Meridian Audio Speaker manual Digital Audio Music to the Ears, Active Loudspeaker

The Meridian Papers - 1 Meridian Loudspeakers: The DSP Path page 3

Digital Audio:

Music to the Ears

Digital audio technology is now widely regarded by industry experts as the best means to hear music in the home. Because music recorded digitally can be transmitted, even over long distances, and played back with no change from the original, like Morse code over a telegraph line.

Why is this important? Traditional ana- logue systems behave much like the child- hood game in which each player repeats a whispered message into the next player’s ear, and so on down a chain. Each ana- logue link – turntable, amplifier, cable, speaker – whispers an analogy of what it hears to the next, but something is always lost or added. What emerges at the end, while charming, may not resem- ble the original message.

By contrast, digital audio first encodes music as digits – ones and zeros – in pat- terns describing specific sound waves. This is the binary language of computers, and it’s very difficult to mistake a one for a zero. No matter how long the chain, at its end digital equipment listens only for patterns of those two digits, which it reassembles into music, ignoring all other

whispered information as noise.… ➤

However, such an approach – “bi-amp- ing” or “tri-amping” the system – is far too complex and prone to error to be a very practical approach in the consumer field.

The Active Loudspeaker

Back in the mid-1970s, multi-amping was almost unknown. Even more unconventional was Meridian’s first product, the M1 Active Speaker, which placed both active crossover and ampli- fication, plus the associated power unit, in the same enclosure as the loudspeak- er drive units. The principle is illustrated in simple form in Fig. 2.

This method delivers a number of important benefits. First, there is a sim- ple line-level connection between the preamplifier and the loudspeakers: a large, heavy box and associated cabling disappears at a stroke.

Second, the crossover is operating at line level, so the considerations as far as components are concerned are the same as with, say, a preamplifier, and the quality delivered by such a crossover should be equivalent. The highest quali- ty components can be employed, such as metal-film resistors and plastic capaci- tors, for example.

But there are not simply benefits on the component side. The designer of an active crossover can design each ele- ment of the crossover – including inde- pendent adjustment of phase and amplitude, and filter curves as complex as are required by the acoustic system – without having to be concerned with issues such as matching driver efficien- cies or the impedance of the configura- tion.

In addition, there is another major bene- fit in that the amplifiers are connected directly to the drivers: there is one power amplifier per crossover band. The directness of the connection means that the amplifier can control the driver over

its entire range. DC coupling between amplifier and driver results in a high damping factor.

In simple terms, this means that if the speaker cone makes a movement other than because of an input signal – as a result of a resonance, for example, the electrical energy generated by this movement is fed back to the amplifier and restrains the motion of the cone – allowing the amplifier to control the driver.

This electromagnetic damping reduces resonance, cone effects and spurious responses. Not only that: the direct con- nection between amplifier and driver means that the amp can control cone movement beyond the range of the crossover band assigned to it – impor- tant, because a tweeter’s resonant fre- quency, for example, can often be out- side the frequency band supplied to it by the crossover. In an active system, the amplifier can deal with this; in a passive system, it can’t.

This tight control also allows a Meridian to sound excellent at any level, from a whisper to a surprisingly loud shout. It’s also important to consider the loud- speaker as a complete system. For the designer, this gives a great deal more possibilities than simply multi-amping an existing passive design.

Less power, more sound

On the face of it, there is a downside to this approach: the system requires a power amplifier per crossover band, rather than just one.

The truth is, however, that the active loudspeaker is much more efficient. When a single amplifier is used in a pas- sive system, apart from the power wast- ed producing heat in the crossover, there also have to be allowances in the power amplifier for all manner of extraordinary unknowns: strange imped- ances at certain frequencies, a wide