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How much power do I need?

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revans

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My project on the go at the moment is a portable boombox. It uses an amp which runs off 12v. This will be supplied by AC/DC adapter or SLA battery.

I want to internalise the power supply instead of having a big, black power brick; so I'm considering this series of Meanwell power supplies: **broken link removed**

I'll be using the 15V versions but I'm wondering how much power I'll need (they range from 15-150W).

I've done an amazing sketch of my setup; it would be great if anyone could give some advice as to how much power is necessary with regard to the PS.

Thanks

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• Project2.JPG
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Power in = power out + losses + headroom. So if I read your diagram correctly...

60W -> Audio
13.5V*500mA = 6W -> Maximum Charging Power as per picoUPS specs.
Plus a bit so actually power the picoUPS.

So greater than >70W. WIth headroom, the 75W supply is cutting it close. I'd go 100W.

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What impedance are the speakers?

The maximum RMS power you can supply to a 4Ω load from a 12V supply is 18W, with lots of distortion and clipping, to get more you need a boost converter.

The impedance of the speakers is 8 ohms

Also - not that I'd argue with 5,632 posts ;D - but could anyone second dknguyen's reasoning?

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@dknguyen: Again - not looking to argue with 5,632 posts ;D - but I know from my rudimentary physics knowledge that P=VI ...so, if the Amp is drawing a max of 0.3A at about 13V, then shouldn't it only need 4 watts?

I think taking the rms of the speakers into account might be wrong, as they are actually only using as much power as the amp can supply... speaking of which, the amp's website says: "The output power is around 2x25W RMS maximum, into 4 ohms at 10% THD+N. Up to about 2X15W, the THD+N is below 0.1%"

The webpage also shows the amp being used with a 35W SMPS. I think the power requirements of the amp are a bit below 60W. Sorry for misleading you with the speaker specs.

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For 60W out, I'd go for 100W based on 60% efficiency.

If the speakers are 8Ω you can knock that down to 9W per channel.

Assuming 9W per channel giving a total of 18W which is 30W of power in plus 3.6W for the Amp6-basic, I'd go for the 35W converter.

Check out the specification for those 25W driver amplifiers: are they specified with 15V in and a 4Ω load? Unless they have an internal DC-DC converter or a bulky audio transformer there's no way they'll give 25W into an 8Ω load at 12V.

RMS power is half the peak power.

You can calculate the theretical maximum peak power from the supply voltage and load using Ohm's law.

P = V²/R = 12²/8 = 144/8 = 18W

That's the peak power, the RMS power is half which is 9W.

This is only a theoretical calculation which gives the maximum output power, in practise the output power will be lower than 9W. The amplifier will be clipping at the maximum power output so will sound terrible so the useable output power will be lower still.

@dknguyen: Again - not looking to argue with 5,632 posts ;D - but I know from my rudimentary physics knowledge that P=VI ...so, if the Amp is drawing a max of 0.3A at about 13V, then shouldn't it only need 4 watts?

I think taking the rms of the speakers into account might be wrong, as they are actually only using as much power as the amp can supply... speaking of which, the amp's website says: "The output power is around 2x25W RMS maximum, into 4 ohms at 10% THD+N. Up to about 2X15W, the THD+N is below 0.1%"

The webpage also shows the amp being used with a 35W SMPS. I think the power requirements of the amp are a bit below 60W. Sorry for misleading you with the speaker specs.

I didn't even see the Amp6 until I read your post and looked back. I just saw the speakers and the PicoUSP. I was thinking "that's odd...the speakers seem to be all by themselves...what's driving them?"

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You posted whist I was writing the above.

I think taking the rms of the speakers into account might be wrong, as they are actually only using as much power as the amp can supply... speaking of which, the amp's website says: "The output power is around 2x25W RMS maximum, into 4 ohms at 10% THD+N. Up to about 2X15W, the THD+N is below 0.1%"
With an 8Ω load you should divide that by 2. 10% THD sounds terrible 0.1% THD is near perfect. The maximum usable power output into 8Ω is only 7.5W per channel.

The webpage also shows the amp being used with a 35W SMPS.
Looks like I was right.
I think the power requirements of the amp are a bit below 60W. Sorry for misleading you with the speaker specs.
Not your fault, amplifier manufacturers often like to exaggerate.

So essentially you guys reckon a 35W supply should cover: the amp + the UPS charge power (6W) + running the UPS?

EDIT: Also, do you think it would be to much to push it down to 25W?

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It depends on the surge rating of the PSU, a 25W will be fine as long as it can handle 40W surges.

Hi revans, are you using class D amps? Since you are after efficiency that would be the best bet, start with amps that have double-triple the energy efficiency of linear amps at moderate listening levels.

Then the power supply choice gets easier.

I agree, class D is a good idea but I wouldn't know where you can get hold of the modules and woudln't recommend building a class D amplfier from scratch.

Class D amplifier IC's are common place and not any higher in price than any other amplifier IC. The additional component count is typical as when compared to most other good amplifier IC's as well.

Here is just one example of a class D amplifier IC that falls into the as calculated speaker power ranges being mentioned so far.

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• 40 Watt Class D amplifier IC.pdf
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Hi guys, thanks for the help

The amp I've got is actually a Class T amp (it's a type of a Class D amp, but with higher efficiency and audio quality)... Regarding hero's comment:

I agree, class D is a good idea but I wouldn't know where you can get hold of the modules and wouldn't recommend building a class D amplifier from scratch.
My amp is an Amp6-Basic from www.41hz.com. They have a load of different kits which you make yourself... or you can get a professional, experienced forum member to do it for you (like me )

I think I'm probably going to go for Meanwell's 35W supply...

In the mono mode, the class-D amplifier has an output of 30W at clipping into 4 ohms with its max allowed supply voltage of 18V. Its output into 8 ohms is less, maybe only 18W.
With a 12V supply, its output at clipping into 8 ohms is only about 7.5W.

Its distortion is quite high for a modern audio amplifier.

@ audioguru: I'm not sure what Class D amp you're talking about but I don't think it's the one I'm using... the Amp6-Basic I'm using is a stereo amp (not mono) and its max allowed supply voltage is 14.5VDC (not 18V). Either way, its got a good rep as a high quality amp that's great for projects like mine.

I was talking about the Maxim "40 Whats" class-D amplifier.

The Amp6-Basic amplifier kit isn't made by 41Hz anymore so I couldn't find its spec's.
But with a 12V supply its power into 8 ohms will be low, maybe only 3.5 Watts (not bridged) or 6 Watts (bridged) at clipping.

What do you mean they don't make it any more?

and I believe you can't bridge the Amp6

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What do you mean they don't make it any more?

and I believe you can't bridge the Amp6

Many of the Tripath designs are already bridged, and you can't do it twice.

The AMP6 looks pretty nice;
(quote)
* AMP6 and AMP6-BASIC are both based on the Tripath TA2020 chip
* Two analogue inputs/outputs (stereo).
* The output power is around 2x25W RMS maximum, into 4 ohms at 10% THD+N. Up to about 2X15W, the THD+N is below 0.1%.
* Audiophile sound quality
* For both models hole mounted components are used, and a few, relatively large, surface mount power diodes.
* All resistors are metal film type.
* The boards are high quality, made with double weight (2OZ) 70um thick copper, holes plated through, and component print on both sides
(end quote)

It looks optimal to run from a 12v SLA battery. As for using an internal mains supply you need to keep it below 14.5v which for safety should be a regulated mains supply as an unreg 3+ amp 12v supply may go as high as 18v when only lightly loaded.

For Audioguru, many of these class D amps run internally bridged output from the amp chip, they are designed to drive reasonable power into 4 ohms from 12v supplies.

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