hi everyone. I landed here after watching a video of a MH-M38 bluetooth receiver. if I get it right the receiver in questions gives the ability to connect two speakers only. it seems if I were to try co connect in parallel a third speaker to it the chip would blow. in my case I have an old camper with three speakers which I’d like to connect to a phone using bluetooth. any way I can do that?
help is greatly appreciated.
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Good to have you with us.
Recently, some of us did a deep dive into this device here which you might find useful.
As I understand the only issue here is having two parallel negatives. I wonder if that’s because no two speakers have the same ohms, leading to two competing waveforms, which damages the amp on the return. 
I THINK that means you could run two or more speakers in series providing the load is appropriate. That is the sum of the ohms of the speakers in your caravan. Only one waveform. The M38 shouldn’t be able to tell the difference between two 4ohm speakers and one 8ohm speaker… I think… 
I’d love someone else to weigh in here to make sure I’m not missing something. 
Pix 
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Hi Pix
You are sort of missing everything and adding a bit of confusion.
I remember that post. It was established that you cannot connect the speaker negatives together.
If this third speaker is considered being connected to the stereo outputs in a bridging configuration to combine the stereo into a mono signal in the third speaker forget it. Can’t be done successfully.
Couple of things need to know about the speakers in this camper.
What is the impedance of the speakers.
Are they all the same.
You are right that 2 4Ω speakers in series is the same as 1 8Ω unit. But there is a thing called phasing. That is if the speakers are not phased correctly you will hear almost nothing. Only a bit of treble and no base.
That is another story. Find out what the speakers are and the intended use first.
Cheers Bob
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hi Pix, Bob. thanks for your feedbacks.
as far as speakers are concerned they’re all the same. can’t tell impedance at this point, I should take em out and see if there’s any indication on em but I’m traveling without the camper.
what I can tell is the three speakers were hooked up to an old car stereo, all speakers are the same, and powered by 12V battery which provide electricity to the unit.
couple of notes worth to mention. what I call camper is a truck camper, the stereo was slotted onto a wall and I wanted to get rid of it since day one. and I don’t want another stereo with bluetooth capabilities to replace the old one.
my goal is to stream music from my phone to all three speakers (if possible) by using some electronic “contraption”. and that’s how I’ve found about the MH-M38.
any way I ca do that in an easy way?
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Are these powered speakers? That is, they expect a very low-level signal (possibly 0-1V) from a preamplifier and they have their own main amplifier? In that case they cannot be used with the MH-M38. See the product page:
sunrom.com/download/705.pdf
“5W power amplifier on board :This product has its own dual 5W power amplifier circuit, and must not be connected to another amplifier!”
Also, it seems that the setup in the van may be for a single mono output to all three speakers. The MH-M38 is specifically designed for stereo. Even if you fed the same mono signal to both inputs you still couldn’t connect the two outputs together to feed a single string of multiple speakers.
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Hi George
That seems to imply the 12V powered the stereo unit and the speakers are passive. That is they just connected to stereo amplifier outputs. This probably was decoded in a “2+1” format, Left, Right and Centre by the original amplifier.
If streaming from a phone you will just have Left and Right, what would have come out of the centre will now come out of both Left and Right in phase.
If I were you I would just connect Left and Right speakers to L+, L- and R+, R-. Note the phasing, the speakers should be marked “+” or maybe a red dot or something on the terminals. Ignore the centre speaker. If you leave it in situ you will probably imagine it is working anyway and will do no harm and save having to block up a spare hole. The sound that would come from a centre speaker will now come from both L and R and appear to be in the centre anyway.
DO NOT attempt to bridge a speaker across the Class D outputs in an attempt to get a centre signal. Like Jeff said, if you want a mono signal connect L and R at the input which in this case would probably be inaccessible (as it is primarily a complete Bluetooth device) so just go with the stereo speaker connection.
Cheers Bob
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hi Bob.
“That seems to imply the 12V powered the stereo unit and the speakers are passive. That is they just connected to stereo amplifier outputs” that’s the setup.
I think I’ll go with just two speakers as you’re suggesting. will have to add something to bring 12V down to 5V but that’s ok.
as probably you might have guessed I’m not knowledge about these things, so I have one last question I have to ask. what if the speakers were four rather than three, same rule would apply?
Hi George
Basically yes.
There could be a situation where they might all be 4Ω units then you could have 2 lots of 2 speakers in series to make 2 sets of 8Ω. You then would have the question of whether you had enough power to drive them. That would be pretty much dependant on speaker sensitivity.
You could have 4 units of 8Ω in which case you could connect in parallel and finish up with 2 sets of 4Ω. The comments on power and sensitivity still remain unchanged.
There are a lot of factors here some of which you would have trouble answering.
My advice is just go with the simple solution and connect 2 speakers. After all you are not dealing with any sort of home theatre situation. You just want a bit of entertainment in your camper.
There are plenty of 12V to 5V converters out there. Just make sure you have enough current capability. Work out what you need then double it.
Cheers Bob
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that is correct
Bob can you please expand your thoughts here?
Hi George
I don’t have any idea what sort of current consumption this unit has nor do I seem to be able to easily find out. Par for the course I am afraid.
Here is a sample of the sort of thing you need. Core part numbers
DFR0831
DFR0929
DFR0884.
Which one would suit depends on how much current you need so I am not making any definite suggestions.
Leave yourself plenty of head room. Like up to 100% more than you need. Will save trouble later.
Cheers Bob
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got it. thanx Bob!
Hi @George272824, Welcome to the Forums!!!
I think the others have done a good job covering this pretty well so far.
@Robert93820 is right on the money about the MH-M80 being able to power the speakers.
The AMP is only designed for 3-5W speakers and most speakers I’ve come across in caravans and campers are usually 10-30W.
Once you can get the specs of the speakers you want to power I’m sure we can find a solution that will work for them.
Hi Aaron
I think that power figure quoted means the speakers will handle that level of power
NOT the level of power it takes to drive them. So a speaker that is rated 10W will handle 10W of power but might only require a few hundred mW to provide adequate listening level.
THAT number is the sensitivity figure which is indicative of how much audio power is required to produce a specified sound level or pressure. I have only seen this figure quoted in higher quality speakers. I forget the actual numbers now but that is obtained by supplying a specified power at 1kHz to the speaker and measuring the sound pressure at 1 metre in front of the unit. I am not sure if the speaker is mounted in a baffle or anything or free standing for this measurement. All the criteria are defined so to provide some sort of comparison and enable any design criteria to be met.
The actual practical sound level you finish up with will depend on a lot of factors which I would think in a camper van would not be very controllable.
Cheers Bob
Add on: This is not a bad explanation of all I have said above
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