Amplifiers (and/or preamplifiers) are essential components in most practical audio signal chains and typically require electrical power to function properly. They’re used to drive loudspeakers and headphones; boost mic and instrument signals to usable levels, and for many other reasons.
Why do audio amplifiers and preamplifiers need power supplies? Audio amplifiers, whether solid-state, tube-based or digital, are designed to increase the amplitude (voltage, current or power) of an input audio signal. Since energy cannot be created, power supplies are required in order for the amp to increase said amplitudes.
Amps/preamps and their power supplies are important. This article aims to discuss their importance and why the two devices are symbiotic.
Why Are Power Supplies Required For Audio Amplifiers?
Audio amplifiers are active devices, meaning they require power to function properly.
They require this external power in order to provide and apply gain to the input audio signals to boost them at the output.
In other words, audio amplifiers need power to amplify. With this line of thinking, it’s clear that without amplification, an amplifier simply isn’t an amplifier (it’s in the name)!
When we think of power supplies (or, at least when I think of power supplies), we think of a separate unit that converts the AC power mains voltage into a specified usable DC voltage to run a piece of electrical equipment.
The power supplies of most amplifiers are built-into the amp itself. If the amplifier or preamplifier is built into a larger device (a mixer, audio interface, powered speaker, etc.), then the power supply is generally also built into the larger device as well.
The power supply still works the same, though. It effectively takes the AC from the power mains (the wall plugs) and converts it to a usable voltage to run the device.
Car audio amplifiers and other amps that run on batteries take their power from, well, a battery (and the alternator). The power supplies of these units do that same thing, converting battery power into a specified voltage to run the amplifier effectively.
Once converter to the proper specs, the power supply will effectively supply the amplifier with the electrical power necessary for its proper functioning.
Again, amplifiers require power, in one way or another, to amplify the input signal and turn it into a stronger version at the output. We’ll get into the specific ways in which this happens in the following sections.
Power amplifier power supplies are often considered as being separate from the power amplifier itself. However, a better way of this about power amplifiers is as an extension of the power supply rather than the other way around.
The valves of a power amplifier (whether tubes or transistors, which we’ll get to shortly) effectively control the flow of current that the power supply is capable of delivering. They do so with a smaller input signal and effectively cause the larger potential power supply currents to mimic the inputted waveform at a higher signal level (power, current and/or voltage).
Audio preamplifiers are also largely defined by their power supplies but in a less obvious way, especially when considering their differential and operation amplifiers holistically. That being said, their gain also comes from the power supply.
Tube amplifiers, as the name suggests, utilize vacuum tubes in their amplifier circuits.
The most basic form of a vacuum tube amplifier is the triode. Let’s discuss the triode tube in a bit of detail to understand how it works and why it requires power. This will effectively answer the question of why tube amplifiers need power supplies.
The Vacuum Tube
Let’s have a look at a simplified illustration of a triode tube:
The 3 electrodes are known as:
- A: Anode (plate) – positively charged
- K: Cathode – negatively charged
- G: Grid
- H: Heater (not an electrode)
The power supply of the tube amplifier sends a current through the heater of the tube in order to bring the tube up to operating temperature. It also applies a positive charge to the anode and a negative charge to the cathode, effectively applying a large voltage across these two electrodes of the tube.
As the tube is heated, thermionic emission causes the cathode (electron donor) to release electrons. These electrons then flow through the vacuum to the positively charged anode plate.
In other words, the heat of the tube and voltage across the anode and cathode causes an electrical current to flow through the tube. This electrical current (and the voltage that causes it) it very high relative to an input signal.
The effective input signal of the tube amplifier is connected to the grid, which is the third electrode.
The grid (aka the control grid) acts as a valve capable of adjusting the current between the cathode and anode. By applying the voltage of the input signal to the grid, the low-level input signal controls the grid, thereby controlling the large current through and voltage across the cathode and anode.
This is effectively how the tube amplifier works. A power supply causes a rather large electrical current and a relatively small input audio signal controls the larger signal. The result, as expected, is amplification.
To help explain, here is an oversimplified schematic of triode vacuum tube amplifier:
Note that, in circuit design, the positive current flow is actually in reverse of the true direction in which electrons flow. The triode in the above schematic still has its cathode at the bottom and anode at the top.
The power supply is also connected to the heater in order to heat it up. However, this is not typically drawn in a tube amp schematic and has been omitted from the simple illustration above as well.
Of course there are other components to a tube amplifier but the tube (or tubes) are the central active components that require power to function.
Tube amplifiers are still used today in audio applications. In fact, audio technology is one of the only disciplines where tubes are still used (and cherished, at that)!
Most modern amplifiers are made with solid-state electronics, which we’ll get to in a moment. Before doing so, it’s important I mention that there are also hybrid amplifiers that use both tubes and solid-state electronics (in which case, the tubes are not the only amplifier elements that would require a power supply).
Transistors & Solid-State Amplifiers
Solid-state amplifiers, as the name suggests, utilize solid-state electronics in their amplifier circuits.
The notable active amplifying components in a solid-state amplifier are the differential amplifier and the transistor. Let’s quickly discuss how each of these elements works and why they require power to work. This will effectively answer the question of why solid-state amplifiers need power supplies.
Let’s begin by looking the transistor, which can be effectively thought of as the solid-state version of the vacuum tube. In fact, transistors have largely replaced tubes in all electronics where cost, size and ease of manufacturing are concerned (practically all electronics).
There are plenty of different transistor types on the market. The transistors used in most solid-state amplifiers are bipolar junction transistors (BJTs). They look like this:
Let’s have a look at the schematic symbols for both types of bipolar junction transistors:
The 3 terminals of a BJT are:
- B: Base
- C: Collector
- E: Emitter
The BJT amplifier works like this: a small current at the base terminal is capable of controlling a much larger current between the other two terminals. To function as an amplifier, an electric potential must be applied across the collector-emitter. The base current, then, acts as a valve to control the larger current (much like the grid of the triode tube).
Transistors are made of semi-conductive materials. BJTs have two junctions and three semi-conductive areas.
- N-type: doped with impurities the provide mobile electrons.
- P-type: doped with impurities the provide holes for mobile electrons.
Bipolar junction transistors are designed as either NPN or PNP.
- NPN: two P-type junctions (at the collector and emitter terminals) that are separated via a thin N-type region (at the base terminal).
- PNP: two P-type junctions (at the collector and emitter terminals) that are separated by a thin N-type region (at the base terminal).
The NPN and PNP transistors are complimentary opposites of one another.
Like the tube, the BJT uses a small current (through its base terminal) to control a much larger potential current (through its collector and emitter terminals).
A solid-state power amplifier output stage features two BJTs (one NPN and one PNP). Each BJT is powered via its own power rail. The NPN has the +V power rail send current from the collector to the emitter. The PNP has the –V power rail send current from the emitter to the collector.
This can be seen in the simplified schematic of a solid-state amplifier pictured below:
We see the two BJTs in yellow. The power supply unit is to the right (complete with a fuse and on/off switch).
Let’s zoom in at the output stage, shall we?
The input signal (from the input or the preamp) is applied to both BJTs.
When the input audio signal (AC) is positive, it allows the +V supply rail to send [positive] current to the output.
When the input audio signal (AC) is negative, it allows the –V supply rail to send [negative] current to the output.
The results is that the amplifier effectively switches between power rails (and very quickly at that). Only allows one power rail at a time can send current to the output at any given time. The +V supply rail is responsible for all positive current in the output signal and the –V supply rail is responsible for all negative current in the output.
Either voltage supply is capably of driving much more current that the input signal and so the dual-BJT configuration acts as an amplifier!
This is an oversimplification but it shows us how solid-state power amplifiers require power to function properly.
The Differential Amplifier
Solid-state preamps (and many power amps) will utilize differential amplifiers to amplify their signals.
A differential amplifier at the input of a preamplifier can effectively accept balanced audio signals and yield excellent common-mode rejection of noise/interference.
The operational amplifier, which is a type of differential amplifier, is a common element in solid-state audio amplifier circuitry. It helps provide gain to the input signal and its negative feedback allows for excellent output impedance specifications that allow power amps to more accurately drive their speakers.
Most differential amplifiers (including those in audio amplifiers) are voltage amplifiers. That being said, current, transconductance and transresistance configurations are also possible.
As we’d expect, the amplifiers require power to functions. Let’s briefly look into why.
Although the schematic symbols of differential amps may look simple, these integrated circuits may have many elements built into their circuitry. These elements include transistors, resistors, etc.
The voltage supplies must be equal but opposite (as is the case with the power rails of the solid-state audio amplifier). This keeps a constant supply on for the internal circuit of the differential and/or operation amplifier.
That’s the basics of it. Understanding differential amplifiers is actually much more complicated and would require a whole other series of articles based in electronics.
For example, an ideal op-amp will have the following:
- Infinite open-loop gain
- Infinite input impedance
- Zero output impedance
- Infinite bandwidth
- Zero offset voltage
It is the circuitry around the amplifier element that allows us to manipulate their functionality to achieve the results listed at the beginning of this section.
For a detailed post on the differences between solid-state and tube amplifiers, check out my article Solid-State Vs. Tube Amplifiers (Pre, Power & Guitar Amps).
Are Power Supplies Required For Anything Else?
The main focus of an amplifier is to amplify the audio signal. However, there are other circuits that the power supplies may be required to power as well.
For example, microphone preamplifiers will generally supply phantom power. This is a +48 VDC voltage applied on pins 2 and 3 (relative to pin 1) of a balanced connection.
Phantom power is used to power the active components of condenser microphones (to power the tube or solid-state impedance converter and to polarize the capsule in non-electret designs). It also powers similar tube or solid-state circuits in other active microphones (including active ribbon mics).
Related article: What Is Phantom Power And How Does It Work With Microphones?
DAC (digital-to-analog converter) amplifier units will require power to convert digital audio signals into analog audio signals. Like phantom power, the DAC circuit is technically separate from the analog signal amplifier circuit(s) but both circuits may be run off the same power supply.
Devices with built-in amplifiers may very well run other active circuits and the amplifier from the same power supply. An audio interface, for example, can run its preamplifiers, phantom power, analog-to-digital converters, digital-to-analog converters, headphone amplifier and more from a single power supply.
Amplifier Power Requirement Specifications
Now that we know why amplifiers need power, let’s have a look at a few amplifier power requirement specifications.
In this sections, we’ll look at the following amplifiers:
- Crown Audio XLi 2500 (solid-state stereo power amplifier)
- Heritage Audio 1084 (single channel microphone preamplifier/EQ)
- Rockford Fosgate Power T600-4 (car power amplifier)
- Rupert Neve Designs RNHP (solid-state headphone amplifier)
- McIntosh 2152 (2-channel tube power amplifier)
Crown Audio XLi 2500
The Crown Audio XLi 2500 (link to compare prices at select retailers) is a popular stereo power amplifier. Its power requirements specification is as follows:
AC Line Voltage and Frequency – Configuration (+ 10%):
- 120V~ 60Hz
- 220V~ 50/60Hz
- 230-240V~ 50/60Hz
Crown Audio is featured in My New Microphone’s Top 11 Best Power Amplifier Brands In The World.
This specification simply tells us what power mains the amplifier can be connected to. To find out the actual power rail voltages of the amplifier (how the amplifier actually converts and uses the power from the power mains), we’d have to find a schematic.
Heritage Audio 1084
The Heritage Audio 1084 (link to compare prices at select retailers) is a Class-A mic preamp/EQ featuring custom polystyrene and polypropylene film tone capacitors. Its power specification is as follows:
Power consumption: Less than 110 mA @24VDC
Here we see the voltage and maximum amperage the 1084 would need to operate effectively.
Heritage Audio suggests its compatible Frame 8 and Rack 2 powered enclosures to power the 1084. These units plug directly into the wall and can supply the 1084 with its power while also holding it in an enclosure.
To learn more microphone preamplifiers, check out my article What Is A Microphone Preamplifier & Why Does A Mic Need One?
Rockford Fosgate Power T600-4
The Rockford Fosgate Power T600-4 (link to compare prices at select retailers) is a power amplifier designed for car audio. Its power requirement specifications are as follows.
- Operating Voltage: 9 – 16 VDC
- Recommended Fuse (not included): 80 A
- Average Current Draw (13.8V Music): 40 A
- Max. Current Draw (13.8V Sinewave): 80 A
- Suggested Alternator: 75 A
These specs are different than the other because the T600-4 is designed to be powered by a car battery (and alternator).
It tells us how much the voltage is required to run the amplifier and how much current the amplifier will draw. Rockford Fosgate also recommends a fuse and alternator to be run in-line between the car batter and the amplifier.
Rupert Neve Designs RNHP
The Rupert Neve Designs RNHP (link to compare prices on Amazon and B&H Photo/Video) is a popular high-end single-output desktop headphone amplifier that goes for a relatively affordable price. Its power requirements are as follows:
Power Supply Requirements:
- 24 VDC @ 0.25 Amp (6 watt) minimum. (use with supplied power adapter, for best output power and noise performance)
- May be used with a properly configured 24 V battery as well
This unit comes with its own power adapter that will convert the AC power mains into the appropriate 24 VDC required of the headphone amp. The RNHP is an example of an amplifier that have its power supply build outside of its main body.
For more information on headphone amplifiers, check out my article What Is A Headphone Amplifier & Are Headphone Amps Worth It?
The McIntosh MC2152 (link to check out the amplifier at the official McIntosh website) is a 2-Channel Vacuum Tube Amplifier. Its power requirement specification is as follows:
Field AC Voltage conversion of the MC2152 is not possible. The MC2152 is factory configured for one of the following AC Voltages:
- 100V ~ 50/60Hz at 6.6 Amps
- 110V ~ 50/60Hz at 6.0 Amps
- 120V ~ 50/60Hz at 5.5 Amps
- 127V ~ 50/60Hz at 5.5 Amps
- 220V ~ 50/60Hz at 3.0 Amps
- 230V ~ 50/60Hz at 2.75 Amps
- 240V ~ 50/60Hz at 2.75 Amps
- Standby: Less than 0.3 watts
McIntosh is also featured in My New Microphone’s Top 11 Best Power Amplifier Brands In The World.
Note: Refer to the right side panel of the MC2152 for the correct voltage.
In the case of this amplifier, the buyer must select, upon purchasing, which power mains the 2152 should be designed for. McIntosh would then produce the amplifier with an appropriate power supply.
What About Passive Preamplifiers?
Ah, the exception to the rule. Passive preamplifiers, as the name suggests, do not require power to function.
Why is that? What is a passive preamplifier?
A passive preamplifier, like a typical preamplifier, is put in-line between an audio source (a record player, television, etc.) and a power amplifier.
However, unlike a regular preamp, a passive preamp has no way of supplying gain to the signal.
Rather, it acts as a simple switcher circuit (if the model is designed with multiple inputs and/or outputs) with a potentiometer to control volume.
Switching is essentially the routing of a specified input to a different output.
Note that, since there is no gain, the passive preamp can really only turn the input signal down.
In other words, the passive preamplifier is a volume control and, perhaps, a switcher device.
Its simple circuitry generally gives a clean sound, which is an advantage over an active preamp which may colour the sound slightly. This advantage is particularly present if the active amplifier is set at a position where it is not applying any gain to amplify the signal.
The Nobsound Mini NS-05P (link to check the price on Amazon) is an example of a passive preamplifier. It has a single volume control and 2 switches to switch its input (RCA or XLR) and output (RCA or XLR).
The NS-05P does not require power nor does it supply any gain to the signal.
To learn more about active and passive amplifiers, check out my article Passive Amplifiers Vs. Active Amplifiers (Sound & Audio).
Why use a preamp and a power amp? An audio preamplifier applies gain to a low-strength signal to bring it up to line level. Power amplifiers take line level signals (from live or pre-recorded audio) and amplifies them to speaker level to effectively drive speaker and reproduce the audio as sound. Using both can bring a mic, phono or low-level instrument signal up to speaker level.
Is an audio amplifier output AC or DC? Audio signals are AC and so audio amplifier outputs are also AC. These alternating currents typically span beyond the frequency range of 20 Hz – 20,000 Hz (the range of human hearing). The amplified electrical audio signals are used to drive speakers/headphones or other audio devices (mixers, recorders, processors, etc.).