High-End-Preamp （1）

Published:2011/7/22 2:27:00 Author:Phyllis From:SeekIC

By Benjamin Hinrichs

The modern design of this preamplifiers yields audiophile specs, convenient operation and an attractive price. This is made possible by using a top-end digitally controlled attenuator/amplifier IC.

Analogue audio electronics appears to have entered a dormant stage. In our present age of CDs, DVDs and MP3, traditional preamplifiers have been relegated to the role of signal distribution and, primarily, adjusting the volume level.

Our expectations for a modern high-quality preamplifier are that in addition to being easy to use, it should perform these signal distribution and volume adjustment functions with the greatest possible accuracy and the least possible distortion. And that is exactly where things start to get difficult.

Volume controls are commonly implemented using potentiometers, which are available in a wide variety of price ranges and types. Since we are normally dealing with a stereo signal, we need two mechanically coupled potentiometers. The decisive factor is the tracking of the two potentiometers, since this determines how closely the volumes of the right and left channels will match each other. Poor tracking is especially noticeable (and disturbing) at low volume settings. If we also want to have a balance control, we need an additional set of coupled potentiometers, and the tracking errors will add together.

The maximum permissible tracking error for ’audiophile’ sensibility is 3 dB, but ideally it should be less than 1 dB. As can easily be seen from Table 1, these values are clearly exceeded by normal carbon-film potentiometers, and even high-quality carbon-film potentiometers have difficulty maintaining adequate tracking accuracy with increasing age.



An audiophile alternative to potentiometers is to use high-quality, multi-position rotary switches with close-tolerance resistors. However, the right-hand column of the table shows that suitable special rotary switches having extremely low crosstalk and contact resistance cost around £80-90, which is rather expensive.

The disadvantage of this solution involves more than just the price (we will need two or three such switches for the volume and balance controls and the input selector switch, if present), since rotary switches cannot be remotely controlled.

If we wish to have the convenience of remote control as well as excellent tracking, there’s no getting around a design using conductive-plastic potentiometers with a motor and the associated control electronics and mechanical parts. This also involves considerable effort and expense, and just about everything must be duplicated for a balance control.

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