Audio Projects

I've received a lot of inquiries about what inspired
my audio designs. Coley Audio was intended
to be a retirement tube audio business... until I lost most of
my hearing. It was fun and technically challenging but
a very difficult business if you are hearing challenged.

Contact: craigjcoley@gmail.com

Coley 215SE Stereo Amplifier

The Coley 215SE is an original Single Ended Triode (SET) design, using a pair of KT-150 tetrodes wired in the triode configuration. Low level amplification is handled by a single classic 6SN7 per channel. Extensively modeled in LTSpice, this amplifier is one of my most popular minimalist designs.

This design was intended from the start to be reproducible, use readily available components, produce a usable amount of power, and use a power supply voltage no greater than a similar guitar amplifier. By using Hammond transformers and current production Tung-Sol tubes, reproducibility was greatly improved.

The stereo design uses a conservative self-biased Class A circuit that idles at 60W per channel for 15 watts audio without the use of negative feedback. When paired with the Coley OB-12 open baffle speakers, the sound is astounding for classical and Jazz music reproduction.

This design has been successfully built and enjoyed by DIY audio builders all over the world, including the United States, Great Britain, Germany, Italy, and India.

The prototype amplifier that appears in the photographs below is owned by Craig Morgan of Fort Worth, Texas.

Coley 125SE Monoblock

The Coley 125SE is an original Single Ended Triode (SET) design, using a Soviet era GM70 modulator triode. Preamplification is handled by a single classic 6SN7. Built largely as an experiment to answer conflicting questions about the GM70, the sound quality is surprisingly good.

A lot has been written about the plentiful and rugged GM70, originally built for the Soviet military. Most designs use a tube almost as big as the GM70 as a driver which was perplexing. In reality the problem many designers were having was low plate voltage and trying to make up for it by driving the grid positive. This minimalist design can easily drive the GM70 to full power with only a single 6SN7 when running the GM70 at 1100V on the plate. Its simplicity has garnered considerable online interest from hobbyists.

The amplifier uses a self-biased Class A design, idling at 120W and easily producing 30 watts audio output. The use of modest negative feedback lowers the amplifier's output impedance enough to be usable with a wide variety of speakers. Not many DIY builders have attempted this design I suspect partly due to the 1100V power supply.

The prototype design shown in the photographs below is owned by Steve Vargo of Bluff Dale, Texas.

Coley 203SE Stereo Amplifier

The Coley 203SE is an original Single Ended Triode (SET) design, using a 6BM8 triode/pentode wired in the triode configuration. The design was intended to create a good performing but economical desktop tube stereo. It is a minimalist design, simulated in LTSpice, and easily reproduced using inexpensive components.

The amplifier uses a self-biased Class A design, idling at 9W and producing 3 watts audio output per channel.

The prototype design shown in the photographs below is owned by Jonathan Coley of Rohm, Texas.

Coley W225 Stereo Amplifier

The Coley W225 is an original 25W per channel Williamson Amplifier, based on a design first published in the late 1940s. Using a pair of 6L6 tetrodes per channel with ultra-linear output transformers, the amplifier achieves a measured THD less than 0.25% at full power. Preamplification and phase splitting is performed with 6SN7s. All of the tubes used in the build were NOS Soviet equivalents of US tubes, keeping the vintage look and feel.

In a mix of old and new technology, this amplifier is fitted with a wireless receiver, relays, and motorized volume control that allows remote control from a small, four button key fob. It also has a Bluetooth 4.0 receiver that automatically switches from rear accessory jacks when connected to iPhone or iPad.

As a system, the Coley W225 amplifier was paired with Coley OB-15 open baffle speakers and is currently owned by Trang Myers of Keller, Texas.

Coley W210 Stereo Amplifier

The Coley W210 is an original design based on a scaled down Williamson amplifier meant to serve as a small desktop stereo. Using a pair of 6V6 tetrodes per channel with ultra-linear output transformers, the amplifier achieves 10W per channel. Preamplification and phase splitting is performed with a single 6SL7 per channel. The layout and wiring were a bit of a challenge due to the small size.

As a system, the Coley W210 amplifier prototype was paired with custom desktop speakers using ribbon tweeters and is owned by Jonathan Coley of Rhome, Texas.