Tube Design Tips To Save A Writer’s Project

Most of the stories we cover here are fresh from the firehose, the newest and coolest stuff to interest you during your idle moments. Sometimes though, we come across a page that’s not new, but is interesting in its own right enough to bring to your attention. So it is with our subject here, because when faced with a tube circuit design problem, we found salvation in a page from [The Valve Wizard].

Do you need to apply negative feedback to a triode amplifier? The circuit is simplicity itself, but sadly when we were at university they had long ago stopped teaching the mathematics behind the component values. Step forward everything you need to know about triode amplifier negative feedback.

Negative feedback is a pretty simple idea: subtract a little of the amplifier’s output from the input. It reduces the amplifier’s gain with a flat response, so it’s useful for removing humps in the frequency response and reducing the tendency for distortion. In a single-ended triode amp it’s done with a resistor and capacitor from anode to grid, but the question is, just what resistor or capacitor?. Here the page has all the answers, taking the reader through calculating the desired gain, and picking the value of the capacitor to avoid affecting the frequency response. We wish that someone had taught us this three decades ago!

The website is full of really useful info about valve or tube amps, and it’s worth mentioning that he’s made it available in book format too. There’s no reason not to have a go at vacuum electronics. Meanwhile in case you are wondering what project prompted this, it was a quest to improve upon this cheap Chinese kit amplifier.

A High-End Studio Multiplexer Surrenders To An Arduino

The equipment used in professional radio and TV studios is both extremely high quality and very expensive indeed, and thus out of the reach of an experimenter. Happily as studios are refurbished there’s a steady supply of second-hand equipment which can be surprisingly cheap, but as [Nathan] found out with a Quartz audio router, comes with no control software. What’s to be done with what’s essentially a piece of junk? Remove its brain and replace it with one that can be controlled, of course!

On the PCB alongside a bank of switch matrices is an FPGA which does the heavy lifting. That’s “heavy” in a limited sense, because all it does is handle the chip select lines for the matrices and write data to their registers. This is a task that can be handled by a microcontroller, so in goes an Arduino Nano, which along with a few other board modifications delivers a serial-controlled studio router.

The interesting part for us in this project comes from a look at the date codes on the board, they’re from the early 2000s. This is (roughly) contemporary with the ATmega chip on the Arduino, so we’re curious as to why the designers saw fit to use an FPGA when the microcontrollers of the day were clearly up to the task for much less outlay. We suspect a touch of millennium-era price inflation, but we can’t be sure.

Meanwhile, old broadcast kit has featured here before.

Raspberry Pi Pico Becomes MIDI-Compatible Synth

ECE 4760 is a microcontroller course that runs at Cornell every year, and it gives students a wide remit to pursue various kinds of microcontroller projects. [Pelham Bergesen] took the class and built himself a MIDI-controllable synthesizer out of a Raspberry Pi Pico.

[Pelham] coded a library to parse MIDI messages on the Pico, with the microcontroller’s UART charged with receiving the input data. MIDI is basically just serial at a baud rate of 31.25k, with a set message structure, after all. From there, the Pico takes the note data and plays the relevant frequencies by synthesizing square waves using a PWM output. A second PWM channel can also be blended with the first to generate more complex tones.  The synthesizer is designed to be used with a source of MIDI note data such as a keyboard controller; [Pelham] demonstrates the project in use with a Roland JD-XI. It’s a fairly basic synthesizer, but [Pelham] does a good job of explaining all the steps required to get this far. If you’ve never done an audio or MIDI project before, you might find his guide very helpful for the way it steps through the basics.

[Pelham] didn’t get to implement fancier features like direct digital synthesis (DDS) or analog audio effects before the class closed out. However, that would be an excellent project for anyone else developing their own Pico synthesizer. If you whip up something that sounds good, or even just interesting, be sure to notify us on the tipsline. Video after the break.

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It’s An Audio Distortion Analyzer, Just Not The One You Were Hoping For

An audio distortion analyzer is a specialist piece of analogue test equipment that usually costs a lot of money and can be hard to track down on the second hand market. Finding one is a moment of luck for the average engineer then, but [Thomas Scherrer OZ2CPU]’s discovery isn’t quite what he might have hoped for. Nonetheless, his Bang and Olufsen K3 Distortion meter DM1 from 1979 is still an interesting and high quality piece of test equipment, and the video below the break makes for a worthwhile watch.

Bang and Olufsen are best known for high-end design Hi-Fi units, thus it’s a surprise to find that in the past they also manufactured test equipment. This distortion meter isn’t a general purpose one, instead it’s designed to measure tape recorders in particular, and it uses an elegant technique. Instead of injecting a sine wave and removing it from what comes out in order to measure the products of the distortion, it records a 333 Hz sine wave onto a tape, then measures the strength of its 3rd harmonic at 1 kHz as an indication of distortion. It’s a working distortion meter made with clever analogue circuitry for a fraction of the cost of the more conventional models that HP would have sold you at the same time, even if it doesn’t give the same THD figure you might have been looking for.

If distortion interests you, it’s a subject we’ve looked at in the past.

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Keeping A Mazda’s Radio On After The Engine Shuts Off

Have you ever pulled into a car park with your favorite song blaring, only to lament the fact that the music cut out when you stopped the engine? Some modern cars are smart enough to keep the radio on until you open the door. [ssh16] decided to hack that very functionality into their Mazda MX-5.

The device uses a microcontroller to read the CAN bus of the vehicle. The microcontroller also has the ability to keep the vehicle’s ACC (accessory) relay energized at will. Thus, when the engine is turned off, the microcontroller keeps the ACC relay on, maintaining power to the stereo and infotainment system. Then, after ten minutes, or when it receives a CAN message that the driver’s door has been opened, it cuts power to the relay, shutting the accessories off. It’s a simple build, but one that [ssh16] executed cleanly. By putting the microcontroller on a neat PCB with a harness that can clip into the stock Mazda one, it’s possible to install the hack without needing to cut any wires. Plus, with a small modification, it was even possible to use the same hack with a Mazda CX-5.

Whether you’re jamming out to a cool song, or you just want to finish a phone call over Bluetooth, it’s a nifty feature to have in a vehicle. We’ve seen some other neat infotainment hacks before, too. Video after the break.

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Stream Vinyl To Your Sonos Without The Financial Penalty

One of the unexpected success stories in the world of hi-fi over the past decade has been the resurgence of the vinyl LP as a musical format. What was once old hat is now cool again, but for freshy minted vinyl fans there’s a snag. Hi-fi itself has moved on from the analogue into the digital, so what can be done if your listening comes through a Sonos system. Sonos will sell you a box to do that of course, but it’s as overpriced as 2023-pressing vinyl. [Max Fischer] has a far better solution, in the form of a Raspberry Pi loaded with open source software.

At the vinyl end is a Behringer audio interface containing a pre-amp with the required RIAA response curve. This acts as the source for the DarkIce audio streamer and the IceCast2 media serer, all of which even with the cost of a Pi and the interface, is considerably less than the commercial device.

We’re guessing that a more humble interface coupled to an older RIAA pre-amp could cut the cost further, and we’d be hugely curious as to whether a simple mic pre-amp could be used alongside some DSP from the likes of Gnu Radio to give the RIAA response.

Either way, he’s made a handy device for any 21st-century vinyl fan. Meanwhile if you’re one of the streaming generation seduced by round plastic discs, we’ve gone into some detail about their audiophile credentials in the past. And if you have found yourself a turntable, of course you’ll need to know how to set it up properly.

Open Source Ear Monitoring Platform Listens To Your Ears

All sorts of exciting things happen in your ears, and now there is a good open source way to monitor them. Open Earable is a new project from a group of researchers and companies that monitors and records what is going on in your ear.

The project is designed as an easy-to-build, cheap way for audiologists and others to capture data about what is happening inside and around the ear. It’s a clip-on device that looks like a small hearing aid but has a six-degree Inertial Measurement Unit (IMU) and several other sensors to measure things around your ear and inside the ear canal. A pressure and temperature sensor measures the air pressure and temperature just inside the ear canal, and a small speaker can squirt sound right in there.

A button on the outside allows the user to control the device, and it can play back or record sound to the internal SD card memory. These are all controlled by an Arduino that includes Bluetooth Low Energy. The existing design only allows you to play a stored WAV file, not streaming audio. That’s a solvable problem, though, so it could also be turned into a set of hacker headphones.

Joking aside, this looks like an exciting research project and a useful tool for researchers. The GitHub repository for version 1.3 of the project lays it all out, including a full BoM and code, and the STL files for the case and PCB designs are in the Resources section of the site.

[Updated 18/10/2023 to correct IMU to Measurement, not Management. Intertial management needs a different set of devices]