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.

Home stereo basics for Engineers, Part 1

I prepared this for my son, and he suggested a series of blog posts, so here it is. This information is designed to understand the signal flow and signal levels inside of a stereo receiver, or a sound system with multiple components. This is for consumer retail stereo. There are different electrical standards for Pro Audio–which is sound systems for recording studios or live performance.

For working with consumer stereo, here are some essential concepts to memorize and understand. In subsequent posts, I will talk about signal routing.

Definitions:

Turntable output–output of a coil that detects the movement of a magnet attached to the needle. Its in the millivolt range. Some audiophiles use a moving coil needle where the magnet is stationary. These have a much lower output and we are not using these. If a pre-amp has inputs for both moving coil and moving magnet, the moving magnet input is the correct one unless you know your device is moving coil.

Consumer audio Line Level (the standard for things that have RCA sockets and cables). It is described as (-10 dBV) which translates to full signal as .316 volts peak to peak.


Pro audio (typically balanced cables between equipment used in recording studios or for band live performances). +4 dBV. Note a different unit of measurement is used here. This just reflects that pro audio was entirely distinct to consumer audio and has different standards. +4 dBV translates into .775 volts peak to peak for maximum signal. It is possible to drive pro audio gear with consumer signal levels. One does need to understand how to get the unbalanced -10 DBV signal connected to the balanced pro audio signal. This is a discussion for another day.


Consumer audio -10 DBV–Line level is the standard for the outputs of pre-amps and the inputs to power amps in consumer stereo.

A pre-amp is something that has an input that is less than line level For example, phono for consumer audio and microphone preamps for pro audio.


Signal to power speakers: A signal that has enough voltage and current to drive an 8 ohm speaker to the designated volume level.


Power amp: Takes line level input and puts out speaker power output.


The home stereo receiver has all of these components in one chassis, and the routing of line level signals from the pre-amp to power amp, or from a line level input like CD to the power amp are performed by multi-position selector switches. There is typically a switch that controls which input is sent to the power amp, and another switch that selects the source of a signal sent to the Line Output rca sockets in the back of the stereo.


Consumer stereos often have controls to change relative strengths of different frequency bands–typically treble, and bass. There is also a “loudness” control. It provides a bass boost to compensate for the human ear’s loss of ability to hear bass at lower volumes. (See Fletcher Munson curves) These can be thought of as equalizers that take line level in and emit line level.


The first goal should be to memorize the voltage for consumer line level–.316 volt and that its called -10 DBV. Second, understand what a pre-amp does and what a power amp does. Some people build their sound system with components typically with the pre-amp and power amp in separate chassis, connected by the standard -10 DBV RCA cables.


Next post will cover the subject of signal routing using the rotary switches.