70/25 Volt Audio by Al Whale

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Connecting a speaker is a fairly easy task
You just connect the amplifier output to the speaker.

Amplifier to Speaker     Amplifier to Speaker


Impedance is the resistance in an AC environment

Impedance can vary widely with the frequency of the AC Voltage

Therefore a Speaker is rated with a Nominal Impedance

Consider an 8 ohm Speaker connected with a wire that has a total of 2 ohms resistance.

The following chart shows what happens if the impedance varies 20%.
8 ohms x 80% = 6.4 ohms, 8 ohms x 120% = 9.6 ohms

 Speaker Impedance   % voltage across Resistor   % Voltage across Speaker 
8 Ohms 20 % 80 %
6.4 ohms 23.81 % 76.19 %
9.6 ohms 17.24 % 82.76 %

Therefore, part of the audio signal is wasted across the connecting wire AND
the amount of signal to the Speaker varies with frequency .
This is an extreme case (hopefully larger wire would be used), but it shows what can happen.

What wire do you use to interconnect with?

The resistance of the wire should be maximum 5% of the speaker impedance.
(5% of 8 ohms = 0.4 ohms)
Therefore for an 8 ohm speaker, the line resistance must be 0.4 ohms or less

Conductor Resistances

(It takes 2 conductors to interconnect to the speaker,
so the distances of the chart are 50 feet and 500 feet)

 AWG   Resistance/100 Feet   Resistance/1000 Feet 
(Size) (Distance=50 feet) (Distance=500 feet)
2 0.016 0.156
4 0.025 0.249
6 0.040 0.395
8 0.063 0.628
10 0.100 0.999
12 0.159 1.588
14 0.253 2.525
16 0.402 4.016
18 0.639 6.385
20 1.015 10.150
22 1.614 16.140
24 2.667 26.670

From the chart, if you wanted to interconnect to an 8 ohm speaker,
you could go up to 50 feet with #16 wire (close enough).
for 500 feet #6 wire or larger would be needed

For more information checkout this site
Gateway to the home pages of Roger Russell
Look at the article entitled "Speaker Wire"


What happens when you try to connect several speakers to the amplifier?

Speakers in parallel     Speakers in Series

Three speakers in Parallel gives a load on the amplifier of 2.66 ohms.
Three speakers in Series gives a load on the amplifier of 24 ohms.
Neither situation loads the amplifier correctly.

Sometimes you can mix and match to come up with the correct impedances, but this isn't always possible.

(an example of a series/parallel combination)

Speakers in Series/Parallel

Multiple Speakers

Often situations arise that require multiple speakers (eg. halls, class rooms).
Groups of these speakers may need to be controlled separately,
without affecting the other speakers.

The wire runs to these speakers are often fairly long.

70 volt audio Distribution

70 volt systems allow multiple controls without interaction, and allow long wire runs

Constant Voltage


70 volt History

If using 70 volt system, insure all components are rated and wired for the 70 volts.
Similarly If using 25 volt system, insure all components are rated and wired for the 25 volts.

Transformers are used at each speaker location to convert from the 70/25 volt system to the speaker impedance (eg 8 ohms).

70 volt transformer

The 70/25 volt line from the amplifier is applied to the input of the transformer.
The input selected is based on the maximum power needed from the speaker.

All of the speaker locations come from this same 70/25 volt line source (in parallel).
The sum of the power setting (of all of the transformers used), should be less than the maximum power of the amplifier.

(If the total is over the maximum setting, the amplifier will be overloaded and will no longer be a constant output. Switching a group of speakers in this situation will then affect the other speakers.)


W= Voltage squared / resistance

W= Voltage squared / resistance       W= Voltage squared / resistance

From the above calculations, in 70 volt systems, the 10 watt tap will be 500 ohms,
and the 5 watt tap will be 1,000 ohms.

Note: for wiring ... 5 % of 500 ohms = 25 ohms, thus smaller gauge wire can be used
and the wire can go furthur without effecting the audio.

In 25 volt systems, the 10 watt tap will be 62.5 ohms, and the 5 watt tap will be 125 ohms.

similarly, 5 % of 62.5 ohms = 3.125 ohms, thus larger gauge wire must be used
for the 25 volt system to gain the same effect as the 70 volt system.


if the total load on the 70 volt line was 100 watts,
from the above formula, the impedance would be 50 ohms.
Using the practice of 5% max, the wire would have to be under 2.5 ohms.
Checking the table, for 50 foot run, the wire would only need to be #22 gauge.
For 500 feet, the wire could be #12 gauge (#14 guage would also be fairly close).
This is far easier than using 8 ohm lines.


Mixing 70 volt and 25 volt equipment

Questions have been asked about using 70 volt transformers
and/or speakers on 25 volt systems.

There is nothing special about these transformers and speakers,
except their inputs are marked in power based on the system
they were designed for (most other audio transformers and
speakers are marked for their input impedance).

By calculating the impedance, you can calculate the effect which
would be obtained when using the device in the other system.

 Input Marking
for 70 volts 
 Calculated Impedance
R = 5000/W 
 Calculated Power @
25 volts   W = 625/R 
10 Watts 500 Ohms 1.25 Watts
5 Watts 1000 Ohms 0.625 Watts
2.5 Watts 2000 Ohms 0.3125 Watts

Thus using 70 volt transformers and/or speakers on 25 volt systems
will reduce their power to 1/8th the labeled input

Using 25 volt equipment on 70 volt systems would not work,
and may be dangerous (fire) because the equipment would be trying to
work at 8 times the power (vastly over the design ratings).


Switching and controlling the speaker zones

Speakers can be individually switched (on/off) or switched in zones (such as several speakers in a hall).
It is a good practice to switch both conductors going to the speaker.

Switching Speakers

If designed properly, switching speakers on/off should not affect the levels in other speaker locations

Special Controls are used to adjust the volume.

70 volt pot

These controls will reduce the volume to the systems that they feed (still at 70/25 volts).

Although not usually done, use of series/parallel circuit can
still be used on the 8 ohm output side of the transformer,
to limit the number of transformers used in low power situations.

three speakers


70/25 volt systems are designed for multiple speakers in remote locations. For higher power systems, use of the standard impedances (16/8/4 ohms) is still the best course. The 70/25 volt system's frequency response is affected by the quality of the transformers used. A transformer rated at 10 watts maximum, probably will saturate (gives distortion) at a far lower value if the frequency is 60 hertz. This will only be evident for loud low frequency situations. If the system is designed with plenty of head room, this probably will not be an issue.

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