Suggestions for amp?

Hello all,
As my question shows, I am looking for suggestions of amps I should use to drIve my speakers. There are 2 speakers RMS rated at 3.7W and 4ohm. Any suggestion is appreciated.
Many thanks in advance
EDIT:
it would be best for the amp to use less than 9v so that it is still portable. 
Please suggest 

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jduffy542 years ago

The LM386 is VERY popular with low-wattage amplifiers, but far from the only choice. Most electronics stores will stock some small amp kit, so check what you have available to you. If you want to make it entirely yourself, here's a very simple one that I've had success with. The exact transistor and resistor values can be changed however you want, I've used 2n3904 and 3906 transistors myself, and it worked fine. Using 2n4401 and 2n4403, should work at the full 3.7W you specified, though I have not personally tried it with those two. All of the parts should be available at radioshack for just a few dollars, or something similar at any decent electronics store.

If you're into something interesting, I've had some success using a 555 timer as a class D amplifier, by setting it up in PWM mode, and putting a power transistor on the output. It's not the best way to do it, but it does work. If you're using a battery, it is also more energy-efficient.

Fig54aa[1].gif
michalk1 (author)  jduffy542 years ago

Hi, this looks like a solid design, and I have a lot of these components already. I have 2 issues with it; how do I incoporate volume control and it says the speaker is 8ohm does that mean i will have to use this one amp to power two 3.7w 4ohm speakers?

Volume control is trivial with a potentiometer, use a double-gang pot (has 6 leads instead of 3, basically two separate pots stuck together), wire ground to the 2 far-left or far-right leads, the R and L inputs to the other far leads, and the R and L output (feeds to the INPUT on this amp) to the middle leads.

You should built two of these amps, one for right and one for left. If you don't have enough resistance within the speaker already, you can do a few things.

1) add some small resistors to the base of the transistors, will lower the gain of the amp, this giving you less power to the speaker.

2) reduce the capacitor value inline with the speaker. In an AC circuit, caps act like a resistor, this will change the frequency response of the amp, though. The equation is Rc=1/2*PI*f*C plugging in 100uF, and a 1kHz signal (normal for testing), gives effectively no resistance, so...never mind, the cap has virtually inconsequential resistance, even down to 20Hz (less than 1/6000 ohm).

3) Add a resistor inline with the speaker. It will waste some power, but is easy, without changing anything about the output other than power. The base-emitter voltage of T3 will be the difference between 0.7 and the positive rail, with a 5.5 ohm impedance in the way (assuming your speaker, and an average 9V battery (about 1.5 ohms internal resistance)). The CE drop of a 4401 is 0.7V at maximum, leaving 8.3 volts over 5.5 ohms, which is too much current.
Limiting you to a spike current of 500mA (~4W peak, also the max of the 4401), we need a 8.3 / 0.5 resistance total, 16.6-5.5=11.1 ohm. A ten ohm resistor here should be fine, or anything around that, as the real circuit will respond slightly differently (it will never be all the way on, the cap will increase in resistance instantly after a change, so the current can't be that high for more than a tiny fraction of a second). This will, however, give less than the 3.7W to the speakers you want, so I'd go with 1, or just limit the input volume. (add resistors inline with the actual audio source input, before the pot).

Audio amplifiers, capable of a few watts of output power, can often be found in module form, usually with the speakers included.

Amplified speakers sold for personal computers, can be found, new, for less than 20 USD. For example,

http://www.newegg.com/Product/Product.aspx?Item=N8...


Typically these are made to run from the 5V supply of a USB port, but I am confident batteries would work too; e.g. a series stack of 4 "C" or "D" sized cells to give an input voltag of around 4*1.5V = 6V

If you want something that's already in the shape of a boom-box, try looking on the used market. If you have thrift stores near where you live, I think that would be the place to check first. If this box has a auxillary audio input. Then that would make it easy to put your signal into it. If not, then you have to do some hardware hacking, that is taking it apart to find out where, internally, the inputs to its amplifier are located. Again the price limit is around 20 USD. If the price of a the used boom-box is more than that, then you're probably paying too much.

Also, check the dumpsters too, because people often throw away stuff like this all the time, e.g. old computer speakers, old radios, boom boxes, old computer monitor (with amplified speakers attached).

Regarding building your own amplifier, like from op-amps, and resistors, and capacitors, and such, I think that would be the hardest way to go. Even if you totally know what you're doing, it is still going to take several hours to put it together, not including the work to put it in an enclosure (a box) of some kind, since you said you wanted it to be portable.

michalk1 (author)  Jack A Lopez2 years ago

Hi, thanks for the answer. I will not be making a box as I am using speakers from an older 5.1 system which includes a case with enough space for my features. So the only things I will be needing are:battery, charging circuit, tone control circuit, amplifier, speaker. As I don't think this is that much, I am planning to construct as much of it as possible.

iceng2 years ago

If you like building....

gainvol386.bmpaudioa13.bmp
Wired_Mist2 years ago

There are FAR TOO MANY Options avalible to you. If you have a good electronics store nearby Just see what they have in stock. Hopefully they will have a Pre-Assembled "Breakout board" you can use.

Some other options from Sparkfun are This Kit:

https://www.sparkfun.com/products/9612

OR This IC:

https://www.sparkfun.com/products/9557