Phono Stage Power SupplyIn our modern world we cannot rely of our incoming power being a clean sinusoidal AC voltage of 50Hz or 60Hz. Dimmer switches, fluorescent bulbs, LED light sources or computer power supplies introduce transients, harmonics and high-frequency voltage spikes that can make it past the AC/DC conversion inside the power supply resulting in "dirty power". Furthermore specifically single-ended designs show poor power source rejection ratio (PSRR) and thus are highly susceptible to transients, noise and ripple on the power supply rails.Problem solvers may be line filtering and (audiophile = expensive) power cords, Faraday shielding between transformer coils, magnetic shielding, careful L-C filtering and / or shunt regulation. Holger Barske (text in German language) designed a sophisticated balanced power supply comprising six chokes followed by shunt regulation. Rod Elliot wrote a comprehensive article on the pros and cons of serial and shunt regulation in small current power supplies. However, if requested voltage and current levels are low as for example in a solid state phono preamplifier you may consider to chemically generate you own power from batteries and elegantly eliminate the "dirty power"problem.Figure 1 below shows the schematic of the power supply. It comprises four 12V lead-gel accumulators and four monostable relays with two change-over switches each. In the release state of the relays the accumulators are completely separated from from their charging circuit and connected in series to deliver 48V in 12V increments (operating mode). The accumulators are connected with 2200 µF capacitors at the power supply outputs that stay charged through 5k resistors. This avoids high current peaks between the accumulators and the capacitors when the power supply is switched from charging mode back to operation mode. Furthermore in the unlikely event of an internal shortcut in a capacitor currents are limited to a comfortable safe region and just slowly discharge the accumulator. For operation mode the resistors are bypassed by switches S2 to S5.Fig. 1: Schematic of phono stage power supplyFig. 2: Phono stage power supplyThe 24V voltage source in figure 1 includes a cheap 18V transformer and a bridge rectifier. If S1 is switched on the relay coils are energized and the relays connect the accumulators to four PB137 voltage regulators which are specifically designed to charge lead acid batteries. The diode D1 is essential to prevent accumulator voltage from being fed back into the relay coils which then would not allow the relays to switch back into their release state once S1 is switched off. The 10Ohm resistors are simply charging current limiters. The above figure 2 shows the power supply with its cover removed.
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