Amplification of audio

[DG_Silva]

Hi all, I'm wondering if someone could help me please?

I'm building a bar from a replica telephone box, and I want to build a stereo into it. The idea is to run one of those cheap Bluetooth receivers from Ebay (see below), but I don't think that it carries enough power to drive a couple of 6" car speakers. I'm thinking of running it through a 20w Class D amp, but my electronic engineering is a bit rusty.

Would I get 100w from the output or 25w?

https://www.ebay.co.uk/itm/5V-Wirel...DIY-Car-Speaker-Circuit-Board-NP/153101452349

https://www.ebay.co.uk/itm/5V-20W-2...o-Class-D-Amplifier-Board-amp-UK/223557845182

Before anyone asks, I don't want to run it from mains electric with an adapter, because the phone box will be outside and I don't want to run a mains electric into the garden. Due to this, I'll be running it from a USB power bank providing 5v.

 

[sesselpupser]

@DG_Silva there's not enough info in the first linked description, but the second one says 5W into a 4 ohm speaker on the left and right channels and 15W into a 4 ohm subwoofer. It also says it's 5V at 2A which is only 10W, though, which is contradictory, you can't get 25W out of 10W.

Not sure a battery bank would be good for this application unless you're going to turn it way down, they tend to be limited to 2A so to get any more than a total of 10W out you'll need something beefier. Also car speakers tend to be 2 ohms which could blow the amp. You might also find the crossover frequency is no good as this is a 2.1 amp but car speakers tend to be built for a wide frequency response, so you might find the bass response is weak.

Personally for an application like this I'd probably just get a car stereo, battery and trickle charger. They're cheap, purpose-built for battery operation and robust.

I'm a bit drunk and may have misunderstood something, but why do you think you'd get either 100W or 25W out of these amps? I mean, how did you arrive at those numbers?


Edit: now that I'm less drunk I see the first is just a Bluetooth receiver!

 

[DG_Silva]

It was probably me hitting a few too many gins too! I think I better start at the beginning again lol.

I've already built a Bluetooth speaker, using one of these and a pair of Vibe Slick 6 Component speaker:

https://www.ebay.co.uk/itm/TDA7492P-AUX-50W-2-Wireless-Bluetooth-Audio-Receiver-Digital-Amplifier-Board-EF/332913907214

I made it portable by making a small battery pack out of wood and 10 AA rechargeable batteries. It can also run from a 12v mains adapter which is suitable for inside use.

I just wanted to understand the basics of amplification and how it worked. Using the two units in the first post, we have 2 x 5w initial output, going into 2 x 5w through the amplifier. Would this multiply to 25w, or simply total to 10w across both items?

I can't use a car stereo (mainly because there isn't the room to hide it away sufficiently, unlike these boards which take up minimal room), so I'm thinking that instead of using two separate boards, I might reuse the one above and use this as a power supply instead:

https://www.ebay.co.uk/itm/12V-DC-R...ck-6800-20000mAh-Battery-UK-Plug/163227287156

 

[BobK]

If you run the output of a five watt amplifier into another five watt amplifier, the final output is five watts. If you want a 25 watt output then you need a 25 watt amp.

Keep in mind that you might be getting into pretty large currents; if you want 2x25 watts then you're talking ten amps current draw at five volts. With a 12-volt supply it's still 4.8 amps. And that's assuming 100% efficiency which of course you won't get.