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 Blue Common side from amplifier 70 volts and common for 24 volts A/C for the lamps Red 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. It is also wired wrong as is the Simplex box because they are using the RED wire for the commons side of the audio.
The problem with this is assuming everything else is wired correctly, when the amplifier puts a positive voltage on the blue lead of the transformer, it will cause the speaker cone to move in and not out as it should be. This is often called a phasing problem and results in thin or tinny sound.
I will have to take new photos of the corrected wiring. NOTE: For years, the old Drive-in Theater Manufacturing company has been shipping junction boxes witht he Red and Blue wire swapped. I finally ran a test using some audio equipment and discovered why the sound was often thin and tinny. Reversing the two wires corrected the problem.
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 BLUE wire in the lists above and below.
So, looking at the terminal strip below, the connections for a real drive-in should be as follows, reading left to right:
Every pole was just wired in parallel with every other pole, like colors going to like colors. Often they had junction boxes at the ends of rows so they could isolate a row if a wire became shorted underground.
- Yellow = White wires on both speakers.
- Green = Black wire on both speakers.
- Blue & lamp socket = Blue or common for both 70 volt sound and 24 volts A/C.
- Black = Black or high side of 24 volt A/C transformer.
- Red = high side of 70 volt audio amplifier.
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 100% 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.
Shown below is the inside of a new RCA speaker showing the internal wiring. The black wire from the cord attaches to the left speaker terminal and the left terminal of the volume control. The center terminal of the volume control attaches to the right speaker terminal. The white wire from the cord attaches to the right terminal of the volume control.
This diagram below is for historical purposes only. It shows how the speakers were originally wired in a drive-in sound system.
Click to enlarge
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.
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 quarter wave 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.
Due to questions about various junction boxes, I am adding more information here.
Reed and Koropp junction boxes were almost identical. Both had a metal cap and base with slots to hang the speakers.
Some of the Reed boxes had narrow slots in the top of the bottom casting to allow the speaker cords to come out while others used the holes in the bottom casting for the speaker cords.
The Reed and Koropp junction boxes often had a plastic diffuser ring which was illuminated to see the boxes in the dark. The diffusers were commonly red but also came in green and white.
The Reed boxes sometimes had a metal boss cast in the top to allow a clip to hold the 24 volt lamp. Some Reed boxes and none of the Koropp boxes had this and the clip mounted on the terminal stip to hold the lamp over the hole to illuminate the pole and ground.
The above image shows a Reed junction box with a clip in the top to hold the lamp to illuminate the box.
/center>The above picture shows the clip mounted on the terminal strip.
This was also common with the Koropp junction boxes.
The lamps were a C7 candelabra size powered by a 24 volt transformer in the projection booth. They often used a 6SB 30 volt bulb as it prolonged the bulb life tremendously while only reducing the brightness a bit. The RCA, Circlite, and Projected Sound junction boxes used a miniature bayonet socket for the lamp. Usually the bulb was a 1864-24 volt bulb or a 313-28 volt bulb for longer life.
There were at least three different styles of RCA junction boxes.
The first one was the long box without the fins on top. This box required the wire baskets to hold the speakers. The one shown is over 60 years old and I powder coated it and the baskets for a "4th of July" theme.
The second type was almost identical but had fins on top so the speakers could hang from the box or the drive-in could use the optional wire baskets. The one shown is over 50 years old and has the optional aluminum wire baskets.
The later type was called the RCA-Circlite box and was a smaller rectangular box that could also use the optional wire baskets of a similar design. The baskets were originally plain aluminum but the later production was powder coated black steel. The set shown has powder coated aluminum baskets in red.
The wire baskets were much more secure and kept the speakers from falling on the ground and getting run over by patrons.
The Simplex junction box was round with arms to hold the speakers and an illuminated plastic dome. The domes came in red, white, green, and blue.
While not a junction box, this is an interesting piece of drive-in memorabilia. It is a wire bracket to hold electric drive-in heaters on the speakers posts.
It is made of steel and the wire is 1/4 inch diameter and each leg is about 6 inches long and about 3 - 1/4 inches wide at the ends The metal band is 1 inch wide and sized to fit a 2 inch id pole..
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Revised May 8, 2013 GWC