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.

Tale of two op amps

Dipping my toe into solid state electronics after all these years of working with tubes… I ordered a few op amps and a non-inverting amp circuit board with a socket which can accomodate various 8 pin dual op amps. The circuit board is set for a voltage gain of something like 5/6 times the input voltage. I have the traditional +-15 volts powering it. One characteristic of an op amp is how fast it can raise the output voltage based on changes in the input voltage. This is called the slew rate and is specified in volts per microsecond. I placed a couple of dual op amps in this circuit and put the output on my scope.

The first is an NE5532–a very traditional ‘audiophile’ op amp in the 1990s–this may or may not be the genuine article, as in this circuit, its slew rate is far below its spec. The second op amp is a far higher performing OPA2134A. The 5532 has a bipolar transistor input while the 2134 has a FET input, so its not automatically a suitable plug in upgrade for circuits containing the 5532. I note the 2134 is at least 10 times more expensive than the 5532–as befitting its higher performance.

When working with these, I ran a 13khz square wave in. At this frequency, the slew rate limitations of the 5532 start to show. I was surprised at how dramatic the difference was, as the 2134 did not appear to exhibit any slew rate limitations at this frequency, only exhibiting a tiny ringing on the leading edge. Each square on the screen represents 10 microseconds. The output voltage here is 16 volts peak-to-peak.

A nod to Siglent for this wonderful SDS1104-XE scope that has the ability to save screenshots to a usb drive, making these kinds of notes very easy to prepare.

13khz Square wave from a NE5532
13khz square wave output from the OPA2134A