A popular question that we get all the time is ‘How do I choose a speaker or amplifier to go with my project?’. It’s a good question, and if you’re not familiar with the terminology, it can seem a bit daunting to know whether the parts you’re getting and compatible. Fortunately, when makers talk about speakers, they’re rarely referring to the heavy-duty, high-performance speakers that are used in PA gear, guitar amps, and home theatre systems. Most of the time, you just want to add a sound effect, or notification element to your project and pristine audio quality is vital.
Once you understand the terms and ratings which are used in audio systems, it’s fairly easy to pick the correct speaker and amplifier combination, so let’s start by taking a look at what it all means. Bear in mind that when you’re talking about matching amplifiers and speakers in larger applications, there’s a huge amount of complexity and depth involved with all kinds of crazy maths and knowledge required. The goal today is to sidestep that and provide a simple guide for connecting small amplifiers and speakers for your next maker project.
Amplifier wattage is fairly straight forward, it is the amount of power the amplifier can produce on the output channels. Most amps will have a rating per channel, so if it’s a stereo amp, it could provide 5W per channel (aka. 5W for the left, and 5W for the right). It’s important to match the amplifier wattage with the speaker wattage.
This is where people tend to get bogged down when choosing an amplifier for your project. You may notice that amps are defined in classes: Class A, Class AB, Class D, etc… These amplifier classes tell you what type of topology the amplifier is using, which is a product of the efficiency (power used vs. power output) and the sound quality. Class A amplifiers, for example, are extremely inefficient, but deliver premium sound reproduction, whereas Class D amplifiers offer far better efficiency, with somewhat worse quality. Bear in mind though that the quality differences we’re talking about here are only noticeable when you’re comparing high-quality audio through high-end speaker gear. I doubt you’d notice the difference, if any, for an everyday maker project.
So, the TLDR of all that is: don’t really worry about it.
Most amplifiers are straight-up analog amplifiers; you give them a small signal, they output a larger signal. Some, however, are designed to interface directly with devices such as the Raspberry Pi, which doesn’t have any analog output on its GPIO pins, so a specific board may have a Digital-Analogue Converter (DAC). These are very specific, so most amplifiers just have solder pads or sockets for the input.
Something to keep in mind is that some amplifiers accept a differential input, which means that they’ll have + and – inputs rather than a single pad with GND connected. But amplifiers that do have differential inputs will usually be ok with non-differential input, but just check the particular board that you’re using.
As with any electronic device, you need to make sure you've got the correct power supply for your amplifier. It doesn't have to be capable of any crazy current output, but just make sure you've got the correct voltage. Most amplifiers (particularly power efficient, Class D ones) will happily run off batteries or plug packs, so take your pick.
Speaker wattage defines how much power the speaker can handle. A larger speaker can handle more power, and a smaller speaker can handle less power (*mind blown*). The general rule of thumb is that your speaker wattage should always be greater than the amplifier wattage. The reason for this is that whilst an amp might be rated at 3W, it could produce peak power slightly above that, and like most things in electronics, running components right up to their ratings will usually result in failure. So for most small projects, you’re dealing with around 5W or less, so make sure you’ve got at least 0.5W spare on your speaker just to be safe.
Again, we’re simplifying a lot here, but follow these examples, and you’ll be fine.
Passive vs. Powered Speakers
This one’s pretty easy. Powered speakers have amplifiers built into them and accept a standard line-level input from devices such as computers and phones. Passive speakers are just a speaker, and require an amplifier to drive them (unless they’re extremely small such as earphones, which can be driven directly from these devices). Fortunately, powered speakers are as easy as plugging in and turning on, so we’re focusing on passive speakers and amplifiers today.
**Line-level is just a definition for a signal which is commonly used for CD players, phones, computers, etc…
Speaker Impedance – Ohms
The impedance rating of a speaker and amplifier output is important. A traditional speaker is just an electromagnet which converts electrical signals into linear motion to move the cone of the speaker back and forth to produce pressure waves. The key part of an electromagnet is the coil, which is what we are running the signal through. And like any piece of wire, the coil has a resistance, which in AC signals, is known as Impedance (still measured in Ohms).
The most common impedances are 4, 8, and 16 Ohms. Amplifiers will have an impedance rating which speakers must match in order to avoid releasing the magic smoke (breaking stuff). If an amplifier is rated for an 8 Ohm speaker, and you connect a 4 Ohm speaker, that’s like shorting the output, which can damage the amplifier. You probably won’t damage anything if you connect a higher impedance speaker to an amplifier, however, the sound quality may not be ideal.
So, whilst it’s good to understand the reason behind it, stick to the rating, and you’ll be fine.
Here’s another easy one. When you’re connecting your speakers to the amp, just stick to one speaker per channel, otherwise, things can start to get messy. Stereo/multi-channel amps will have separate connections for each speaker, and mono amps will have a single connection.
So that’s a run down on the terminology you need to worry about when choosing an amplifier and/or speaker for your next project, so here are a couple of products and combos that I like to use in projects:
- Mono 2.5W Class D Audio Amplifier - PAM8302
This is a particularly great product from Adafruit. A $7 2.5W amplifier with differential inputs, thermal control, and onboard volume trimmer is pretty fantastic. It's super easy to use, and I've paired it in a recent project with Adafruit's 3" 4 Ohm, 3W speaker, with great success.
- Stereo 3.7W Class D Audio Amplifier - MAX98306
This is the natural step up from the mono amp I mentioned above. With 3.7W of power for each channel, it can handle stereo audio for almost any maker project.
- Stereo 20W Class D Audio Amplifier - MAX9744
If a smaller amplifier doesn't quite do it for you, then this 20W monster should. It packs 20W of power on each channel and also has some nice features such as an I2C interface for digital volume control, or you can use the included potentiometer.