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The Vault: Watching The Signal Flow

In sound reinforcement systems, distribution is everything.

By Barry McKinnon January 11, 2016

PSW Top 20 presented by Renkus-Heinz

 
Editor’s Note: This article was featured in the very first issue of Live Sound International, which in 2016 celebrated its 25th anniversary. Published in January 1992, the article is understandably a bit dated, yet it still provides a wealth of valuable information on an important topic in sound reinforcement.

Signal distribution can get unwieldy at times. One of the worst situations is the slow evolution of a sound reinforcement system.

The growing demands on the system require the addition of components until some critical point is reached where it just seems to get more and more awkward to hook everything up and make it work. We’ll take a look at the often talked about G’Zintas and G’Zouttas of a sound system (as in: it G’Zinta here and G’Zoutta that thing there).

Although there are some technical limitations on how many devices you can put on the output of a crossover or limiter or EQ, let’s take a look at the philosophical side of the problem first. It will be the philosophy chosen that determines the technical approach taken.

As the sound system grows from a single rack of amplifiers and a couple of loudspeaker stacks to multiple racks of amps and larger stacks, some decisions have to be made. Placement of the crossovers and limiters in relation to the mixing console and amps, and the amps in relation to the loudspeakers, will determine wiring methodology.

If you have a sound system that is of a modular nature, using increments of processor or non-processor based loudspeaker systems, all identical, and able to be added as the job demands, you have likely already made the decision to use several self-contained racks. These racks would each contain the signal processing and amps for each modular increment of amp/speaker.

If your system is more component-like, where you add blocks of low, mid and high components as required, and add amps with each block, your signal processing might be located in the parent rack, and additional low, mid or high groups of amps are daisy-chained as needed. Either way, there are some things to consider when laying out the signal distribution to and from the amps that can make the system quieter, cleaner and more reliable.

Arrangements
Modern electronic crossovers are capable of driving a large number of amp inputs. Most professional power amps have a nominal input impedance of 20,000 ohms, actively balanced, and typically half that when connected unbalanced. A quick survey of electronic crossover spec sheets from the binders on the wall here show typical output impedances of 20 ohms to 25 ohms. At least one that is popular in the pro-sound field has an amazing 0.05-ohm output impedance!

A quick refresher on impedance matching vs. bridging: if the load impedance matches (=) the source impedance, there is a 6 dB loss in level delivered to the load; if the load impedance is significantly higher than the source impedance, the source virtually behaves as if it were unloaded. In passive devices this is not good, as they don’t perform at all as expected, but in active devices such as crossovers and limiters, this is not a problem.

The electronic crossovers mentioned are rated to meet their performance specs driving 600-ohm loads, and with the output impedances shown, should be quite happy with that load. Let’s just grab a calculator and see what that means. Let’s see, an amp is 20,000 ohms, divide that by 600 ohms for the minimum load on the crossover. That’s approximately 33 amp inputs in parallel to get down to the minimum rated load of 600 ohms.


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