Electronics project notes/Audio notes - amps and speakers

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This is for beginners and very much by a beginner. It's meant to try to cover hobbyist needs, and as a starting point to find out which may be the relevant details for you, not for definitive information.

Some basics and reference: Volts, amps, energy, power · Ground · batteries · resistors · changing voltage · transistors · fuses · diodes · capacitors · inductors · transformers · baluns · amplifier notes · frequency generation · skin effect

And some more applied stuff:

IO: IO and wired communication · localish communication · wireless (ISM RF, GSM, RFID, more) · 802.11 (WiFi) · 802.15 (including zigbee)

Sensors: General sensor notes, voltage and current sensing · Knobs and dials · Pressure sensing · Temperature sensing · humidity sensing · Light sensing · Movement sensing · Capacitive sensing · Touch screen notes

Actuators: General actuator notes, circuit protection · Motors and servos · Solenoids

Some stuff I've messed with: Avrusb500v2 · GPS · Hilo GPRS · Bluetooth serial · JY-MCU · DMX · ESC/POS notes

Audio notes: basic audio hacks · microphones · amps and speakers · device voltage and impedance, audio and otherwise ·

Less sorted: Common terms, useful basics, soldering · Microcontroller and computer platforms · Arduino and AVR notes · ESP series notes · Electronics notes/Phase Locked Loop notes · mounts, chip carriers, packages, connectors · signal reflection · pulse modulation · Unsorted stuff

See also Category:Electronics.

Design side

On amplifier and speaker rating

Division of work in speakers

Open versus closed cabinets; bass reflex ports

Amplifier classes

This article/section is a stub — probably a pile of half-sorted notes, is not well-checked so may have incorrect bits. (Feel free to ignore, fix, or tell me)


  • Most of these can be made to work well, so you generally don't need to know this unless you're designing things yourself.
  • While higher-quality amps have correlations with classes, a particular class won't guarantee quality - the quality of a specific design does.

Class A

  • input is AC on top of DC bias, and all-positive
bias tries to stay above the nonlinearity of transistors (that happens near 0V)
  • the simplest quite-decent design
  • but inefficient. max efficiency can theoretically be 50%, but typically 25-30%
  • uses most if its power always, regardless of input signal

Class B

  • input AC biased to 0V
  • A transistor only amplifies one in one polarity, so this is typically seen in a pair, each doing half of the wave (or rather across the barrier that is often 0V), then often called a push-pull amplifier
  • the transistor is nonlinear / does not trigger below some voltage, so both will be a little off when the signal passes near or through 0V. (called crossover distortion)
  • This distortion is not present in class A, and B was mostly abandoned for AB,
  • max efficiency is ~80%, typically more like 50-60%

Class AB

  • a compromise between A and B that is
    • more efficient than A
    • avoids B's crossover distortion (depends a bit on proper design - the nonlinear part is entered, but controlled)
  • lower cost than A at comparable power
  • efficiency beween that of class A and B
  • decent-quality amplifiers have used AB for a pretty long time now (verify)

Class D

  • basically an analog imitation of PWM signal (plus some output filtering)
  • quality varies. For example, low distortion depends on good timing.
  • max efficiency can be ~90%
  • (also easier to produce small )

Power efficiency: D is best, A is worst.

Class C

  • biased well below the cutoff
  • it amplifies little of the input signal, and gives a distorted version
good at pulses, good for functions that use those (oscillators, some RF)
  • not worth mentioning in audio context

Classes E and further were settled later and sometimes just by single companies. Many of them are some variation on D.

Tube amplifiers, other special cases

Weather-resistant speakers

This article/section is a stub — probably a pile of half-sorted notes, is not well-checked so may have incorrect bits. (Feel free to ignore, fix, or tell me)

For example for

  • Terrace (deals with moisture, not necessarily rain)
  • Marine (deals with water in general)
  • Sauna (deals with moisture.
The heat is not that much trouble if you place them low -- it won't be much hotter than e.g. the average car on a sunny day)

Those three terms should help start your search.

These tend to have plastic cones, and generally be made of plastic, and maybe have metal contacts coated.

Getting great sound is a little more complex, mostly because there is much less choice / competition.

Use side

Speaker placement

single/few listener / on spatial sound



Speaker repair

Foam surrounds

This article/section is a stub — probably a pile of half-sorted notes, is not well-checked so may have incorrect bits. (Feel free to ignore, fix, or tell me)

Surrounds are the soft rings that connect the (often paper) cone to the (often metal) frames.

Its function is mainly just to keep the edge from flapping, without limiting the cone's movement much. As such, they are ideally thin, and light.

Which is why they're often foam, which makes them one of the first things to go brittle and wear out after a decade or two.

And easy to fix, given a new ring, some glue, and some patience.


  • get some new surrounds
buying foam surrounds of the right size is cheap and often easy to find.
if you find them too expensive, you can DIY it from fabric or so (fabric surrounds will often last longer, though are also a little more work)
  • get some glue
the ebay-style kits will typically contain some.
When you have paper cones, look for acid-free glues. Your basic wood glue seems a decent option(verify)

Procedure is roughly:

  • optional: take the speaker out of its cabinet.
not necessary, but often easier to work on.
If you disconnect it, remember the polarity.
  • Remove the trim ring - the thing inside the outer part's metal edge that holds down the foam.
This is often a solid cardboard or thick foam, and may well be reusable
if it's foam and has also disintegrated, or when you can't remove the ring without destroying it, make some new ones. Its only function is to solidly hold the surround, arguably just to help be part of the glue process, so you could make it from whatever you have handy. I did one out of generic cardboard.
  • Clean anything that seems to want to come off, or otherwise does not look like a good glue surface
if there's such muck on the cone, friction may be enough. Sometimes you may prefer a sharp knife to avoid large forces on the cone.
avoid twisting the cone. Patience is good here.
If the foam comes off in lots and lots of tiny particles, moistening it with something like rubbing alcohol may help if you wish.
This works on the cone too (and rubbing alcohol won't hurt the paper).
  • Plan the gluing step
You can glue inside and outside at once, but on your first repair you may wish to do them separately, to get a feel for the things to pay attention to
Lay the surround on the speaker to get an idea of positioning.
Laying the surround upside down can help remind you of which parts need to get glue later
  • apply glue to the foam surround's inside edge (it's easier to tell how far to go than it is on the cone)
put the two parts together, and for the first few minutes make sure they are in contact (so all the surface takes - after that you can assume the it won't let go)
you may e.g. like to make some rings that gently hold all parts down while gluing.

Amplifier repair

This article/section is a stub — probably a pile of half-sorted notes, is not well-checked so may have incorrect bits. (Feel free to ignore, fix, or tell me)

On protect mode

Most amplifiers have a protect mode that will cause it disable high-power parts (mostly the power amp) whenever it sees more current than sensible (and sometimes also monitors temperature)

This protects amp parts against breaking/burning, and also the speakers, particularly when you do something silly.

Your amp may have a LED showing protect state, most do not. If it does, consider that it may, or may not, show during the regular startup mute delay.

Reasons for protect mode to trigger:

  • shorted speaker wire (e.g. in cars)
check: if it always goes into protect mode if speakers are connected, and never without
  • transistors are broken (often mean it does this at startup - never comes out of the mute delay)
You can check transistors with a multimeters diode test. That is, you can see if there are shorts between pairs.
  • noisy (dirty/worn) pots can trigger it -- though only if the protect mechanism is highly sensitive or the pots dirty enough to effectively send spikes
check: if it generally only happens when changing volume knobs, regardless of level
(or always at the same place on the pot)
  • thermal protection can trigger
in the sun, near a radiator, if something designed to vent is completely covered
if overdriven too long (where possible)