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Old 07-12-2013
mariush mariush is offline
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Those are switching power supplies, just like the power supply in your computer.

If you want to see if the amplifier works better with a different power supply, just get 12v from one of your computer's molex connectors (the yellow is 12v, the black is ground).

The hum is most likely from the emi interference and lousy switching power supply it came with.
Another switching power supply may not kill the noise completely.

By the way, that Lepai amplifier uses a Tripath TA2020 amplifier chip, which doesn't even get close to 100w per channel (maybe pmpo but that's pointless).

here's the datasheet for the amplifier IC :

The amplifier chip can tolerate a maximum of 16v but they recommend not going over 14.6v! .

In addition, looking at a review of the amplifier posted here some important capacitors are only rated for 16v, so you definitely don't want to get higher than 15.5-15.8v or you'll blow the capacitors or get smoke out.

So you can't use that 36v power supply anyway on an amplifier with the IC above, because the most you can use is about 14.6-15v. Other audio amplifiers, in particular those that are class AB can work with up to 35-50v (and actually prefer to use higher voltage, they can't output much with low voltage).

Then you go further in the datasheet :

“Audiophile” Quality Sound
0.03% THD+N @ 10W 4Ω
0.1% THD+N @12W 4Ω
0.18% IHF-IM @ 1W 4Ω

High Power
25W @ 4Ω, 10% THD+N, VDD=14.6V
22W @ 4Ω, 10% THD+N, VDD=13.5V
13W @ 8Ω, 10% THD+N, VDD=13.5V

10% THD is horrible, so those values are meaningless.

If you look further on the page at that graph, you can see the curve for 4ohm speakers that it crosses the 0.5% thd at about 16 watts of audio power, when powered from 13.5v
With 8 ohm speakers, the distortions go over 0.5% thd at about 9 watts.

So basically, this is a honest very good 15w per channel audio amplifier.

Moving on, the amp chip has about 80% efficiency, so to produce 30w of audio (2x15w) it will waste about 6 watts as heat.

So overall, this amplifier will not consume more than 36w (but let's say 50w just to be sure) unless you really up the value so high that it will distort everything to hell.

50w is basically as much as how much a video card with a single 6pin pci express connector uses, so you can safely power the amplifier from a computer power supply.

I would recommend getting one of those 12v 4-5A monitor or Asus laptop power bricks, overall they are much better made than those crappy power adapters coming with the amp.

Ideally, you'd want to get a classic transformer (8-12vac 75-100va+ comes to mind), a bridge rectifier and about 10.000 uF (3x3300uF 25v for example) capacitors and you get a smooth power supply for your amplifier.

A 12v AC transformer will produce 12v AC which gets rectified by the bridge inverter into 1.414 x 12v = 17v . There's going to be about 2v loss on the bridge rectifier so you're left with about 15v with huge amount of ripple.
That's where the capacitors come in place, smoothing out everything and producing a very good 14-15v on average, with less noise than the adapter bricks.

The maximum output would be abut 0.62x75va = 46w

So a 12v AC 75VA will produce about 15v at 3A (46w/15v), which should be more than enough for your amplifier).

Regular transformers are simply not used anymore because a 12v AC 75-100va weights about 1.5-3Kg so they're expensive to ship around and they're expensive because they have a lot of copper in them.

For example of such transformer, see here:

Last edited by mariush; 07-12-2013 at 02:52 PM.
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