Repairing Burnt Speakers with a Steady Hand

[Martin] seems to have a knack for locating lightly damaged second-hand audio gear. Over the years he’s collected various types of gear and made various repairs. His most recent project involved fixing two broken tweeter speakers.

He first he needed to test the tweeters. He had to remove them from the speaker cabinet in order to gain easier access to them. The multimeter showed them as an open-circuit, indicating that they had likely been burned. This is an issue he’s seen in the past with this brand of speaker. When too much power is pumped through the speaker, the tiny magnet wire inside over heats and burns out similar to a fuse.

The voice coil itself was bathing in an oily fluid. The idea is to help keep the coil cool so it doesn’t burn out. With that in mind, the thin wire would have likely burned somewhere outside of the cooling fluid. It turned out that it had become damaged just barely outside of the coil. [Martin] used a sharp blade to sever the connection to the coil. He then made a simple repair by soldering the magnet wire back in place using a very thin iron. We’ve seen similar work before with headphone cables.

He repeated this process on the second tweeter and put everything back together. It worked good as new. This may have ultimately been a very simple fix, but considering the amount of money [Martin] saved on these speakers, it was well worth the minimal effort.

Holiday Cheer From The ATtiny13

There are smaller microcontrollers than the ATtiny13. Some ARM chips will fit on the head of a large pin, and even in Atmel world, the ATtiny10 comes in a tiny SOT-23-6 package – a size normally reserved for surface mount transistors. The ‘tiny13, though, can be programmed with just about any ISP and comes in an 8-pin DIP. It’s the bare minimum if you’re looking to break out of the world of Arduino, and you can do some pretty cool things with it, like playing some holiday audio with an SPI Flash chip.

[Vinod] tried opening up a cheap camera pen, but in the course of disassembly a few traces broke. He was now left with a 4Mbit SPI Flash chip. This was obviously the time to investigate what could be done with a small microcontroller and a huge amount of Flash. and the Attiny13 audio player was born.

The circuit uses one PWM for audio out, and reads audio directly from the Flash chip. The UART on board the ‘tiny13 is used to update the Flash, and there’s also a switch to select between play and record. If you’re counting, that means there are 4 pins for the Flash, 2 pins for the UART, 1 for the switch, one for the audio output, and the power and ground rails, all in an 8-pin package. That’s a pretty cool way to use one pin for two different functions.

You can check out a video of the project in action below.

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The Four Thousand Dollar MP3 Player

[Pat]’s friend got a Pono for Christmas, a digital audio player that prides itself on having the highest fidelity of any music player. It’s a digital audio device designed in hand with [Neil Young], a device that had a six million dollar Kickstarter, and is probably the highest-spec audio device that will be released for the foreseeable future.

The Pono is an interesting device. Where CDs have 16-bit, 44.1 kHz audio, the Pono can play modern lossless formats – up to 24-bit, 192 kHz audio. There will undoubtedly be audiophiles arguing over the merits of higher sampling rates and more bits, but there is one way to make all those arguments moot: building an MP3 player out of an oscilloscope.

Digital audio players are limited by the consumer market; there’s no economical way to put gigasamples per second into a device that will ultimately sell for a few thousand dollars. Oscilloscopes are not built for the consumer market, though, and the ADCs and DACs in a medium-range scope will always be above what a simple audio player can manage.

[Pat] figured the Tektronicx MDO3000 series scope sitting on his bench would be a great way to capture and play music and extremely high bit rates. He recorded a song to memory at a ‘lazy’ 1 Megasample per second through analog channel one. From there, a press of the button made this sample ready for playback (into a cheap, battery-powered speaker, of course).

Of course this entire experiment means nothing. the FLAC format can only handle a sampling rate of up to 655 kilosamples per second. While digital audio formats could theoretically record up to 2.5 Gigasamples per second, the question of ‘why’ would inevitably enter into the minds of audio engineers and anyone with an ounce of sense. Short of recording music from the master tapes or another analog source directly into an oscilloscope, there’s no way to obtain music at this high of a bit rate. It’s just a dumb demonstration, but it is the most expensive MP3 player you can buy.

SingLock Protects Your Valuables from Shy People

Two Cornell students have designed their own multi-factor authentication system. This system uses a PIN combined with a form of voice recognition to authenticate a user. Their system is not as simple as speaking a passphrase, though. Instead, you have to sing the correct tones into the lock.

The system runs on an ATMEL MEGA1284P. The chip is not sophisticated enough to be able to easily identify actual human speech. The team decided to focus their effort on detecting pitch instead. The result is a lock that requires you to sing the perfect sequence of pitches. We would be worried about an attacker eavesdropping and attempting to sing the key themselves, but the team has a few mechanisms in place to protect against this attack. First, the system also requires a valid PIN.  An attacker can’t deduce your PIN simply by listening from around the corner. Second, the system also maintains the user’s specific voice signature.

The project page delves much more deeply into the mathematical theory behind how the system works. It’s worth a read if you are a math or audio geek. Check out the video below for a demonstration. Continue reading “SingLock Protects Your Valuables from Shy People”

Using a Headphone Jack as a UART

We’ve seen audio ports being used to establish a communications channel between a computer and a microcontroller before, but nothing quite as slick as this. [Gordon] is using a webpage running on a tablet to send Javascript to a microcontroller where the entire program is interpreted.

[Gordon] is using the Espruino Pico, a board that’s on Kickstarter right now. This tiny board is built around a javascript interpreter, allowing code to be written and updated on the fly without mucking around with bootloaders.

This technique can be expanded to provide bidriectional communication between a microcontroller and a computer. On the project Github, [Gordon] used the microphone pin on a TRRS jack to sent data to a computer. It needs two more resistors, but other than that, it’s as simple as the one-way communications setup.

[Gordon] put together a few demos of the program, including one that will change the color of some RGB LEDs in response to input on a webpage.

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Low-Voltage Tube Amp is Great for Beginners

If you ever wanted to build your own tube amplifier but you were intimidated by working with high voltages, [Marcel]’s low-voltage tube amp design might spark your interest. The design operates with a B+ (plate) voltage of only 40v, making it less intimidating and dangerous than many other amps that operate over 300V. It’s also incredibly easy to build—the whole design uses only 11 components.

The amplifier is designed around the ECL82 tube, which includes both a triode and a pentode in one package. The ECL82 is practically an amplifier in a tube: it was designed for low-cost electronics like record players that needed to be as simple as possible. The triode in the ECL82 is used as a pre-amplifier for the incoming signal. The pentode is controlled with the pre-amplified signal and acts as a power amplifier.

[Marcel]’s amplifier also uses a PY88 tube rectifier instead of semiconductor diodes, making it an entirely silicon-free design. Although [Marcel] hasn’t posted up detailed build instructions yet, his simple schematic should be all you need to get started. If you want some more background information about tube amps but you don’t know where to start, check out our post on basic tube amp design from earlier this year.

Beverly-Crusher, the Greatest Name for an Audio Effect

Image is © aliceazzo [].
Image © aliceazzo [].
When it comes to audio effects, you have your delay, reverb, chorus, phasing, and the rest that were derived from strictly analog processes. Compared to the traditional way of doing things, digital audio is relatively new, and there is still untapped potential for new processes and effects. One of those is the bit crusher, an effect that turns 8- or 16-bit audio into mush. [Electronoob] wanted to experiment with bitcrushing, and couldn’t find what he wanted. Undeterred, he built his own.

There are two major effects that are purely in the digital domain. The first is the sample rate reducer. This has a few interesting applications. Because [Shannon] and [Nyquist] say we can only reproduce audio signals less than half of the sample rate; if you run some audio through a sample rate reducer set to 1kHz, it’ll sound like crap, but you’ll also only get bass.

The bitcrusher is a little different. Instead of recording samples of 256 values for 8-bit audio or ~65000 values for 16-bit audio, a one-bit bitcrusher only records one value – on or off. Play it through a speaker at a decent sample rate, and you can still hear it. It sounds like a robotic nightmare, but it’s still there.

[Electronoob] created his bitcrusher purely in software, sending the resulting bitcrushed and much smaller file to an Arduino for playback. Interestingly, he’s also included the ability to downsample audio, giving is project both pure digital effects for the price of one. 1-bit audio is a bit rough on the ears, but 2, 3, and 4-bit audio starts to sound pretty cool, and something that would feel at home in some genres of music.

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