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walterm 07-12-2013 01:55 PM

SURE-HIFI 36 volt power supplies
Stumbled over this site on ebay, very tempted to "play" with one of their amps and power supplies.
IE a 100 watt per channel amp and 350 watt power supply for $69?
The Lepai Tripath LP-2020A+ hums at my favored listening volume for my speakers.
Hum disappears when listening to music, sound tracks, etc.
At these prices the power supplies appeal, or would I be better off modifying a computer power supply?
Amps will function on 12-36 volts.

mariush 07-12-2013 02:41 PM

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:

Digerati 07-13-2013 09:20 AM

Hum, assuming you mean through the speakers and not directly out of the power supply, may also indicate poor system or facility grounding - although considering the price and source of those supplies, I suspect mariush is right and it is EMI from the supply.

walterm 07-13-2013 07:04 PM

Thank you for the prompt replies.
The Lepai is what I own (about $20-24 on amazon), I've used the supplied power supply, a laptop supply and a monitor brick, same result.
I suspect my speaker load is not ideal for the Lepai amp. Also I am hard of hearing so run more volume than most.

Sure-HiFi offers what appear to be "big brothers" to the Lepai amp, without a case.

This one interests me but I think the 36 volt switching amp they sell would "pair" well with it. Just wondering if at their prices they are likely to be junk?

Actual components(yes look at shipping charges)

It is supposed to run on 12-36 volts
Power supplies

combo deal

Quality of these kind of purchases vary a lot, so any input would be helpful.

mariush 07-13-2013 07:55 PM

That one with STA508 can indeed work with up to 36v ... the absolute maximum is 40v.

The STA508 is a 4 channel amplifier - but each two channels are paired together as a single channel to simplify the design of the PCB.

Unfortunately, there's VERY LITTLE information in the datasheet regarding how many watts the amp can produce with minimal distortion - that figure of 100w could be with 10% thd which would be horrible.

Anyway, a 24v input would be enough for that amp to output some serious music power, probably about 2x60w ... but that means at about 80% efficiency, you would need about 150 watts to get those 2x60w.
The 24v x 5A would still work if you don't plan to get some reasonable volume at low distortions.

At 36v input, if I were to guess I'd say that amp can do about 2x80w with minimal distortions

But you'd need better cooling for such high power (for example put a 120mm to blow air over the whole board because that small fan and heatsink wouldn't be able to keep the amplifier chip cool at such high output)

The 36v power supply would be better and the 5.9a model should be enough, but if the price is only a few dollars more, I'd go for the 9a model, simply because it may have better heatsinks and won't be so loaded during use so the fan inside won't be as noisy as the lower end model.

walterm 07-14-2013 02:47 PM

Thanks, what I need to know.
The Lepai only hums when the volume control is past the half way point, which should mean I wouldn't need much from a more powerful the amp.
Whether these sound good is the question.
A better cooling fan makes good sense.

walterm 07-16-2013 09:09 PM

Project on hold, low bid on an integrated amp came thru.
Audio Source Model Amp 100, cost less than above amp and power supply.
new too.

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