Audio distribution

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1935 moving coil loudspeaker 0222-7.jpg

Sometimes its desired to distribute an audio feed around the house. This makes it possible to listen to the radio or a recording wherever you go, without carrying equipment around. It also makes it easy to listen to and record from the stereo when that and the computer are in different locations, without the inconvenience and downsides of digital distribution. Other uses are also discussed.


Purpose

Audio distribution can do several related things:

  • transfer digital files to a computer
  • transfer audio to a remote speaker
  • transfer audio to speakers all round the house
  • transfer audio from analogue sources (eg stereo, tv) to computer
  • transfer audio from TV to hifi
  • transfer audio 5 feet or 500 feet

All of these are audio distribution, but they don't all share the same goals or best solutions.


Analogue or Digital

At first sight it might appear that the installation of home computer networking would make an audio feed redundant, but this isn't really so. Computer networking is the ideal choice for transfer of files between computers, but for other audio distribution tasks it runs into significant problems:

  • Sound can't be transmitted from hifis, TVs, radios, etc
  • Each sound receiver requires an entire computer system
  • The resulting audio is not synchronised between computers, so whole house sound works very poorly
  • Sound is only available where computers are located, making coverage patchy
  • Computer reception & playing isn't automatic, making such a system often inconvenient
  • Often some of the networked computers are switched off, giving worse coverage & more inconvenience
  • Wireless data links give poorer synchronisation and sometimes breakup

Analogue distribution solves all the above issues.

The good news is now that people are installing wired computer networks, audio distribution can be installed at the same time with almost no extra work or cost, making physically distributing audio pretty simple. This enables this or various other applications to be used later if desired. See Low Voltage Wiring for more info.


Analogue's issues

Computer networking is a set of fully pre-designed systems. You just do what it says and it works.

Analogue isn't a predesigned system, or fully standardised, and there are a few options, each with its own design issues that sometimes need checking. The quick selector below aims to suggest options that won't leave you having to understand and calculate any potential issues.


Quick selector

Good distribution options for each job.

  • Computer file transfer: digital computer networking
  • Analogue signal transfer all around an average house: bell wire, speaker wire or cat5e cable connected to speaker output of a small audio amp (eg 1watt). At receiving end, connect to line input sockets and/or active speakers (eg PC speakers). Can also connect headphones directly.
  • Analogue signal transfer to appliance: small screened cable for upto 10', large co-ax cable upto 25'. Connect to line input at receiver & line output sockets at sender. For 1970s or older equipment, small screened cable upto 4', large co-ax upto 10', but keep cables shorter if possible.
  • Speaker distribution upto 100m: bell wire or speaker wire, feeding speakers directly. 2.5mm^2 T&E can manage 300m.
  • Long distance distribution around a site with more than one building: low impedance differential feed with a differential input at each building. A bridged output audio amp can be used to provide the differential output, feeding the line with 10v or so.
  • Large flexible distribution systems where sound quality isn't critical: 70v line or 100v line over any cable rated for 150v+. Use a PA distribution amp, and PA speakers with built in matching transformers.


Avoiding hum

The main potential issue with analogue audio distribution is hum. There are various ways to avoid hum infecting the audio signal, and 2 simple options are presented here.

Screened wire

The simplest option is to use screened wire for the feed. The cable's outer conductor, the screen, connects to the chassis of the source stereo. This screen shields the inner conductor from hum pickup. Distributing stereo requires 2x screened wires.

This approach means buying a reel of screened cable, when one could instead use cat5e, which you already have when networking, is cheaper and can also carry other signals for other jobs.

When using this approach, the cable needs to be fully screened. There are screened cables on the market that are very light on screening copper, resulting in a partially screened cable. These are not suitable.

One issue with screened cable is capacitance. When not using a low impedance feed, most screened cables have too much capacitance to preserve the high frequencies properly over the sort of lengths in use here. Low capacitance screened wires are physically larger, around 1/4" thick, and include TV co-ax and RG59. For some of us, a convenient source of such cable is to re-use old TV co-ax that was replaced with more modern cable.


Low impedance feed

This option works with any type of wire, and cat5e is generally the most convenient. No screening is used, instead the signal is driven from a low impedance source. This is most easily achieved by feeding the audio output to a small audio amplifier (eg 1 watt), and connecting the output of that to the cable. Such amplifiers have output impedances of under an ohm*, and this low impedance eliminates hum.

* note an amplifier's real output impedance is not the same as its intended load impedance. (And its not unusual to see one described as the other.)


Headphones

Headphones can be run directly from a low impedance feed, meaning the speaker output from an audio amp. Headphones can't be connected directly to a distribution system driven from a line output connection.

For low impedance distribution networks, simply fit a headhone socket, and connect a 33ohm resistor from network L to headphone socket L, and another from network R to headphone socket R. Headphones use TRS sockets in 2 sizes, either 3.5mm or 1/4". Resistor value isn't critical, but the pair should be the same. The resistors prevent plugging and unplugging or a faulty heapdhone from affecting the sound on the network.

In principle there's a limit to how many headphones can be run at once, and this limit depends on the characteristics of both headphones and feed amplifier. However in practice it would be unusual to encounter a domestic setup where the feed amp's abilities were exceeded.

Power distribution

Where its wanted to use small compact active speakers, such as computer speakers, distributing the power required to run them down the same cat5e can in principle avoid the need for a number of local wallwarts, or the need to run wires from the speakers to mains sockets.

However there are significant problems amd gotchas with this approach. If you don't understand them or aren't willing to spend a little time debugging then this topology is better avoided.

Note this option is only for low impedance audio feeds, its not suitable for high impedance screened feeds.

Reliable topology

A fully reliable bug free system can be realised if the 2 points below are followed.

First, the audio ground wire must not be used to distribute the power ground, 2 separate conductors are required for the power distribution.

Secondly the active speakers should have a differential input to ensure good behaviour. This is more likely to be found on speakers intended to run from a USB connection; speakers that come with their own wallwart normally don't have a differential input.

There is no sure way to tell differential input amps from ones that aren't, unless you have electronics skills and can examine the circuitry. But a quick test can tell in some cases. Checking the resistance from input ground to power ground (on the amp or active speaker):

  • if the 2 are connected, you have an amp without diff input
  • if they aren't connected (not zero resistance), the test is inconclusive

Speakers without differential input

Most computer speakers don't have differential input. These will work fine when each is run off its own local wallwart. Running such speakers from a single central supply is a recipe for trouble, though it can be done.

Speakers without differential input on distributed power

Its possible to use speakers without differential input and feed them all from a central power source, but with this combination problems are likely, debugging time is necessary, and even after solving them the setup is not ideal.

If its not known whether speakers have a differential input, they can be tried. If they don't have it, a problem may occur, which shows up as unwanted noises on the sound, typically plopping, farting, growling, motorboating, or squealing noises. This problem is most likely to happen at maximum volume, so turn them up to check.

There are various possible solutions to parasitic noises described below, but the use of non-differential input speakers with power distribution always runs the risk of affecting sound quality at low frequencies, and while this often won't be noticed with computer speakers, it may be noticeable when listening through a hifi system. So if you want to ensure best sound quality, avoid any hassle and you use non-differential input speakers, then each such speaker should be fed from its own wallwart rather than using distributed power.

To obtain optimum performance for recording on this system, switch all the fed speakers off during recording time.


Solutions

If using speakers without differential input and feeding them all from a central power source, there are several methods to reduce power/audio feed interaction and the risk of noises it causes. Whether such a topology causes any low frequency distortion depends on how thoroughly the problem is tackled, and the problem is not always 100% solvable. However its quite practical to try to run a system this way, knowing that if issues should persist one can simply revert to using a local wallwart for each speaker to solve it.

Problem noises can often be eliminated by connecting a large capacitor across the speaker's power connections, 2200uF or larger. If some active speakers on the system exhibit a problem and some don't, its best to connect a capacitor to every speaker. This minimises the chance of one speaker affecting another. Capacitors should be used with all systems using this topology. If 2200uF doesn't fix it, try a 4700uF cap with a 0.1uF capacitor (preferably ceramic) in parallel with it. (Note such capacitors are polarised, and need to be connected + to the positive wire, - to the negative.) The 3rd option, a bit more effective again, is to use the following CRC filter (the diode behaves as a non-linear resistor):

+ o----+-----|>|---+-----+----o + to speaker
       |           |     |
      C1      D   C2     C3
       |           |     |
- o----+-----------+-----+----o - to speaker

C1 = 2200uF
C2 = 2200uF
C3 = 0.1uF
D = 1A diode, eg 1n4001


If capacitors don't eliminate it the problem, another option is to parallel conductors for the negative power feed wire. The downside is this then uses 3 or 4 wires instead of 2. Another possibility is to fit bell wire or speaker wire when installing network wiring, and use this thicker bell wire to distribute the speaker power. This will much reduce power/signal interaction. (Bell wire has a small 3 dimensional stripe on one side used to tell + from -, you need to look closely to get the polarity right at each termination.) Another option is to fit an additional single wire during installation, and use this for the power feed ground wire.

Positioning the power supply used for the feed in the geographic centre of the network minimises the length (and hence resistance) of cable from power supply to speakers, and also reduces the effect of one speaker on another.

Using an unearthed power supply for the feed reduces interaction further. Most plastic cased supplies are unearthed, and metal cased earthed.

Where one speaker is on a long run compared to the others, it will be much more prone to problems than the others, plus each active speaker contributes to some extent to the risk of trouble with the others. One option is to either use a local wallwart for that one or use a passive speaker. A passive speaker may give sufficient volume, but sometimes it may not, as the audio power is limited by the power output of the feed amplifier, and further restricted by the resistance of the cable run.

If in spite of these measures, parasitic noise or distortion at low frequencies occurs, then lose the power feed and use local wallwarts on each speaker, and it should all behave perfectly.


Phasing

In practice there are areas covered by sound from more than one speaker or pair of speakers. If both are not in phase with each other, partial loss of bass occurs, and less even sound distribution. It isn't necessary to phase adjacent sets of speakers, but doing so does improve the evenness and quality of the sound.

When speakers are out of phase, the problem is solved by either reversing the phase of one set or by pointing them in the opposite direction.

Phase reveral is easy with passive speakers (just swap the wires over) but not so easy with active speakers.

Speakers with differential inputs normally work fine if the audio input wires are swapped over.

Non differential input speakers can have the audio input wires swapped if powered by local wallwarts, and usually work ok.

Never swap input wires over with:

  • non differential input speakers if powered from a central power feed
  • any earthed appliance, such as a computer, tv, stereo system, etc

If one of these is done, the speaker may not work, and other problems can sometimes occur.

For non differential input speakers powered from a central power feed there is only one way to reverse phase: open the speakers up and swap the 2 wires going to the speaker driver. Pointing the speaker in the opposite direction is often easier.


Terminology

Active speaker
a speaker with built in ampifier. These require a source of power. Nearly all modern computer speakers are active, and nearly all hifi speakers passive.
Differential input
an amplifier (which may be built into an active speaker) that responds to the difference between the 2 audio wire inputs. This has a few advantages, but most amps lack this feature.
Distribution line
aka distribution cable, feed line etc. The cable that runs round the house to distribute the signal.
Passive speaker
Speaker with no built in amplifier. These don't require any power source, just the audio signal itself. Small computer speakers from the win3.1 era were mostly passive. Hi-fi speakers are normally passive.
S/N ratio
The ratio of wanted audio signal to unwanted noise. The noise usually consists of hiss, hum & distortion products, hopefully at very low levels.


Perfection

No audio system is perfect, but for recording, a few tips can help this analogue approach get as close to perfect as equipment will allow.

Run the feed amplifier at around 1/3 or 1/2 maximum volume. This optimises S/N ratio while maintaining good headroom. Max volume is just under the point at which the amp begins to distort, and is not determined by volume knob position.

Its best not to route your network wiring with mains wiring.

When using non differential input speakers on central power distribution, its best to switch the central power feed off when recording, as if left on the speakers can affect sound quality in many cases.

When using multiple feed amplifiers driving one distribution line, switching off any feed amps not being used maximises S/N ratio.


External wire runs

Exterior wiring is vulnerable to physical damage, and should be routed where this isn't likely. Wiring at or below ground level benefits from physical protection, and the simplest option for underground signal wires is to thead it through garden hose. Where any form of conduit is used, more cables can be threaded at the same time for eg computer networking.

Wiring mounted above ground is vulnerable to UV damage, and for longest possible life it should either be

  • black
  • shaded from the sun
  • or be painted with oil based gloss paint (topcoat only)

To avoid exterior wiring faults affecting the system or frying the feed amp, the exterior cable should be fed via a pair of resistors (eg 15 ohm). These go in the L+ and R+ lines.


Very long wire runs

For very long wire runs, eg covering several houses or very large buildings, either of the following works:

1. Use co-ax wire for distribution, driving it with a low impedance feed amplifier.

2. Use a bridged output feed amplifier, which gives a low impedance balanced output, and receive the signals with an amplifier with differential inputs. Note that this requires 4 feed conductors, the two -s must not be connected, and non differential input equipment and 3 terminal connectors must not be connected to the feed line. Consequently it may be convenient to arrange the system with a balanced main feed which supplies an amp in each building that in turn supplies a local unbalanced feed for that building.


What plugs in where

The 'record output' (may be labelled 'tape out' or 'line out') from the source (eg a stereo amplifier) is connected to the input of the feed amp using a standard screened lead.

The feed amp's speaker output terminals connect to the distribution line.

  • the L wire in the line connects to the L speaker + terminal, preferably via a resistor
  • the R wire in the line connects to the R speaker + terminal, preferably via a resistor
  • the ground wire in the line connects to - terminal of either speaker connection on the feed amp.
  • Also connecting a 2nd ground wire to the other speaker - terminal makes connecting plugs & sockets to the distribution wiring easier, and reduces problems when using central power feed, but its not essential.

Distribution sockets have 3 or 4 terminals. A resistor goes between the feed line's L+ and the socket's L+, and another resistor goes between the feed line's R+ and the socket's R+. Distribution line's ground wire(s) connect to the socket's ground connection(s).

When more than one feed amp is used, the feed line's L+ is connected to the feed amp L+ through a resistor, and the feed line's R+ is connected to the feed amp R+ through a resistor. Feed line ground goes to one of the speaker - terminals on the feed amp.


Can't I just connect to the stereo's speaker output?

If you do it will work, but all the adjustments the amplifier makes will then affect the sound feed. These include bass & treble controls, volume and balance, and any other sound controls fitted. This is ok for casual listening, but for recording audio on the computer its a real curse. Its also not ideal in that speaker resonances can affect the signal quality.

If you want you can run 2 feeds in the one cat5 cable:

  1. from the main amp output, which feeds standalone speakers, controlling volume etc house-wide from the source stereo system.
  2. from the small feed amplifier - this feed is used for recording.

This would use a minimum of 5 of the 8 wires in a cat5 cable.


Connectors

3.5mm TRS jacks allow computers, headphones or computer speakers to be plugged straight in. They are also conveniently small connectors. However they are not especially good quality connectors for analogue audio use, being prone to poor contact affecting the sound, and they really aren't robust. Where best quality is more important than convenience, a better connector type is preferable, such as phono sockets or 1/4" TRS jack.


Resistors

A series resistor should be fitted between the distribution cable and every socket on the cable (one resistor on each of the L & R lines) for several reasons:

  1. Resistors protect the audio feed against faults in receiving equipment and leads
  2. Resistors increase the number of headphones that can be plugged in at once.
  3. Resistors prevent a local problem from distorting the feed or frying the feed amplifier
  4. TRS jack connectors often short during insertion, and resistors protects the feed from being shorted
  5. Protection against TRS shorting is still needed when no TRS connectors are used on the feed line, as TRS connectors on the other end of a lead plugged into the feed will still cause shorting on insertion.
  6. Resistors protect the feed amp from various scenarios that would otherwise fry it.

When using a low power feeder amplifier (eg 1 watt), low resistance values should be used at the sockets to avoid significant loss of volume with headphone use. There is no specific ideal value, but something in the region of 15-22 ohms would be good for a typical domestic setup. Both resistors should be the same value.

See #Multiple feeds for information on resistors connected to the feed amp's output.

When exterior wiring is used, the exterior cable run should be fed by a pair of 15 ohm or so resistors. Now if faults occur on the exterior cable it doesn't stop the rest of the system working.


Feed amps

A small amp of 0.1w to 3 watts is best. More power is counterproductive, and only increases hiss level. Feeding a signal distribution system off your main 30w stereo amp isn't a great plan, hiss will result.

The amp should be run from a wallwart via a suitable fuse. A 0.3A fuse is fine. The fuse is wise because many small amps have no output device protection, there are many chances to short the distribution cable, and the small power dissipation is concentrated in very tiny dice in the amplifier. The fuse doesn't stop the amp's output stage frying, it helps prevent more serious problems.

Computer speakers

Computer speakers with a headphone socket are about as easy a feed amplifier as is available. They're common, all necesary connections are externally pluggable, and they come with their own wallwart supply. No need to even open them, just plug in. Connecting the 3.5mm jack input to a hifi requires either a 2 phono plug to 3.5mm stereo socket adaptor, or replacing the 3.5mm plug with a pair of phonos. The miserable quality speakers built in to them have no effect on the sound in this use.

New amps

Uncased low power amplifiers are available from electronic component suppliers for a few pounds. Its usually necessary to solder the wire connections to the PCB and provide a power supply.

LM386

Most small amps behave well in these distribution systems, but there is one notable exception. The LM386 is a minimum cost 8 pin audio amp chip with sound quality best suited to answerphones and novelty devices. The LM386 has a fair bit of distortion, and is easily spotted by the mess it makes of treble frequencies. Avoid!

hifi amps

An unused stereo hifi amp could be used, but will not give good performance if connected directly, with high hiss levels. This problem can be solved with a voltage divider:

amp +  -------+
              |
              R1
              |
              +------ distribution cable +
              |
              R2
              |
amp -  -------+------ distribution cable ground

For a 30w/channel (rms) amp, R1 = 33 ohm 5 watt, R2 = 15 ohm 2.5 watt (ignore pmpo power ratings).

when one is used, set bass treble and balance to central position.

Salvaged amps

Most people have a small amplifier somewhere, sitting in an unused appliance. Computer speakers, radios, in fact anything that can drive speakers will contain an audio amp. Appliances that can drive headphones at loud volume, such as a CDROM, also have a usable small amp inside. Identifying the right places to input signals is often required, and often removing the existing signal input.

Computer speakers are easier, as there is nothing but an amplifier on the circuit board, so no need to work out where to solder inputs and power to. A pair with a headphone socket makes life really easy, as everything just plugs in.


Multiple feeds

Multiple feed points are not usually used, but can be. A new feed can be added to a system at any point. Multiple sound sources get mixed on the distribution line, so its necessary to have only one powered up and connected at a time.

One minor modification is necessary when using more than one feed; resistors need to be inserted between each feed amplifier output and the feed line. The value of each resisor should be at least 8 ohms multiplied by the number of feed amplifiers connected to the line, which is 16 ohms for 2 feed amps, or 24 ohms with 3 feed amps. Then add a 50% margin, giving 24 and 36 ohms. When the result isn't a standard resistor value, the next value up is used.

Adding a mic at the feed points enables the system to be used as a PA system. This can sometimes reduce the amount of run around with kids, and can be used as a basic help call system. To do this the feed amp or something feeding the feed amp needs to have a microphone input. A momentary press switch is used to connect the mic, and ideally a relay is added to also silence the local speaker (to prevent feedback).


Doorbells etc

Audible notifications can be fed to the system from anywhere in the house. Probably the most useful of these is the door bell. If the bell output is fed to the distribution network, the bell should still have its own local sounder as well, as its sensible to turn off the bell's feed when recording. When signalling devices are connected, running them all off a central power feed makes it easy to switch them all off.

Note that audio feed devices such as doorbells may have the wrong voltage, impedance and ground levels for direct connection to the feed line. Resistors and sometimes capacitors are used to correct this.

when feeding such signals to the line, the signal sources are treated like another feed amplifier, requiring suitable resistors to be added to all feed sources. See #Resistors for details.

These sources can cause problems with recording if mixed with the main feed. A solution to this is to feed them down another conductor, with both signals mixed at the receiving end for speakers, but not mixed where recording is done.


See Also