Not All 1/4″ Jacks Are Created Equal
OK, here’s something I’ve seen done in at least three churches over the last few months, so I know it happens. Let’s suppose you run a standard 100 ft audio snake from the console position up to the stage. But instead of putting the amp rack up on the stage, maybe you’re using a mixing board with built-in power amps, or the amp rack is already sitting next to the console. So you look over at the audio snake and see the XLR inputs from the microphones, but there’s also two or four TRS 1/4″ jacks that look just like the 1/4″ speaker jacks on the back of your powered mixer or amplifiers.
Here’s a standard Hosa snake with 24 gauge wires for both the XLR mic inputs as well as the 1/4″ phone jack TRS returns. Looks like you can plug the TRS fanout end into the back of a power amp and passive wedge speakers into the phone jacks on the stagebox, doesn’t it? We’ll we know it sort of works, but there’s a certain amount of wattage loss from the 100 feet of 24 gauge wire instead of 100 ft of 12 gauge, which is industry recommended for this sort of speaker run.
Just how much loss will you get using 100 ft TRS 24 gauge returns instead of 12 gauge speaker wire? Glad you asked…
Let’s consider a standard situation where you have an amplifier rated with an output of 500 watts at 8 ohms, and 1,000 watts at 4 ohms. Also, let’s hang a pair of 8 ohm passive wedges on each amplifier channel which makes a load of 4 ohms. Assuming no loss in the wiring, that’s 1,000 watts coming from the amp, which splits up to 500 watts for each of the 8 ohm speakers. That’s exactly how it’s supposed to work.
Now, instead of a heavy 10 or 12 gauge gauge speaker cable, let’s substitute with our 100 ft of 24-gauge TRS returns between the amplifier and the passive speakers.
Take a look at this wire gauge resistance chart and you’ll see that 100 ft of 24-gauge copper wire has a resistance of just over 2.5 ohms. And since we have to consider that there are two legs of this wire, it adds up to 200 ft and over 5 ohms resistance in series with the speakers. For a quick review of how speaker impedance works, watch this video: http://livesoundadvice.com/sound-bytes-videos/soundbytes-speaker-impedance/
So first we start with the amplifier output. Instead of driving a 4 ohm load and producing 1,000 watts, it’s now driving a 9 ohm load, which works out to less than 500 watts (the amplifier’s 8 ohm wattage output with a 9/8 impedance factor. We now only have 430 watts coming out of the amp since it’s no longer driving an 4 ohm load. But it get’s worse. We also have to factor in a voltage divider with 5 ohms of series resistance added to a 4 ohm load. That’s 4/9 times the 430 watts of power, so we’re now down to 190 watts of power being delivered to the 4 ohm speaker load. Finally, we have to divide that 190 watts by 2 since there are two 8-ohm speakers hooked in parallel. That’s less than 100 watts being delivered to each speaker instead of the 500 watts we paid for and expected. Yikes…
That shows us that if you use a 24 gauge snake return to drive a pair of 4 ohm passive speakers, you’re throwing away 80 percent of the amplifier power you paid for. So a 1,000 watt amp is only delivering 200 watts to the speakers. Plus we’ve not even discussed how this 5 ohms of series resistance will mess with the crossover points in the speaker which can change its frequency response in a bad way adding feedback nodes.
So what to do? Well, either run a separate 12-gauge speaker wire between the power amplifiers and your passive speakers up on stage, or get something called a “Power Snake” which has 1/4″ phone jack inputs that use 14 gauge speaker wire instead of 24-gauge microphone wire. For instance, Whirlwind makes a Power Series of audio snakes for just this type of circumstance. While the 14 gauge wires will introduce a small amount of loss compared to 12 gauge wiring, the improved flexibility of the snake makes it a reasonable compromise. If you’re running a lot of power over very long runs, I still suggest 12 gauge or even 10 gauge speaker wire.
A quick look at the chart and a little math shows us that 200 ft of 14-gauge will insert only 0.5 ohms in series with the speakers, while 200 ft of 12 gauge adds up to only 0.3 ohms. Much better options with very low wattage loss.
I will admit that my first guess would have been maybe losing 20% or so of the amplifier wattage using 24-gauge twisted pair rather then 12-gauge speaker wire, but numbers don’t lie. If you use the TRS returns on a 100 ft snake to drive a pair of 8 ohm monitor wedges (or any type of passive speaker) then you will lose around 80% of the amplifier output wattage due to series insertion loss plus impedance mismatching. So buy the heavy speaker cables and get the most performance out of your sound system.
As you can see from this example, just because you can plug something into a connector on a stage, doesn’t mean you should. One of the most important things you can know is all the types of connectors you might encounter on a live sound stage, and what signal types are likely to be on them. Oh yes, and understanding the relationship between voltage, resistance, current and wattage is really helpful for figuring this sort of thing out. But more on Ohm’s Law later.
Copyright Mike Sokol 2015
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