The first commercial digital audio recordings were released in 1971, and since then, listeners have been amplifying these recordings to enjoy them through speakers and headphones. Nowadays, most of the audio people listen to is digital (even with the resurgence of analog audio vinyl records), which begs the question of whether amps are digital devices or are they still analog.
Are audio amplifiers analog or digital devices? Amplifiers may have digital audio inputs and/or outputs but are inherently analog devices. Analog-digital audio conversion may be required, and some amps use digital clocks. Still, the amplification process is analog, using analog input signals to control higher-amplitude analog output signals.
In this article, we'll discuss the inherently analog nature of various audio amplifier designs; explain the differences between analog and digital audio, and touch on the role and functionality of digital-to-analog converters (DACs).
Analog Vs. Digital Audio
Before we can really understand the analog nature of audio amplification, we should understand the differences between analog and digital audio.
First, audio is a representation of sound.
What is sound?
Sound itself is made of mechanical wave energy. Sound waves in the audible range of human hearing oscillate in the range of 20 Hz – 20,000 Hz.
The mechanical waves of sound affect the localized pressure variation from the ambient pressure in a medium. They are a continuously variable physical quantity.
The term “analog” refers to measurements and representations of continuously variable physical quantities. They are “analogous” to physical sound wave waveforms. So, sound is a continuously variable measurement of pressure in a medium; analog audio is a representation of this varying pressure in the form of electrical energy (voltage).
Because sound is defined within 20 Hz – 20,000 Hz, analog audio is defined within the same range.
Let's have a look at the similarities between sound and analog audio. For this example, we'll take a 1 kHz (1,000 Hz) sine wave that completes one cycle every millisecond.
As we see in the sound wave pictured below, we have a single wave cycle happening in 1 ms. The amplitude of the wave is measured in pressure deviation from the ambient pressure of the medium. It can be given in pressure units (often in the SI unit Pascal) or as decibels of sound pressure level (dB SPL).
The wave reaches a peak of maximum pressure (max. compression) and a trough of minimum pressure (max. rarefaction).
Any part of the wave above the dotted centre line represents an increase in localized pressure compared to the ambient pressure. Any part of the wave below the dotted centre line represents a decrease in localized pressure.
If we look at a 1 kHz sine wave analog audio signal, we'll see a striking similarity to the 1 kHz sound wave it represents.
The difference between the analog audio signal and the sound wave it represents is the type of energy they're made of.
Sound is made of mechanical wave energy and is measured in pressure. Audio is made of electrical energy, and its amplitude is measured in voltage.
Like the sound wave it represents, the analog audio signal has a peak and trough in each cycle.
The peak represents maximum positive voltage and forward-flowing current. The trough represents maximum negative voltage and backward flowing current.
Any part of the wave above the dotted centre line represents positive voltage and forward-flowing electrical current. Any part of the wave below the dotted centre line represents negative voltage and backward-flowing electrical current.
The key takeaway is that both sound and analog signals are continuously variable.
The voltage/current of an analog audio signal can be amplified to the maximum capabilities of the amplifier tasked with amplifying it. Of course, there are all sorts of issues to be aware of when amplifying an analog signal (the amount of gain, the amount of noise, distortion, source/load impedance, slew rate, etc.), which we'll cover shortly. For now, it's important to know that the electrical nature of analog audio signals makes them amplifiable.
To learn more about the relationship between sound and audio, check out my article What Is The Difference Between Sound And Audio?
Now let's discuss digital audio.
Digital audio is effectively a representation of analog audio in a digital format. Rather than being continuously variable, digital audio takes samples or “snapshots” of the audio across time and assigns an amplitude to each sample.
The number of samples per second in digital audio is referred to as the sample rate. Two common sample rates are:
- 44.1 kHz (44,100 samples per second)
- 48 kHz (48,000 samples per second)
The number of potential amplitudes each sample can have is defined by the bit-depth of the digital audio signal.
Bit-depth is not linear like sample rate but is exponential. For each additional bit in a signal's bit-depth, there is an additional 1 or 0 in a chain of 1s and 0s.
1-bit has two potential values in a 1-word word length: 1 or 0
2-bit has 4 potential values in a 2-word word length: 00; 01; 10, or 11
3-bit has 8 potential values in a 3-word word length: 000; 001; 010; 011; 100; 101; 110; 111
So on and so forth.
The two most common digital audio bit-depths are:
- 16-bit (216 or 65,536 distinct amplitude values)
- 24-bit (224 or 16,777,216 distinct amplitude values)
The same 1 kHz sine wave in digital audio (at 48 kHz 24-bit resolution) would look something like this:
The dotted line represents the intended analog signal. The bars represent the digital samples and the amplitudes of the samples.
So digital audio is non-continuous. It closely approximates analog but has discrete values.
Digital audio is great, but it has a defined ceiling (all 1's across its word length). The dynamic range of a digital audio signal is defined by the difference between its largest and smallest “word” (amplitude), but the digital information itself cannot truly be amplified.
How Does Amplification Work?
To understand whether amplifiers are analog or digital, it's important to understand the basics of how amplification works.
To goal or amplification, of course, is to ideally increase the level of an audio signal without altering (distorting) the shape of its waveform.
To do so, an amplifier will effectively regulate the power it receives (and stores up) from the power source (direct wall socket, power conditioner, external power supply, etc.).
The amplifier will use the input audio signal waveform to control how much or how little of its stored power is outputted. In other words, the input circuit of an amplifier controls the output circuit in real-time.
This is true of simple op-amps, transistors in their linear regions, and vacuum tubes/valves. It's also true of the more involved amplifier designs (class A, A/B, B, D, and so on). There are a variety of amplifier designs based on one or more of these active devices, all of which use the input signal to modulate a power source to produce an amplified output signal.
If we're dealing with a true amplifier (one designed to amplify the signal—to make it louder), the relatively small variations in the input stage produce relatively large variations in the output stage.
So why are amplifiers analog?
Well, as discussed in the previous section on Analog Vs. Digital Audio, digital audio has a ceiling defined by its bit-depth. We can use digital signal processing to adjust the maximum discrete amplitude and/or the minimum discrete amplitude (doing both would affect the dynamic range), but we can't truly “amplify” the signal.
Analog signals, which are essentially alternating currents in a circuit or stored on an analog medium (tape, vinyl, etc.), can be amplified in voltage, current and power. For example, mic level signals are about 1 to 100 millivolts AC (-60 to -20 dBV). They are generally amplified up to line level via a microphone preamplifier, nominally 1.228 volts (+4 dBu). To properly drive a loudspeaker, line level signals (from mixers, DACs, preamps, etc.) are often required to be upward of 50 to 100 volts (34 to 40 dBV) or more.
Furthermore, all the active components listed above are analog. They are modulated in one way or another by the input signal (AC voltage) and use this modulating signal to control the flow of electricity from the power source. The variations of the input signal (the audio) are effectively reproduced at higher amplitudes at the amplifier output.
Both tube and transistor (solid-state) amps use analog components in their amplification stages.
Single-ended and push-pull amps are analog, as are class A, B, A/B, C, and even class D (though there's confusion around this class we must discuss later) along with all the other lesser-known classes.
Microphone and phono preamplifiers, which are nearly all either class A or A/B, amplify their signals by analog means. Power amplifiers are typically either A, B, A/B or D and also amplify audio signals.
Even voltage-controlled amplifiers (VCAs), which aren't true amplifiers since they only offer attenuation, are controlled by analog means (voltage).
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Digital-To-Analog Converters (DACs)
So the amplification process of amplifiers is inherently analog, but many amps have digital audio inputs and outputs. What's the deal?
Well, digital audio has its benefits. Beyond its ease and forgiveness in editing, mixing and otherwise manipulating (there's no undo button when splicing tape), digital audio has become incredibly popular for the consumer. From CDs to digital playback devices like smartphones and computers to streaming, digital audio is king.
So, analog audio is necessary for amplification and is ultimately what is converted to and from sound (microphones and pickups convert sound into analog audio, while headphones and loudspeakers convert analog audio into sound). Conversely, digital audio is much better for working with and storing.
Hence the ubiquitousness of digital-to-analog converters (DACs) and analog-to-digital converters (ADCs).
If an amplifier has digital audio inputs, it will likely have a DAC shortly thereafter to convert the digital audio to analog audio for amplification. This is more common in power amplifiers, which are tasked with amplifying line level signals to speaker level since line level is standard for digital audio conversion.
There are “direct digital” amplifier options that forego this conversion and rather convert the digital audio signal directly into an analog control signal for the amplifier. However, these are rare—more on these amps in the following section.
Similarly, some amplifiers (particularly preamplifiers tasked with bringing phono level signals up to line level) will have digital audio outputs. These outputs will have ADCs to convert the amplified analog audio to digital audio with appropriate resolution.
Headphone amplifiers often have built-in DACs, and standalone DACs are typically marketed specifically for their use with headphones.
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Are Class D Amplifiers Digital?
There is considerable confusion regarding class D amplifiers that adds to the complications in the analog/digital amplifier conversation.
First off, it is a general misconception that the “D” in class D amplifier stands for “digital;” it doesn't. Rather, the D in class D comes as the letter after C (after class A, B and C comes class D).
That being said, there is some truth to the notion that class D amplifiers are “digital” (though not fully). Let's understand how class D amps work to deepen our understanding.
Class D amplifiers utilize pulse width modulation (PWM), often with a triangle or sawtooth oscillator and comparator op-amp, to control their outputs.
The input audio signal is effectively converted to a stream of pulses in a “PWM” signal equivalent to the input audio signal. These pulses essentially make up a high-frequency square wave with varying pulse widths.
The pulses, taken at a fixed frequency, have variable widths. Greater input audio signal amplitudes produce wider/longer pulses, while lower input audio signal amplitudes produce narrower/shorter pulses. In other words, greater input signal amplitudes produce greater duty cycle percentages (the percentage of “on” time in a given pulse).
The frequency of the pulses (switching frequency) of the pulse width modulators in class D amplifiers ranges between 250 kHz to 1.5 MHz for optimal resolution. A factor of at least 12 times the upper audio cutoff frequency is recommended, and 20 kHz • 12 = 240 kHz, so 250 kHz is a safe lower limit for the entire audible range.
The clock that maintains this switching frequency is often digital, which is a possible point of confusion.
At lower frequencies, like those in the audible range (20 Hz – 20,000 Hz), the amplitude of the resulting signal is determined by the amount of time the PWM is in “on” position versus “off” position in a given period.
At rest (no signal), the duty cycle of the switching frequency is 50%, or evenly divided between “on” and “off”. As the input audio signal becomes more positive, the duty cycle increases. As the input audio signal becomes more negative, the duty cycle decreases.
To put it differently, the PWM signal is either maximum or minimum (square wave). However, because its pulses are at such a high frequency, the audible results are the same at low frequencies.
The active elements of a class D amplifier act as switches and are typically MOSFETs (metal–oxide–semiconductor field-effect transistors), though tubes and bipolar transistors may also be used. The PWM signal triggers these switches on and off at very fast rates, which in turn pass and block greater voltage/current from the amplifier's power supply.
The fact that the PWM signal acts as an on/off trigger is another point of confusion since on/off (0s and 1s) is a core concept of digital technology.
So the amplification circuit effectively amplifies the PWM signal that represents the input audio signal. A low-pass filter is used at the output of a class D amp to remove the high-frequency pulses from the output, maintaining only the amplified signal.
Hopefully, that's not too confusing. I did my best to explain the basics of class D amplification without getting too into the minute details.
Now, I believe the real confusion about class D amplifiers comes from the following point: digital audio signals can also be converted to PWM signals. So by that truth, class D amplifiers can also amplify digital audio at their input.
So then, are these class D amplifiers that amplify digital input audio considered digital amplifiers? The case can certainly be made, though they aren't technically digital. Again, the digital audio itself is not amplified and outputted as digital information.
Rather, the digital audio is converted into an analog pulse width modulation signal. Remember that the switching devices (MOSFETs or otherwise) are analog devices, which are triggered by voltage.
So a class D amplifier's amplification stage is analog, whether the input signal is analog or digital.
There are other issues worth addressing before we wrap up this section in regard to analog and digital audio inputs.
Analog signals are continuous, so there's no fundamental limit on the PWM conversion resolution (switching frequency). Furthermore, analog feedback circuits at the output can account for power supply ripple/sag, finite switching delays and other issues in real-time (no latency) with expert design.
Digital signals are discrete, meaning they have their own sample rates. Therefore, the digital audio input and digital oscillator (for PWM comparator) would have to work together (which typically means multiplying the two rates together). The resulting switching frequency is often impractical, so dithering and noise shaping are often required.
Additionally, latency will further complicate the design. Though the initial simplification of removing a DAC before the PWM seems great for “digitizing” class D amps, the resulting design headaches make the process more complicated and expensive than it's typically worth.
Related article: Top 11 Best Class D Amplifiers For Home Audio
What About Digital Modelling Amps?
Perhaps this article would be complete without mentioning digital modelling amplifiers. After all, we've already discussed how the pre and power amplifiers for guitars and other instruments are inherently analog. I figured I'd make a note here anyway.
The term “digital modelling amplifier” doesn't strictly refer to any type of amplifier circuit. Rather, these units are designed to produce specific tones for guitars (and other instruments) with the help of digital signal processing (DSP). In this regard, they can be considered more like effects than amplifiers.
Of course, many modelling amps have built-in pre and/or power amplifiers. However, these amplifier circuits are analog in nature.
The digital modelling amp will effectively take in the analog signal from the guitar/instrument, pre-amplify it (preamplifier circuit), convert it to digital audio (ADC), process it via a modelled amp tone (DSP), convert it to analog audio (DAC), amplify it (power amplifier circuit), and drive the internal or external speaker to produce sound.
Amp modelling software doesn't have any type of amplifier. Rather, it's strictly DSP, which can offer a variety of different tones to make your instrument(s) sound superb.
If you'd like my take on the best amp modelling software, check out my article Top 11 Best Guitar Amp Simulator Plugins For Your DAW.
Are speakers analog or digital devices? Though speakers are regularly connected to digital audio devices, they are inherently analog transducers. Speaker transducers convert analog audio signals (electrical energy) into sound waves (mechanical wave energy). Digital audio must be turned into analog audio in order to drive a speaker.
Related article: Are Loudspeakers & Monitors Analog Or Digital Audio Devices?
Are headphones analog or digital audio devices? Though headphones are regularly connected to digital audio sources, they are innately analog devices. Headphone transducers convert continuously variable (analog) audio signals (electrical energy) into sound waves (mechanical wave energy) and often require a digital-to-analog converter to function.
Related article: Are Headphones Analog Or Digital Audio Devices?
Are microphones analog or digital? Microphones convert sound waves into AC electrical audio signals and are, therefore, analog devices. However, some microphones (like USB mics) are designed with built-in analog-to-digital converters and output digital audio, making them “digital microphones.”
Related article: Are Microphones Analog Or Digital Devices? (Mic Output Designs)
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