The decision to use low impedance, 4 ohm or 8 ohm loudspeakers, or a high impedance (often called “Constant Voltage” or “70Volt”) system is not a straight forward matter in system designs that use large format loudspeakers or are distributed over a substantial distance.
One Systems offers both low impedance designs and “high impedance” designs for direct weather installations and indoor applications. The calculator in this article was designed to provide the information necessary to help system designers choose the best solution for a particular application or venue.
The calculator requires simple input parameters and will generate outputs for both low impedance and high impedance situations. The basic intent of this program is to calculate the performance trade-off associated with both low impedance and high impedance designs for a particular situation.
There are often performance trade offs associated with transformer insertion loss, bandwidth limitations and line losses. Of course the low Z speaker can sometimes suffer from extreme line losses, particularly when the resistance of the line becomes an appreciable percentage of the nominal impedance of the low Z speaker. In order to make a good comparison between low impedance loudspeaker systems and “high impedance”, or transformer based loudspeakers several user inputs are required.
NOTE: It is frequently the case that smaller diameter wire is used when high impedance speakers are specified. Entering various gauges of wire can often show that there is no substitute for “copper”. (SAY IT AGAIN, THERE IS NO SUBSTITUTE FOR COPPER, THE LARGER THE WIRE, THE BETTER THE SYSTEM PERFORMANCE!) That is, large diameter wire is always preferable if there is adequate budget, regardless of whether the design is low impedance or high impedance
The “calculator” below will allow you to vary any of the input parameters to see how a low Z system will compare with a high Z system. The last result will allow you to see the SPL loss in each system type.
Enter the required data in the “INPUTS” section below.
The power tap values are as follows for One Systems high impedance loudspeakers:
Transformer Tap Values (Watts)
50 Watt 50, 25, 12.5
150 Watt 150, 75, 37
600 Watt 600, 300, 150
You can enter any power you wish. A good place to begin is to enter the same power level as the high Z speaker. One thing to remember is that the high Z speaker really limits you to the power rating of the transformer, where the low Z speaker power can be increase to near the power rating of the speaker enclosure itself. As an example, you could begin with 150 watts for the high impedance loudspeaker and also use 150 watts for the low impedance loudspeaker. Once this result is analyzed you can easily increase the power input to the low impedance speaker and compare new results to the high impedance case. Remember, you do NOT need to use the same input power for the low impedance design as the high impedance design. (but remember that you may need more amplifier channels when you start increasing power levels with this parameter)
The program automatically calculates the return distance so it is not necessary to enter the distance to the loudspeaker and back again. Simply enter the distance from the amplifier to the loudspeaker.
NOTE: If the distance is in meters multiply the metric distance (m) by 3.28 and enter in feet.
This is a fun parameter to play with. Change the wire size and watch the final SPL and associated line losses vary. (At the moment the program only deals with AWG and feet, not metric units). As noted above, BIG wire is always better then smaller wire. The lower the dc resistance of the wire, the higher the system performance, regardless of whether the design is high impedance or low impedance.
NOTE: See Appendix 2 for approximate conversions of AWG wire sizes to metric wire sizes.
High Impedance Outputs
Low Impedance Outputs
What we mainly care about in this analysis is the power applied to the loudspeaker transducers themselves. Does the high impedance design or low impedance design maximize the power to the loudspeaker?
Also, consider the number of loudspeakers that need to be wired to a single channel of an amplifier output.(The high impedance design may still offer a more desirable alternative even if the power to the individual speaker is less if more loudspeakers are required on an amplifier channel then the low impedance design will permit)
Transformers work by increasing the impedance presented to the amplifier. This then allows the amplifier output voltage to be increased, and the current decreased. The decrease in current reduces the power loss in the wiring (I^2*R losses). The impedance of each power tap is listed below. This is important because of the 600 watt transformer.
|Power Tap||Impedance (70.7Vrms)||Impedance (100Vrms)|
|12.5 Watts||399.8 ohms||799.7 ohms|
|25 Watts||199.9 ohms||399.8 ohms|
|37.5 Watts||133.2 ohms||266.6 ohms|
|75 Watts||66.6 ohms||133.3 ohms|
|150 Watts||33.3 ohms||66.6 ohms|
|300 Watts||16.6 ohms||33.3 ohms|
|600 Watts||8.3 ohms||16.6 ohms|
Inspection of the table shown above illustrates the limitation of high impedance designs when power levels become large. At a desired power tap of 600 watts a 70.7 Vrms transformer presents an 8.3 ohm impedance to the amplifier. Under these conditions it makes no sense to connect the transformer to an 8 ohm loudspeaker/transducer load. There would be no advantage and the transformer insertion loss would produce a reduced SPL! Using the 600 watt tap with a 70.7Vrms transformer makes only slightly more sense when the transformer is connected to a 4 ohm loudspeaker/transducer load. The impedance presented to the amplifier goes up from 4 ohms to 8 ohms but it is likely that the transformer insertion loss will outweigh any advantage.
Using the 600 watt tap with a 100Vrms transformer makes more sense, particularly if the loudspeaker/transducer load is a 4 ohm nominal impedance. The transformer impedance, from the table above, is 16.6 ohms and this can offer advantages over long cable runs.
Power taps from 300 watts and below certainly offer advantages but the calculator can easily analyze the tradeoffs between the high impedance designs and low impedance designs when all other parameters are factored in.
|AWG||Area (mm^2)||Approximate Metric Equivalent|