by Neil Bauman, Ph.D.
A man wrote,
I am planning to install a loop system in our new house and have been investigating different systems. I’ve narrowed my choices down to the Univox DLS-50 and the Oval Window Microloop III, which are different types of loop amplifiers.
I believe you are saying that in the Univox system, the wire length is not important, whereas in the Oval Window system, the wire length (resistance) is critical. Any light you can shed on this for me will be mightily appreciated.
Actually, both the Univox and Oval Window amplifiers are the same type of amplifiers. Both are current amplifiers—the kind you want to use in loop systems—so there is no fundamental differences between them, just the differences between similar “rival” products.
The length of the wire is not as critical as it sounds for either of these amplifiers. Its just that Oval Window uses the KISS technique—Keep It Simple Sweetheart—whereas I explain things in more detail if people want to know more in-depth information regarding the finer points of loop systems.
Typically, you want to keep the DC resistance of the loop wire to about 1 ohm (+ or – 30%), that is, between 0.7 and 1.3 ohms, no matter what system you use. When you do this the loop amplifier will operate optimally and will not overheat.
The 20 gauge wire Oval Window supplies has a resistance of 1.015 ohms per 100 feet. Thus, if you make a 100′ loop, you’ll be right on target.
We typically use 22 gauge wire which has a resistance of 1.614 ohms per 100 feet. Thus, you can loop a smaller area and still be within the target of 0.7 to 1.3 ohms. For example, a 50′ loop would be be 0.807 ohms, whereas if you did the same with the Oval Window wire, you’d be below target with a resistance of 0.507 ohms.
You’ll also notice that the Oval Window website tells you not to cut the wire, but this is not critical as you could cut 20 feet off and still be in the ballpark for example. You just don’t want to cut more than 30′ off the loop.
Instead of cutting the wire (if you are looping a smaller area), they say to coil the extra wire up—but what they don’t say on the website, again in the interests of keeping things simple—is that coiling the wire will add inductance reactance to the circuit and thus limit the power, especially in the higher frequencies. Personally, I think a better trade off is to cut the wire rather than coil it, but it is not critical one way or the other for typical home loop systems.
There is nothing special about either of the wires used. You just want to use an insulated stranded copper wire of a gauge to give you the resistance you need. You can get your own wire to fit your situation.
For example, if you want to make a double wire loop, then it is a good idea to use a larger gauge of wire to get the resistance down into the 1 ohm range again. For example, if you made a double-wire loop of 100′, you’d need 200′ of wire. To keep the resistance down to around 1 ohm, you’d need to use 18 gauge wire (0.639 ohms per 100′) for a total DC resistance of 1.378 ohms—the upper limit of our target, or even better, a 16 gauge wire (0.402 ohms per 100′) for a total DC resistance of 0.804 ohms—well within the target range. In this case, 16 gauge two conductor speaker wire would do the trick just nicely.