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Thoughts on the Audio Playback Chain


The advent and breakthrough of digital music formats have changed the way music is played and consumed. By working with the new formats instead of palying them through a system devised for analogue sources significant strides can be taken to optimize the audio signal.

To reproduce audio a source is used to feed the sound material to a preamplifier whose job it is to attenuate the signal to a desired level. The attenuated signal goes to a power amplifier which drives the loudspeakers. Inside the loudspeakers a crossover filter splits the amplified signal to the various drivers inside the loudspeaker box.

This system has been used for over a century and works quite well, but it’s filled with compromises that distorts the signal and lowers the efficiency (which in turn degrades the signal even more due to the even larger amplification needed). A couple of these compromises are actually quite easy to remedy.

The source does the exact same thing whether it’s a turntable or a computer; it outputs a low-voltage analogue signal of the sound to be reproduced. This signal is as accurate as the source can output and modern soundcards and compact disc player are extremely accurate, the distortion is in the order of a hundredth of a percent.

The preamplifier is not needed as most sources can output an attenuated signal. The other main purpose of the preamp, to switch sources, isn’t generally needed either due to the flexibility of the modern sources.

Since the signal needs to be split between different drivers, no driver can currently reproduce the whole audible sound spectrum to satisfying accuracy, a crossover is needed. However, it can be positioned before the power amplifier so that the crossover components only have to deal with a low level signal thereby greatly reducing the distortion added to the signal and the varying load otherwise presented to the amplifier.

The power amplifier is still needed, but since there are ways to use high efficiency loudspeakers and the losses from the crossover filter can be minimized a low power output amplifier still gives the same sound output level. A low power amplifier can be made smaller, more efficient and possibly introduce less distortion due to the lesser amplification of the signal (less heat to warm the components etc). A single voice coil represents a very easy load for an amplifier further enhancing the performance.

The loudspeaker is still the link in the chain that introduces the biggest interferences. The drivers contribute but also the enclosure and the room itself. In order to minimize these factors open baffle dipole speakers are preferable, the open baffle eliminates the need for a box, a solid baffle to hold the speakers and separate the front from the rear is all that is needed. The radiation pattern originating differs from the normal directional and omni directional pattern from box speakers, the dipolar pattern produced spreads much less energy to the sides and minimizes room resonances. By removing the box all stored energy apart from the driver is eliminated.

What this all leads to is a slightly different sound reproduction chain from what is normally used, the source unit directly feeds an active dipole speaker internally consisting of the low level crossover filter, power amplifier and the open baffle speaker.

Further Reading

Linkwitz Lab: Dipole speakers and active crossovers.
Bit-Tech’s Pc Audio 101.

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