AV power amplifiers: what and why?

SERIOUS SYSTEMS NEED SERIOUS AMPLIFICATION, AND A POWER AMP GIVES YOU THAT.

YOU’RE PROBABLY ALREADY aware that, in the world of hi-fi enthusiasts, there is a sort of hierarchy of quality: all-in-one units like micro systems and soundbars are thought of as inferior to ‘component’ or ‘separates’ setups, which have a standalone A/V receiver or integrated ampli er to drive your speakers, and to which you connect separate audio and video sources, like Blu-ray players, turntables, media streamers, and so on.

What you may not be aware of, however, is that true audio fanatics go one step further and separate their A/V receiver or integrated ampli er into a further two discreet units: a pre-ampli er — the part that selects the sources, handles the volume and so on — and a power ampli er — a chunky box that does nothing but power your speakers.

ere are two core arguments for this: one is that you can keep the low-voltage pre-amp signals — the sound coming from your source — and the high voltage ampli ed signals — the ones ready to power your speakers — separate from each other, reducing interference. e other, more pertinent, argument is that, by removing the pre-amp components from a chassis, you make room for a much larger power supply and amp – resulting in louder maximum volume, and greater dynamic range for more definition.

Indeed, the extra space in a power amp chassis means that the ampli er stage can then be of the larger and warmer class ‘AB’ or ‘A’, rather than the cheaper, smaller, cooler class ‘D’ amps seen in most of today’s receivers, and which are held by many audio enthusiasts as inferior for critical listening.

Now, while this setup requires the separation of pre-amp and power amp, the pre-amp doesn’t have to be a new unit: your existing integrated ampli er or A/V receiver will continue to do its job as pre-amp. By adding an external power amp, all you’re doing is bypassing its ampli er section.

All of these bene ts are well and good, but there is one formidable hurdle to switching to a setup like this: the price. Power amps, for the reasons outlined above, are enthusiast-grade equipment; they are built better, and they cost more – much more – than your standard A/V receiver, despite the receiver coming with a laundry list of features and nominally (if not really) the same output power. So, you can expect to pay thousands for the good stu , and tens of thousands and beyond for the crazy ‘audiophile’ stu .

However, unlike your A/V receiver, a good power amp will last you decades; as it’s just amplifying a signal, it isn’t a ected by leaps in technology standards and features. us, once

purchased, you don’t have to worry about the next big audio or video standard making it obsolete; to get the latest features, you need only upgrade your A/V receiver or processor, just as before.

Armed with the knowledge that a good power amp of old is still a good power amp, you can actually nd some real bargains on the second-hand market. While they do retain their value pretty darn well, you can get a solid power amp only a couple of decades old (or less) for under $1000, perhaps well under.

But before you get that far, let’s talk a bit more about why power ampli ers are used, and gure out if you would bene t from one.

WHY USE AN EXTERNAL AMP AT ALL?

As mentioned earlier, the primary reason that you’d add an external power ampli er (or two) to your system is for MORE POWER. More power doesn’t just mean you can drive larger speakers, or crank your existing ones up a little louder; it also means that you’ll have enough power to reproduce the more di cult lower frequencies (important for systems without subwoofers), or, at higher volumes, enough headroom to deal with sudden increases in volume (we’re talking ‘peaks’ like explosions in movies or surprise trumpet solos in music) without clipping.

Further, in multi-channel surround systems, adding a power amp to drive even just your front speakers will not only improve their performance — o en resolving user’s complaints that their A/V receiver isn’t “musical” enough — but also reduce the burden on your receiver’s internal ampli er and allow it to give more oomph to those speakers still attached to it. Given that rear/ surround speakers are usually smaller than the front ones, there’s no real need for further ampli cation beyond what your receiver generally provides.

However, you may need to run a test tone and check that your speakers are all running at equal volume, and adjust your power amp’s gain/trim controls, or your receiver’s pre-amp output levels, so that they match. You’d be best using a dB meter for this, but an app on your phone should su ce.

If you’re a real power end, desperate to further amplify all of your surround speakers, you may be pleased to know that you can buy multi-channel power amps, too – one well-priced-but-well-built US company that makes them (and ships to Australia) is Emotiva. Alternatively, you can use multiple power amps – and, if money is no object, most power amps can be bridged to run in mono mode, meaning you can have one power amp per speaker. May your tinnitus quake at the very prospect.

IS IT WORTH IT?

As money is an object for most of us, however, you’ll want to be sure that investing well upwards of a grand in a power amp will make a useful di erence. Gene DellaSala from Audioholics ( www.audioholics.com/ home-theater-connection/connecting-an

external-amp-to-a-receiver) has four main reasons why you would want more power in your system: You like your music or movies loud. Like sustained 90dB+. Your hi- is in a large room, say above 25m2. Your speakers are ine cient: either 4 ohm, <90dB e cient, or both. You don’t use a subwoofer to lighten the load on your speakers, and don’t want one. Now if you meet only one of these requirements, the investment into a power amp isn’t likely worth it (unless you can nd a used one for a bargain price, that is). But if you meet two or three, then you may see value in adding more power to your system.

Before you do, however, you’ll need to check that your existing equipment can work with a power amp: that is, if your A/V receiver or integrated ampli er has ‘pre-amp outputs’ (sometimes called ‘pre/pro outputs’).

Now, what’s interesting – if not surprising – about power ampli ers is that they only do one job: amplify. What I mean is that they don’t even control the volume; your existing receiver will continue to do that, and every other task, too. You really are just replacing its internal ampli er with an external one.

Given the xed ampli cation of power amps, there are some considerations that you now need to make: namely, how much signal will your existing receiver give to your power ampli er, and how much volume will that result in?

I’ll skip over the maths here (see the ‘Voltage, ampli cation, and output’ box for a link to it, though), but there is one thing that you must understand: your power amp can only reach its maximum power output if the pre-amp outputs provide it with enough signal (voltage, in this case). If your receiver can only output one volt or less on its pre-amp outputs, then your power amp may not be able to reach its full potential, even with your receiver’s volume knob turned to maximum.

What this translates to is this: if you have a crappy, noisy, or low-level pre-amp, then a power amp is only going to make those faults more audible, and/or provide ampli cation only marginally better than your existing integrated ampli er. With this in mind, it makes no sense to pair a $200 A/V receiver with a $2,500 power ampli er. e old adage always remain true: garbage in = garbage out.

In saying that, however, the average quality of A/V receivers has been steadily rising over the years, and if your receiver is of a reputable brand and was of a price-point high enough to include pre-amp outputs at all, then you’re probably ok to add a power ampli er to it – a er all, that’s the reason the outputs are there. To be sure, you can search online for reviews that went to the e ort to measure the pre-amp output, such as those at audio enthusiast sites like Audioholics. An output of around 2v or more should be enough to drive your chosen power amp. Further, you should also check with the seller if there is a return policy if you’re not happy with your new power amp.

HOW MUCH POWER?

So, how much power should you go for when choosing a new power amp? It’s generally held that the output of A/V receivers isn’t quite as good as it says on the box, so – and this is usually a bit surprising – using a 75w stereo power amp to drive your front speakers will likely be better than using your 100w per channel A/V receiver to drive them. In short,

don’t feel that you need to fork out the big dollars for a 200w+ model – unless, of course, you’re driving exotic, ine cient, highimpedance loudspeakers.

Further to this, a doubling of ampli er output does NOT mean a doubling of maximum volume. In fact, a doubling of ampli er power – whether it’s from 10w to 20w or 100w to 200w – only results in an increase of 3dB to the maximum volume, whereas a doubling of perceived volume requires somewhere between a 6 to 10dB increase, meaning between 4x and 10x more power is needed. So, if you’re looking at two power amps from the same manufacturer, but one has a 75w output and the other a 100w output, and the price di erence is more than a couple hundred dollars, then it’s probably only worth the extra if it comes with some other features, such as a di erent ampli er design or better power supply – there won’t be any noticeable di erence in the volume that the two can put out.

However, if you nd a reputable-brand, high output power amp at a cost you’re willing to spend, don’t feel bad for opting for more power. As I’ve said before: it’s almost always better opting for too much power than too little, as having more maximum power will give more headroom to maintain dynamic range at volume without clipping.

And, as you’ll want your power amp to last decades, buying one with more power means that you can always upgrade to hungrier, larger, and generally better sounding speakers later down the line. If your chosen power amp has gain/trim controls, or your receiver’s pre-outs have adjustable output, you can always dial them down a touch to make sure that you’re not sending too much power to your speakers before then, anyway, or else refrain from spinning your receiver’s volume control too far to the east.

But enough preamble, let’s talk numbers. e technical speci cations section of your speaker manual will tell you how much your speakers can handle: look for the “continuous power RMS” (or “long term” or something similar) value and their impedance (in ohms), and if the power amp you’re looking at is signi cantly above that value (say, double) at the same impedance, then it’s not a great idea to use it with your speakers, as a mistake with the volume knob may cause damage (they’ll be perfectly ne at lower volumes, however). If it’s equal to that value, or only 25% or so above it (say 200w speakers with a 250w amp), then they’ll be ne together – just remember that the further above your speaker’s peak RMS the power amp is, the more careful you need to be in how far you turn it up and for how long. Finally, if the power amp you’re looking at is less than the power rating of your speakers, don’t bother.

Power amps, then, really are for the audio obsessed – those that want to get the very best out of a very good set of speakers. When paired with a modern A/V receiver, they can provide the means for a well-engineered, traditional style ampli er to be used for your music (where it matters most), without missing out on the latest features like digital crossovers, room and speaker calibration, and surround standards. And while the entry price is considerable, the investment is over a longer period than your standard A/V receiver, which goes someway to justifying it. Oh, and they’re sick for parties, too.

A/V receivers with “pre out” connections are ready for a power amp – there’s no need to add a separate pre-amp.

A/V processors, like this flagship . channel Marantz AV (RRP $ , ), are e ectively high-end A/V receivers without the ampli er stage – for those obsessed with getting the best sound. The Emotiva XPA- is a x w (@ ohm) surround sound...

Audio purists will often use multiple mono power ampli ers – one per speaker – to maximise the power in their system.

Exotic, ohm speakers – usually those with large or multiple woofers – like these Vienna Acoustics Klimts bene t greatly from the extra oomph of a power amp.

Separating the pre-amp (above) and power amp (below), like this NAD C BEE and C BEE pair, can result in superior — and louder — audio.

Big setups need big power — the kind of power that only power ampli ers can provide. Picture: Kipnis Studio Standard/Facebook.

Power amps often provide three-pronged XLR inputs for balanced audio connections, which eliminate interference during transmission.

By removing the pre-amp section, power amps can accommodate extra large transformers, capacitors, and heatsinks.

The Azur w is Cambridge Audio’s flagship power amp, delivering w per channel (stereo), or w bridged (mono), at ohms for $ . Note the ventilation holes in the top: this thing runs hot.