Tillo, J., Tressler, S., and Hanson, J., "New Approaches for Low Voltage Audio System and Circuit Design," SAE Technical Paper 2012-01-0037, 2012, doi:10.4271/2012-01-0037.
New vehicle systems that stop and restart the engine to avoid prolonged idling place new requirements on audio systems. In particular, there is a need for the audio system to continue to operate during the engine start sequence. In the past this was not an issue, as audio systems could be turned off during the engine start sequence.Among the ramifications for this “operate while starting” requirement is that the audio system must operate at a lower voltage, say 6 volts, instead of 10 volts. Audio system operation in this context means continuing to play music without interruption or spurious noises.The main focus of this presentation is on audio circuits, both line-level and speaker level.In the past, line level outputs could be fed by linear regulated power supplies that developed 8 to 10 volts. The signal levels sent from one module to another could thus be in the 3 volt RMS range. Given DSP integrated circuits with 3.3 or 5 volt power supplies, it was often necessary to “gain up” the DSP audio outputs to meet system requirements. Running line level input and output circuits on 3.3 volt power supplies implies that gaining up is no longer necessary, and in fact it may be necessary to gain down DSP audio outputs. The challenge is to perform the gain reduction without degrading signal to noise in the signal processing chain. Circuits that accomplish this are shown.A prototype amplifier that operates with output IC power supplies as low as 6 volts has been successfully demonstrated. The design and test results from the prototype amplifier are discussed.