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Drive-In Speaker Wiring
Originally, drive-ins fed the audio from the projector into a large tube type audio amplifier. The output of the amplifier was typically 70 volts at maximum volume.
This was done because it is easier to transmit a signal at a higher voltage and step it down at the point of use with a transformer. This allows the use of smaller wires and lower line losses. It also allows you to simplify the wiring because every pole was just wired in parallel with every other pole.
The power company uses this principle every day, often transmitting power on lines at thousands of volts and then stepping it down to the 110/220 volts we commonly see in our homes with a nearby transformer.
The drive-ins often advertised that their speakers were “Special High Voltage” to discourage people from cutting the wires and taking them. In reality, the speakers were nothing more than common 4 inch square, 8 ohm speakers as used in many table radios of the era.
The pictures below will help illustrate how the junction boxes were wired.
We need to establish some standards first.
This chart is based on the typical 70 volt transformer found in most drive-in junction boxes.
FYI, Commercial office type public address systems often used 70 volt transformers with many taps on them to select different wattages or volume levels for the speakers.
Since drive-in speakers had a volume control, only one pair of wires at a fixed wattage or volume were supplied for the output side of the 70 volt transformer.
Also note that drive-ins often used any kind of wire available for their underground cable, including regular Romex type with a black wire, white wire, and copper ground. I've also commonly seen three conductor twisted direct burial wire similar to the wire used for low voltage landscape wiring. They simply assigned a function to each wire and stayed consistent througout the drive-in field.
| Wire Color | Wire Function | |
|---|---|---|
| Red | Common side from amplifier 70 volts and common for 24 volts A/C for the lamps | |
| Blue | High or 70 volt side of audio amplifier | |
| Black | High or 24 volt A/C from the lamps transformer | |
| Yellow | High side of 8 ohm speakers | |
| Green | Low side of 8 ohm speakers |
Below is a Circlite drive-in junction box. This box is lighted as it has a socket and 24 volt bulb in it.
Lighted junction boxes were often used to mark the location of the poles so the patrons would not run over them in the dark.
Many junction boxes also has a colored plastic dome or lens that was lit up from the same lamp.
In order to minimize the number of wires needed to be run to each pole, the common grounds for the 70 volt sound and 24 volts A/C were tied together at the projection booth. This is the RED wire in the lists above and below.
So, looking at the terminal strip below, the connections for a real drive-in would be as follows, reading left to right:
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Notice that only three wires (3, 4, and 5) actually came from the projection booth. (1 and 2) were the wires for the speakers that hang on the box.
The picture below shows a Simplex terminal board. Notice that it uses the same colors except the lamp socket actually has two black wires. The bulbs used in these sockets did not have a connection to the shell, just the two pins on the bottom of the bulbs, therefore the two black leads.
If you would like to know more about how 70 volt sound systems are wired, just search YouTube for 70 Volt Speaker Wiring. There are a number of useful videos that explain the concepts. Remember though, drive-in speaker 70 volt transformers only have one set of output wires because the speakers themselves have volume controls.
Now, how does one go about using these at home?
Unless you are using an old fashioned juke-box as your sound source, it is highly unlikely that you will have a 70 volt audio output. These are usually only found on commercial public address system amplifiers and old juke boxes that fed speakers at every booth.
Just remove and tape off the Yellow, Green, Red, and Blue transformer wires from the terminal strip.
Connect your two 24 volt transformer leads to the two black leads for the lamp or the metal socket base and black lead in the case of the Circlite box. This will power the lamp.
Use any free screw terminals to terminate your speaker leads directly to your sound source. If you are using a stereo with an 8 ohm output, connect the black wires from both speakers to ground and one white from one speaker to the right channel and the other white on the other speaker to the left channel.
This way your speakers will be stereo with one speaker for left channel and one for right channel.
A note about the 24 volt transformer.
The lamps consume very little power. The largest ones consume about 5 watts. That means that they consume 24 volts at approximately 200 milliamps. One ampere = 1,000 milliamps.
So you can power multiple lamps from one transformer. Just add up the current for each bulb and make sure that it does not exceed the rated amps of the transformer.
For example, the usual 24 volt transformer that I supply is rated at .8 amps (800 milliamps) That means you could safely power at least 2 lamps off of one transformer and leave a safety margin.
CAUTION! - Do not short the output leads of the wall-wart transformer while it is plugged in. The transformers usually have an internal fuse and shorting the lamp leads with the transformer plugged in will blow the fuse. The fuse is not replaceable.
This is a hand drawn schematic of how to wire your speakers for a stereo or sound card. Just use a three conductor cord from the junction box to your stereo and connect as shown.
This diagram below is for historical purposes only. It shows how the speakers were originally wired in a drive-in sound system.
Enjoy your speakers and keep the drive-in memories alive. If you still have a drive-in in your neighborhood, attend them often and enjoy movies under the stars on that giant screen.
Drive-in speakers were always a maintenace issue. They were usually exposed to the weather 24/7 and the cords and speakers cones deteriorated from exposure. There was also the issue of the volume controls getting dirty and creating scratchy sound.
Also, patrons often stole the speakers by cutting the wires. Many drive-ins went to a "theft-proof" cable between the speaker and the pole. This cable had a length of stranded steel cable inside the jacket. It was attached inside the speaker and inside the junction box. This type of cable is not easily cut with the usual pocket knife.
Some drive-ins even added an external steel cable to prevent theft but it meant drilling holes in the speakers and junction boxes which just increased the possibility of water damage.
Drive-ins started experimenting with radio sound to eliminate the maiteneance issue. Some drive-ins used a low power AM transmitter tuned somewhaere in the AM radio band. Other drive-ins went with FM sound and in the later years, very good low power FM stereo trasmitters were available.
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One issue with radio sound is the patron needed to use their car radio and would sometimes run down the battery, requiring a jump start to leave after the show.
Another issue was that although many drive-ins were expert at projection, they lacked the knowledge to properly implement radio sound. Our last four screen tried to use yagi TV antennas to directionalize the sound for each lot. Because the transmitters were 50 ohm output and TV antennas are typically 75 ohms, this created an impedance mismatch and most of the signal was wasted, negatively affecting what they were trying to accomplish.
As an experiment, I took a low powered FM transmitter with 25 milliwatts output just like the ones our last drive-in used. I connected it to a home made ground plane antenna cut to the exact frequency of the transmitter and of the proper impedance. I found that I was able to pick up the transmission with excellent sound for several city blocks with my car radio while driving around the neighborhood.