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Old 6th December 2012, 09:11 AM   #11
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Quote:
Originally Posted by Leejin View Post
Aren't the Headphone amplifiers measured in mW though? Like around 800mW? This thing would at least give me 6W (6000mW) if I bridged it to the single driver.. right?
A bridged amp running from a +5V supply won't give more than 3W into a 4 Ohm speaker. That simply the old V^2/R law of physics!

Now this is more than a USB port can supply (5V at 0.5A). So it won't give a full-power continuous sine wave, but if you have enough reservoir capacitance and your music has the 10dB crest factor that it should have (and not the 2dB crest factor that so much over-compressed material has these days) then it can work fairly well.
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Old 6th December 2012, 02:52 PM   #12
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Quote:
Originally Posted by Ouroboros View Post
A bridged amp running from a +5V supply won't give more than 3W into a 4 Ohm speaker. That simply the old V^2/R law of physics!

Now this is more than a USB port can supply (5V at 0.5A). So it won't give a full-power continuous sine wave, but if you have enough reservoir capacitance and your music has the 10dB crest factor that it should have (and not the 2dB crest factor that so much over-compressed material has these days) then it can work fairly well.

Ok... I have no ide.... I'll see myself out. Sine Waves..? dB crest factors.. ?

*WHOOSH*

Maybe my question should have been simpler; Will these 2 things I linked make sound come out that someone would hear ~3 feet away?
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Old 6th December 2012, 06:07 PM   #13
AndrewT is offline AndrewT  Scotland
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Have you got the message yet?
It appears not.
To get 3W from 5V you would need to use a 1ohm speaker.
The maximum output of a rail to rail opamp using a single polarity 5V supply is ~2.3Vpk (or 4.6Vpp) in either direction from the mid biased voltage of 2.5V.

2.3Vpk into 1r0 is equivalent to 2.6W, well not quite 3W but getting close.
Go and do your research and learn how to assess the lies told by retailers in trying to con folk into buying their useless product.
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Old 6th December 2012, 06:51 PM   #14
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This is a commercial USB power speaker from a year ago.

http://www.theaudiocritic.com/plog/

It's all about energy harvesting during the quite sections.
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Old 7th December 2012, 07:03 AM   #15
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As the device quoted by the original poster was a class-D part, then the max output from an op-amp isn't really relevant. You can get an almost 10V pk-pk PWM switching signal from a properly designed full-bridge MOSFET output stage. Now the audio after the output filter won't quite manage 10V pk-pk, but I have managed over 9V pk-pk of demodulated audio from a bridged-output, class D amplifier running off a single 5V rail. It's not that difficult.
That is 3.2v rms for a sine wave, which gives 2.5W into 4 Ohms.
A year ago or so there was a company developing amps which would give over 10W peak from a USB power feed, having an efficient class-D amp, with a dc-dc boost stage which made the amplifier supply rail track the envelope of the music. No use unless the music had a large peak to mean power ratio, of course.
They were pushing their technology on this site, but seem to have disappeared now.
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Old 7th December 2012, 09:34 AM   #16
AndrewT is offline AndrewT  Scotland
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.................... You can get an almost 10V pk-pk PWM switching signal from a properly designed full-bridge MOSFET output stage. ......................
From single polarity 5Vdc?
Are you sure?

Or,
have you included a DC to DC voltage converter in there to get the doubled voltage?
If so, then that doubled voltage comes at the expense of a slightly more than doubled current from the 5Vdc source.
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Old 7th December 2012, 10:09 AM   #17
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From single polarity 5Vdc?
Are you sure?

Or,
have you included a DC to DC voltage converter in there to get the doubled voltage?
If so, then that doubled voltage comes at the expense of a slightly more than doubled current from the 5Vdc source.
Yes, it's a bridged output amplifier, so the pk-pk output voltage is (theoretically) twice the supply voltage.

Still twice the current from the supply of course.

(EDIT! 4x the supply current for the same impedance load. That's why the big reservoir caps to cope with the avareage power demands of the music)

Last edited by Ouroboros; 7th December 2012 at 10:12 AM.
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Old 7th December 2012, 10:23 AM   #18
AndrewT is offline AndrewT  Scotland
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No.

The peak to peak voltage approaches the rail to rail voltage.
If the rails were +-5Vdc then 10Vpp is the theoretical maximum output. i.e one output would be at +4.9V and the other output would be at -5V. The load would see the 10V

But the USB is a single polarity 5Vdc. One pole would be at +4.9V while the other pole would be at +0.1V. That is 4.8V across the load.

I got it.
The 4.8V swaps to opposite polarity for an effective 9.6Vpp across the load when bridged.

And that is equivalent to 2.9W into 4r0.
BUT !!!!!
each half of the bridged amplifier is delivering 1.45W into a 2r0 load.
The USB sees an effective 1r0 load since the two 2r0 loads are in parallel. The 2r0 loads are effectively in series, but the amps are drawing current from the same source.
In stereo the four amplifiers show an effective 0.5ohms load to the USB 5Vdc source.

Do I need to add a comment on what I think of the audio quality that will ensue?
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Last edited by AndrewT; 7th December 2012 at 10:46 AM.
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Old 7th December 2012, 10:43 AM   #19
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Certainly not! Full bridge Class-D amps if anything are higher quality than half-bridge ones.

Yes, the powering issue from USB is very real. Modern music with low peak-to-mean ratios won't work too well. You need a good few thousand uF of reservoir capacitance to supply the peak currents while re-charging on the quiet bits. (Except that most music these days doesn't have any quiet bits!!)
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Old 7th December 2012, 05:02 PM   #20
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Quote:
Originally Posted by AndrewT View Post
Have you got the message yet?
It appears not.
To get 3W from 5V you would need to use a 1ohm speaker.
The maximum output of a rail to rail opamp using a single polarity 5V supply is ~2.3Vpk (or 4.6Vpp) in either direction from the mid biased voltage of 2.5V.

2.3Vpk into 1r0 is equivalent to 2.6W, well not quite 3W but getting close.
Go and do your research and learn how to assess the lies told by retailers in trying to con folk into buying their useless product.
I agree.. I need to do tons more of my own research.. I thought this was it.. But I'm not going to go out and buy an oscilloscope and jot down wave patterns.. Again.. Tons of respect for those that do. But I stick to my gogle-fu.

Speaking of.. I found this.

30W from 2.5w (.5A/5V USB)

Then it led to THIS!

Last edited by Leejin; 7th December 2012 at 05:23 PM.
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