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The development of artificial reverberation in the audio industry has been aimed at duplicating the sound of real rooms, more and more accurately.  No one ever considered the possibility of developing reverberation more desirable than real. Richard Burwen discovered that your ears really like to hear the high frequencies missing in room reflections above 5 kHz.  Their most important characteristic is not the spatial and echo effect, but thousands of peaks and valleys in the frequency response that tend to ‘clean up’ high frequencies in the music. Signal processing to achieve a wiggly frequency response having a carefully tuned trend can greatly extend the potential of an audio system and program material for realistic reproduction. This article discusses the thinking behind the system and explains the various processing options. Example sound files with different processing are available on-line at http://www.burwenbobcat.com .

A low-noise preamplifier with variable-frequency tone controls Vol 5

In 1996, Douglas Self published a design for what became known as the Precision Preamplifier, which included a treble/bass tone control with variable turnover frequencies. Starting with the idea to revisit the design with the benefit of years-long experience and newer parts, the design developed into much more than ‘just’ a tone control preamp. The necessary additional stage to correct the phase reversal of the tone control stage was called up for a Baxandall-type active volume control duty.

Then, reminding himself that we don’t all use symmetrical listening spaces, Douglas realized that a ‘vernier’ balance control would be very useful – but of course without interfering with either the tone or the volume controls. Add to that the need to maintain very low noise and distortion levels, and you have a article that is more like a tutorial on preamplifier design.

In his search for a good sounding headphones amplifier, Rob Scheepens has experimented with, and listened to, several different amplifier topologies. These were all tube based, like Single-Ended, Balanced, Standard Cathode Follower or White Cathode Follower, with the latter proving to be the best sounding of the lot. Eventually, he felt the all-to-familiar urge to refine the design. Having worked with tubes in amplifier designs for years, he decided to consider the use of semiconductors, and try to design around whatever was available in his parts bin. This article is the result of the design process which resulted in good sounding amplifier (at least to Rob’s ears) with some interesting features and parts, like a dual gate MOSFET as input stage.
 

Bruno Putzeys maintains that one of the causes for many, many ground problems in audio is the (subconscious) way of thinking of voltages as being a property of a single node, rather than as the potential difference between nodes. The usual method to try to solve it all is to use fully symmetrical balanced circuitry, and accept the additional expense in parts and board area. Yet, as Bruno shows in this article, it is perfectly possible to ‘get your audio off the ground’ with the usual single ended non-symmetrical circuitry. All it takes is a change of perspective and being consistent from start to finish.

The article closes with a ‘demo project’ that demonstrates the discussed principles. Thanks to the kind people at www.eurocircuits.com , a free PCB for this project is included with every purchase of Volume 5 (as long as stocks last).