The amplifier is the core of any hifi system. All your source devices feed their signals to it and you use the amplifier to choose which one of them you want to play. It takes the modest signal that they provide and turns it onto the muscular power required to drive loudspeakers.
As such, choosing your amplifier is a vitally important part of building an exciting high-fidelity system. Here’s what you need to know to make the best choice for you.
What is an Amplifier
First, let’s dig in a little more to what an amplifier does. The output signal from, say, an audio streamer or a CD player is only a couple of volts at most, and they are designed to connect to devices with an input impedance of tens of thousands of ohms. Which means that at most they could deliver only a fraction of a milliwatt to your loudspeakers.
Loudspeakers do work. Quite a lot of it. They have to vibrate the air to make the sound. To do that work, they have to be powered. I’m sure that you’re familiar with the kind of power audio amplifiers normally deliver – it’ll be anything from twenty or thirty watts, all the way up to hundreds of watts, with most probably falling in the range of 50 to 125 watts. When an amplifier is pouring one hundred watts into an 8-ohm loudspeaker, it is delivering 28 volts and 3.5 amps of current. That’s a lot.
Powering loudspeakers is only one of the jobs to be performed by an amplifier.
Another is to select between your sources. There are three kinds of analogue sources you’re likely to come across. The most common are line level sources. These almost always connect using RCA sockets and plugs. Sometimes they are called “single-ended” or “unbalanced”. Line level means a signal level which is typically between around 0.5 volts to 2 volts maximum. Think CD player, old-fashioned tape deck and so on. They will also work with the headphone output of an iPod or iPad or phone (if your phone has one).
Some high-end gear employs balanced connections. These use three-pin XLR plugs and sockets, adopted from the pro-audio world. Much of the time they double the voltage, so the maximum voltage may be as high as four volts. The main advantage these connections offer is a reduced chance of the connecting cables picking up noise.
The third kind of analogue source is the turntable. Most of these produce an extremely low-level signal. If fitted with moving magnet cartridge the signal level will be somewhere around 5mV. That’s one hundredth of the usual line level. Furthermore, the frequency balance of the recording on the vinyl has to be adjusted significantly, with the bass being boosted by up to 20dB and the treble similarly cut. If the turntable is fitting with a moving coil cartridge, its output is likely only one tenth of that of a moving magnet cartridge, requiring even more of a boost.
Some amplifiers include a phono preamplifier to properly treat signals from a turntable, but not all. For some you will need an external phono preamplifier, which you can then plug into a line input.
Some amplifiers also now feature digital audio inputs for connecting digital sources. And some incorporate a source. For example, an amplifier with a built-in radio tuner is called a receiver. In recent years an increasingly number of amplifiers have incorporated network streamers.
And then there is a multitude of other features sported by different amplifiers, such as circuits to adjust the frequency balance or perform other processes. In general, we high fidelity types prefer our amplifiers to avoid most of those things. Still, you may have a particular need for such things as balance and tone controls, perhaps a mute button.
Types of amplifier
Amplifiers come in two different forms: one is where you have a separate preamplifier and power amplifier, and the other is where they are combined. Those are called integrated amplifiers.
Most preamplifiers and power amplifiers are designed with broadly compatible inputs and outputs, so you need not always stick with the same brand for both devices. That’s one advantage of separates over an integrated amplifier: you can upgrade your power without having to change the preamp. Some power amplifiers have a “trigger” connection, usually a 3.5mm socket. When connected to a compatible (not necessarily the same brand) preamplifier, switching on the preamp will trigger the power amp to also switch on.
A pre-amplifier is primarily responsible for selecting which source you want to listen to. Often – but not always – they also boost the level of the signal. What they always do is allow you to control the level of the signal that is ultimately passed through to the power amplifier. Since they are not driving the loudspeakers, they do not need the massive power supplies and heatsinks often employed by power amplifiers, so they can often be slighter than the power amplifier.
It is the power amplifier that’s doing the heavy lifting, taking a the incoming signal – which is typically up to two volts and a few hundred microamps – and boosting it to potentially 28 volts and 3.5 amps for 100 watts output. It typically has just one set of inputs and of course the speaker terminals. In most cases it will be heavily built and feature heatsinks to help dissipate the internal heat it produces while working. As with a lot of gear, an imperfect but useful proxy for its quality is simply how heavy it is.
A significant part of the weight of a power amplifier is in its transformer (if that’s what it uses) and associated electronics. Power amplifiers are almost always rated to work properly with speakers of both 8 ohms and 4 ohms nominal impedance. One mark of unusual quality is if the rated power output for 4-ohm loudspeakers is double that for 8-ohm loudspeakers. That suggests a high quality internal power supply.
Power amplifiers come in multichannel form – often for home theatre use – stereo or mono. Mono power amplifiers are often called monoblocks. I should note that some stereo power amplifiers can be bridges, which turns them into mono amplifiers. Obviously you’d need to purchase two of them if you plan to use them in this mode in a stereo system.
The advantage of this method of operation is that usually the power output into 8 ohms is more than double what it was in stereo mode. One disadvantage is the a minimum impedance supported is doubled. So most bridged amplifiers require speakers of at least 8 ohms impedance.
An integrated amplifier simply has both a preamp and power amp in the same box. The great majority of amplifiers are integrated. Some straddle the integrated/separates division by including pre-amp outputs and power amp inputs, so you can go to a separate for either function.
What is an Amplifier Class?
Any discussion of amplifiers – well, integrated and power amplifiers primarily – must include talking about their “Class”. Audio amplifiers have been around for a very long time, first in the form of designs based on vacuum tubes, and since the 1950s increasingly on solid-state electronics. I’ll leave valve or vacuum tube amps out of this discussion since they really should be judged on their own individual merits by enthusiasts seeking the particular character to the sound that these devices can impart.
As far as high-fidelity solid-state amps go, there are three main types of design that are employed. As is so often the case, there is a little overlap and variation.
Transistors are largely DC devices, but audio is AC. That’s a problem. One design solution to this was to use two complementary transistors. One would handle positive sections of the signal while the other would handle the negative sections. Then the two would be stitched together.
Unfortunately, that stitching part is hard to do seamlessly, and it resulted in “zero crossover distortion”. In that instant when the signal was at zero – neither positive nor negative – both transistors were off and there’d be a brief non-linearity. Which we would hear as harmonic distortion, generally of the unpleasant odd-order kind. Those designs were called Class B and they aren’t used in any hifi amplifiers worthy of the name.
Class A amplifiers solve all this by applying a significant bias to the signal. Bias is one of those scary electronic terms, but in this case all it means is that some DC is mixed in with the signal, which shifts the whole signal into either the positive or the negative. So instead of it being an AC signal centred on zero voltage, it becomes a varying DC signal, centred on the bias voltage. And that means the signal never crosses the positive-to-negative boundary, which results in inherently low distortion. The DC bias is filtered out before the signal is delivered to the speakers.
But this comes at a cost. The main one is inefficiency. The bias voltage is set at half the maximum voltage. So, for a 100-watt amplifier, it would be running at around 14 volts. And that 14 volts is running all the time that the amplifier is switched on, even if there’s no signal. So they even idle hot and consume a lot of power.
And given the large amount of heat sinking required, they tend to be large and heavy. Nonetheless, many audiophiles aspire to Class A when possible, simply because of the complete lack of crossover distortion.
Class A/B amplifiers are a compromise. They still use a bias voltage, but only to lift the crossover a little away from zero. That reduces much of the distortion inherent in Class B, while avoiding the extreme inefficiency of Class A. Class A/B remains the most common design method for high fidelity amplifiers and with modern careful design they often deliver magnificent performance.
There are lots of design choices available to designers, including the level of the bias voltage. The brand Moon by Simaudio, for example, sets its bias voltage relatively high so that the first five watts of power in its higher end models are pure Class A.
Class D – also known as a switching amplifier – is radically different, and quite hard to explain. Don’t worry if you don’t understand this bit. We’ll get to the practical stuff in a moment. Although the D in Class D does not stand for digital, the confusion is not surprising. In Class A and Class B designs, at every point through the electronics the signal is pretty much the same. It comes in rising and falling as the music does its thing. That’s what it does in the transistors, and that’s how it’s delivered at the speaker terminals.
With Class D, instead the incoming signal is converted to pulses. It kind of looks like digital (particularly Direct Stream Digital), but it is still analogue in the sense of not having a specific number space defining where the edges of the pulses must fall. The pulse might be wide, or frequent, to represent a high point in the wave form, and narrow or infrequent for a low point.
At the speaker terminal, a simple filter is applied and this converts the pulses back into an analogue signal.
This seems like a very weird way of doing things, but it does have some real benefits. The main one is efficiency. Instead of the signal ramping up and down to different levels in the output transistors, they are at all times simply on or off. The pulses determine when they are on or off. Transistors are highly efficient when they are fully on, so they’re not wasting a whole lot of energy as heat.
So, if you need a lot of power but in a lightweight package, with little need for heavy heatsinks, Class D is a viable solution. So viable that a lot of subwoofers use Class D. The technology is still not common in high end audio amplifiers yet – but it is in some. And don’t be surprised as you see more of them about.
What features do you need?
Whether a pre/power combination or an integrated amp, you should consider what features you’d like in your amplifier. Here are a few that may appear on amps, along with my views on them. Yours may differ.
- Remote control – this is a no brainer, really. At the least you should be able to change the volume at a distance. All that said, there are different ways this can be achieved. My preference is for the volume knob to either front a high-quality potentiometer, or act as a control for a ladder of attenuating resistors. And that means the remote should control a motor that rotates the knob.
- Line level subwoofer output – if you’re starting out on the high-fidelity path with a tight budget, this RCA socket could prove very useful. I usually recommend that for your first speakers, aim for the best stand-mount speakers you can afford rather than going for full-range floorstanders. The quality will typically be better, but chances are they may be a little deficient in the bass. The sub output – on those stereo amps that offer one – sums the left and right channels, but usually doesn’t filter it. When you can afford to, you can plug in an active sub, twiddle its level and crossover knobs and fill in the bass of your main speakers.
- Pre-out on an integrated amp – this stereo output provides a straightforward upgrade path: you can switch to a higher quality power amplifier and use your integrated as a pre-amp.
- Power in on an integrated amp – this adds a little flexibility if the amp also offers pre-outs. For example, you can put some kind of audio processor – that’s not always a bad thing – between the pre and power amps.
- Tape loop – some pre and integrated amplifiers till offer tape out and tape in connections. If you can see yourself buying a device to record analogue signals, this may prove useful to you.
- Tone controls were ubiquitous back in the stereo days … and frequently frowned upon by hifi purists. They really are of limited utility.
Have a budget in mind? Our amplifier picks
Integrated under $1000
Rega IO Integrated Amplifier – $849
Pre/Power under $1000
Integrated from $1000 to $5000
Pre/Power from $1000 to $5000
Integrated from $5000 to $10,000
Pre/Power from $5000 to $10,000
Integrated more than $10,000
Bryston B135³ Integrated Amplifier – from $11,200
Pre/Power more than $10,000
How much output power do I need?
As a general rule, the more the better. But as an also general rule, you’ll need to double the power to even start to notice a difference. Doubling power only adds – potentially, not always actually – three decibels. A three-decibel level difference is slight.
So, if choosing between fifty and a hundred watts, all else being equal go for the higher output. But if your choice is between 115 and 130 watts, ignore the power differential – it makes only 0.5dB difference – and concentrate on other aspects of the devices.
What is Damping Factor?
The damping factor is sometimes specified for amplifiers, and it’s kind of a measure of the amount of control the amplifier exercises over the loudspeakers. It’s inversely related to the internal impedance of the amplifier output. Want to learn more? Read about it here.
Can I put my preamplifier on top of my power amplifier? That’s the way I always see it in advertisements.
No. I’ve never understood why these illustrations appear. I’ve been to product launches and demonstrations in which this happens. And it’s bad. Power amplifiers need plenty of ventilation. Preamplifiers do not need to have excessive heat pumped into them. Don’t do it.