Hi, we are a professional sound rental company.

We are having a show along the streets of a city and we need to place speakers all around the streets. Now i have already seen something like this but i don't know exactly what it is. Basically the wire gets into a homemade box before entering the speaker and than it goes into the speaker from the box, eventually with one run of wire more than 15 speakers are connected. This does only mean that only one amplifier channel was used. How can this be? i don't believe they are all in parallel, i 've seen only amplifiers going down to 2ohms. Can someone explainn this to me.

 

Typically such installations use what is called a "constant voltage system". They use higher voltage and lower current then the low impedance design used in standard home audio. A transformer on each speaker converts the high voltage signal to the low voltage, higher current required by the speaker.

 

Hi, we are a professional sound rental company.

Hello Jason, what is your role in this company?
It's good to learn more, that is the way to rise in the business.

I expect the audio distribution is via what is known as a 100volt line system.

The amplifier outputs a relatively high voltage at its output terminals.
Each speaker is not connected directly to this output, but instead connected across the secondary of a transformer, known as a '100 volts line transformer'.

The secondary of the transformer matches the (lower) impedance of the speaker.
The primary of the transformer is of higher impedance, suitable for the 100 volt line.

Yes you are correct that all the transformer primaries are in parallel, but they have a high impedance so that is OK.

 

It may be that I am way too old, but I think I saw this described as a 70 volt system. And yes, a transformer is used at each speaker location to tap a few watts off the high voltage line.

Sound System Engineering by Don and Carolyn Davis, 1975, published by Howard Sams & Co., Inc
4300 West 62nd Street, Indianapolis, In
Page 118: "Impedance Matching 70-Volt Lines"

 

hi, thanks for your replies. This is relatively new stuff for me. So basically that cannot damage the amplifier output? And at this point how is the amplifier ohmage calculated? And basically you need to build up these transformers custom or you can find off the shelf?

 

"cannot damage?"
You haven't been reading this site very long! Done properly, anything can be reduced to a smoking heap.

The printing date of the book I referenced means you are dealing with technology that is over 30 years old...probably a LOT older than 30 years, and finding parts for it is going to be an adventure. I suggested that book because it has the answers you need, except for where to buy parts. Then again, trying to buy that book might be an adventure, too.

 

This is relatively new stuff for me. So basically that cannot damage the amplifier output? And at this point how is the amplifier ohmage calculated? And basically you need to build up these transformers custom or you can find off the shelf?

That was why I asked about what you do, so that more explanation could be provided at a suitable level.

I called the system a 100 volt line, and #12 a 70 volt line.

The lowest I have seen is 50. 70 is common in small distribution systems, 100 in larger ones.

#12's book may be old but the system is still in normal use as it overcomes other problems such as cable losses in addition to multiple speakers.
Are you familiar with calculating cable losses and efficiency?

If you look at the back of the distribution amp you will probably see that in addition to normal (optional) speaker terminals it has something labelled 50V line output or 70 Volts line output or 100 volts line output.

 

Hi, thanks for your information. I am quite familiar with cable loss and efficiency calculations. So by this system you will almost eliminate the delays than? Regarding the transformer present before every speaker thus it must be a 100 to 8 ohm transformer, with what power rating?

 

thus it must be a 100 to 8 ohm transformer

Why do you say this?

100 is the line voltage.

The line impedance is often 600 ohms.

So by this system you will almost eliminate the delays than?

The line impedance affects the frequency response of the system, rather than delay time which is insignificant at audio frequencies.

Here are some parameters.

The higher the line impedance the finer the transformer winding wire, making for increased fragility (and cost) and decreased reliability.

Line leakage appears as a shunt across the line. The higher the line impedance the more significant a given value of shunt offers.

High frequency (audio) attenuation effects due to line capacitance increase with line impedance.
A 10nanofarad capacitor has an impedance of 2000 ohms at 7kHz.

 

Public address audio amplifiers with constant voltage outputs are commonly available.

You don't worry about amplifier impedance, just the total wattage you need.

The speaker transformers are also commonly available and usually have taps for different wattage (volume) level output for the speaker. The correct tap is generally experimentally determined when the speaker is installed.

 

Don't forget that your aplifier output must match the speaker system. For example you cannot connect speakers wired with 70 volt transformers to a amplifier with an 8 ohm output.

 

For the benefit of those to whom the subject of audio distribution systems is new here are a couple of photos of the back of one of my PA amps.

Note that the output is between a common connection and a tap at

4Ω ; 8Ω ; 70 volts and 100 volts.

The (mains) earth is a separate connection that may or may not be connected.

Note also that although the inputs may be stereo or mono, the output is single channel.

Feel free to discuss this further.