Preamp Muting Circuit

Oftentimes a muting circuit for the output of a preamp is left out of DIY applications as an unnecessary or annoying part of the project. This is usually due to the lack of awareness that a simple circuit can perform such a duty. Anyone who occasionally forgets to turn the power amp on last, or switch it off first, endures the annoying thumps and noises that often occur from the preamp upon power cycling. This article proposes a simple circuit for preamp muting. The article was originally published in audioXpress, March 2010.

In this amazing project by Gregory L. Charvat, two pre-amplifiers are used with a battery powered high voltage supply. The high voltage supply works by creating a square wave with a 555 timer. The square wave is fed into a small audio power op-amp. The output of this op-amp is back-fed into the secondary of an audio output transformer, generating high voltage AC 170V at 40 kHz. This signal is rectified, filtered, and regulated in a conventional sense. The entire system runs on 8 AA batteries and should operate for approximately 4 hours continuously. This article was originally published in audioXpress, March 2010.

audioXpress December 2011 introduced readers to Willow, a “proof-of-concept” project featuring a PMBF4393 JFET and HA5002 buffered preamp, built as a companion to the author's 5002 power amp, and the first to use this typology. After considerable positive DIYer feedback, the Willow was revised and updated, and this article, Willow Revisited was published in February 2016.

This simple project provides a convenient method to remotely power audio equipment. Many manufacturers make audio equipment and power products that can be remotely switched with a low DC trigger voltage. With the trigger supply, my entire system can be turned on and off with one low-current switch. This article was originally published in audioXpress, November 2013.

Tubes or transistors? Whatever your prejudices, this article may make you change your mind about how you view hybrid designs. audioXpress featured this modular hybrid amplifier project in February 2001, which was referenced multiple times by other authors in many subsequent projects. The project features a shunted regulated push-pull (SRPP) stage design, using the ECC82/12AU7 tube combination, followed by a power stage, using two to eight complementary power MOSFETs in parallel.

Tube amplifiers can benefit from regulated and stabilized power supplies, especially sensitive preamplifier stages, but also (single-ended) power amps. But traditional high-voltage regulators add several extra tubes, complexity, and power losses, while good performance is not easy to obtain. This article, published in audioXpress, April 2009, describes a novel design that is very simple yet offers excellent performance. Output voltage is fully adjustable with a single resistor without any compromises in performance.

Ethan Winer explains how to "Build the Mojo Maestro," a simple device that allows "subtle distortion" or saturation from any recording or track. Just using two diodes and a few resistors, Winer designed this simple box using the free LTspice simulation software and only half a dozen parts. And the results can be heard on the audioXpress website, with Winer playing multiple instruments, with and without the "Mojo effect." This article was originally published in audioXpress, October 2017.

Keeping things small and inexpensive — and at the beginner’s level — Rick Spencer shows you how to build your own single-ended amp. For about $10 worth of tubes and some small transformers, you can build this neat little amp in one weekend. This article was originally published in audioXpress, April 2004.

In this classical tube amplifier project, author Bob McIntyre offers what is effectively the foundation for many designs that we can find currently in the market. This is not surprising. The Mighty Mouse is indeed a good sounding and inexpensive project that is also a good introduction to tube amplification. This article was originally published in audioXpress, April 2006.

In this article for audioXpress, Bill Reeve discusses “An Alternative to Linear Regulators.” A project were he searches for equivalent power-line ripple rejection with less power dissipation, using a source follower-based (SFB) low-pass filter. Because no one wants to listen to an audio amplifier that hums. This article was originally published in audioXpress, April 2014.