HD-2: A Single-Ended Class-A Headphone Amp

This Class A tube headphone amp project, which the author created for Vintage Audio, includes a 6SL7GT for the input stage and a 6AS7G for the driver, and is able to drive headphones with impedance at or above 64Ω. The article provides a full description of design fundamentals and some useful guidance on the construction process. The result, as the author describes, should be “a satisfying headphone amp that will provide years of enjoyment.” This article was originally published in audioXpress, June 2008.

I built this amplifier to run my Dynaudio X12 speakers. The X12s are not known for efficiency, but these little mono blocks run the accurate X12s with ease. Don’t be fooled by their low power output. The limiting factor is voltage, not their ability to drive a load. This article was published originally in audioXpress, February 2012.

In this often mentioned project published in audioXpress, April 2001, the author explores the advantages of composite amplifiers and provides a detailed description on how he built his 50W per channel unit. A composite amplifier (also called nested-feedback amplifiers) consists of two or more operational amplifiers cascaded together with a negative-feedback loop around the entire network. Read the complete article now available online.

In this article from audioXpress archives, originally published in July 2003, Charles Hansen describes four hybrid power amps that anyone might consider for their own designs. Starting from the idea that an op amp provides both a differential input stage and high overall gain in one package, Hansen explores the ramifications of using an op amp as a combined input and voltage amplifier stage in power amplifiers. As he writes, “I did not build the four amplifiers shown in this article, but I did run a SPICE simulation to verify that they did perform as audio amplifiers."

This project is based on Erno Borbely’s EB-602/210 single-ended pure Class A amplifier and is capable of driving headphones between 32 and 600Ω. It consists of two identical but separate amps laid out on the same PCB. The schematic is the same as Erno’s extremely popular hybrid tube/MOSFET line amp, published in Glass Audio 1/98. However, the dual triode at the input is replaced by a dual JFET (SK389). This article was originally published in audioXpress, July 2004.

Speed up your crossover prototyping and preserve your crossover components with this true plug-and-play approach. As the author, Ramkumar Ramaswamy explains, today’s computer simulation software allows you to design passive crossovers extremely accurately. “Is there a way to effortlessly and reliably plug and play with a set of (fixed) capacitors across crossover prototypes without the fear of ending up with broken leads? This question drove me to design this passive crossover prototyping system”, he explains. This article was originally published in audioXpress, March 2007.

One of the most important speaker building projects published by audioXpress, THOR was designed by Joseph (Joe) D'Appolito, who is world-renowned for his pioneering work on MTM-based (mid-tweeter-mid) loudspeaker configurations. The original THOR transmission-line design was published in May 2002. For this online publication, audioXpress kept the original article unchanged, which has since been updated with availability of the original Madisound/SEAS kit and the publication of a complete kit project on the SEAS website. Read the complete article here.

In 2008, Nelson Pass submitted an article on its new F5 power amplifier design to audioXpress. At the time, he wrote: “I feel that it’s important to support AudioXpress, a magazine with which I have a 35 year history, and which also serves as a mainstay for the DIY community. Lately they have kindly printed projects that I had already posted on the web, but I decided that they deserve a little better than that.” Effectively, the F5 project was published in May 2008 and had a huge impact on the global DIY community and certainly contributed for the fact that audioXpress is still around today.

Many articles have appeared regarding projects using one of the most sturdy, versatile, and trustworthy audio tubes ever made — the 6550. Most of them show these audio beauties used in the push-pull configuration in order to obtain the greatest efficiency and the most power output. In this article, the 6550 is used in a clean, sweet-sounding, single-ended design. The amplifier has very few parts, is easy to build and repair, and the cost of the project is nominal when you consider the kind of sound that is produced. This article was originally published in audioXPress, September 2001.

In the February 2015 issue of audioXpress, Mark Driedger discussed a tube power amplifier controller based on the Arduino open-source microcontroller. In this follow-up article, Mark Driedger explains how to use some of the Arduino’s untapped capabilities to incorporate the bias meter into the amplifier controller with virtually no additional hardware. A temperature sensor and over-temperature shut-down capability are also included. This article was originally published in audioXpress, May 2015.