Audio Distribution

premkumar9 · Nov 13, 2008

Hi,
I want to take the Audio O/P from a TV receiver and distribute it to different points in a hall. At each point there has to be separate volume control. The immediate solution came to my mind is to take the Audio O/P through a cable and use standard speaker system with amplifier (used along with PCs) at each point.
Here my doubts are
- What is the source impedance of the audio source from TV? is it 600 Ω
-What cable to be used? 600 Ω audio cable?
- What is the normal input impedance of the PC speaker system? is it 600 Ω.
- Should I use an audio distribution amplifier to give separate output for each speaker system.
-Or can I use passive tap off?

Nigel Goodwin · Nov 13, 2008

Only professional audio systems use 600 ohm feeds, and not really that much of those - you don't usually use impedance matching in audio.

The audio line output of a TV is a lowish impedance, probably a couple of kilo-ohms?, if you're using PC amplified speakers you need to check (or measure) their input impedance, as they are designed to be fed from a headphone output. Assuming it's sensibly high (10K or greater), you should be able to just tap off the TV using a resistor (say 5K) to each one.

premkumar9 · Nov 13, 2008

Nigel Goodwin said:
Only professional audio systems use 600 ohm feeds, and not really that much of those - you don't usually use impedance matching in audio.

The audio line output of a TV is a lowish impedance, probably a couple of kilo-ohms?, if you're using PC amplified speakers you need to check (or measure) their input impedance, as they are designed to be fed from a headphone output. Assuming it's sensibly high (10K or greater), you should be able to just tap off the TV using a resistor (say 5K) to each one.

Can these source and amplifier input impedance be measured with a multimeter OR multimeter reading may not give the correct value?

audioguru · Nov 13, 2008

A multimeter measures DC resistance. Impedance is AC resistance and the amplifier probably has an input capacitor that blocks the DC from your multimeter but passes AC.

premkumar9 · Nov 13, 2008

audioguru said:
A multimeter measures DC resistance. Impedance is AC resistance and the amplifier probably has an input capacitor that blocks the DC from your multimeter but passes AC.

So the practical method is to make connections and see the signal levels with an oscilloscope. Right?

Externet · Nov 13, 2008

premkumar9 said:
Hi,
I want to take the Audio O/P from a TV receiver and distribute it to different points in a hall. At each point there has to be separate volume control.

What will you feed that audio signal to? Speakers; recording inputs; amplifier inputs?

premkumar9 said:
The immediate solution came to my mind is to take the Audio O/P through a cable and use standard speaker system with amplifier (used along with PCs) at each point.
Here my doubts are
- What is the source impedance of the audio source from TV? is it 600 Ω

From its earphone jack probably around 200 to 600 Ohms.
From its speaker probably 4 to 16 Ohms

premkumar9 said:
-What cable to be used? 600 Ω audio cable?

Answer the first one

premkumar9 said:
- What is the normal input impedance of the PC speaker system? is it 600 Ω.

PC speaker system? What is that ?. Are you planning to use PC speakers or PC amplified speakers at the 'different points of the hall' ?
Will that be stereo ?

premkumar9 said:
- Should I use an audio distribution amplifier to give separate output for each speaker system.

Depends on how many and how loud you want them and how powerful the TV speaker amplifier is.

Miguel

premkumar9 · Nov 13, 2008

Externet said:
What will you feed that audio signal to? Speakers; recording inputs; amplifier inputs?

From its earphone jack probably around 200 to 600 Ohms.
From its speaker probably 4 to 16 Ohms

Answer the first one

PC speaker system? What is that ?. Are you planning to use PC speakers or PC amplified speakers at the 'different points of the hall' ?
Will that be stereo ?

Depends on how many and how loud you want them and how powerful the TV speaker amplifier is.

Miguel

1- Amplifier
2- from earphone
3-External speakers with amplifier normally used with PC. provision for stereo play back to be there.

Externet · Nov 13, 2008

You may use two conductor + shield cable similar to Belden 8451 for L, R, gnd; from either the TV headphone jack or speakers outputs, and probably not from the TV L, R line outputs ---> feeding in parallel to several amplified PC speakers or to a distribution amplifier inputs.
Those amplified speakers should already have the volume control you want.

PC amplified speakers may have any input impedance the manufacturer decided to design, probably around 10K Ohm

Miguel

premkumar9 · Nov 13, 2008

Thank you very much.

premkumar9 · Nov 13, 2008

Externet said:
You may use two conductor + shield cable similar to Belden 8451 for L, R, gnd; from either the TV headphone jack or speakers outputs, and probably not from the TV L, R line outputs ---> feeding in parallel to several amplified PC speakers or to a distribution amplifier inputs.
Those amplified speakers should already have the volume control you want.

PC amplified speakers may have any input impedance the manufacturer decided to design, probably around 10K Ohm

Miguel

Normally what is the impedance of head phones? Just to know the possibility of directly plugging in headphones in the place of PC Amplifiers if required.

premkumar9 · Nov 14, 2008

Externet said:
You may use two conductor + shield cable similar to Belden 8451 for L, R, gnd; from either the TV headphone jack or speakers outputs, and probably not from the TV L, R line outputs ---> feeding in parallel to several amplified PC speakers or to a distribution amplifier inputs.
Those amplified speakers should already have the volume control you want.

PC amplified speakers may have any input impedance the manufacturer decided to design, probably around 10K Ohm

Miguel

So what I plan is
- take out audio output from the AV output of TV (which may be a high impedance source).
- Give it to a distribution amplifier (which has to give a low o/p impedance)
- Take the signal to different locations (parallel tap off.)
- Sockets will be provided at these points where either the amplifier or head phone can be plugged in.
The above is OK? For your suggestions.

premkumar9 · Nov 15, 2008

To feed the stereo input to a mono amplifier, can I connect the two inputs together and give?

Mikebits · Nov 15, 2008

premkumar9 said:
To feed the stereo input to a mono amplifier, can I connect the two inputs together and give?

Then you just have mono anyways. Also not a good idea to have to driving devices feed into each other.

premkumar9 · Nov 15, 2008

Mikebits said:
Then you just have mono anyways. Also not a good idea to have to driving devices feed into each other.

Any better method? In this case some of my amplifiers are mono only and for them I want mono O/P only. My input is stereo.

Nigel Goodwin · Nov 15, 2008

Mikebits said:
Then you just have mono anyways. Also not a good idea to have to driving devices feed into each other.

As long as they are line level signals it's pefectly fine, they are designed to be used like that - but NEVER do it with speaker outputs, as it's going to kill the amp.

premkumar9 · Nov 15, 2008

Nigel Goodwin said:
As long as they are line level signals it's pefectly fine, they are designed to be used like that - but NEVER do it with speaker outputs, as it's going to kill the amp.

So you mean L and R signals from high impedance sources can be connected together while from low impedance higher power sources should not be connected together. Right?

Nigel Goodwin · Nov 16, 2008

premkumar9 said:
So you mean L and R signals from high impedance sources can be connected together while from low impedance higher power sources should not be connected together. Right?

Pretty well, 'high' impedance sources feed their output via a resistor, so joining two together makes a simple mixer using the two output resistors.

For example, a stereo keyboard - the line outputs will be fed from an opamp, but these will feed the socket via a series resistor (usually a few kilo-ohms), this is to prevent damage to the opamp should the output be shorted to ground.

premkumar9 · Nov 17, 2008

Nigel Goodwin said:
As long as they are line level signals it's pefectly fine, they are designed to be used like that - but NEVER do it with speaker outputs, as it's going to kill the amp.

What is the difference between line output and head phone output?

Externet · Nov 17, 2008

Line output level is not varied by the volume control and its impedance is typically 10K; much higher than earphone output.
Earphone output is typically 200 to 600 Ohms output impedance and its amplitude varies by the volume control

Nigel Goodwin · Nov 17, 2008

Usually it's perfectly safe to connect headphone ouputs together, as they have similar resistors to line outputs (but lower values).

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