Preface

This was my contribution to the „Hack A Day“ design contest. The task was to develop a useful circuit to fit on a business card of given size

Check out the original site:

http://www.hackaday.com/dc/

 

Update

Since I received some emails from people interested in buying a PCB or kit of the amp I have decided to compile a kit.

It is available now!!!

I have created a new page for the project, which you can find here. You can dowload the manual for the kit there.

I have now added info on the prices of the kit and PCB. Thanks to everyone who wrote me!!!

 

Description

This is a business card sized full-fledged vacuum-tube multipurpose audio preamp.

The tube is a standard ECC83/12AX7 type. The amp consists of a standard gain stage with an approximate gain factor of 10. By adding a cathode bypass capacitor gain can be increased further (approximately 30 with a 4.7µF capacitor), if required.

The second stage is a cathode follower; it reduces the output impedance while maintaining output level. No solid-state components are used in the signal-path.

The circuit can be powered by a wall-mount 12-14 VAC power-supply , power consumption should be around 350 mA or below.

AC power is transformed by a 230V/9V transformer connected in "reverse" to approximately 190 VAC. After rectification and ripple-reduction the amp is supplied with approx. 140 V DC. To reduce humming, the heaters are connected in serial, they are powered by rectified and stabilized 12.6 Volt supply (LM7812 with1N4148 diode).

 

What makes this design qualify for the HACK A DAY contest?

• The amp can be used to be hacked into any kind of audio gear. You can add tube-power to your solidstate guitar combo, your active pc- or multimedia-speaker-set, your TV or even stuff them in your PC.
• It is a single layer PCB, so it can be easily fabricated by DIY enthusiasts in their garage or at the desk.
• Finally, tubes rule!!!

Schematic