To feed modular system, you need bipolar power supply giving voltages from ±9v to ±18v depending from used system. Additional power lines might be needed. Eurorack standard suggests symmetrical ±12v line and additional +5v line which is useful for MCUs and some another digital ICs. It is possible to use computer power supply for modular systems, but PWM supplies have quite unstable voltage in fluctuating environment. Because of that, tune and volume of the synthesizer powered by PWM supply may have undesirable modulations. In some cases it can be used as artistic effect but to get completely predictable sound you need traditional power supply that uses a transformer.
Here is the one supply of this kind — Regulated Bipolar PSU. It gives enough power for almost any system.
Voltage of the regulated power supply can be precisely set to obtain ideal symmetry for the oscillators, stable tune and low noise floor. This power supply consists of bridge rectifier, filter and three identical parts of voltage refulation, protection and indication circuitry. The positive line works as following: Bridge (D1, D3) rectifies AC, then it being filtered with two 3300uF capacitors (C1, C3) and 100n ceramic disk capacitor (C5) which are connected in parallel. Then goes LM317 regulator working as Adjustable regulator. (U2, R2, R5, D6, C8) The load resistor (R8) and LED (D9) provide both indication and minimal load required for adjustment. The resistor value should be between 2K—10K, so it’s better to find LED which is not so bright.
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Any use beyond this license must be discussed with author.