Preamps and Networks
Preamps
In an audio system, “pre-amp” functions are performed on signals which have not yet been amplified. The preamplifier is in between the source or sources and the amplifier in the signal chain. It might be a standalone component, or it might be a section within another component (the “preamp section”).
The two most important preamp functions are source selection and volume control.
Source selection lets the user choose which source is routed to the amplifier.
Volume control lets the user control the listening level. This is usually done by changing the voltage of the pre-amplified signal before it reaches the amplifier.
In home audio, a “preamp” is a component which accepts signals from various sources, switches between them, and steps up the preamp signal’s voltage appropriately in response to the volume control setting. Record-playing turntables usually emit very low-voltage signals which need special handling by a properly-equipped preamp.
A home-audio preamp component in a system. Arrows indicate the signal path.
A home audio “integrated amp” combines the amp and the preamp in one component. A “receiver” combines a radio tuner section, a preamp, and an amp into one component. Most aftermarket source units for car audio are receivers.
Front/rear fade and subwoofer level control are also common preamp functions in the car-audio environment.
A block diagram of a modern aftermarket in-dash receiver for a car (orange arrows indicate the analog signal path, the green arrow indicates control inputs)
Since the 1970’s, the aftermarket car audio industry has offered source units and amplifiers. The source units may have had amplified outputs (with very limited power output), but many had low-voltage preamplified outputs. These outputs could be routed to an external amplifier. The first amplified OEM sound systems used the same architecture - the preamp was in the source unit.
Block diagram of a sound system - a source unit with an internal preamp, and an external amplifier
Early OEM systems used analog connections to carry preamp signals. The data connection - whether it was K-Line, CAN, LIN, or something else - carried preamp commands, but not audio. Even if the connection was “digital” SPDIF rather than analog, a separate data connection was used to carry preamp commands to the preamp inside the amplifier.
Fully-Networked Sound Systems
The first automotive sound system to combine program content, commands, and other data over a single digital network used the Domestic Data Bus network technology, or D2B. This network could have been implemented over either optical or coaxial cables, but only optical cables were used in actual production systems by Mercedes and Jaguar.
Block diagram of a fully-networked OEM system - the Source Unit is connected to an amplifier via a network
The original networks connected communication interfaces, optical disc players, amplifiers, tuners, and telematics devices on a common network. D2B used a ring networking configuration to do this. The functioning preamp is located inside the “sound system”.
Zen Audio Preamps
Zen Audio preamps connect to this OEM infotainment network, and "pull" pure digital audio from it before any processing takes place. Then it is available in a pure, unmodified form - either high-quality analog, or SPDIF digital - for aftermarket audio upgrades. For more information, see this article on Zen Audio Network Preamps.
