Cheapo Chinese Amplifiers

For the past few years, I've been seeing very cheap - under £10, posted - amplifiers on eBay. Of course they've got outrageous power ratings given in that lovely PMPO wattage, which basically means the manufacturer can say clock radio has a 1000w amp inside...PMPO of course. Anyway, curiosity got the better of me and I bought the cheapest one I could find on there.

 

Lots of pics of the guts of this little thing after the jump.

Total cost: £8 including postage to the UK. They must be turning these things out like children. Oh wait...

It arrived just over a week later, double-boxed in a sturdy cardboard shell. I've seen UK eBayers ship 30lb amps with poorer packaging than this featherweight which I could lift with my little finger. It works off 12v DC via a 2.1mm barrel jack with 3.5mm (labelled "MP3") and RCA (labelled LINE) inputs. Both can't be used at the same time though. Speakers are attached with spring clips as you'd expect.

First thing before connecting it to a speaker was to check the offset to make sure it's not throwing out 12v DC. All was in good order so I hooked it up to a cheapish Sony speaker. Sounded not too bad...next thing was to try it on my main Goodmans speakers. Again, the midrange is pretty muddled but there's a decent amount of bass and it's not too muffled.

And here's its juicy, juicy guts. Inside it's all very neat, especially for a Chinese made cheap product. The PCB appears to be of a reasonable quality and the soldering is as neat as you'd expect from something costing a lot more. No splashes and dry joints here at all...ok, maybe there's a couple of joints where the solderer has gone a bit nuts, but it's nothing to worry about.

All the electrolytics are 105 degree C brand names and the resistors have a gold 5% tolerance band. Surprising given the price, and how it barely gets warm inside.

Component values are silk-screened on and unlike some Chinese stuff, the correct value bits are fitted.

The empty component locations are for another product like this but with an FM radio - because the FM version has a dual throw dual pole switch to select between FM and line inputs, it would be fairly simple to modify this PCB so either RCA or 3.5mm inputs can be selected with a switch. Currently there's just jumpers where the switch normally is.

To prevent it from blowing up if connected wrong, it has reverse polarity protection with a chunky diode. I'm guessing they think that MP3 players have a lower gain as the RCA inputs are put through 47k attenuating resistors whereas the 3.5mm MP3 player jack isn't.

There's three ICs inside - two TDA7056A mono BTL amp modules and a 4558 dual opamp. Each of the amp chips is good for 6w into 8 ohms at 5% THD according to the datasheet...so 12w altogether isn't too bad really if you ignore the inflated specs on the advert. The opamp is for the tone controls with a basic RC circuit.

The tabs on the power ICs have been folded over so they can be clamped to the aluminium case for heatsinking. On the other side there's a metal bar with threaded holes which is hot-glued in place to make assembly less fiddly. When screwed together, the hot glue serves no purpose. Neat.

So yeah, I recommend these little things for small projects or even for using on your PC with a pair of little hi-fi speakers. It's happily run off the 12v PSU rail inside if you're happy soldering up a Molex to 2.1mm jack cable. Not nearly enough power for a car though, and don't expect massive current out of it.