Know Audio: Amplifiers And Distortion

July 29, 2021 by Jenny List 31 Comments

As we’ve traced our no-nonsense path through the world of Hi-Fi audio, we’ve started with the listener, understood the limitations of the human ear, and thence proceeded to the loudspeaker. We’ve learned a bit about speaker cabinets and their design, so it’s time to venture further down the chain to the amplifier that drives those speakers.

The sharp-eyed will be ready to point out that along this path also lies the speaker cables, but since we’ll be looking at interconnects at a later date we’ll be making the dubious and simplistic assumption for now that the wires between speaker and amplifier are ideal conductors that don’t have a bearing on listening quality. We’ll be looking at amplifiers in enough detail to warrant more than one piece on the subject, so today we’ll start by considering in a slightly abstract way what an amplifier does and where it can fall short in its task. We’ll be introducing probably the most important thing to consider in any audio system, namely distortion.

The job of an audio amplifier is to take an audio signal at its input and present the same signal on its output at a greater amplitude. In the case of a preamplifier it will usually be designed to work with high impedances in the order of 50 kΩ at both input and output, while in a power amplifier designed to drive speakers or headphones it will drive a much lower impedance. Commonly this will be 4 Ω or 8 Ω for loudspeakers, and 32 Ω for headphones. Continue reading “Know Audio: Amplifiers And Distortion” →

How To Modify Your Car Stereo For Bluetooth Or Aux-In

July 28, 2021 by Lewin Day 42 Comments

If you’re an automotive enthusiast of taste, you can’t stand the idea of fitting a janky aftermarket stereo into your nice, clean ride. Flashy, modern head units can spoil the look of a car’s interior, particularly if the car is a retro, classic, or vintage ride.

Thus, we’re going to look at how to modify your existing stock car stereo to accept an auxiliary cable input or even a Bluetooth module. This way, you can pump in the latest tunes from your smartphone without a fuss, while still maintaining an all-original look on the dash.

Fundamentals

A simple Bluetooth module designed for wiring into car audio systems. There are two wires for 12 V power from the vehicle, and the audio signal is sent out over the RCA plugs. The RCA plugs can be cut off and the module hard wired inside your stereo if you have room. Cutting off the plastic case can help too.

Depending on your choice of audio player, you may prefer a 3.5 mm aux jack, or you might want to go with Bluetooth audio if your smartphone no longer has a headphone port. Whichever way you go, the process of modifying the stereo is largely the same. To achieve your goal, you need to find a way of injecting the audio signal into the head unit’s amplifier stage, while making sure no other audio sources are getting sent there as well.

Whether that audio source is a 3.5 mm jack or a Bluetooth module doesn’t matter. The only difference is, in the latter case, you’ll want to buy a Bluetooth module and hardwire it in to the auxiliary input you create, while also splicing the module into the stereo’s power supply. In the case of a simple headphone jack input, you simply need to wire up an aux cord or 3.5 mm jack somewhere you can get to it, and call it done.

This guide won’t cover every stereo under the sun, of course. Edge cases exist and depending on the minute specifics of how your original car radio works, these exact methods may or may not work for you. However, this guide is intended to get you thinking conceptually about how such mods are done, so that you can investigate the hardware in front of you and make your own decisions about how to integrate an external audio input that suits your usage case. Continue reading “How To Modify Your Car Stereo For Bluetooth Or Aux-In” →

The Hidden Sounds Of The Past

July 18, 2021 by Matthew Carlson 20 Comments

If you stop to think, the number of pre-recorded voices and sounds you might hear on an average day might number in the hundreds. Everything from subway announcements, alerts on your phone, to sound effects at Disneyland is a sound that triggers in response to an event. [Techmoan] came across a device that many of us have interacted with, but likely never seen: a humble Sontranic 9A Announcer.

In their heyday, these sorts of devices formed the backbone of audio feedback. Messages from Father Christmas were recorded and could be reached when calling a number. Sound effects in theme parks that were activated when a ride activated some hidden switch. Anything where the sound effect needed to play on some sort of trigger.

An interesting thing to note is that this is not a reel-to-reel system. The tape is of the standard 1/4″ magnetic variety, perhaps a little thicker for extra durability. It instead sits in the top of the machine; coiling and uncoiling like a two-dimensional lava lamp. Additionally, there’s nothing clever about detecting the beginning or end of the audio loop (as there were four copies of the same recording on this particular tape). In fact, everything about this machine speaks of reliability as the most important design consideration. A reel-to-reel system would just add more points of failure.

After a little bit of diagnosing, [Techmoan] managed to get the device running again and found the message on the tape to be from the phone system, informing the listener that the line is no longer in service. This banal message is perhaps the best testament to the ubiquity of devices like these.

Perhaps in the future, we’ll see an instrument like this magnetic tape-based one created from a similar machine to the one [Techmoan] found.

Continue reading “The Hidden Sounds Of The Past” →

Digital Audio For Microcontrollers Doesn’t Come Much Simpler Than A WART

July 11, 2021 by Jenny List 11 Comments

Adding an audio channel to your microcontroller project can mean a pile of extra components and a ton of processing power, as a compressed stream must be retrieved and sent to a dedicated DAC. Or if you are [rdpoor], it can mean hooking up a low-pass filter to the UART that’s present on even the simplest of devices, and constructing a serial data stream that mimics PWM audio.

Sound on your microcontroller, it’s this simple!

WART is a Python script that converts a WAV file into a C formatted byte array that can be baked into your microcontroller code, and for which playback is as simple as streaming it to the UART. The example uses a Teensy and a transistor to drive a small speaker, we’re guessing that better quality might come with using a dedicated low-pass filter rather than relying on the speaker itself, but at least audio doesn’t come any simpler.

The code can be found in a GitHub repository and there’s a few recordings of the output in the files section Hackaday.io page, one is embedded below. It’s better than we might have expected given that the quality won’t be the best at the PWM data rate of even the fastest UART. But even if you won’t be incorporating it into your music system any time soon we can see it being a useful addition for such things as small warning sounds. Meanwhile if persuading serially driven speakers to talk is of interest, there’s always the venerable PC speaker.

Continue reading “Digital Audio For Microcontrollers Doesn’t Come Much Simpler Than A WART” →

Your PC Sound Card As A Sensor Input

July 3, 2021 by Jenny List 21 Comments

The commoditised PC is the most versatile tool many of us will own, and since it has been around for a very long time it is also something that can be found for free or very cheaply if the latest components aren’t a concern. It’s not without limitations though, while it’s designed for expansion it no longer has any ports that can easily be repurposed as GPIOs for reading sensors. A solution for some sensors comes courtesy of [Ruslan Nagimov], who shows us how the PC sound card can become a measurement interface.

The idea is that simple resistive or capacitive sensors can be read through their AC characteristics by sending out a sine wave on one channel of the card and reading the result on the other from a divider circuit. He goes extensively into the code, both for the resistive example and for reactive components, and we can see that it forms a handy extension to the PC capabilities.

We’re sure this technique will find applications for some readers, but it interests us for another platform. Measurement using a mobile phone’s audio jack doesn’t have an inspiring history, but perhaps this could be used as well for mobile sensors.

Know Audio: A Loudspeaker Primer

June 25, 2021 by Jenny List 27 Comments

As we’ve started out on our journey through the world of Hi-Fi audio from a strictly practical and engineering viewpoint without being misled by any audiophile woo, we’ve already taken a look at the most important component in any audio system: the listener’s ear. It’s time to move down the chain to the next link; the loudspeaker.

Sound is pressure waves in the air, and the purpose of a loudspeaker is to move the air to create those waves. There are a variety of “exotic” loudspeaker technologies including piezoelectric and electrostatic designs, here we’ll be considering the garden variety moving-coil speaker. It’s most usually used for the large bass or smaller mid-range drivers in a typical speaker system. Continue reading “Know Audio: A Loudspeaker Primer” →

Hi-Fi Combines Best Of 60s And 90s Technology

June 7, 2021 by Bryan Cockfield 55 Comments

The 90s were a dark time for audio equipment, literally and figuratively. Essentially the only redeeming quality from the decade of nondescript black plastic boxes was the low cost. Compared to the audio equipment of the 60s, largely produced in high-end enclosures with highly desirable tube amplifiers, the 90s did not offer much when it came to hi-fi stereo sound. However, those cheap black boxes from the 90s turn out to be surprisingly perfect for project enclosures for other amplifier builds, such as this 60s-era tube amp recreation.

This mesh of the best of two distinct decades comes from [Alvenh] and begins by preparing the old enclosure for its new purpose. This means a lot of work fabricating a custom metal face plate for the new amplifier and significantly modifying the remaining case. After the box is complete, the amplifier build began. It uses a tube-based preamp and a solid-state power amplifier since [Alvenh]’s experience suggested that the warm tube sound was generated mostly in the preamp. This means that his design is a hybrid but still preserves the essential qualities of a full tube build.

The build also includes a radio module that has the ability to cover the 2m and 70cm bands popular in ham radio. This module also has been found to have much better audio quality than the standard AM/FM receiver typically used in projects like this. With the radio module added to the custom enclosure, as well as a phono amp and a power supply, [Alvenh] has an excellent audio amplifier in an inexpensive case which preserves the tube sound from the true hi-fi eras of decades past.

Continue reading “Hi-Fi Combines Best Of 60s And 90s Technology” →