Current Vs Voltage.

[fugyaself]

After checking out JBL's website and watching their "What the..." video I had a fiew questions.

They bring up power = current x voltage or watts = amperage x voltage.

They said that typical car audio amplifiers work on a high current design at low impedance and that they amps have the ability to work on high voltage at higher impedances to give the same power and that the benefit is you dont need as vulky of a coil to handle the extra heat. I this true? I thought the heat was generated by the power running through it and not just either the voltage or current. This would give them absolutely no advantage when running under higher voltages at a higher impedance.

[fugyaself]

OK after a bit of reeducation I understand that current is the driving force as well as the source of the heat.


So why are things rated in watts and not amps of output?

[Steven Kephart]

That is very interesting. And you know, they are right. We actually discussed the advantage of higher voltage, lower current amps from a distortion point of view in the George thread. Here's a quote by Dan, that fits this discussion as well:

Hi guys,

Stephen and Geo, you're both right! Force truly is BLi, so given all else being equal, you need the same current to generate the same SPL in the drivers you're talking about.

Now, if you had one driver that needed X amps of current to generate Y Newtons of force, and a second driver that needed 0.5X amps to generate that same Y, then the second driver would have lower distortion. Of course, it's also 3 dB more efficient, meaning either BL doubled or mass was cut by 41%.

All else being equal, current is current, BLi=ma, and how you get that current is immaterial. In car audio we tend to have lower impedance because back in the dark ages (the 80s - were some of you even alive then? ) high voltage switching power supplies were expensive. So you used the least amount of voltage possible, meaning that to maximize current flow at that low voltage rail you used low impedance drivers.

Now days, with switching FETs being cheap and plentiful, a 3 kW switching power supply is quite affordable. We stick to low impedance mainly because of market inertia. This is one area that I have a lot of hope for 42V car systems - higher voltage systems allows use with higher impedances.

Why higher impedances? Go back to the top - that whole effiency thing. What is efficiency? BL and Mms. Guess how I can increase BL for really cheap - add more L... Go to an 8 Ohm voice coil, I can double the BL for free (as compared to a 4 Ohm voice coil). Meaning I don't need as much current to generate the given force, but I do need more voltage. Of course, with a 42V rail, we can get a clean +/- 40V into the speaker without a switching power supply, and that is a solid 200W into an 8 Ohm load! Into a 4 Ohm load, we have 400W of power. Imagine a head deck that can do 400Wx4 - yes, that's the benefit we get...

So, bottom line - current is the key, it does increase BL nonlinearities as current increases, but for two drivers, if all else is equal, it doesn't matter if it's a high impedance or low impedance. Key being "all else being equal".

Dan Wiggins
Adire Audio

Steven Kephart
Adire Audio

[PolkMM]

i have a question kind of related to this
when you have one pair of RCAs going from your HU to TWO amps, using y-splitters to go to the amps, are cutting the voltage in half, or???

[hobbes26]

Essentially you're creating a parallel circuit, so voltage should stay the same. I believe you may run into problems created by the different input impedances to the amplifiers. I'm not sure as to the magnitude of the problem though, and it may work fine.