In addition to the attenuator, phono preamp and the battery charger/power supply, a complete preamplifier requires inputs for a CD player and other auxiliary equipment. This prototype is a triple differential input board built for testing purposes. Each of the three inputs uses 12 operational amplifiers in three quad packages. The circuit diagram for the board can be found here. The OPA1664AIPWR opamp is used in the design. This opamp is part of the Texas Instruments SoundPlus series and has low noise, low distortion and bipolar inputs.

The reason for so many opamps for each input is to reduce noise. The use of multiple opamps to reduce noise is described in references like Doug Self “Small Signal Audio Design”. Differential inputs using XLR connectors are preferred over single-ended inputs using RCA connectors because the differential input can reduce the hum caused by ground loops. While single-ended inputs with RCA connectors have been the standard for consumer grade audio equipment, differential inputs and XLR connectors are ubiquitous in professional audio equipment. Noise is one area were single-ended inputs can have a leg-up over differential inputs. Using multiple opamps can reduce the noise of the differential input down to the levels achieved with single-ended inputs.

DÆ v0.1 Differential Input Profile