Electronics project notes/Audio notes - basic audio hacks and DIY

✎ This article/section is a stub — some half-sorted notes, not necessarily checked, not necessarily correct. Feel free to ignore, or tell me about it.

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

A switch connects one source, or the other.

The sources are isolated from each other, mechanically, and you will only ever connect and hear one.

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 quick but 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 listening device will be high-impedance, so that input is fine with this. Worst case the combination of signals is loud enough that it distorts a bit.

However, the two (or more) outputs will interact. Which is not intended, and which will usually affect audio quality, which is already enough reason not to do this.

If passive (e.g. two guitars pickups) this wouldn't always be so bad.

If the output is active, that means they have a small amp at the end, and you are connecting multiple of those directly together - which means they treat each other as loads. This can push in enough current to break things, so this is probably a bad idea.

In other words

a splitter is fine to put one source into multiple listeners
a splitter that puts multiple sources together, intended for one listener, may be trouble.

Y splitter with resistors

The most basic fix to the last-mentioned issue is a passive summing circuit, which puts a resistor on an input lines before you join them.

It doesn't completely isolate the signals, but you can dimension the resistors so that it can't put out enough current to damage another of itself.

(again, instrument pickups are a little different - you generally wouldn't do this)

It will lower volume by a bunch of dB, so you'll turn the volume up, bringing up the existing noise floor.

In some situations you may not have input gain, just the amp's main volume, and the total possible sound produced is also lower. Whether this is an issue depends on context.

See/read:

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).

This mostly the same as the previous, except that you can change the volume. It's arguably a little worse, because you can turn the pot to zero ohm for each channel, and probably fix that by adding a little more resistance in series.

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 "I could probably make this" level $20 active mixers are not the best quality and are often they're two-channel (or 4-channel mono, almost the 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 own (see below).

Build active mixer

The "passive adder plus potmeter" option is functionally half of a real mixer - the most important next step is a buffer and a little amplification right after, e/g/ so that you avoid attenuating the signal into the next device's noise floor. So, an op amp.

This does require a power supply.

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

http://www.learningaboutelectronics.com/Articles/How-to-build-your-own-audio-mixer

http://web.archive.org/web/20150220140529/http://www.mattkrass.com/?page_id=880

http://forum.allaboutcircuits.com/showthread.php?t=26654

Multiple headphones on one output

✎ This article/section is a stub — some half-sorted notes, not necessarily checked, not necessarily correct. Feel free to ignore, or tell me about it.

A Y-cable used to put two loads, in parallel, on one output.

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 the headphones.

doubles the load

(...if equal impedance. If not, one will be louder than another.)

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

Many headphone outputs have some leeway, as they're typically designed to cope with real-world variation in headphone impedence.

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

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

You can usually avoid that by keeping the volume lower. Under half is safer.

If there e.g. is a distinction between line out and headphone out, then think twice and read the manual - but as far as I can tell this is rare.

http://boards.straightdope.com/sdmb/showthread.php?t=624492

Amplifying a weaker signal

Adding a line/aux in

Line out from speaker out?

Avoiding pops

Car audio

Tape related

Cassette speed mod

Tape loops