70-Volt Speaker System

Life gets complicated when you have several speakers that you want to hook up together to put the sound in more areas, or handle more power. Rather than wrestle with the nightmare of speaker impedance matching, many large haunts use a 70-Volt speaker system.

It won't improve the quality of the sound over a properly impedance-balanced set of speakers. Think of it as a different way of wiring things, where the impedance-balancing is magically done for you.

We have a bunch of information about 70-Volt speaker systems...

What is this?
Why do this?
What you need
Wiring
Looks like a waste of money!
Wiring specifics
Cautions
Related Pages

What is this?

There are two types of audio amplifier that you may encounter.

Constant Current Amplifier
This type of amplifier is most common in home stereos. They have a low impedance output of around 8 ohms or 4 ohms.
Constant Voltage Amplifier
This type of amplifier is commonly used for commercial "distributed" audio systems with speakers in numerous locations. These amps have a high impedance output identified by a voltage, like "25 Volt", "70 Volt".

This page is intended for Constant Voltage Amplifiers. For information on Constant Current Amplifiers, you will have to deal with speaker impedance matching.

Note: 70 Volts is commonly used for commercial sound distribution systems in the United States, but other levels are also used. 25 Volts is often used in intercom systems in the U.S.. Europe tends to use 100 Volts. In any case, the theory still applies, although the voltage levels differ.

Why do this?

There are several reasons why you might want to use a 70-Volt speaker system:

You won't need to do any calculations or fancy wiring in order to string together a bunch of speakers.
The system allows you to mix speakers of different impedance and power without much work.
Despite dissimilar charactersitics, all speakers can be made equally loud, or you can make specific ones louder or softer than the others.
The 70-Volt system is more efficient at transmitting electricity to speakers, saving power and allowing the use of less expensive components.

What you need

Think of 70-Volt as a means of wiring. So if you want to do this in stereo, you need two sets of everything, just as a stereo speaker system needs two sets of cables to connect the speakers to the amplifier. From here on, we will just discuss one channel of spooky sound.

a bunch of speakers

a big transformer at the amplifier that turns 8-ohm or 4-ohm signals into 70-Volt signals (a step-up transformer)
- Radio Shack #32-1032 - handles up to 100 Watts total
a little transformer for each speaker that turns 70-Volt signals back into 8-ohm or 4-ohm signals to run the speakers (a step-down transformer)
- Radio Shack #32-1031 - handles up to 10 Watts; impedance 4, 8, or 16-ohms
wire
- For 50 feet (15m) or less of total length, with 50 Watts or less of power - use 18 gauge wire.
- For 50 feet (15m) or more of total length, or over 50 Watts of power - use 16 gauge wire.
- It's always OK to use thicker wire than suggested; it's not a good idea to use thinner wire. The smaller the gauge number for wire, the thicker it is.

Wiring

The wiring is quite simple, as indicated by this schematic enclosed with the #32-1032 transformer: [photo]

Just to make this amply clear:

at the amplifier, where you would normally connect the speaker, you put the big transformer
at each speaker you put a little transformer
then you just wire all the transformers in parallel.

And that's the neat bit - when you want to add another speaker, just bolt a transformer on it and add it in parallel. No math, no mess, no fuss.

Looks like a waste of money!

The astute reader is probably scratching his head right now. All we have done is spend money - adding transformers here and there. In fact, the little transformers all over the place just un-do what the big transformer does. Why bother?

What we are doing is trading off some cost for convenience.

Speaker impedance matching can be a real pain:

Let's say that you have two 8-ohm speakers and two 4-ohm speakers and you want to run them all off a single amplifier. How do you hook them up so that you don't damage your amplifier or speakers?
What changes must you make if one of the 8-ohm speakers is rated for 10 Watts and the other only 5 Watts?

You can do this without the 70-Volt system. It just takes a bit of thinking and some math. But if you get it wrong, you might have speakers catching fire, or damage your amplifier.

The 70-Volt system makes it easy to set things up the right way.

Wiring specifics

The main step-up transformer is simple: the side marked "input 4 ohm" goes to the speaker output of your amplifier. Keep the transformer as close to the amplifier as possible. Use wire as short as possible; the thicker the better.

Despite the "4-ohm" marking on the transformer, it can be used with 8-ohm and 16-ohm output amplifiers, too. But you won't get the full power of the amplifier.

The connectors marked "output 70-Volt line" goes in parallel to all speakers.

Then, each speaker (or speaker cabinet) gets its own step-down transformer.

The only part of the wiring that is the least bit tricky is selecting the right connectors on the little transformer that you put on each speaker. That transformer bristles with connections, like a porcupine. It's really easy, though.

[photo] When connecting the little speaker transformers to the big parallel line that services everybody, look for the side of the transformer marked "Primary" or "Pri". Connect one wire to the common (marked "C") and the other wire to the tap marked with the amount of power you want to allocate to this speaker. Thus a 10-Watt speaker will get connected to everybody else via the terminals "C" and "10".

This particular transformer only goes up to 10 Watts, but goes down to .62 Watts.

If you have an area in your haunt that wants slightly softer sounds, use a lower wattage tap.

[photo] When connecting the speaker to the transformer, Look for the side of the transformer marked "Secondary" or "Sec". Connect one wire to the common (marked "C") and the other wire to the tap marked with the impedance that matches the speaker. Thus an 8-ohm speaker gets attached to the terminals "C" and "8".

There are also taps marked "4" and "16", in case your speakers have impedences other than 8 ohms.

It's not that impedance matching is not being done. It's that the impedance matching calculations were done by the guy who designed the transformer!

[photo] By the way - save yourself a lot of trouble by permanently attaching the transformer to the speaker. Then you can stop thinking of it as an 8-ohm speaker. It is now a 70-volt speaker.

Of course, we'll paint it black before Halloween.

Cautions

There are only a few warnings:

Do not allow the two conductors of the 70-Volt line to touch each other.
As far as building codes are concerned, 70-Volt audio lines are usually considered "high voltage", just as if you were running 110 VAC to your compressor. This may require special conduit or installation by a professional electrician - check!
You have a power budget, which is the smaller of (a) the power of your amplifier and (b) the capacity of the big 70-Volt transformer attached to it. In the case of the #32-1032 transformer, it can handle 100 Watts. Thus the sum of the Watt taps on all the speakers used can not exceed 100 Watts. If your amplifier is capable of more, say 200 Watts, you are still limited to 100 Watts because of the transformer.

You may be interested in these related pages:

Thank you for visiting. Your comments are welcome.
. . . . . .