Know Audio: Amplifier Nuts And Bolts

August 24, 2021 by Jenny List 31 Comments

As we’ve followed a trail through Hi-Fi and audio systems from the listener’s ear towards the music source, we’ve reached the amplifier. In our previous article we gave a first introduction to distortion and how some amplifier characteristics can influence it, and here we’ll continue along that path and look at the amplifier itself. What types of audio amplifier circuits will you encounter, and what are their relative merits and disadvantages?

A Few Amplifier Basics

Horowitz and Hill's Transistor Man — Horowitz and Hill’s Transistor Man

If you know anything about a transistor, it’s probably that it’s a three terminal device whose output pin forms part of a potential divider whose state is dependent on what is presented to its input pin. The Art of Electronics had it as a cartoon of a man standing inside a bipolar transistor and adjusting a variable resistor between collector and emitter while watching an ammeter on the base.

Properly biased in its conducting range, a transistor can behave as a linear device, in which the potential divider voltage moves in response to the input in a linear relationship, and thus the voltage on the output is an amplified version of the voltage on the output. This is the simplest of transistor amplifiers, and because different types of amplifier are referred to by lettered classes, it’s known as a class A amplifier. Continue reading “Know Audio: Amplifier Nuts And Bolts” →

Miller (Effect) Time

July 21, 2021 by Al Williams 10 Comments

While the Miller effect might sound like fun, it is actually the effect of parasitic capacitance in amplifiers. What do you do about it? Watch the video below the break from [All Electronics] and find out. We like how the test circuit it uses has a switch to put the mitigation circuitry in and out of the test for comparison purposes.

Actually, the Miller effect can refer to any impedance but in practice that is most often parasitic capacitance because of the construction used for tubes and transistors. The sometimes tiny capacitance gets multiplied by the inverting gain of the stage and increases the amplifier’s input impedance. This, in turn, reduces the bandwidth of the stage.

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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.

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Otters Deliver A High Power Stationary Audio Experience

May 12, 2021 by Kerry Scharfglass 20 Comments

Our favorite raft of otters is back at it again with another display of open source audio prowess as they bring us the OtterCastAmp, the newest member of the OtterCast family of open source audio multitools. If you looked at the previous entry in the series – the OtterCastAudio – and thought it was nice but lacking in the pixel count or output power departments then this is the device for you.

The Amp is fundamentally a very similar device to the OtterCastAudio. It shares the same Allwinner S3 Cortex-A application processor and runs the same embedded Linux build assembled with Buildroot. In turn it offers the same substantial set of features and audio protocol support. It can be targeted by Snapcast, Spotify Connect or AirPlay if those are your tools of choice, or act as a generic PulseAudio sink for your Linux audio needs. And there’s still a separate line in so it source audio as well.

One look at the chassis and it’s clear that unlike the OtterCastAudio this is not a simple Chromecast Audio replacement. The face of the OtterCastAmp is graced by a luscious 340×800 LCD for all the cover art your listening ear can enjoy. And the raft of connectors in the back (and mountain of inductors on the PCBA) make it clear that this is a fully fledged class D amplifier, driving up to 120W of power across four channels. Though it may drive a theoretical 30W or 60W peak across its various outputs, with a maximum supply power of 100W (via USB-C power delivery, naturally) the true maximum output will be a little lower. Rounding out the feature set is an Ethernet jack and some wonderfully designed copper PCB otters to enjoy inside and out.

As before, it looks like this design is very close to ready for prime time but not quite there yet, so order at your own risk. Full fab files and some hints are linked in the repo mentioned above. If home fabrication is a little much it looks like there might be a small manufacturing run of these devices coming soon.

Signal Conditioning Hack Chat This Wednesday

February 15, 2021 by Dan Maloney 4 Comments

Join us on Wednesday, February 17 at noon Pacific for the Signal Conditioning Hack Chat with Jonathan Foote!

The real world is a messy place, because very little in it stays in a static state for very long. Things are always moving, vibrating, heating up or cooling down, speeding up or slowing down, or even changing in ways that defy easy description. But these changes describe the world, and understanding and controlling these changes requires sensors that can translate them into usable signals — “usable” being the key term.

Making a signal work for you usually requires some kind of signal processing — perhaps an amplifier to boost a weak signal from a strain gauge, or a driver for a thermocouple. Whatever the case, pulling a useful signal that represents a real-world process from the background noise of all the other signals going on around it can be challenging, as can engineering systems that can do the job in sometimes harsh environments. Drivers, filters, amplifiers, and transmitters must all work together to get the clearest picture of what’s going on in a system, lest bad data lead to bad decisions.

To help us understand the world of signal conditioning, Jonathan Foote will drop by the Hack Chat. You may remember Jonathan as the “recovering scientist” who did a great Remoticon talk on virtual modular synthesizers. It turns out that synths are just a sideline for Dr. Foote, who has a Ph.D. in Electrical Engineering and a ton of academic experience. He’s a bit of a Rennaissance man when it comes to areas of interest — machine learning, audio analysis, robotics, and of course, signal processing. He’ll share some insights on how to pull signals from the real world and put them to work.

Our Hack Chats are live community events in the Hackaday.io Hack Chat group messaging. This week we’ll be sitting down on Wednesday, February 17 at 12:00 PM Pacific time. If time zones have you tied up, we have a handy time zone converter.

Click that speech bubble to the right, and you’ll be taken directly to the Hack Chat group on Hackaday.io. You don’t have to wait until Wednesday; join whenever you want and you can see what the community is talking about.

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It Costs WHAT?! A Sounding Into Hearing Aids

February 1, 2021 by Bob Baddeley 153 Comments

We are accustomed to medical devices being expensive, but sometimes the costs seem to far exceed reasonable expectations. At its most simplistic, a hearing aid should just be a battery, microphone, amplifier, and speaker, all wrapped in an enclosure, right? These kinds of parts can be had for a few dimes, so why do modern hearing aids cost thousands of dollars, and why can’t they seem to go down in price?

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Circuit VR: Even More Op Amps

January 15, 2021 by Al Williams 23 Comments

In the last Circuit VR we looked at some basic op amp circuits in a simulator, including the non-inverting amplifier. Sometimes you want an amplifier that inverts the signal. That is a 5V input results in a -5V output (or -10V if the amplifier has a gain of 2). This corresponds to a 180 degree phase shift which can be useful in amplifiers, filters, and other circuits. Let’s take a look at an example circuit simulated with falstad.

Remember the Rules

Last time I mentioned two made up rules that are good shortcuts for analyzing op amp circuits:

The inputs of the op amp don’t connect to anything internally.
The output mysteriously will do what it can to make the inputs equal, as far as it is physically possible.

As a corollary to the second rule, you can easily analyze the circuit shown here by thinking of the negative (inverting) terminal as a virtual ground. It isn’t connected to ground, yet in a properly configured op amp circuit it might as well be at ground potential. Why? Because the + terminal is grounded and rule #2 says the op amp will change conditions to make sure the two terminals are the same. Since it can’t influence the + terminal, it will drive the voltage through the resistor network to ensure the – terminal is at 0V.