Home Stereo Basics for Engineers, Part 2

Last post we laid some groundwork, the most important concept is to think about different types of amplifiers connected together internally or with RCA cables. Let’s start thinking about the input selector knob at a deeper level, which is essential for teeing up Part 3.

When we have a simple receiver, its simple to operate and it doesn’t take any under-the-hood insight to use it. We have a tuner built in, and maybe we plug in a cd player or record player into their designated sockets. To select what we want to hear, we turn the input selector and it just works.

To use the features of a more advanced receiver, we need to think about the input selector more deeply. What the input selector is doing is connecting an internal or external component to the input of the power amplifier. These signals are routed at the consumer standard -10DVB whether internal or external.

The chassis which is often called the pre-amp, contains a function in addition to the signal routing. It has a signal processing component that typically has frequency based gain controls–typically Treble and Bass, a high and low pass filter, respectively, and often a “loudness” control which boosts low frequences to match the human ear’s lower sensitivity to bass frequencies at low volumes, and of course an overall gain control called the volume control. We can consider the signal processing unit to take the line level in (-10DBV) and emit a line level signal.

For those who have seen a component stereo system, this is more visually obvious as the power amplifier is physically separate from what we call the pre-amp, and is connected to the power amp by RCA cables. That which is not the power amp is often nicknamed the pre-amp though it may or may not actually contain a phono preamp, and always contains signal routing capabilities and a gain control (audiophile components often do not contain any frequency shaping to maintain a higher fidelity signal).

When a chassis does not contain a tuner, but it has the signal switching and gain control and the power amplifier, it is often called an Integrated Amplifier. An integrated amplifier may or may not contain a phono preamp.

The main point here is that I defined a pre-amp in the first post as an amplifier that takes a signal lower than line level and boosts it to line level–that is an engineering or electronic definition. In common parlance, however, the component that does the signal switching and gain control is referred to as a pre-amp, whether it actually contains a phono preamp or not.

Well, thats a lot of discussion, and aside from some vocabulary lessons, we are not yet very far from where we started. This discussion does present or re-enforce fundamental concepts that help us understand what’s going on when we take it to the next level. What if we add some signal output sockets to the back of the receiver and add a second selector knob. This additional selector determines what line level signal from the various line level inputs, including a tuner and phono preamp, is sent to a line output in the back, and one of the options of the power amp input selector is getting its signal from an external line level socket that is not connected to any line level sources in the receiver.

Well, the options and possibilities multiply, and that takes us to the pinnacle of the discussion in part 3.

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