This week, with a little help from a Roomba, [Dino] built a guitar pickup and preamp that sounds marvelous. A pickup takes vibrations from the guitar and turns them into an electrical signal which can then be amplified and broadcast. He grabbed a long-dead Roomba which has slowly but surely been donating its organs for his weekly projects. After plucking out a piezo element he grabbed a bag of Junction gate Field-Effect Transistors (JFET) and built a preamp circuit around one of them.

JFETs operate in much the same way as MOSFETs (which we took a look at last week). [Dino's] design adds a few resistors and capacitors to tune the gain and decouple the circuit from the input and power rails. He epoxied the piezo element inside the guitar and connected leads between it and a jack mounted in the body. As always, he does an excellent job of explaining the concepts behind the design and outlining the build techniques that he used. We’ve embedded his video after the break.

The most recent installment of [Dino Segovis’] Hack a Week covers the construction of a simple NPN transistor audio preamp. Some time ago, he built a small audio amplifier using an LM386 which worked well, but didn’t quite get his music as loud as he would like it. He decided to build a preamp to complement his amplifier, and demonstrates how you too can build one with just a small handful of components.

As the name probably suggests, the cornerstone of this amplifier is an NPN transistor. He explains that a forward bias is applied to the base-emitter junction, which results in the transistor operating halfway between its cut-off and saturation regions. Both halves of the input audio signal are superimposed on this bias voltage, resulting in a decent amount of gain across both channels from a relatively small package.

The preamp isn’t going to win any awards among audiophiles, but it is definitely a great beginner project. Its a novel way of demonstrating how transistors work, while producing a useful takeaway piece of audio equipment at the same time.

Continue reading to see a video showing just how big an effect [Dino’s] NPN preamp had on his music.

[Dan Hemingson's] been refining a design for building a tetrahedral ambisonic recording system. This is a set of four microphones used to record audio that can later be mixed down for a three-dimensional listening experience. His goal is an easy and inexpensive build while maintaining the highest fidelity standards possible. Lucky for us he’s made a set of extremely detailed build instructions you can use to make your own. In addition to the mounting bracket seen above he has also developed a pre-amp module that connects to the four mics; it’s part of the build instructions with schematic and board layout files available as well.

[Thanks Isaac]

If you’re curious about tube amps but don’t have a firm enough knowledge base to dive right in you might want to try a kit. [Mark Houston] reviewed one such kit and we enjoyed reading about his experiences. It comes with everything you need save soldering tools, an enclosure, and the final connectors ([Mark] used RCA connectors). There is a full schematic available and the assembly instructions take you through tube matching and using that piece of copper coil you see in the picture to wind your own inductor. Consider trying this primer before you jump into building a single tube, multiple tube, or an amplifier of your own design.

[Thanks Gio]

On a side note, This is the kind of crap that gives hackers a bad name. I hope the jerks behind it end up as Soylent Green.

[Darkrom] has set a new standard for Hack-A-Day readers… I haven’t seen it in person, but that looks like a legit Hack-A-Day tattoo.

[null] sent in a new use for a frequency generator, a spare car amp, a sub-woofer and a plastic coffee can – brass cartridge polishing.

[LoopyMind] sent in this Game Boy Advanced Movie Player IDE hack. It’s pretty much a direct CF to laptop drive cable with an external battery supply.

[Dingolishious] sent in a POE UPS/remote power control solution. Could be handy if you’re using many POE devices, or if you’re having power issues. He added an inexpensive remote power monitor/switch solution behind his UPS. It senses power outages and kicks out an email – and allows remote power cycling of his POE devices. Of course, if you’ve got a linux box behind the UPS, it can monitor the output from the UPS and send notifications.

[William] added a preamp stage to his iKEY usb recorder. looks like an interesting toy – it’ll record audio directly to a USB flash key. The pre-amp allows him to record in more challenging environments.

[Andrew] noted a simple mod to increase the deadly fire power of the ubiquitous airsoft pellet gun. It’s just a matter of reducing some extra space in the spring compression area.

Last but not least, [VIPER] modded his projector to use a 12v halogen headlight bulb. Not a bad idea – at one point I was pondering a 550 watt source four halogen as a possible replacement.