I'm trying to make a custom PA system for my car, but I am confused about what type of amplifier I might need, or do I need one at all?

I'm wanting to take the main sound unit out of a "personal soundtrack shirt" like this http://www.thinkgeek.com/tshirts-apparel/interactive/a5bf/ but remove the speaker from the shirt and replace it with a PA horn.

I already have a pa horn that came with a "coga pa system" http://www.cogapa.com/.

I actually have already hooked up the sound unit from the shirt, to the pa horn, just by simply cutting the audio output wires, and hooking them to the wires of the PA speaker, and it does work, and is louder than I would have expected, but I don't think its nearly as loud as its potential. The reason I think this is simply because the PA speaker is so much bigger than the cheap little speaker that came with the shirt, and the little sound unit amp in the shirt is run off 4 AAA batteries, whereas the Coga PA system the horn is coming from would run off 12 volt.

Couldn't find any specs on the shirt, but supposedly the Coga system would output 40 watts RMS 50watts Max to its speaker, so I'm guessing the speaker can handle 50 watts, and I just need to find a way to bump up whatever output is coming from the shirts sound unit, up to 50 watts. But then, I really don't know anything when it comes to audio and amplification.

Can anyone help?

  • 1
    \$\begingroup\$ Car-radio type amplifier IC's are fairly available and do not require much support circuitry. Another option might be to try to find the right place in the existing PA amp circuit to couple in the signal from the t-shirt generator, via a small series capacitor (and if needed, a resistive attenuator) \$\endgroup\$ Jan 5, 2012 at 23:52

1 Answer 1


As I understand it you are using just the horn and not the amplifier that comes with the COGA.
If this is the case, then it won't be driven correctly from the T-shirt amp as (as you guessed) the power output is too low. I would say it's probably a couple of watts at best (as opposed to 40W)

What you need is either the original COGA amp or something of a similar output rating.
Checking the manual it says the horn is 4 ohms, so most simple amps up to 40W capable of driving 4 ohms will work.
For example, something like this looks suitable.

  • \$\begingroup\$ Thanks for the reply. Is there any easy way to measure how many watts is coming from the shirt amp? I don't own an oscilloscope, but I do own a multimeter. Also, any idea if this particular amp in the link below would work good? Its cheap, looks very user friendly, and available from hong kong (I believe the amp that you linked to was in UK if I'm not mistaken, never ordered from there, but always get things from honk kong no problem, I'm in the USA) \$\endgroup\$
    – Carl
    Jan 7, 2012 at 17:15
  • \$\begingroup\$ To measure the power, you will need a sine source and load resistor to replace the speaker. Then you can trust your multimeter's AC RMS range and calculate power as V^2/R \$\endgroup\$
    – markrages
    Feb 5, 2012 at 2:38
  • \$\begingroup\$ An easier way to estimate power, and good enough for your purposes, is to measure the supply voltage and current. Power in is V*I. By the first law of thermodynamics, the amplifier can't put more power into the speaker than it draws from the battery. So watts from the battery puts an upper bound on the output power. You might assume 70% efficiency and multiply the answer by 0.7. If this seems awfully sloppy, consider the ear's logarithmic response. An answer within a couple dB of the true power is probably accurate enough. \$\endgroup\$
    – markrages
    Feb 5, 2012 at 2:44

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