Many of us have been tasked with coming up with some kind of speaker system to play our station throughout the building. These systems operate somewhat differently from other broadcast chains, but still have to be created nonetheless. Many systems I have encountered simply have all the speakers wired in parallel, which is the wrong thing to do.
Recently, I had to upgrade the speaker system at WGLS(FM) and thought an updated “how-to” on this might benefit other engineers. There are several types of systems out there, so let’s dig right in.
THE 70-VOLT DISTRIBUTION SYSTEM
Fig: 1: Typical 70-volt speaker.
Courtesy Atlas Sound
The most common speaker system out there where a lot of speakers have to be connected to a common source is the 70-volt distribution system.It works on a 500-ohm distribution feed, and this is done by either using a PA amplifier or a standard 8-ohm output amplifier and a special step-up power transformer (see Fig. 1).
Most PA amps have 70-volt outputs built in. Since this system works on voltage with a fairly high impedance, large diameter wire is not needed. I have used 24-gauge wire to do these systems before.
At each speaker, there is a step-down transformer, usually attached directly to the speaker, which steps down the 500 ohms back to 8 or 4 ohms (depending on which output taps you use). On the primary of the transformer there are “loudness taps,” which allow you to do a rough control of the volume each individual speaker puts out. One simply runs a line out of the amp and parallels the speaker transformers across it.
Fig. 2: L-Pad.
Courtesy Atlas Sound
When sizing the amp, you simply need to make it slightly larger than the total desired wattage of all the speakers. If it is necessary to control the volume of each individual speaker, you can put an L-pad (see Fig. 2) in between the secondary of the transformer and the speaker, or simply wire your 8-ohm speaker to the 4-ohm output tap.
A trick that I have used in the past where I only need one or two speakers is to hook the lowest power tap of one of these 70-volt speakers directly to an output of a distribution amp. The typical DA using 5532s as output devices can put at least 30 milliwatts into a 500-ohm load, which is enough power to play one of these speakers to a decent volume, especially if it is just for background audio. This is the way to go if you have either a very large or small system to do.
THE 25-VOLT DISTRIBUTION SYSTEM
A 25-volt distribution system is similar to a 70-volt system in that it uses transformers.It is an older system that does require larger-gauge wire because of the lower impedance (around 120 ohms) used. Seventy-volt systems have largely replaced 25-volt systems, but I mention them here for reference in case you encounter one.
PA amplifiers and speakers that work with a 25-volt system are still readily available. One of the last large systems I did for a radio station in a TV station building was done 25-volt on purpose, specifically so it could not be easily interfaced with the building’s main 70-volt PA system.
SERIES DISTRIBUTION SYSTEM
In this system, the speakers are simply wired in series. If you have to rough set the volume of a particular speaker, simply put a resistor across that particular speaker. You can also use an L-pad on each speaker. This system works well with up to about 15 speakers (120+ ohm load on the power amp, which makes the typical solid-state power amp very happy).
I usually use 18-gauge wire with this type of system. This is what I did at WGLS with five speakers. I had to reduce the volume on the speaker over the entry door, so I put a 22-ohm resistor across that speaker. That system works quite well. Whatever you do, never wire more than two 8-ohm speakers in parallel or you risk blowing out the driving amp!
USING STUFF YOU ALREADY HAVE
You may have equipment that can be re-purposed for use with a monitoring system. For example, many older DAs are actually power amps with each output built out by resistors. Usually, the clue is that they only have one output control (as opposed to one for each individual output).
At WGLS, I found several old LPB mono DAs from the 1980s of this type in a closet, and sure enough, they used two 2.5-watt power ICs running in balanced bridge mode with 125-ohm resistors on each individual output. After changing out the old electrolytic capacitors and shorting out the output resistors on the #1 output, I had the perfect amp to drive their five speakers. It even has balanced inputs. Many TV DAs are also of this type.
Fig. 3: Standalone 70-volt step-down transformer.
Courtesy Atlas Sound
Fig. 4: Inexpensive BTL power amplifier.
If you have speakers already, you can find just the 70-volt transformers (see Fig. 3) at places like Amazon or even locally. In smaller 70-volt or series distribution systems, you could also use a “BTL” (bridge tied load) amplifier such as is used in most car audio systems (or even repurpose an old BTL car stereo and simply tune its tuner to your station).
As I mentioned, any BTL type amplifier can be used to drive a smaller system. This includes both car stereos and/or cheap audio amplifiers. An example of a suitable amplifier is shown in Fig. 4 — available for under $10 on Amazon. This amplifier can drive up to ten speakers quite comfortably, especially if the audio is strictly background.
Most of these monitor systems are designed as mono, but there is no reason why they can’t be stereo or even intermixed mono and stereo. If you intermix, a neat trick of getting mono is to connect your load to the “+” terminal of one channel and the “–” terminal of the other channel. I even use this trick with DAs to get mono outputs.
Though it is generally not necessary, if you want to be extra careful, put two 1000 uF 16-volt capacitors in series with the BTL outputs with the “+” terminal of the capacitors connected to the amp output(s). These will block the DC offset that most of these amplifiers have (since both of the amps’ outputs are biased at half the power supply, the potential between the outputs equals zero volts, but both to ground will measure half the power supply voltage — in the case of the typical 12 volt BTL amp, 6 volts).
VARIATIONS ON A THEME
Since many studios now have multiple stations, how does one determine which station goes to what speaker? In the case of a 70-volt system, this is easy: You can use a multi-position switch to simply switch each speaker across multiple amplifiers. I would definitely do this in bathrooms, for example — that way, the DJ knows how much time is left on his “record.”
The receptionist could switch the single amplifier system between the various stations with either a multi-position switch on its input, or as was done in one station, use the output of a router port. One local studio has a timer on its speaker system that switches between the various stations every couple of minutes (though frankly I found this a bit schizophrenic).
I hope that this information is useful to you, and as always questions (and criticism) are welcomed and encouraged. Email me via RW Engineering Extra at firstname.lastname@example.org.
Dana Puopolo is chief engineer at WGLS(FM), Rowan University in Glassboro, N.J.