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What speakers can go with a 7W mono amplifier?

I have a bunch of speakers lying around, and I recently bought an amplifier kit from Velleman.

Specs:

Music output: 7W/4ohm
RMS output: 3.5W/4ohm or 2W/8ohm
Total harmonic distortion: 0.05% (1W/1KHz)
Frequency response: 20Hz-20KHz (-3dB)
Input sensitivity: 40mV/150Kohm
Signal/noise ratio: 86dB (A weighted)
Power supply: 8-18VDC/0.5A

I have two 8ohm 2W speakers, and two speakers that I can't seem to find the specs for, but it's the same speakers in the picture. I was just wondering if I'd be able to wire up the two 8ohm speakers together and connect it to the amp, but from the specs of the amp, I'm guessing I can't do that? And out of curiosity, what happens when you wire up speakers that don't suit the amp?

Also, would 4AA batteries be sufficient enough to power this?

And any extra advice/input would be greatly appreciated!


Picture of What speakers can go with a 7W mono amplifier?
NachoMahma4 years ago
.  According to the specs, your amp will handle four ohm loads. Two eight ohm speakers in parallel will give you ~four ohm* load.
.  Power rating is not that important as long as it's over about 1½-2 W (1/4 of your 7 W max output)

*Because of the way speakers work, there will be a dip below four ohms at certain frequencies, but your low-power amp probably won't care.
+1 NachoMahma.

I have the black version of those speakers for my laptop (thanks to WM after Xmas clearance). The ratings are as you say, 8 ohms, 2 watts. In parallel, they would be 4 ohms. Your amp can drive them just fine.

As for the power supply, you need a minimum of 8 volts to properly operate. 4 AA batteries is only 6 volts (1.5 volts per battery, 1.5 x 4 = 6). You will need at least 6 batteries (9 volts) to make it work. and at 0.5 amps, you'll drain AA batteries pretty quick. I suggest either D cells (for portability) or an actual power supply (when at home or near a wall outlet). One is available from Velleman for this amp.

Qa
=D (author)  Quercus austrina4 years ago
Sounds good. So for wiring the two speakers together, how would I go about in doing it? Would I solder the positive ends of the two speakers together, likewise with the negative ends, then connect the joined wires to the appropriate ends on the amp?

As for the power supply, I'll probably end up doing both: having a power outlet, but also a battery pack for a portability option.

And could you just explain how/why there will be "a dip below four ohms at certain frequencies?" Just so my knowledge will be broadened, and add anything else that would help me.

Thanks again for your inputs guys.
The voicecoil of the speaker is just that - a coil of wire wound around a former (tube). When the signal from the amplifier is sent to the coil, it creates an electromagnetic field that alternates between positive and negative, in time with the music. This then interacts with the magnet on the speaker, causing the cone to move. This creates the sound you hear.

From this description, you can see that you have an electro mechanical system that is subject to outside forces, in addition to it's own set of parameters. I think that ESP's explanation can do better justice than I can.

Qa
=D (author)  Quercus austrina4 years ago
Thanks for all your help!

I didn't realize that a power source MIGHT be a problem... as I'm sure that I can't simply hook up an old cell phone charger to it? (since cell phones only draw in 5V) Could you possibly give me some pointers on what I could do as my power supply?

And yes, I've looked into a suitable mains supply for my amp. However, I'm not sure yet how much it'll cost since I'm not looking into spending a lot of money on this project, but if I have to I will.Low cost alternatives would be nice!
I just realized that I didn't answer part of your question. The cell phone charger won't be a satisfactory unit because it cannot provide the minimum required voltage. As stated in your OP, the unit requires anywhere from 8 volts to 18 volts (at a minimum of 0.5 amps) to operate correctly. The closer to the maximum ratings suggested that you get, the closer you will be to the circuits maximum power output, but the range suggested will give you satisfactory performance. That is why the manufacturer provides that information. Follow their instructions and you will be good to go and their product will survive for a long time.

Qa
Do you have any GoodWill stores near you? I picked up a couple of 12 V, 1 A wallwarts from my local store for less than a buck apiece. You could always scavenge also. Look for old broken paper shredders. I found one that has a 12V 1.6A wallwart that works fine. Also look for other unwanted computer products such as printers or scanners. They usually have wallwarts or bricks you could use (unless that was the reason they were thrown out). You could also look at All Electronics or Electronics Goldmine for possible solutions. Keep your eyes peeled and you'll find more opportunities than you could ever have guessed were there.

Just remember to use one that fits your range of requirements (8-18 VDC, 0.5 A minimum - more is OK)

Qa
Yes, the wiring is correct.

As for a box, just try different things. Cardboard is great for testing ideas. Once you find something you like, make it out of wood. For the size you will most likely end up with, 1/4" luan underlayment is probably good enough.

Qa