Today a friend of mine gave me two old mono speakers with two separate 2,5mm mono jacks that i think connect as L and R to an amp. He asked me if its possible to connect them together to an 2,5mm stereo jack to make them play music from his smartphone, i searched the internet to find if that's possible but i didn't find anything, is there anyone that can help me?

 

Welcome to AAC.

Stereo has two channels, each with their own output. A speaker could have two drivers in it and provide both channels, or just one but provide a stereo plug to combine the channels.

In your case you could use one speaker for each channel and get stereo, this is the ordinary case.

As far as using them for with a phone the problem isn’t that they are mono, or how to wire them to the 3.5mm plug, but that the ut put of the smartphone is unlikely to be enough to drive them.

There is likely an impedance mismatch. Impedance is a measure of the complicated resistance that components (like speakers) have to the signals put into them. There are various reasons for differing impedance, no particular impedance is “better” than another expect when you try to use it, it has to match.

The phone‘s amplifier is an additional problem. It is likely the phone can’t produce enough power to drive The speakers to a useful listening level. That’s why plug in speakers for phones usually include an amplifier to increase the signal to the speakers. If your device uses batteries or other powered than it is amplified. This is often called active as opposed to passive. In the passive case no additional power is added.

In the past there were small speakers designed to be run directly from phones as passive devices, but they couldn’t get very loud without distorting.

In the end, the answer to your question is:

Yes, “mono” speakers can be used for stereo if wired properly.
Yes, those speakers could be wired to the L and R signals on a 3.5mm plug to act as a stereo pair.
Yes, you could plug it into a phone with the same connector (headphone jack).
Yes, you could use them on the output os some amplifier assuming the impedance match was good.
No, it will probably not work as useful speakers for the phone.

 

I tried exactly that with small speakers plugged into an earphone jack and yes it did sort of work but no it was not nearly loud enough to be useful at all.
If you get a cheap decent small stereo amplifier then it could work quite well. BUT before spending money or effort make sure that the speakers are in good condition and sound OK.

 

In the past I've run speakers directly from portable devices successfully. I brought my Sanyo pocket AM/FM/cassette player and a patch cable with alligator clips to the store and found a no-name pair of 5" surface-mount car speakers that sounded tolerable and were plenty loud enough.

 

In the past I've run speakers directly from portable devices successfully. I brought my Sanyo pocket AM/FM/cassette player and a patch cable with alligator clips to the store and found a no-name pair of 5" surface-mount car speakers that sounded tolerable and were plenty loud enough.

Some portable devices are far more powerful than others. And consider that the portable player battery pack is probably far more able to support a power amplifier much bigger than for a cell phone. There is probably no comparison between the two.

 

Loudness is perceived logarithmically, so 10 mW is just half as loud as 100 mW. 100 mW was enough power for many portable/table-top radios and tape players to be audible in noisy workplaces and homes. My portable (powered by two AA cells) didn't get as loud, but for desktop use it was fine. I just tried my Sansa Clip Sport's headphone output with a vintage Realistic Minimus-7, and, it's clearly audible in a quiet room. That's an MP3 player the size of a matchbook, and a relatively insensitive (86 dB) speaker.

 

My Minimus 7 speakers got their woofers burnt out. Radio Shack rated them at 40W.
They were powered with my 15W per channel amplifier.
The original woofers were stamped 5W Korea. The replacement woofers were a little different and lasted for many years.

 

My Minimus 7 speakers got their woofers burnt out. Radio Shack rated them at 40W.
They were powered with my 15W per channel amplifier.
The original woofers were stamped 5W Korea. The replacement woofers were a little different and lasted for many years.

Probably the original speakers were rated for "IHF" watts, which depend heavily on one's imagination.

 

Today a friend of mine gave me two old mono speakers with two separate 2,5mm mono jacks that i think connect as L and R to an amp. He asked me if its possible to connect them together to an 2,5mm stereo jack to make them play music from his smartphone, i searched the internet to find if that's possible but i didn't find anything, is there anyone that can help me?

I've done that, no big trick just as long as the speaker impedance is not terribly low for the amplifier. Probably anywhere from 8Ω and up should be a low risk for most devices that drive earphones. @Audioguru again is the expert in this area.

 

The short answer is "YES, BUT".
The volume of the sound from the speakers will probably not be anywhere near loud enough to enjoy listening to. The reason is that the audio amplifier in a smart phone is not intended for speakers like that, neither matching the speaker impedance nor having enough power. So what will work is to use that smart phone to connect to a stereo amplifier to drive the speakers. THAT WILL WORK. The smart phone is intended to feed ear-plug headphones, and they do that rather well.

I tried driving a pair of small speakers from an earphone jack a few weeks ago, and while it did work the sound level was so low that I had to use the two speakers like headphones to even hear. So it was not useful.

 

One other thing: Make sure the speakers are phased the same. You will know because when they are the "sound image" will be right in front of you. If one speaker is out of phase with the other, the image will be spread out in space. If you have a phase problem, the general solution is to swap the leads on one speaker.

This additional information might be useful to you: https://www.audiocheck.net/audiotests_polaritycheck.php