Update a Pair of MTX 5i 5.25” Bookshelf Loudspeakers

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.

This article focuses on the Poddwatt Series II stereo integrated valve amplifier, a project with roots that go back several years. The design continues to be relevant and to evolve. I call the basic design an “Oddwatt” because of its “odd” configuration, using two stages. This article was originally published in audioXpress, May 2013.

Recently, I got bitten by the nostalgia bug and picked up a pair of old factory-wired Dynaco ST70s in pretty nice condition. On one of the Dynaco ST70s, I installed a new 6GH8 driver board instead of dealing with the rare 7199 issue, and I designed a small (3” × 3”) PCB to combine the bias supply and added the LED bias-setting circuit. This article was originally published in audioXpress, April 2013.

This versatile control circuit is pure Class A analog, allowing control over the ultra-high transparency SACD (Direct Stream Digital) or DVD-A multichannel surround formats with up to six power amp channels. As the author wrote when he experienced multichannel music in SACD, “With a good recording, this transparent naturalness extends to an enveloping soundfield that - at its best - can make you think you’ve been listening from just outside the concert hall doorway, and now you’ve walked inside!”

When this great tube amp project was published originally in Glass Audio magazine, Ed Dell wrote “Joseph Norwood Still is pushing the envelope. His most recent triode design, which features the Svetlana 6550s and Hammond transformers, establishes new output limits—up to 100W.” Indeed, this would become a very popular project at the time, and still is, considering the frequent references. This article was originally published in Glass Audio 3, 2000.

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.