Principals of Audio: System Building


The Golden Rule of System Building

Audio systems tend to begin with seeds: a hand-me-down from an older brother, a gift from a parent, a component rescued from a skip – maybe a couple of items bought with scraped-together savings or a first pay packet. Occasionally a new home, or a moment of peaking disposable income, allows the audiophile to start with a clean sheet, but it’s rare that an entirely new system lands in one piece without having inherited components from a predecessor.

Rather like Trigger’s Broom (or Theseus’ Ship), I’ve known aged audiophiles go to their graves with the same system they bought as a teenager: 17 heads and 14 handles down the line.

It’s also common to build a system ‘around’ a component (for instance, a cherished valve amp) or loyalty to a specific brand.

All three approaches are pitfalls that lead to circular buying and compromised outcomes, and should be flushed from our thinking without delay.

It’s important to understand which specification decisions are yours to make, according to subjective taste, and which are not: either because they are dictated by earlier choices, or because a component is objectively good or not good, and the issue is not whether you like it, but whether you can afford it. We’ll present this as an overview, then break it down.

FIRST: Let the room help choose speakers.
SECOND: Let the speakers dictate amplification.
THIRD: Choose a volume control and input switch according to preference
FOURTH: Choose a DAC or turntable according to preference
FIFTH: Let the DAC dictate the processor

CHOOSE SPEAKERS FIRST

You may have limited choice over the room: it will either be a living room, or a dedicated listening space (see chapter X), but it’s vital to follow its lead: before considering any other aspect of the system, choose speakers that fit the room. Speakers are rarely too big for a room; they are often too small. Rooms mock under-sized speakers. Bear in mind the difference between creating a listening triangle and energising an enclosed space: the speakers need to drive the whole room (not just the part you sit in) with convincing dynamics at frequencies below 300Hz.

The most important decision you make according to personal preference is the speaker topology: listen widely to, then choose between horns, cones (full range and multiway), membranes and domes (soft and hard) – ported and sealed cabinets – active and passive – dipoles, bipoles and monopoles. There you will discover your true audiophile loyalty: not to a brand, but to a tone of voice deriving from a specific speaker technology. You may find the effortless spatial rendition of full range electrostats pushes your buttons, or you may find them lacking punch when asked to rock out. You may find the trouser-flapping qualities of ported speakers bracing, or overdone and unmusical compared to the precision and purity of a sealed box – which, to some, sounds anaemic. Horn-lovers love horns. Open-baffle enthusiasts will have it no other way. Some find all metal domes shrill; others swear by Beryllium.

Once you’ve settled on your preferred speaker type, or hybrid of types, narrow in on a particular brand according to your budget. But bear in mind that your room (especially if it’s a living space) won’t allow you free rein. There’s no point buying a dipole unless you can locate it a third of the way into the room. It makes little sense to put a 6-inch two-way sealed box in a room larger than 25m3. If you can’t locate a rear-ported speaker a metre away from a rear wall, choose a different speaker. Consider how many subwoofers you can sensibly install: two is a minimum, four is preferable. Bear in mind, too, than bass is a room function entirely separate from the listening triangle . . .

The room/speaker interaction is perhaps the most important part of an audio system: one of the most common mistakes is not to allow the room a say in the buying decision. And of course if you move to a different room, don’t expect to retain the speakers: they may not be appropriate to a different space.

LET THE SPEAKERS CHOOSE THE AMP

Once you’ve established your baseline preference to speaker type, and involved the room in the conversation, choosing an amplifier is easy – especially if you’ve fallen in love with active speakers: no amp is needed.

One of the counter-intuitive truths audiophiles learn early on is that little cones tend to be much harder to drive than big ones, and that array speakers using lots of little cones can either be very efficient, or rather inefficient, depending on the wiring and design objective.

Understand that if you’re preferred speaker type is inefficient (ie less than 90dB) or hard to drive (ie, dipping below 4 ohms) you’ve set the bar higher not just for amplification, but for the dynamic properties of all other items in the system. The conservative nature of line stages in DACs often requires the extra muscle of a dedicated preamp in such cases.

But whether the source is analog or digital, power is a must for insensitive speakers. You can’t have an amp too powerful for speakers. In fact, you’re much more likely to damage a speaker with an amplifier underpowered for its load. To a live sound engineer the goal of power is primarily volume; in the home the goal of power is mainly control. More control over a speaker is a good thing: again, it may seem counter-intuitive to buy a more powerful amplifier specifically because of its properties at low SPL, but an amp that can grip an insensitive speaker with a surfeit of watts and amps is absolutely necessary. In the case of magnetostats, for instance, there’s practically no limit to their appetite for power.

However, high-power amps tend to be either expensive or crude; sometimes both. If you don’t need lots of power, spend your money on quality instead of quantity: high efficiency speakers thrive on very simple (ie Tripath), or very pure (ie, high quality valve) amplification.

In the past, a great deal was made of the importance of choosing an amp with the right character. Again with the branding. However, buying in 2019 isn’t like it was in Grandad’s day. Amplifier technology – thanks largely to PWM / switching technology (sometimes called Class D) – has democratised high performance in a way that was unimaginable in 1989. Of course, characteristic differences remain between amplifier technologies that system-builders will choose according to taste.

Mindful of what a fool’s errand it is to make meaningful generalisations about amplifier characteristics – implementation matters most – we might describe family traits of common amplifier technology as follows:

Pure Class A amps run hot

Class D switching amps (made by manufacturers such as Hypex and B&O/ICE) have very low distortion, high efficiency and high power. They all sound clean and truthful. They share a peculiarity of throwing a wide soundstage of shallow depth. They are generally admired by audiophiles for correct presentation more than loved because they tug at the heart. They are rarely expensive and are well matched to hard loads: many of the pro-derived models are 2 ohm stable.

Either way, if you let your speakers choose your amplifier, you won’t go wrong. The next choice is rather more open . . .

TO PRE OR NOT TO PRE?

Obviously not an factor if you’ve chosen an integrated amplifier – or if your system is all-digital: DAC-direct connection to a power amp or active speaker offers several advantages – not least the omission of a set of cables and connectors. The question is whether a preamp brings enough to the table to justify the cost . . . .

Several issues are in play here.

  1. Gain: does the system require gain at the pre- stage, or is it comfortable as it is?
  2. Impedance: does the output impedance of the source match the input impedance of the power amp or active speaker?
  3. Attenuation: in a digital system, for instance, how does the quality of the DAC’s volume control compare to a given preamp’s?
  4. Connectivity: how many analog / digital inputs are required? Does the design of the power amp favour RCA or XLR inputs?
  5. If a preamp isn’t indicated by any of these practical grounds, is it desirable sonically? And which is better – a passive or an active design?

In a digital system, the preamp decision is sometimes a decision about which DAC to buy – both of which are subjective decisions according to taste: analog stages inevitably have sonic characteristics that differ subtly.

But again, everything makes sense if work backwards from decisions already made about upstream components: we’re asking what kind of preamp stage suits the power amp – which is the right question. As well as matching its impedance, we need to know whether the power amp is a true balanced design (in which case the DAC or pre output should also be differential and connected with balanced XLR/XLR cabling) or single ended (connected via RCA/RCA cabling). Many power amps have both RCA and XLR inputs and


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