX7 Tube Pre-Amplifier PCB
Figure 2: Angled view of the PCB, showing component and trace sides
4. Specifications
4.1 General Specifications
- Compatible Audio & Video Equipment Style: Professional Audio Equipment, Amplifier
- Origin: Mainland China
- PCB Size: 84x96x1.6mm
- PCB Copper Weight: 1oz
- PCB Finish: HASL (Hot Air Solder Leveling) with lead
4.2 Bill of Materials (BOM)
The following table details the components required for assembly. Please refer to the PCB markings for exact placement.
Figure 3: Bill of Materials (BOM) for the Pre-Amplifier PCB
| Component Type | Part | Quantity | Specifications/Description |
|---|---|---|---|
| Resistors | |||
| Resistor | 200Ω | 1 | 0.25W |
| Resistor | 820Ω | 1 | 0.25W |
| Resistor | 5KΩ | 1 | 0.25W |
| Resistor | 1KΩ | 2 | 0.5W |
| Resistor | 3KΩ | 2 | 0.5W |
| Resistor | 27KΩ | 2 | 0.5W |
| Resistor | 47KΩ | 2 | 0.5W |
| Resistor | 1MΩ | 2 | 0.5W |
| Resistor | 22KΩ | 1 | 3W |
| Capacitors | |||
| Bi-Polar | 1uF | 2 | ≥100V, metallized polyester capacitor, Axial Leaded, Diameter=12mm, Length=30mm |
| Electrolytic | 47uF | 3 | ≥160V, aluminum electrolytic capacitor, Radial Leaded, Lead Spacing=5mm, Diameter=10mm |
| Electrolytic | 100uF | 2 | ≥16V, aluminum electrolytic capacitor, Radial Leaded, Lead Spacing=5mm, Diameter=10mm |
| Electrolytic | 1000uF | 4 | ≥25V, aluminum electrolytic capacitor, Radial Leaded, Lead Spacing=5mm, Diameter=10mm |
| Diodes | |||
| LED | Your choice | 1 | Superbright LED |
| Bridge Rectifier | KBP307 | 2 | 3A/700V Bridge Rectifiers |
| Transistors | |||
| Adjustable Low Dropout Pos VR | LT1085 or LM317T | 1 | TO-220 Package |
| Miscellaneous | |||
| Terminal | 2P-7.62mm | 2 | |
| Thermally Conductive Insulator | 1 | TO-220 | |
| Insulating Washer | 1 | ||
| Heat Sinks | 1 | 42x25.4x35(H)mm with pin extrusion TO-220 | |
| Other | 1 | Circuit board standoffs or spacers; metric M3 machine screws, hex nuts; other hardware as needed | |
| Tubes | |||
| Tube | 12AU7/12AX7 | 1 | |
| Tube Bases | 9 pin valve base | 1 | |
| PCB | |||
| PCB | L1-12AU7 Pre-Amp | 1 | SIZE: 84x96x1.6mm, 1oz Cu, HASL with lead |
4.3 Power Requirements
- Heater Voltage: AC 7.5V-10V, ≥0.8A (Best: 8VAC @ 1A)
- High Voltage: AC 90V-105V, ≥0.02A (Best: 100VAC @ 0.03A)
5. Setup and Assembly
Follow these steps carefully for proper assembly of your pre-amplifier PCB:
- Prepare the PCB: Clean both sides of the blank PCB with a paper towel and isopropyl alcohol or electronics flux remover. This removes any residue that could interfere with soldering.
- Component Placement: Solder components to the board, starting with the lowest profile parts first (e.g., resistors, small diodes), then moving to taller components (capacitors, tube sockets).
- Through-Plated Holes: The PCB holes are plated through. Solder parts from the bottom of the board. Do not drill or enlarge the holes as this will damage the through-plating and compromise the circuit integrity.
- Verify Components: Before soldering, measure each resistor with a multimeter to ensure it has the correct value. Double-check all component values against the BOM.
- Polarity and Orientation: Pay close attention to the polarity of electrolytic capacitors, diodes, and transistors. Ensure they are oriented correctly as indicated on the PCB silkscreen. Incorrect orientation can cause component failure or circuit malfunction.
- LT1085/LM317T Installation: When installing the LT1085 or LM317T voltage regulator, ensure the thermally conductive insulator and insulating washer are correctly placed between the component's mounting tab and the heatsink/chassis to prevent short circuits.
Figure 4: Correct installation of LT1085/LM317T with heatsink
- Solder Quality: Inspect all solder connections carefully, preferably using a magnifying glass. Look for cold solder joints (dull, grainy appearance) or solder bridges (unintended connections between pads).
- Post-Soldering Cleaning: Clean up any solder flux residue from the board with isopropyl alcohol (or electronics flux remover) and a brush.
- Short Circuit Check: Use a multimeter in ohms scale to check for any unintended short circuits on the board before applying power.
6. Operating Instructions
Once the PCB is fully assembled and verified, it can be integrated into your audio system. Ensure your power supply meets the specified requirements (AC 7.5V-10V for heater, AC 90V-105V for high voltage).
Connect your audio source to the 'IN-PUT' terminals and your amplifier/next stage to the 'OUT-PUT' terminals. The PCB is designed to function as a pre-amplifier, boosting the signal level and adding the characteristic warmth of tube amplification.
6.1 Debugging and Test Points
The PCB includes test points to assist in debugging. After applying power, measure the voltage at these points and compare them to the expected values. A deviation of +/- 20% is generally acceptable.
Figure 5: Schematic diagram with key voltage points (12AX7/12AU7)
Figure 6: PCB layout with component markings and test points
Refer to Figure 5 and Figure 6 for the schematic and PCB layout with indicated test points and expected voltages (e.g., 1.1VDC, 2.9VDC, 97VDC, 120VDC).
7. Maintenance
- Cleaning: Periodically inspect the PCB for dust accumulation. Use a soft brush or compressed air to gently clean the board. If necessary, use isopropyl alcohol to remove any stubborn residue.
- Visual Inspection: Regularly check solder joints for any signs of cracking or corrosion. Ensure all components are securely seated, especially the vacuum tubes.
- Tube Replacement: Vacuum tubes have a finite lifespan. If you notice a degradation in sound quality or unusual noise, consider replacing the tubes. Always replace tubes with compatible types (12AU7 or 12AX7).
8. Troubleshooting
This section provides general troubleshooting tips for common issues encountered with DIY audio circuits. Due to the nature of DIY projects, comprehensive troubleshooting support is limited.
| Problem | Possible Cause | Solution |
|---|---|---|
| No power/No sound | Incorrect power supply connections, short circuit, faulty component, incorrect tube insertion. | Check power supply voltage and current. Inspect for solder bridges or cold joints. Verify component placement and polarity. Ensure tubes are correctly seated. |
| Distorted or weak sound | Incorrect component values, faulty tubes, incorrect wiring, improper test point voltages. | Double-check all resistor and capacitor values. Try swapping tubes. Verify all connections against the schematic. Measure voltages at test points and adjust if possible (if adjustable components are present). |
| Hum or noise | Grounding issues, poor shielding, power supply ripple, faulty components. | Ensure proper grounding. Check power supply filtering capacitors. Inspect for loose connections. Consider shielding sensitive areas. |
If you encounter persistent issues, carefully re-examine the PCB for any missed connections, incorrect component placements, or poor solder joints. Refer to the schematic and PCB layout diagrams (Figure 5 and Figure 6) for guidance.
9. User Tips
- Component Sourcing: Always use high-quality components for the best audio performance and reliability.
- Soldering Practice: If you are new to soldering, practice on scrap boards before working on your pre-amplifier PCB. Good soldering technique is crucial.
- Documentation: Keep a record of your assembly process, including any modifications or specific component choices. This can be invaluable for future troubleshooting or upgrades.
- Enclosure Design: Plan your enclosure carefully to ensure adequate ventilation for the tubes and proper shielding to minimize noise.
10. Support
Due to time zone differences and language communication restrictions, MOFI-Acoustics can provide only limited technical support for this PCB. Users are expected to possess a high level of electronics knowledge and experience to complete this project.
For further inquiries or to explore other products, please visit the official store: OnifealiMajeson HiFi Official Store.