Portable speaker question

[mattko]

Hello!

Yesterday I received a portable speaker that I'm not sure how it receives its audio input. It has a battery, audio amp, speaker in a plastic case.
How it works it that u place your smartphone that is playing music on top of the plastic case and the sound gets amplified on the speaker quite well. With no audio jack or microphones.
I attached a picture of the circuit. It has 2 transformers that I'm guessing receive audio signal and aplify it. Does that make sense? Can transformers be used as audio receivers?

[SeanB]

Can use the changing magnetic field of the smartphone speaker, inducing a voltage into the transformer by induction, as used in hearing aids for telephone use. Most smartphone speakers are too thin to have any magnetic shielding, so make quite effective if very lossy transformers. The transformers there have no magnetic parts in the top, so they are quite capable of making this lossy air gapped transformer with the typical phone, the use of 2 so that there is going to be some pick up irrespective of where the phone is placed. Speakers used on the device have to be magnetically shielded of course, but just adding the drawn steel cup and extra magnet is cheap enough, and available off the shelf as a regular magnetically shielded speaker that was very common in CRT TV sets.

[Zero999]

What's on the other side of the PCB?

Is the sound quality decent?

[Brumby]

Cute exercise.

The transformers appear to use standard E-I core components - but are missing two thirds of the top section.  There are some shadows which indicate there are 7 E core pieces at the top, which I expect is for mechanical stability, but the gaps in between are vacant, when you would expect the I core pieces to be for completeness of the magnetic circuit.



With those missing, it really opens up the top of the transformer for magnetic coupling.

[mattko]

Can use the changing magnetic field of the smartphone speaker, inducing a voltage into the transformer by induction

That makes perfect sense! It is super dependent on the position of the phone. It should have one of the two speakers over one of the transformer. If both speakers are each above one transformer the sound is not so loud.

What's on the other side of the PCB?

Is the sound quality decent?

I attached a picture, but basicaly nothing, just connectors to the battery and speaker output. Oh and I soldered a 3.5mm jack, to directly plug it in my phone. I prefer that way if I will ever use it
Sound quality is quite good. It is a bit annoying because, the phone has to be placed exactly on one point to have music loud enough and it picks up some noise of course. But for a cheap potrable speaker not bad (I got it for free actually).

Cute exercise.

The transformers appear to use standard E-I core components - but are missing two thirds of the top section.  There are some shadows which indicate there are 7 E core pieces at the top, which I expect is for mechanical stability, but the gaps in between are vacant, when you would expect the I core pieces to be for completeness of the magnetic circuit.

With those missing, it really opens up the top of the transformer for magnetic coupling.

Good observation! I made another picture with no yellow tape and it is missing the I pieces. Also, music is much louder if the phone is on top versus if it is from the side.

The amplifier chip is an MD4123 3W class-D. It has differential input and output. Is the differential output better for low THD? Usually I saw only push-pull and ground connection on a speaker.

[Zero999]

Can use the changing magnetic field of the smartphone speaker, inducing a voltage into the transformer by induction

That makes perfect sense! It is super dependent on the position of the phone. It should have one of the two speakers over one of the transformer. If both speakers are each above one transformer the sound is not so loud.

What's on the other side of the PCB?

Is the sound quality decent?

I attached a picture, but basicaly nothing, just connectors to the battery and speaker output. Oh and I soldered a 3.5mm jack, to directly plug it in my phone. I prefer that way if I will ever use it
Sound quality is quite good. It is a bit annoying because, the phone has to be placed exactly on one point to have music loud enough and it picks up some noise of course. But for a cheap potrable speaker not bad (I got it for free actually).

Cute exercise.

The transformers appear to use standard E-I core components - but are missing two thirds of the top section.  There are some shadows which indicate there are 7 E core pieces at the top, which I expect is for mechanical stability, but the gaps in between are vacant, when you would expect the I core pieces to be for completeness of the magnetic circuit.

With those missing, it really opens up the top of the transformer for magnetic coupling.

Good observation! I made another picture with no yellow tape and it is missing the I pieces. Also, music is much louder if the phone is on top versus if it is from the side.

Sean and Brumby are right: it's using a transformer with some of the laminations missing to inductively couple the signal from the phone speaker's voice coil.

The amplifier chip is an MD4123 3W class-D. It has differential input and output. Is the differential output better for low THD? Usually I saw only push-pull and ground connection on a speaker.

The differential/bridged output doesn't make any difference to the THD. It enables four times the power to be driven into the same load impedance, given the same supply voltage. Another advantage of a bridged output, is a single supply can be used, without the need for a large AC coupling capacitor between the output and the speaker.

The MD4123 is a filterless class D amplifier, which means the inductance of the speaker's voice coil is enough to block the high frequency carrier signal, without the need for a filter, made from expensive capacitors and inductors.