Quad Power Supply For Hybrid Amplifier

This power supply was designed for use with the Simple hybrid amplifier published elsewhere in this issue. It is suitable for various applications. A cascade generator is employed for the 170 V output, a switch mode supply for the 16 V, a series regulator for the 12 V, and a separate transformer for the 6.3 V filament supply. An LT1074CT (IC1) is chosen for the regulator, allowing the circuit to be built with standard components and achieving high efficiency. The power loss is minimized with this device compared to a linear voltage regulator, enabling the use of a higher transformer voltage and a smaller cascade section to generate the 170 V necessary for the SRPP stage in the amplifier. The lower input current also results in smaller losses in the bridge rectifier (D1 to D4). A standard 12 V regulator (IC2) converts the 16 V into a stabilized voltage for the buffer stage. When an ECC83 (12AX7) is used in the hybrid amplifier, this 12 V can power the valve filaments, although 12.6 V is preferred. The current drawn by the valve is approximately 150 mA, necessitating a heatsink for the 12 V regulator, which can be a small version of an SK129 heatsink from Fischer (38.1 mm, 6.5 K/W). To increase the voltage by 0.6 V, diode D7 is added to the ground connection of the regulator. If a 12 V output is required, JP1 should be closed, shorting D7. IC1 and D5 require additional cooling, and the 63.5 mm version of the SK129 heatsink (4.5 K/W) is sufficient. Both components should be mounted on opposite sides of the heatsink, ensuring electrical isolation from each other and the heatsink. It is advisable to refer to Linear Technology's website for layout recommendations regarding the use of the LT1074. Standard chokes rated at 5 A can be used for L1 and L2. To further reduce residual 100 kHz switching frequency, an additional LC filter can be added at the output. The diodes in the bridge rectifier are B10100 Schottky rectifiers, chosen for their low forward voltage drop (0.7 to 0.8 V at 10 A). Diodes with a reverse voltage rating of 100 V were selected, allowing the option to use an LT1074HVCT, which can handle input voltages up to 60 V, enabling the use of a 40 VAC transformer. This setup allows the cascade circuit to generate 220 VDC easily. The standard LT1074CT can manage up to 45 V, thus operating IC1 near the limits of its specifications in this circuit. The cascade circuit generates the high-tension supply for the valve. While it would have been feasible to use a separate transformer with a bridge rectifier and smoothing capacitor to achieve this voltage, it would require a non-standard 4.5 VA transformer with a 40 V secondary, which was deemed impractical. Consequently, an 80 VA transformer is used as the source for the cascade generator. The capacitors in the cascade circuit are rated higher than necessary, simplifying the calculation of the expected output voltage, which is approximately 4 x 30 x √2 V for the no-load voltage, yielding nearly 170 V. L3 and C22 filter out high-frequency interference from IC1. When the cascade supplies 20 mA, the output voltage drops to 140 V. For heavier loads, it is recommended to utilize a smaller cascade circuit with a higher transformer voltage, and to employ the LT1074HVCT due to the increased input voltage. The filament voltage for the valve is generated by a 4.5 VA transformer, which typically outputs slightly above 6 V, thus approaching the required 6.3 V. An alternative solution includes using a specialized transformer or a stabilized 6.3 VDC supply. Any of these options are viable, allowing for personal preference in the choice of supply. It is theoretically possible to use this power supply for two channels; however, if the ECC88 is used in the amplifier, it may be necessary to implement a separate cascade generator for each channel.

This power supply circuit comprises several key components and configurations that ensure optimal performance and efficiency. The LT1074CT serves as a highly efficient voltage regulator, allowing for a compact design while minimizing power losses. The use of a cascade generator for the high voltage output is particularly advantageous, as it enables the generation of the necessary 170 V for the SRPP stage without the need for bulky transformers or additional components. The bridge rectifier, utilizing Schottky diodes, provides a low forward voltage drop, enhancing overall efficiency.

In terms of thermal management, careful consideration has been given to the placement of heatsinks for both the 12 V regulator and the LT1074CT. Ensuring adequate cooling is crucial, especially when operating close to the component's maximum ratings. The inclusion of diode D7 for voltage adjustment demonstrates a thoughtful design approach, allowing for flexibility in output voltage requirements.

The choice of transformers and chokes is critical in maintaining the integrity of the power supply. Standard components have been selected to ensure availability and ease of assembly, while the option to add additional filtering stages can further enhance performance by reducing high-frequency noise that could affect circuit operation.

Overall, this power supply design is robust and versatile, suitable for a variety of applications beyond the Simple hybrid amplifier, and exemplifies best practices in electronic design by leveraging standard components, efficient voltage regulation, and effective thermal management strategies.This power supply was designed for use with the Simple hybrid amp` published elsewhere in this issue. It is of course suitable for use in other applications as well. We`ve used a cascade generator for the 170 V, a switch mode supply for the 16 V, a series regulator for the 12 V and a separate transformer for the 6.

3 V filament supply. We`ve chosen an LT1074CT (IC1) for the regulator, which means that the circuit can be built with relatively standard components and will have a high efficiency. The power loss is less with this device compared to a linear voltage regulator. This allows us to use a higher transformer voltage and a smaller cascade section to generate the 170 V (which is required for the SRPP stage in the amplifier).

The lower input current also results in smaller losses in the bridge rectifier (D1 to D4). A standard 12 V regulator (IC2) turns the 16 V into a stabilised voltage for the buffer stage. When an ECC83 (12AX7) is used in the hybrid amp we could use this 12 V to power the filaments in the valve as well, although we really need 12. 6 V. The current taken by the valve is about 150 mA, which means that the 12 V regulator needs to be fitted with a heatsink.

This can be a small version of an SK129 heatsink from Fischer (38. 1 mm, 6. 5 K/W). To increase the voltage by 0. 6 V we`ve added diode D7 to the ground connection of the regulator. If an output voltage of 12 V is required you should close JP1, which shorts D7. IC1 and D5 require a little more cooling and for this the 63. 5 mm version of the SK129 will suffice (4. 5 K/W). Both components can be mounted on opposite sides of the heatsink. You have to make sure that they are electrically isolated from each other and the heatsink! You should take a look at the website of Linear Technology ( and take note of the layout recommendations regarding the use of an LT1074. You can use standard chokes for L1 and L2, rated at 5 A. If you want to remove more of the residual 100 kHz switching frequency you could always add an extra LC filter at the output.

The diodes in the bridge rectifier are B10100`s. These are Schottky rectifiers, which have a low forward voltage drop (only 0. 7 to 0. 8 V at 10 A). We have chosen diodes with a reverse voltage rating of 100 V so we have the option of using an LT1074HVCT instead. This can work with an input voltage of up to 60 V, which means we could use a 40 VAC transformer. The same cascade circuit can then easily generate 220 VDC. The standard LT1074CT can cope with up to 45 V, so we`re using IC1 fairly close to the limits of its specifications in this circuit.

A cascade circuit generates the HT supply for the valve. It would also have been possible to use a separate transformer with a bridge rectifier and smoothing capacitor to generate this voltage. But then we`d have to find a 4. 5 VA transformer with a 40 V secondary and connect it the wrong` way round. As this isn`t exactly a standard transformer we dropped that idea. The source for the cascade generator is now an 80 VA transformer. The capacitors in the cascade circuit have higher values than are strictly necessary. This makes it easier to calculate the expected output voltage. In our case this is 4 x 30 x v2V for the no-load voltage, which comes to nearly 170 V. L3 and C22 filter out any HF interference coming from IC1. When the cascade supplied 20 mA the output voltage dropped to 140 V. At heavier loads we recommend that you use a smaller cascade circuit and a higher transformer voltage (and also use an LT1074HVCT because of the higher input voltage).

The filament voltage for the valve is generated by a 4. 5 VA transformer, which in practice had an output a bit above 6 V and therefore came closer to the required 6. 3 V. Another solution is to use a special transformer or a stabilised 6. 3VDC supply. Any of these will work, so it`s down to your own preference which of these you`ll use. It is in principle possible to use the supply for two channels. However, if you use the ECC88 in the amplifier you may find it`s necessary to use a separate cascade generator for each channel.

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