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By Jon Wong |
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When digital audio appeared 10 years ago, it has greatly improved the fidelity of signal sources. There was a greater requirement of line amplifier, as the CD output increased to about 2 volts. The line amplifier has to be able to handle higher voltage swing, and to deliver better current. It also has to cope with the higher dynamics and the better signal to noise ratio of CD audio. Unfortunately, the transistor, which has rapidly replaced the vacuum tube, was a less ideal candidate to handle truly hi-fi signals. The IC modules were even worse than discrete components, and usually was the cause of poor sound, even though there has been great improvement lately.
The traditional vacuum tube sounds transparent, nice, smooth and pleasant. Although it lacks the punch and power of the BJT, it has better definition, lifelikeness and presence. Its exotic and outstanding sound makes the vacuum tube amplifier still widely popular today. It is almost impossible for modern electronics to deliver such sound. Also, the tube can remove the harsh and zaggered edges of digital sound, and can open up the sound stages. That’s why many are returning to tube pre-amps. Actually, many expensive audio gears’ designs were taken from or developed from classic circuitries. Although there are many excellent transistor amplifiers in the market, they are usually less impressive than a tube device.
Actually, there is no absolute design in this world. Every design is a compromise. A compromise between the reality and the ideal. It is possible to analyse the successful designs of well known pre-amps, and deduce what their success factors are, so as to corporate the success factors into one design. |
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Digital audio has improved tremendously in the past ten years. This has to give credit to the ever rapidly advancing computer development. In comparison, analog circuit design seems to be lagging. Apart from the designs from Pass Lab, there are few announcements of new topologies or break through. Actually, regardless of how fanciful digital technology is, it still must rely on analog means to reproduce the music. There is a need for better analog design optimisation. As the communication industry is rapid developing as well, there will be more demands of mixed signal integrated circuits. As such, there will be demands for the better ideas and designs. As far as audio is concerned, many factory amplifiers still employ traditional circuitry. In actual fact, there is a general trend in the deterioration of sound quality with the popularisation of PCs.
The transistor amplifier, due to the BJT switching characteristics, sounds harsh, dry and lifeless, unless care and consideration are given in its properly design. Although it sounds powerful and punchy, and widely accepted by the mass public, power does not mean good sound. MOSFET is gaining wider popularity as it can mimic the sound of vacuum tube, but it retains the drive capability and strength of the transistor.
For a pre-amplifier that is both analytical and powerful in sound, one has to examine the circuit from two approaches: circuit design and listening. There was a long time the author was listening to music using a passive pre-amplifier only. That was based on the believe that CD has a 2V peak-to-peak output. Although the passive pre-amplifier has the facility of switching sources and volume control, it lacks the drive capability and enhance the sound quality of a properly designed pre-amplifier. In actual fact, the author believes that the pre-amplifier is the soul of a good system.
