Drive-in Movie Speaker

A unusual inquiry I am sure. I am attempting to restore an old drive-in movie speaker. The original inside parts were totally rusted and beyond identification or function. Found a 4" waterproof marine speaker (100 watt) that fits well. I am unable to find a potentiometer with a shaft length that will accommodate a knob when properly mounted inside the casing. It needs to be about 1 inch or 2 cm. I have no idea what ohm rating should be required. My intention is to tie into a vintage Sharp 10 W stereo amplifier / receiver. Not enough knowledge to know what I am doing - Please advise!

Hi Mehirenner,

We may have one of that length. What ohms is the speaker you will use? For example if it is 4 ohm speaker then a 16 ohm potentiometer can give full volume down to 1/5th the total volume. Once we figure this out and what percentage you need the volume lowered then we need to make sure the potentiometer power rating will be sufficient.

Thanks for the response. I was not sure my inquiry went through because I somehow deleted my comments and was unable to retrieve. Really glad to get some advice.

My speaker is a Pyle Hydra, 100 W peak power, 4 ohm. Do not know what percentage the volume needs to be lowered.


It’s probably not a potentiometer in the speaker. The traditional speaker mounted volume control is called a speaker L Pad. It maintains a constant impedance on the input while varying the output from 0 to infinite dB reduction. You select the L Pad based on the speaker impedance and output power level of the amplifier.

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The potentiometer is mounted separate from the speaker inside the housing and must be a certain shaft size to accommodate an outside volume control knob.

See attached photo. Made mount bracket for new potentiometer, but do not know proper ohms and shaft too short.

I just realized the Wiki snippet shows a L Pad for an antenna connection. A speaker L Pad looks like a potentiometer and they are available with many different shaft and bushing lengths. This Google search shows a wide variety of speaker L Pads.

Nice find PaulHutch,

I found one L-pad that we carry, appears to be 100 watt also, please verify specifications: Click here for 2056-LC95-ND.

If you are stuck on still getting a potentiometer, please take a look at CT3023-ND, click here.
This would also have a built in switch to turn the speaker off completely. This would be our best option, please verify datasheet to see if these will work for you since the wattage is much lower than needed, however may work if you are running a much lower volume than the actual 100 watts.

Not sure the L-pad meets my requirement for a 1/4 or 3/8 β€œ diameter threaded mount with shaft to protrude outside the casing. The potentiometer looks right, but what is the shaft length? That is critical for knob to attach. Amp source is 10 watts, so much lower than 100 watts.

Photo here is of drive-in movie speaker casing front with old potentiometer shaft length protruding to accommodate volume knob.

Hi Mehirenner,
It looks like the shaft is 1’’ in length, which includes the threading.

Yes, that is correct! - The new potentiometer I purchased in prior photo will not work because of short shaft and I forgot what ohm rating it might be (Not knowing what I really need).

Another note: The old potentiometer was rusted to base plate and despite a vise, vise grips and WD40, it could not be removed. A restoration challenge for a simple volume control!

Sounds like a project! A penetrant like 3M156305-ND may help. :slight_smile:

The prior photo shows a new base plate I was able to make out of a mending plate, So, what is the shaft length of the potentiometer you suggested? Hope it is a potential fix.

Hi Mehirenner,

This one looks to have 1’’ shaft length including the threads. Basically the pot being mounted flush, it will extend 1’’ outwards.

Great, I am thankful it may meet my requirement! - THANKS!

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