Power Amp in a Box

[PRR]

> What about the LM384? ... will something like this work from a 9vdc (perhaps doubled with a 1044)?

RTFM. The maker's datasheet is 99% correct and 98% complete, unlike the average blog-page.

Page 5 shows power versus voltage. The 8 ohm graph does not go as low as 9V, but the trend is clear: you will get less than 3 Watts at 9V. (Simple math tells you that a perfect amp on 9V will give 9V peak-peak which is 3.18Vrms which on 8 ohms is 1.26 Watts rms Sine.)

The "5W" in the datasheet title is an example of "99% correct and 98% complete". Obviously you have to treat it right to get ANY power out, and just-right to get 5 Watts. Looking at the graphs we see 4 Watts in 4 ohms (the plot rolls-back due to current saturation), _9_Watts in 8 ohms (at absurd dissipation), 5W in 16 ohms.

This chip is heat-sunk through the leads. Just naked it can do 1.5W dissipation on a cool day. Several square inches copper PCB-stuff gets you to 2W or 3W dissipation. You need a LOT of copper on the center leads for 4W dissipation (no, you can't get the V-7 today; you can approximate it with heavy hobbyshop copper/brass).

Accepting 2W-3W dissipation we see that 18V in 8 ohms gives 5 Watts audio output with 2.5W dissipation.

> 9vdc (perhaps doubled with a 1044)?

Remember, no free lunch. 5W output with 2W dissipation means 7 Watts total electricity. 7Watts at 18V is 7/18= 0.389 Amperes or 389mA. MAX1044 is suggested for 10mA loads, graph shows 40mA with significant sag.... this is literally a 1/2-ton pickup trying to haul a 5-ton load.

Seriously. Tiny Giant. Either kit, or study the idea and adopt it. The bridge-mode amplifier is like a doubler without useless action. The laptop supply is over-ample yet *cheaper* than a good 9V pedal supply. These car-audio chips make a very-honest 16 clean Watts in 4 ohms and are easily "22 Watts" in guitar application.

[aresaudio]

I'm looking to build a power amp in a stompbox sized form factor, with 9v input. What chip should I use (I'm guessing it has to be opamp to maintain simplicity of the circuit and smaller board size, but am open to other options) to get the maximum wattage and output? Onboard voltage doublers and inverters are required,I suppose (lets say, to get +18v and -18v) but I don't think this will suffice for power amp chips like LM3886 etc which will almost certainly need a transformer - which I'm reluctant to work with at this stage. So are there any alternatives I can look at? Maybe a bridged configuration or something?

Look here: http://www.aresaudio.com/uniamp-v2.html
9v=10w

;-)

[R.G.]

Look here: http://www.aresaudio.com/uniamp-v2.html
9v=10w

9V pk-pk = 4.5V peak
(note: assumes a rail-to-rail output amplifier, which does not exist at any power)

4.5 Vpk = 3.18Vrms

P = V**2/R

10 = (3.18**2)/R

R = (3.18*3.18)/10 = 10.128/10 = 1.013 ohms

Volts is not equal to watts.

Even the article docos show a 2 ohm load.

[aresaudio]

Look here: http://www.aresaudio.com/uniamp-v2.html
9v=10w

9V pk-pk = 4.5V peak
(note: assumes a rail-to-rail output amplifier, which does not exist at any power)

4.5 Vpk = 3.18Vrms

P = V**2/R

10 = (3.18**2)/R

R = (3.18*3.18)/10 = 10.128/10 = 1.013 ohms

Volts is not equal to watts.

Even the article docos show a 2 ohm load.

Guarda bene sia lo schema che il grafico della potenza VS tensione (http://www.aresaudio.com/images/Power-Curve.jpg) e tieni presente che l'amplificatore lavora a ponte (due canali contemporanei per un solo altoparlante) BTL MODE! (9V, 4Ohm = 6,579 W RMS = 13W potenza musicale)

[R.G.]

Yes, if you use two amplifiers running out of phase to double the effective voltage across the load, you get more power. This is what "BTL" means.

In that case, you get Vpeak = 9V (again, if the amplifier is perfect; in real cases, it will be less), so Vrms = 9/1.414 = 6.36Vrms. The power into a resistor R is then (V**2)/R,
so into 4 ohms you can get (6.36*6.36)/4 = 10.12Wrms. Note that what is happening is that conceptually, this is two amplifiers, each driving a 2 ohm load to 5W.

One could also say that 9V = 500W. All you have to do is to have one of the 9V/5Wrms amplifiers driving a two ohm speaker, and replicate that 100 times. Same concept.

[jishnudg]

wow...okay, I'm just going to take some time out and try to grasp the topologies active in the Tiny Giant and AresAudio designs I guess.Lot of homework.
Thanks, all

[aresaudio]

http://www.aresaudio.com/nanoamp.html    ...... is better!   

[R.G.]

http://www.aresaudio.com/nanoamp.html    ...... is better!   

Is it? What *exactly* does "better" mean here?

Smaller? Cheaper? Hotter? Cooler? Heavier? Able to leap taller buildings?
One important issue here is that as noted in the thread above, getting a power amp to work is almost trivial, maybe inconsequential compared to getting enough and the right kind of power to it, and getting that power OUT through a speaker, which will almost invariably be larger and more costly than the amplifier itself.

I didn't notice any power supply connected to it at all. Is there one? How big it that, how heavy, and what does it cost? And what speaker cab does this thing use?