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​How to add a loud speaker to (the most basic) landline phone? Answered

How would you connect a loudspeaker to a basic landline phone (so several people can hear the conversation)? (Basic phones don't have loudspeakers, only the one handling the headset can hear).

The loudspeaker doesn't have to be super loud, it could be like the ones powered by USB on computers, or much smaller, as far as we could hear the conversation.

It's not that easy since theses phone are merely powered by the little current inside the landline phone line. So I guess I need another power source, like a USB charger, and maybe to amplify the sound signal, to use the loudspeaker. I started to learn electronics for a few months, but I have no idea how to handle this problem. Any advice would be great!

Thanks for your help!


Jack A Lopez

2 months ago

There is such a thing as an amplified speaker, or at least there used to be.

You know it was an audio amplifier, a speaker, and a DC power source (e.g. 9 volt battery), all in the same box. Also there was a knob to adjust the gain.

In the bad old days, there was this company called, "Radio Shack," who sold such a gizmo. It had just one speaker, and relatively high input impedance, like maybe 10 Kohm, or so. That way it would not draw a lot of signal current from whatever you wanted to connect it to.

Unfortunately, I think Radio Shack went out of business sometime in the late twentieth century.

There is a picture of this gizmo, RS part number 277-1008, in an instructable I wrote long ago, here:


I am guessing it was called, "amplified speaker", but it might also have been called "mini amplifier speaker," since those words are printed on the artifact itself. I just noticed this by looking at the pictures again.

Similar hardware exists inside, "amplified speakers", the kind made for providing sound for a desktop computer. Those have a 3 mm, stereo, headphone and plug into the a sound card's, uh, "line out" ? It might also be called "green out", since that is usually what color it is.

I guess for those, for the desktop computer speakers, just tie the right input and left input, together, and connect them to the same signal, which, I guess, is the wires going to the speaker in the handset of your POTS (plain old telephone service) telephone.

Another thing I was going to mention. There might be danger of annoying feedback whine, if you turn the gain up too high.

Also I recall there used to be a kind of POTS phone called a "speaker phone", and those had a speaker loud enough for the whole room to hear. Curiously, the "speaker phone," could also listen to the whole room too. Sort of. Or at least it would work from about a meter away, from the user's mouth, in contrast to the usual handset or headset, for which the mouth-to-microphone distance is just a few centimeters.

I always wondered how those speaker phones worked, without annoying feedback whine, since the mic was certainly capable of picking up sound coming from the speaker.

Downunder35mJack A Lopez

Reply 2 months ago

I forgot about those weird speakers!!
At some point and right before the first cordless phones came out my grandparents had one.
Was a little box, screwed to the wall and with a junctions box on the hadset cable.
In said box a switch to turn the speaker on or off.
When a proper phone finally arrived I got the extra wall speaker but couldn't make much use of it.
It was somewhere in the region of 500 Ohm and the only good use I found was to connect it where otherwise headphones were used.
And you are right, once switched on the other side could hear everything in the room.

So I looked up how old phones actually worked to confirm my theory.
Speaker and microphone were quite differently.
The microphone was carbon based so to say.
The resistance changed with the compression of the carbon dust.
For the speaker the high resistance of close to 600 Ohm meant it works perfectly fine with the quite high line voltage that could be up to 60V.
Like modern PA systems that use 100V and high resistance speakers that wouldn't make a beep on a normal amp.

I think there might be very nice and weird way to connect and amplified speaker to an old analog phone :)
The speaker usually is not shielded, just an aliminium cover pressed onto a plastic housing to hold the parts together.
The backside was in most cases just plain plastic or better Bakelite.
That means we should be able to tap into the magnetic field by means of a tape head from an old cassette player.
It reacts to very tiny changes in the magnetic field and should be sensitive enough to steal the signal.
The rest of the tape player can then be mutilated to drive just the speaker.
Back in the day you could even get glue on thingies for that purpose - they went on the back of the handset and not even in contact with the speaker itself.
I am certain better options than finding an obsolete but still working tape recorder exist but you get the picture ;)

Damn, we really need to start a collection thread for old tech and the uses we forgot about LOL

Jack A LopezDownunder35m

Reply 2 months ago

It is called a, "pickup coil", or maybe, "telephone pickup coil". Here is an link into an image search for, "telephone pickup coil"


There is no need to use the tape head from old cassette player, or roll your own from magnet wire, because there are sellers who still sell that thing, "telephone pickup coil", cheap.


2 months ago

Thanks a lot for these informations, I 'll first try to connect to the headphone jack (I did not think about that, I thought it had to be much complicated). I'll keep you updated!


2 months ago

Oldschool comes to mind.
Back in the day you could get crystal ear pieces that had a tiny transformer.
Slide it over the line to the handset and be able to hear in the earpiece.
600Ohm is the standard for old landline phones, so whatever is added should have a higher resistance.
This however is only good for one additional listerner.
A similar approach uses a small audio transformer, probably 10:1 or 100:1.
The primary of this transformer goes in series with the speaker of the handset.
The secondary is to be connected to a small audio amplifier.
You might get away with connecting the audio input (headphone jack) of a small computer speaker that has an amplifier build in.
What you want for an amp is an output from the transforer in the range of 0.5 to abovt 1.3V as the max.