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Headphones to radio speaker?

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NutmegCT

NutmegCT

Great Pumpkin
Bronze
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Is there a way to use headphones instead of the radio speaker on my 1928 RCA Radiola 18?

I figure it *can't* be as simple as detaching the two lead wires from the speaker and using them as headphone input ... or is it?

Thanks.
Tom M.
 
I would think the power needed to run a speaker would blow headphone drivers immediately. I think you'll need a headphone amp coming out of a preamp on the radio.
 
First, Is the speaker magnetic or field coil? If it's a field coil type, be careful. It can knock you on your butt. If it is magnetic, The short answer is yes. Assuming the radio is fairly low power.
 
Thanks Greg. I figured that might be the case. As the old radio has no "headphone jack", maybe a resistor in the line might reduce current enough for headphone use? or maybe just keeping the volume control very low?

Tom M.
electronics dummy
 
George - we posted at exactly the same moment it appears.

There is no permanent magnet for the speaker. Speaker line from chassis has four terminals (male plug).

It's a 1928 RCA Radiola 18, running on 120v AC, but I don't know how I'd determine if it's "fairly low power".
Thanks.
Tom M.
 
OK, You have a field coil speaker. 2 wires go to the "field" They are DC and normally somewhat high voltage. You DO NOT want those. It will instantly destroy your headphones.(and shock the heck out of you) The other 2 are the "voice coil" wires. That is what you need. I would think if you kept the volume low you would be fine. Worst case is you would need a few resistors and make up a voltage divider circuit.
 
Thanks George. Sadly I have no idea how to create that "voltage divider circuit", altho' I'm guessing I'd put two new leads on the voice coil wires (if I can figure out which ones those are), then put resistor(s) on one of those leads, then run to the headphones?
 
Well no, but lets cross that bridge if we need it. If it was mine, I would just go direct to the voice coil wires with the volume low and see what happens. What I'm not sure of is the resistance (The correct term is Impedance) of a voice coil in 1928. Modern voice coils are 4-8 ohms. Does it say anything on the speaker?
 
Tom, I did a little checking on this. Assuming that you are using the original type speaker(100A?) Those had high impedance. Maybe 2000 ohms.( not 4-8 ohms) That means it would not be able to drive modern headphones correctly without a audio matching transformer. Basically a step-down transformer that will work in the audio range. I hope that you are not too far down the rabbit hole with this. George
 
Thanks George. Yes, an RCA 100A.

Is there any way for me to know details of an audio matching transformer that I might find for sale?

Also, I'm going to (unplug and) poke around underneath to see if just by chance there's an earphone jack, or at least something marked "phones" or such. I found schematics online, but I don't understand them well enough to interpret symbols.
 
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