Welcome to Linear Audio, your tech audio resource! Linear Audio Volume 1 is our second publication. Below you find the Table of Contents and Article Abstracts. You can get to know our authors better through the Authors tab. Enjoy!

Articles in this Volume:

A flexible 2/4/8 channel remote level control
Jan Didden

Project21, described elsewhere by Jean-Claude Gaertner, needed a flexible level control system for up to 4 amplifiers actively-driving speakers in at least 2 locations. Cabling having too low a WAF and IR remote control having synchronization issues, the choice fell on RF remote control. This article describes a small preamp-like control unit with an RF link to two 4-channel level controls integrated in Project21. It describes the level control circuitry, control concept and implementation. The article concludes with a universal 2/4/8 channel level board that can be used to control multi-channel levels in multiple locations using the same system components.


Vol 1

Square-Law class-A: a family of efficient class-A power amplifiers

Ian Hegglun

altIt is well known that Class-A gives the best sound quality in audio power amplifiers but the efficiency is so low that it is impractical to build much more than around 20 watts per channel. The Class-A amplifier here seems to break this rule – it can deliver 50 watts in Class-A using the same parts as other 15 watt Class-A amps. This breakthrough is achieved using Square-law curve devices in push-pull. The article gives the information to build your own efficient 50 watt Square-law Class-A amplifier as well as efficiency- and distortion calculation methods. Ian also gives all the information you need to build even bigger and better Class-A amplifiers and still be ‘green’.


Project21 - Part II - subwoofer and DSP system
Jean-Claude Gaertner

In L|A Vol 0 Jean-Claude described the development of the P21 mid-high satellites with optional passive cross-over. The satellites are now complemented with an active subwoofer. A DSP cross-over and correction unit integrates the subwoofer and satellites into a very high quality audio reproduction system. Jean-Claude describes the driver- and DSP selection process and gives guidelines how to get the most out of the DSP system. The article also describes a completely built unit and is finalized with extensive tests and measurements. Level- and balance control of all amplifiers in the active enclosures is achieved with an RF remote control system, described in another article in Vol 1 by Jan Didden.




A Universal Continuous-Time Active Filter
Ramkumar Ramaswamy

For loudspeaker design and prototyping you’d want a simple, flexible system to ‘play’ with the crossover. You would want a simple way to vary cross-over frequencies, filter characteristics and levels. Cheap and flexible DSP solutions are available for the final implementation, but for prototyping it would be better to just be able to turn some controls and switches and listen to the changes in real time rather than input changes in a software application and then listen to the incremental changes. RR as he prefers to be called developed exactly such a flexible analog cross-over that’s a great tool for the speaker designer, allowing continuously-varying cross-over frequency, on-the-fly transfer function change and custom-implementation of your choice of transfer functions. The author can also supply PCBs and a professional-looking front panel to help building this versatile unit.


Low-noise Microphone Preamplifiers
Scott Wurcer

Scott Wurcer provides an in-dept analysis of what it takes to build a state of the art mic preamp. An extensive discussion on the noise characteristics of mic capsules is followed by selection criteria for low noise JFET devices and trade-offs in circuit topologies. JFET Spice model modifications to make the models correspond better with the actual device parameters are next, and are used to verify theoretical predictions with simulations, leading to some prototype preamps. (This article will be followed in the next issue with a very high performance, state of the art all JFET class A mic preamp).

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