An Accurate Bias Meter for Tube Output Stages

Accurately setting the bias level for push-pull output stages is important to maximize amplifier performance and tube life. This circuit measures error relative to the target bias current and error relative to a balanced condition in the push-pull pair. The meter is compact, low-cost, simple, and accurate thanks to the use of a window comparator IC - a Linear Technology LTC1042. This article was originally published in audioXpress, July 2012.

In this high quality project, Robert Nance Dee designs a potentiometer for accuracy and flexibility. A pot with 64 levels and precision metal film resistors that is hard to beat for sound or facility, as he describes it. The project started with a clean sheet, looking to address potentiometer design with a quality that rivaled existing designs while maintaining a reasonable cost. This article was originally published in audioXpress, September 2011.

Published in two separate parts in audioXpress October and November 2007, this hybrid amp design is a detailed and interesting project, frequently referenced by other DIY audio enthusiasts. We thought it was time to make it available online in its entirety, in the best possible quality. The article discusses the unity-gain, single-ended MOSFET output stage, the valve voltage amplification stage and wraps up with the power supplies, printed circuit boards, and construction.

Simon Brown rethinks what he needs as the amp that will be the centerpiece in his own audio system and perfects a previous design with a true hybrid design. As he writes, "This amp is a “hybrid” in many ways. There is the obvious way in which it uses both valves and MOSFETs. But it’s also a hybrid in its use of enriched Class A/B, which means it is running Class A up to a few watts. This is deliberate." Read the complete project now. This article was originally published in audioXpress, March 2007.

This article describes an improved design for a power supply dedicated to audio power amplifiers that exhibits ecological and economical qualities, ideal for audio enthusiasts. In this truly DIY project, easy to build, the author, Vincent Thiernesse, proposes a power supply dimensioned for 100 W RMS/4 or 8 Ω power amplifiers, that behaves like a resistive load with respect to the mains network. This project was originally published in audioXpress, June 2016.

This project is based on a previous audioXpress article: “A Solid-State Single-Ended Power Amp” by Ed Simon, published in the April 2006 issue. Ron Tipton was deeply impressed with this amplifier project which used only five transistors per channel, two of them being current sources. This article was originally published in the old audioXpress website as a PDF, but was never reposted. Now we make available online with updated references and links.

Here’s an amp project especially for first-time builders featuring the 6BQ5 tube. As Rick Spencer writes in its introduction, "In order for this beloved hobby of ours to thrive and survive well into the future, we must always strive to make it interesting to newcomers." With this in mind, Rick Spencer created this single-ended design using the 6BQ5/EL84 tube in triode mode. This article was originally published in audioXpress, October 2008.

In this simple amplifier project, Ed Simon describes a Class-A amplifier design that mainly intends to reproduce music with reinforced emotional expression. That's what you normally get with a quality design, but to achieve it, the author shares the complete concept, power requirements, circuit topology, construction and testing of this single-ended amp... in a solid-state design. This article was originally published in audioXpress, April 2006.

"There is nothing quite like a quality tube amplifier," states the author in the introduction to this directly heated triode amplifier project. The idea for the amplifier started from the author's discovery of the Loftin-White circuit. The author chose the 2A3 directly heated triode power tube to implement the Loftin-White circuit. The 2A3 vacuum tube is a power triode capable of outputting 3.5 W in a single-ended Class A configuration. This article was originally published in audioXpress, August 2013.

I recently purchased a pair of MTX Audio Monitor 5i 5.25″ bookshelf speakers because I needed small monitors for a new computer. I do some audio restoration, so I wanted good sound quality when I wasn’t using headphones. My tests determined that the reviewers were right about the MTX 5i’s weakness in the midrange frequencies. I measured the response and it was not especially flat. This convinced me to update the drivers. This article was originally published in audioXpress, June 2013.