Electronics project notes/Audio notes - basic audio hacks

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This is for beginners and very much by a beginner.

It's intended to get an intuitive overview for hobbyist needs. It may get you started, but to be able to do anything remotely clever, follow a proper course or read a good book.

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

And some more applied stuff:

IO: Input and output pins · wired local IO · wired local-ish IO · · · · Shorter-range wireless (IR, ISM RF) · RFID and NFC · bluetooth · 802.15 (including zigbee) · 802.11 (WiFi) · cell phone

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 · JY-MCU · DMX · Thermal printer ·

Noise stuff: Stray signals and noise · sound-related noise names · electronic non-coupled noise names · electronic coupled noise · ground loop · strategies to avoid coupled noise · Sampling, reproduction, and transmission distortions

Audio notes: See avnotes

Microcontroller and computer platforms Arduino and AVR notes · ESP series notes · STM32 series notes · · · ·

Less sorted: device voltage and impedance, audio and otherwise · electricity and humans · power supply considerations · Common terms, useful basics, soldering · PLL · pulse modulation · signal reflection · resource metering · SDR · Project boxes · Unsorted stuff

See also Category:Electronics.

Combining multiple signals (to one input)

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)

You may want this when you want to

connect multiple inputs to e.g. a speaker set with only one input plug
wire an AUX port to somethat that doesn't have it

Manual switch

This isolates the inputs, mechanically. The simplest version is a switch (DPDT for stereo). Could do this in a cable, or make a small box with some sockets.

You'll get a pop whenever you switch. There are some fairly quick-and-dirty ways around this.

But often you don't want to have to do this manually in the first place, so...

Y splitter

A wire with three plugs, that connects everything to everything.

The device that senses this signal is high-impedance so is fine with this. Worst case the combination is so loud that it clips.

However, the things producing sound wont' be quite as happy. In practice, most things you'll connect have active output, meaning they are powered and have a lowish-impedance amp at the end. Sometimes they'll be passive, but can be reactive themselves, e.g. guitar pickups.

In both cases, these will interact, which is unintended. It will usually affect audio quality, which is already enough reason not to do this.

In some cases the current pushed into the other thing can break things. So this is probably a bad idea.

To avoid most of these problems, the inputs need some resistance between them.

Y splitter with resistors

The most basic alleviation is a passive summing circuit, which puts a resistor on an input lines before you join them.

It means going to the other sound source sees more resistance than going . It doesn't completely isolate the signals, but assuming active outputs, it limits current enough to avoid damage and almost all distortion. (again, instrument pickups are a little different)

It will lower volume. For example, 10kOhm, which is probably wise, will attenuate around 6dB(verify), which is half. Since there's a noise floor somewhere, and you'll turn the volume up, this means more noise. And if the amp only volume and no input gain, then the total possible sound produced is also lower. In cars you'll probably notice neither and this is a fine solution.


Passive potmeter mixer

Instead of just a resistor, you could add a potmeter so that you can fade between the two channels (or mix each input into the output).

The combination and values are a little different between these two cases.

You may want a two-channel pot, to separately do the same thing with both stereo channels.

Same notes on less signal and more noise as before.

Buy active mixer

Adding more channels would attenuate down some more, and the noise starts to come closer, so you'll soon want an active summing circuit instead (recognizable by that they take DC power input).

Your cheap $20 active mixers are not the best quality ("I could make this" level) and are often they're two-channel (or 4-channel mono, same thing).

Halfway decent ones are easily $50, and if you find that a little expensive for what they are, you can consider soldering your their own (see below).

Build Active mixer

The "passive adder plus potmeter" option is functionally much of a real mixer.

The main thing still to add is amplification right after - so that you no longer lessen the signal, and so it doesn't fall into the next device's noise floor.

This does require a power supply. At this point you probably want to build something more solid than a pot on a wire.

Note that adding more channels to what you have is now a fairly simple and cheap option. (planning for a number of channels, and/or stereo, is still handier to do up front though)




Multiple headphones on one output

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)

A Y-cable puts two loads in parallel.

This does two things:

Makes the headphones interact

...because they are not simple loads. However, they won't do so very much, you probably won't notice or care, and it can't damage them.

doubles the load

(...if equal impedance. If they're different it's something roughly 2-ish, and one will be louder than another.)

If the output can handle that, it's perfectly fine.

Most any headphone amp will be fine, as they're typically designed to cope with real-world variation in headphones.

Computers, mobile phones, and such will often be fine for similar reasons.

Though in case they have separate speaker out and line our, then line out may not, this is fairly rare.

Things that can't deal will distort the sound, in extreme cases overheat.

You can usually avoid that by keeping the volume low. Half is safest, more may be okay depending on the combination of devices.


Adding a line/aux in

Line out from speaker out

Avoiding pops

Car audio

Tape related

Cassette speed mod

Tape loops