Drive-in movie speaker via Bluetooth and mini amp

Question

I bought some drive-in movie speakers which I restored by cleaning up and replacing the old 4" drivers.The two drivers are 4" in size, 4 Ω and 5 W (dee pictures).

I bought a mini Bluetooth amplifier to hook up these two speakers so that I could play '50s tunes from my phone via Bluetooth cast out in mono.

It worked great for the short test runs I did but yesterday I lost all output from the mini amp (see picture of mini amp) which had an output of 50 W per speaker. I put both speakers onto just one output on the amp so it would be 10 W on that output leg.

The amp company through a very broken English email is telling me that the speakers are not large enough and it backed up the amp and overheated it. That may be true, but the amp was only on for less than 15 minutes and it was not hot when I went to see what was up with it.

The only advice from the amp company (they say they won't warrantee my purchase) is to buy another amp but hook bigger speakers up to it. I can't do that due to the nature of what I am trying to do with the drive-in speakers.

My question here is: Is there a way to put some kind of circuit/resistor in this mix somewhere that could fool the amp into thinking there are larger speakers and thus dissipating any output power that might otherwise back up and blow out the amp?

Please note with this request that I know NOTHING about wiring and circuitry so your answer would have to be very specific of what I should buy and where to place it in the mix (a drawing would be helpful). If any of you with a more technical side would wish to help me I would be so appreciative. I was supposed to show off this system at the end of the month at a drive-in which no longer uses real speakers. I was going to Bluetooth the radio broadcast of the movie onto my speakers.

Answer 1

The amp company through a very broken English email is telling me that the speakers are not large enough and it backed up the amp and overheated it.

That's baloney. As long as the speaker resistance is greater than or equal to the minimum load the amplifier can handle the amplifier will not be damaged. Typically those amplifiers can drive down to 4 Ω. If you confirmed that by measurement then that should be OK. The "40V" stamped on the rear is strange so I don't know what that means.

That may be true but the amp was only on for less than 15 minutes and it was not hot when I went to see what was up with it.

^{simulate this circuit – Schematic created using CircuitLab}

Figure 1. Connecting equal resistances in parallel halves the resistance. Connecting them in series doubles the resistance.

That seems as though the load resistance is too low. If you connected the speakers in parallel on one channel then you have provided a load of only 2 Ω and this could be the problem. Instead you should have connected them in series.

The problem is further compounded by the speakers' own volume controls. We need a photo to debug that further.

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Your new photo shows that the speaker volume control is a series-connected variable resistor. The problem now is that if we series connect both speakers the volume controls will interact. (One customer increasing the resistance on their speaker will increase the resistance on the series circuit, reduce the current and reduce the volume.)

^{simulate this circuit}

Figure 2. A recommended wiring scheme.

The wiring of Figure 2 solves a couple of problems.

• The circuits are independent.
• Adding the series 10 Ω resistor ensures that you never overload the amplifier and also reduces the risk of damaging the speakers if the amplifier volume is cranked up. Any value > 4 Ω will work here. You should choose a wire-wound resistor type with a power rating (watts) of several watts - 10 W would be safe enough and readily available.

One more thing: if your audio source is stereo and you only use one channel you will miss anything that is on the other side of the audio panorama. The solution to this is to bridge out the left and right channels of your 3.5 mm stereo lead. Ideally you would mix them with a few more 1/4 W resistors as shown in Figure 3.

^{simulate this circuit}

Figure 3. A simple resistor mixer to convert stereo to monaural (mono).

Answer 2

I guess you are using 2 speakers because they can feed mono audio to 2 cars. The NoSound amplifier says minimum 4 ohms per channel but you connected two 4 ohm speakers in parallel to only one channel that overladed that channel with 2 ohms. Why not connect each speaker to its own channel on the amplifier? Then each channel gets a 4 ohm speaker and works normally. Use the wiring in figure 3 to convert your phone's stereo into mono at the amplifier's input.

You never said the power supply voltage and current that you are using. The voltage and speaker impedance determines the output power into the speaker. NoSound says to use 19V at 4.7A then the amplifier will produce a max power at low distortion of 35W per channel into 4 ohms which will blow up your cheap little speakers when the volume is turned up higher than 5W. You can reduce the maximum output power by reducing the supply voltage but then the Bluetooth receiver might not work.

Reducing the volume with a variable resistor in series with a speaker makes a speaker sound boomy like a bongo drum. Drive-in and gas station speakers sound boomy.

If the amplifier chip is a REAL Texas Instruments chip and not a cheap Chinese copy then it is protected from damage when overloaded.

Answer 3

The amp company through a very broken English email is telling me that the speakers are not large enough

When that's the case, it's the speakers that overheat and get damaged, not the amp.

and it backed up the amp and overheated it.

Nonsense. Audio amps are neither plumbing nor antenna drivers. They can't "back up".

This is not legal advice and I'm not a lawyer! If I personally bought it on Aliexpress or Amazon or even eBay, I would start a return of a faulty product. In case of Aliexpress, if they won't cooperate, then probably I would do a chargeback via the credit card company, but I could end up "banned" from the platform.

In any case, this thing failed through no fault of yours, as far as I can tell.