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A Detailed Guide To Constant-Voltage Audio Systems
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With this background in mind, let’s proceed to installation practices. Here’s a basic procedure that neglects transformer insertion loss:
1. Do NOT connect the 70V loudspeaker line to the power amplifier yet.
2. Install a transformer at each loudspeaker location, or use loudspeakers with built-in transformers.
3. Connect each loudspeaker to its transformer secondary tap. The tap impedance should equal the loudspeaker impedance.
4. Connect each transformer primary to the 70V line from the power amplifier. Choose the tap that will deliver the desired wattage to that loudspeaker.
5. Add the wattage ratings of all the primary taps. This sum must not exceed the amplifier’s wattage rating. If it does, change to a lower-wattage primary tap of one or more transformers, or use a higher power amplifier.
6. Connect the 70V loudspeaker line to the 70V output of the amplifier.

As an example, suppose you are setting up a 70V system with 8-ohm loudspeakers and a 60W power amp. Connect the 8-ohm secondary taps to each speaker. Suppose the total loudspeaker wattage is 55 watts. This is acceptable because it does not exceed the amplifier power rating of 60 watts.

Here’s a more detailed procedure that emphasizes impedance matching:
1. Compute the minimum safe load.

The minimum safe load impedance that can be connected to the amplifier is given by:

Z = minimum safe load impedance, in ohms.
E = loudspeaker line voltage (25V, 70.7V, 100V, etc.)
P = maximum continuous average power rating of power amplifier, in watts.

An example: For an amplifier rated at 100 watts continuous average power, the minimum load impedance that may be connected safely to the 70.7V output is:

2. Choose transformer taps.
Tap the primary at the desired power level for the loudspeaker, and tap the secondary at the impedance of the loudspeaker. The sum of all the power taps for all the loudspeakers should not exceed the power output of the amplifier.

Note: Changing the power tap also changes the load impedance seen by the amplifier. Raising the power tap lowers the load impedance, and vice versa.

Also, changing the power tap changes the SPL of the loudspeaker. Reducing the power tap by half reduces the SPL by 3 dB, which is a just-noticeable difference in speech sound level.

If a particular loudspeaker is too loud or too quiet, you can change its power tap. Just be careful that the total power drain does not exceed the power output of the amplifier.

3. Connect the loudspeakers together.
Connect all the loudspeaker-transformer primaries in parallel. Run a single cable, or redundant cables, back to the power-amplifier transformer secondary. But DON’T CONNECT IT YET.

4. Measure the load impedance.
Before connecting the load, first measure its impedance with an impedance bridge (a simple low-cost unit is adequate). Here’s why you must do this: If the load impedance is too low, the power amplifier will be loaded down and may overheat or distort. It’s a myth that your can connect an unlimited number of loudspeakers to a 70V line.

If the load impedance measures too low, re-tap all of the loudspeakers at the next-lower power tap. This raises the load impedance. Measure again.

Usually, it’s no problem if the load impedance measures higher than the matching value (the calculated minimum safe load impedance). The system will work, but at reduced efficiency. Typically there is more than enough power available, so efficiency is not a problem.

If for some reason power the power is limited, then the system should be wired for maximum power transfer. This occurs when the measured load impedance matches the calculated minimum safe load impedance. If the load impedance measures above this value, you can re-tap all the loudspeakers at the next-higher power tap and measure again. This tap change lowers the load impedance.

Many people don’t realize that a transformer labeled for use with a specific voltage will work just as well at other voltages. See the constant voltage calculator here. It determines the power delivered from a transformer tap when driven with other than the rated voltage.

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