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Description Although the modular Portable Mixer design available on these web pages has become a hit for many amateurs, some correspondents required a much simpler device, mainly for mixing mono signals. This design should fulfil their needs, featuring three inputs with switchable high/low sensitivity and unusual level-control circuits, providing high overload margins and low-noise figures, proportional to gain-level settings. Low current consumption due to a simple, five-transistor circuitry, allows the Mini Mixer to be powered by a common 9V PP3 battery for many hours.  Circuit diagram:  Parts P1 = 5K P2 = 5K P3 = 5K R1 = 180K R2 = 2M2 R3 = 750R R4 = 1K R5 = 15K R6 = 220R R7 = 1.5K R8 = 820R R9 = 150R R10 = 100K R11 = 180K R12 = 2.2M R13 = 750R R14 = 1K R15 = 180K R16 = 2M2 R17 = 750R R18 = 1K C1 = 1µF-63V C2 = 100µF-25V C3 = 220µF-25V C4 = 100µF-25V C5 = 220µF-25V C6 = 1µF-63V C7 = 100µF-25V C8 = 1µF-63V C9 = 100µF-25V Q1 = BC550C Q2 = BC547 Q3 = BC557 Q4 = BC550C Q5 = BC5...

This circuit generates three supply voltages using a minimum of components. Diodes D2 and D3 perform full-wave rectification, alternately charging capacitor C2 on both halves of the ac cycle. On the other hand, diode D1 with capacitor C1, and diode D4 with capacitor C3 each perform half-wave rectification. The full-and half-wave rectification arrangement is satisfactory for modest supply currents drawn from -5 and +12-V regulators IC3 and IC2. Three Rail Power Supply Circuit Diagram: You can use this circuit as an auxiliary supply in an up-based instrument, for example, and avoid the less attractive alternatives of buying a custom-wound transformer, building a more complex supply, or using a secondary winding, say 18 Vac, and wasting power in the 5-V regulators.