Hackaday Prize Entry: High End Preamps

While compact disks are seeing an uptick in popularity thanks to a convenient format that offers a lossless high-quality 44.1 KHz sample rate with 16-bit depth, some people are still riding the vinyl bandwagon of 2010. With that comes a need for the best hardware, and that means expensive cartridges and preamps designed by someone who knows what they’re doing.

For this year’s Hackaday Prize, [skrodahl] is building a really, really good preamplifier for moving coil turntable cartridges. It’s already built, it’s already tested, and the results are good: it produces between 36 and 46dB of gain, -110dB of dynamic range, and a signal to noise ratio of 79.46 relative to a 5mV input. That puts this preamplifier into the same territory as preamps sold with serial numbers, crystal lattices, and other audiophile nonsense.

The quality of this preamp comes from the design, and like any good open hardware project, [skrodahl] has made the schematic, PCB, and layout of this preamp completely open. It’s a great preamp, and a great entry for the Hackaday Prize.

Simple Vacuum Tube Preamp Results in a Beautiful Build

We have no intention of wading into the vacuum tube versus silicon debates audiophiles seem to thrive on. But we know a quality build when we see it, and this gorgeous tube preamp certainly looks like it sounds good.

The amp is an attempt by builder [Timothy Cose] to give a little something back to the online community of  vacuum tube aficionados that guided him in his journey into the world of electrons under glass. Dubbed a “Muchedumbre” – Spanish for “crowd” or “mob”; we admit we don’t get the reference – the circuit is intended as a zero-gain preamp for matching impedance between line level sources and power amplifiers. Consisting of a single 12AU7 in a cathode-follower design and an EZ81 for rectification, where the amp really shines is in build quality. The aluminum and wood chassis looks great, and the point-to-point wiring is simple and neat. We especially appreciate the neatly bent component leads and the well-dressed connections on the terminal strips and octal sockets. There’s a nice photo gallery below with shots of the build.

As much as we appreciate the miracles that can be accomplished with silicon, there’s still magic aplenty with vacuum tubes. For more thermionic goodness, check out these minimalist homebrew vacuum tubes or these artisanal vacuum tubes.

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Beautiful tube amp uses a TV tube


Most audio tube amps we see use common tubes – usually a 12AX7 for the preamp and one of the more common power tubes such as an EL34.[Daniel], on the other hand, decided to build his own audio tube amp with a 13EM7, a tube originally used for a television’s vertical oscillator. The resulting project is a wonderful stereo amp that sounds really good, to boot.

[Daniel] picked up the idea of using a 13EM7 tube from [Fred Nachbaur]’s MiniBlok SET amplifier. This very tiny 1-watt tube amplifier uses a single tube originally designed for use in old, old televisions. The secret behind this build is the fact this tube is actually two triodes in one package; one side of the 13ME7 has tons of gain but not much power, making it perfect for a preamp. The other side has a lot of power, useful for delivering two watts of power into a speaker.

After [Daniel] etched a few boards for his amp, he milled out a piece of wood for the chassis. When everything was mounted he had an awesome looking stereo amplifier that also sounds great.

Making a phono preamp for a first electronics project

Nearly everyone’s first electronic project is something that blinks a LED. There are a million ways to go about this ‘Hello World’ project of electronic design; 555 timers, microcontrollers, or maybe even discrete components if you’re really cool. When [miceuz] was asked by a friend to help with his first electronic project he eschewed the usual blinking LED project and taught him how to build something he actually needed: a phono preamp for an old turntable.

Back in the day when vinyl was king, albums needed to be mastered to play on a record player. The mastering process cuts some of the bass and increases the treble. When the record is played, this process needs to be reversed. It’s a preamp that does this job by attenuating the high frequency sounds and boosting the thumping bass.

[miceuz] found a nice DIY RIAA preamp  project and found a nice little op amp  somewhere in his parts bin. After laying out the circuit, [miceuz] etched a few boards and taught his friend how to solder SMD components.

Of course the project didn’t work the first time around, but after poking around with a meter and checking out the old turntable, the preamp came to life with the clang of chords from an old record. If you’d like to build your own, you can get the files from [miceuz]’s git.

Log guitar uses tube as a bridge, actually is the blues

In the never ending quest to replicate the tone of depression-era blues records, [Valve Child] managed to build the most backwoods guitar ever seen.

The body of [Valve Child]’s slide guitar was taken from the limb of a red gum tree felled during a wind storm. After taking a chainsaw, router, and sander to the guitar, [Valve] sealed it with linseed oil.

The real beauty of this build comes from the bridge and electronics: the pickup is made from six stacks of magnets encased in hot glue and wound with enamel wire. The bridge of the guitar is actually made from a 6GM8 dual triode. Not only does this provide the guitar with a wonderful brassy sound, the tube serves as a wonderful low-tech preamp when powered by a 6 volt battery.

The three strings on the guitar are tuned DAD, perfect for the likes of [Robert Johnson], or, for the younger kids, [Jack White]. Surprisingly, [Valve Child]’s guitar actually sounds really good. as heard in the video demo after the break.

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[Dino’s] JFET guitar preamp with piezo pickup

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.

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Quick and easy audio preamp


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.

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