Hacklet 85: Alternative Audio Amplifiers

When you think of amplifiers, you’re probably thinking of audio or some big ‘ol power amps for radios. While interesting, there are some very interesting ‘alternative’ amplifiers floating around hackaday.io that are more than just power amps, and exceedingly useful, to boot.

1601181393316190625[Ronald] bought an XMS amplifier a few years ago, and although it worked well, every time he changed the input, the loudness had to be toggled. One thing led to another, and he realized this amplifier had a four-channel audio processor that could be controlled by I2C. This was the beginning of his Network Amplifier.

Inside the box is a Raspberry Pi that controls a PT2314-based amplifier. Also included is a 2×16 character LCD, a few LEDs, switches, and a rotary encoder.  There was an Android app that controlled the amplifier, but this was discarded for a better looking web-based solution. Now [Ronald] has every audio source available over WiFi.

973501443636885535What if you want an audio amplifier without a speaker? Wait, what? That’s what [DeepSOIC] is doing with his experiments in ion wind loudspeakers.

‘Ion wind lifters’ have been around for decades now, mostly in the labs of slightly off-kilter people who believe this is the technology aliens are using to visit earth. Nevertheless, ion wind lifters produce an airflow, and if you make that wind variable, you get sound. Pretty cool, huh?

The amplifier for this project uses a tube to modulate kilovolt supply through the ion ‘blower’. Does it work? Sure does. [DeepSOIC] got a piece of 0.2 mm nichrome wire to discharge ions into the air, after which the ions drift into the second electrode. The result is sound, and the entire project is built deadbug style. It really doesn’t get cooler than this.


2981611414932529525Continuing with the tube amp trend, [Marcel] built the cheapest little tube amp around.

The design of an audio tube amp is fairly simple business. First, you start with a big ‘ol transformer, and rectify the AC into DC. This gets fed into a preamp tube, and this is fed into a bigger power tube.

In about 50 years of development, tube designers had the technology down pat by the mid 1950s, and triode/pentode tubes were created. This allowed tube designers to condense two amplifier stages into a single tube. While this setup was usually used for cheap, toy-like electronics, you can still buy the ECL82 tube today.

[Marcel] took one of these tubes, added a rectifier tube, transformer, and big cap to create the simplest possible tube amp. Use it for guitars, use it for hi-fis, it’s all the same. It’s not going to sound great, but it is a very easy amp to build.

All of these interesting audio amplifier projects are curated on this new list! If you have a build that amplifies sound in an interesting way, don’t be shy, just drop [Adam] a message on Hackaday.io and he’ll add it. That’s it for this week’s Hacklet. As always, see you next week. Same hack time, same hack channel, bringing you the best of Hackaday.io!

How To Tell If You’re Installing Foil Capacitors Backwards

It only takes one mistake to realize electrolytic capacitors have a polarity, but if you’re working with old tube gear, tube amps, or any old equipment with those old orange dip, brown dip, or green dip foil capacitors you also have to watch your polarity. These old caps were constructed with a foil shielding, and there’s always one side of these caps that should always be connected to the chassis ground. If you don’t, you’re going to get interference – not something you want in an amplifier circuit.

Old caps that have long since given up the ghost usually have a black band designating whatever side of the cap the ‘foil ground’ is. This is the side that should be connected to ground. If you look at modern foil caps, you might also see a black band on one side of the cap, which should – if we lived in a just world – also designate the foil ground. This is not always the case.

To properly test foil caps and determine which side should be closer to ground, you can construct a small tester box that’s more or less an h-bridge with a single switch and a pair of alligator clips in the middle. Connect the cap to the clips, put the output of the circuit in your scope, and flick the switch: the direction that has the least amount of interference is the denotes the foil ground of the cap. Replace those old caps in your vintage equipment with a new, correctly oriented cap, and you’re well on your way to having a great sounding amplifier.

Video below.

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A Nicely Designed Stereo Tube Amp

tube amp

Most of the work that [Ron] has done in the past with vacuum tubes and solid state electronics has been repair. At 59 years old, he finally put together his own stereo tube amplifier and we have to admit it definitely has an awesome look.

The platform is built around the well-known 6V6 beam-power tetrodes which are mostly used by major audio brands for their guitar amplifiers nowadays. The Dynaco 6V6 circuit based PCB was bought from China and minor changes were made to it. The amplifier uses one transformer to convert the US 120VAC into 240VAC and 9VAC, the first being rectified by a glassware PS-14 power supply while the later is converted regulated at 6.3V for the tube heaters. The output stage consists of two Edcor audio transformers (one for each channel) that converts the high voltage for its 8 ohms speakers. The home-made chassis provides proper grounding and as a result you can’t hear any background noise.

We are very curious to know if some our readers have been experimenting with glass tubes for audio applications. Please let us know your experience in the comments section below.

DIY Headphone Tube Amp Builds On Existing Design

DIY Tube Headphone Amp

[Simon] wrote in to tell us about a headphone tube amp that he just built. It is based on schematics at diyaudioprojects.com that were actually featured on Hackaday in the past. [Simon’s] design adds an on board regulated power supply and a volume control for the input. Effort was made to keep the PCB single sided to facilitate making this at home.

DIY Tube Headphone AmpThe 12AU7 is popular due to its ruggedness and tolerance for low operational voltages. This amp design uses a plate voltage of 12, although the 12AU7 can handle up to about 330.  Since the 12AU7 is of the Twin Triode variety, one tube can be used to amplify both a left and right audio channel.

The case for the amplifier is laser cut plywood. The top piece is kerfed so that it can bend around the radii of the front and rear panels. The top also has a hole cut in it to allow the tube to peek out through.The pieces look nice but, unfortunately, he doesn’t show the case and amp in an assembled state.

If you’re interested in building one of these, [Simon] made all of the Eagle and Case files available. The total cost of the project was £25, about $43 US. To learn more about how tube amplifiers work, check out this Retrotechtacular from earlier in the year.

[Tymkrs] Tombstone Guitar Amplifier


[Atdiy and Whisker], the team behind [The Tymkrs] YouTube channel, are at it again with a tombstone guitar amp project.(YouTube playlist link) Their amp began life as a Philco Tombstone radio which had seen better days. By the time [Tymkrs] got their hands on it, it was just a shell of its former self, as someone had already stripped all the electronics.

The amplifier itself is a disused Leslie tube amp [Tymkrs] had on hand. An LM386 serves as the pre-amp, making this a hybrid solid and vacuum state machine.

The tombstone speaker is especially interesting. [Tymkrs] went with an electrodynamic field coil speaker. Field coil speakers have no magnets, instead using a high voltage (approx 90V DC) coil to create a magnetic field for the voice coil to push against. This sort of speaker was commonplace in the 1930’s, as large magnets couldn’t be made lightweight enough to be used in a speaker. As magnet technology improved, permanent magnets became a staple in speakers.

[Tymkrs] paid special attention to the finish of the amplifier. They brought the tired old radio back to a high shine, then added a Metropolis inspired overlay from aged copper-clad board. The result is an amp that looks great and sounds great!

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Trash to treasure Bluetooth radio and tube amp build


The before image doesn’t look all that bad but we were still impressed with what went into the restoration of this radio. Perhaps restoration isn’t the right word since it didn’t manage to hold on to any of the original internals. This is more resurrection of a retro radio case for use as a Bluetooth radio.

At first look we didn’t notice that the original knobs were missing. The speaker fabric is ripped and the glass on the tuning dial is broken as well. [Yaaaam] happened to have another antique radio with interesting knobs — but he didn’t just transplant them. He made a mold of one knob and cast three replacements for the radio. After refinishing the wood he replaced the fabric and things were really starting to look up.

All of the electronic components were removed and a new tube amp was built on the original metal chassis. It uses a Bluetooth module for input which facilitates using your smart phone as the playback device without involving any wires or other nonsense. Two problems popped up after the project was completed. The first replacement power supply overheated. The second replacement had a different problem, needing some additional shielding to prevent noise from creating unwanted… noise.

This looks so much better than modern injection molded plastic shelf systems. But there are some fun wireless hacks out there for those too.

A really, really tiny tube amp

After building his first tube amp from a kit, he set to work on his next amp build. Since tube amps are a much more experimental endeavor than their solid state brethren, [Jarek] decided to make his next amp unique with military surplus subminiature tubes, and in the process created the smallest tube amp we’ve ever seen.

Instead of bulky 12AX7s and EL34s tubes usually found in tube amp build, [Jarek] stumbled upon the subminiature dual triode 6021 tube, originally designed for ballistic missiles, military avionics, and most likely some equipment still classified to this day. These tubes not only reduced the size of the circuit; compared to larger amps, this tiny amplifier sips power.

The 100+ Volts required to get the tubes working is provided by a switched mode power supply, again keeping the size of the final project down. The results are awesome, as heard in the video after the break. There’s still a little hum coming from the amp, but this really is a fabulous piece of work made even more awesome through the use of very tiny tubes.

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