When you see one of these grilled on the ceiling, your first thought may be “Hey, that’s a fire alarm evacuation speaker!” Indeed, you would be right. In the past few weeks, Wormald Fire Systems have been upgrading the fire protection safety systems in the buildings at my university, and part of the upgrade included replacing all of the public address (PA) speakers with newer units and testing for performance and speech intelligibility index (STI). As a result, many functional PA speakers were removed and replaced in order to keep a clean consistent look and get rid of all these yellowed grilles.
As a result, I couldn’t help myself but to take one home just to take a closer look at it and explain some of the differences between PA amplifiers and “regular” home amplifiers.
The first thing I realized was that the speaker did not seem to be secured to the grille, and merely rested in some protruding studs. As with this older design, it was secured to the ceiling with screws and screw covers which have since gone missing – it is being replaced with springy rat-trap style speakers instead of a very similar visual design. This particular unit had Audio Telex Communications ATC 5400 marked on the grille, along with Made in Australia which is a pretty rare thing to see.
Some researching shows that this model number corresponds to the grille design (see Page 23 of their archived 2005 catalogue). Sadly, a search for their name leads to the Australian Monitor product listing where they have no Audio Telex products at all, so they may not be existent anymore.
The speaker behind it is a rather hefty deep 8 ohm speaker with a large magnet. It is an Australian Monitor ATC5101-3, with an Australian Monitor ATC5006-3 100V audio line transformer on the top of it. While the datasheet for this model isn’t available, the datasheet for the similar, but newer, ATC5101-6 is still online.
The transformer itself is tapped for 5W, 2.5W, 1W, 0.5W and 0.33W output, and features a terminal connector block at the top.
The connector block accepts two wires on the left, and wires in an electrolytic capacitor in series rated at 2.2uF 100V. This serves two purposes – it prevents transformer saturation by stray DC on the transmission line AND it permits DC to be applied to the line for supervisory purposes (i.e. detecting cable shorts) which is necessary with safety-of-life systems like fire evacuation PA systems.
The speaker itself is a 4″ speaker not dissimilar to those found in older cars. The speaker has a foam suspension edge and deep travel. It probably delivers quite good sound, although it’s main purpose is to deliver rather mundane evacuation messages.
Why 100V Line Transmission?
The short answer is basically to reduce transmission losses, reduce cable costs, reduce stray noises and simplify installation and cabling requirements.
The first thing to realize is that many amplifiers for home use are specified for power at 8 ohms impedance, or 4 ohms. Most are not specified for less than that. As a result, when driving multiple speakers in parallel, the impedance will drop and the amplifier may not be able to drive such a low impedance load as it demands high current to deliver the same power and will likely damage the output stage. High currents also increases cable losses.
In a building, it may not be uncommon to need to carry audio a distance of 100m or even further. Assuming the use of a cable similar to CAT5 (14.5 ohms / 100m), and lets say four cables are combined together (3.625 ohms / 100m), putting a single 8 ohm speaker at the end of a 100m results in 7.25 ohms in the cable (to and return). This means that almost half the power put into the system is lost as heat in the cable.
If we were delivering 5W to the 8 ohm speaker, this would be a current of 790mA and a voltage of 6.32V. At a current of 790mA, the cable itself consumes 4.52W. If instead, assuming loss-less conversion, we needed to deliver 5W but at 100V, then we can do so with a current of just 50mA. Using the same cable, the cable now only consumes 0.018W making it much more efficient. Even if you lost 0.5W (10%) in the transformation process, you are well ahead, and it means you can use thinner cables which reduces costs and makes installation easier.
Ultimately, this is a similar reason to why we use transformers and high voltages for transmitting power long distances – you reduce the current and thus reduce your voltage and power loss, provided your conversion process is reasonably efficient.
Another good reason for this is that with low-impedance speakers, it only takes a small voltage over the cable to create some sound. RF interference and crossing over mains cabling can induce small voltages to appear on the wires which can be transformed into annoying hums or noises from speakers. Having higher voltage transmission can make the whole system less sensitive to low-level induced voltage noise.
Unlike regular home amplifiers, PA amplifiers have standardized on 100V transmission (or 70V in America) and instead sell the amplifiers by power. This makes it easy to choose an amplifier as it only needs to have at least enough power to drive the speakers connected in parallel. Higher powered amplifiers can merely supply more current at the same voltage.
The Audio Transformer
The audio transformer pictured above is fairly standard and I remember seeing them available in Dick Smith a long time back as well, not fully understanding the whole 70V/100V line transmission system.
The transformer effectively is an impedance transformer. From the datasheet, it is claimed that the taps correspond as follows from basic resistance calculations:
- 0.33W = 30.3kohms
- 0.5W = 20kohms
- 1W = 10kohms
- 2.5W = 4kohms
- 5W = 2kohms
The output is matched to the 8 ohms of the speaker. With the input lead ending in an insulated spade lug, it allows for the installer to essentially choose the output of a given speaker within this power range at the speaker itself. This can be quite important in PA systems to avoid having signals too loud especially in echoey places which can impair intelligibility. It also allows for not needing several different types of speakers to have different power levels and potentially can allow for more speakers per amplifier if the power output is carefully budgeted.
The transformer appears to be constructed using an E-I formation and with a ferrite core. The series capacitor is important to block any DC, as that would magnetize or saturate the core which would consume energy and create heat in the windings, as well as cause distortion of any audio signal that is riding upon it.
In this salvage, I managed to meet a PA speaker and remind myself all about the 100V “constant voltage” line transmission. I also managed to surprise myself by seeing the Made in Australia product, which is all too rare nowadays. Surprisingly, even though the speaker is made for PA “voice” purposes, it looks like it won’t be too bad of a speaker in other respects and looks quite similar to those used in older OEM car audio.