Saturday, 10 August 2013

Power Supply Build

The custom toroid mains transformer has arrived so I can complete the build of the power supply. This toroid had dual primaries so it can be configured for either 115V or 230V mains and it has secondaries of 250VAC at 150mA for the HT and 50VAC at 100mA for the phantom power supply. It has a GOSS band (Grain Oriented Silicon Steel ) for reducing the radiated magnetic field and an electrostatic screen between the primary and secondary windings for minimum noise. This transformer is made by Terry at Canterbury Windings Ltd. They carry a wide range of standard toroids and will also wind any toroid to your specifications at very reasonable prices.

I also received the remaining components for the two power supply boards. The estimated total HT draw is 90 mA (6 Eurocards at 10mA each plus 30mA for the headphones amp) so to obtain 10V drops across each of the three dropper resistors in the HT supply PCB, each resistor needs to be 110 ohms. See the HT Power Supply Design document for details. The first of the three resistors is a 5 watt type.

I also made some changes to the heater elevation circuit. Normally I use a potential divider consisting 220K and 75K resistors to produce a nominal 75V heater elevation voltage from a 300V HT supply. However, many tube manufacturers specify a maximum resistance value between heater and cathode of rather less than 75K. To date this has never been a problem but I thought this was a good opportunity to update the design. One problem with using smaller value resistors s that the current they draw increases and so they dissipate more heat. As many tube data sheets mention a heater cathode resistance of around 20K, I decided to change the 75K resistor to 22K. With 75 volts across it, this will dissipate 255mW so a 0.5W type is required. The 220K I decided to change to a pair of 33K resistors in series. Each one of these will dissipate nearly 400mW of heat so 1W types were used. The heater elevation voltage is now exactly one quarter of the HT voltage. An unplanned side benefit of the smaller value resistors is that the HT supply does discharge more quickly when turned off.

Lastly I needed to set the SOT resistor in the phantom power supply. There is a nominal 1.25V dropped across the 120 ohm resistor between the output and the ADJ pin of the TL783 regulator which means for a 48V output voltage, the SOT resistor from the ADJ pin to ground needs to be about 4.6K. I made this from a 2.2K and a 2.4K resistor connected in series. The measured output is 49.8V which is due to the tolerances in the resistors and the chip's on board reference. Fortunately the phantom power spec is 44V to 52V so we are comfortably within that. In future I might update the PCB to replace the SOT resistor with a 10 turn pot. However, one of the reasons for using a resistor is that it dissipates 500mW of heat so a 1W type is needed. I used two 0.5W types in series.

Here is a picture of the completed power supply. The white cable is the mains input which will come from a filtered, switched and fused IEC connector mounted at the rear of the mixer.

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