Balanced Input to Unbalanced Output PreAmp

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The PreAmp2 Project is a two channel balanced input with unbalanced output pre-amplifier. The circuit is basically a unity gain differential amplifier, where balanced source signals are connected to both inverting and non-inverting inputs of MC33078. MC33078 was chosen because of its high performance specifications for use in high quality audio applications. MC33078 is a dual high-speed low noise operational amplifier that features a low THD and large output swing with no deadband crossover distortion.

Most professional audio equipments have balanced inputs and outputs usually identified by its XLR or TRS connectors. This preamp will hook up those devices with XLR/TRS only connectors to the unbalanced input of the power amplifier projects.

The PCB layout and PreAmp2 BOM are provided free, as is with no warranty. All component values in the schematic diagram are also shown as Part Ids in the PCB. The PCA shown in the picture is Rev 1 but the PCB layout is the latest revision. An added option of the PCB layout is that, you can connect an unbalanced source into the RCA input header.

The schematic diagram is fairly straightforward; C1 ~ C8 capacitors are used to filter high frequency noise. The resistor values shown in the schematic sets the voltage gain of the amplifier to almost one or unity. All resistors are 1/4W metal film resistors. All capacitors are 2.5 mm(lead spacing) multilayer ceramic, film or whatever is available in your parts bin. An onboard 3-pin positive and negative linear regulators were added to the PCB layout.

PreAmp2 schematic-page-001

Schematic Diagram of Balanced Input to Unbalanced Output PreAmp


PreAmp2 Bottom Layer-page-001 (2)
PreAmp2 Bottom Copper Layer of PCB


PreAmp2 Silkscreen Layer-page-001 (2)
PreAmp2 Silkscreen Parts Placement Layer of PCB


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