If one has a 4-ohm speaker and a car battery that puts out 12.6V, an amplifier can produce:
P = V^2/R = (12.6V)^2 / 4 = 39.7 W.
If one uses two amplifiers in a bridged configuration, that power then becomes:
P = V^2/R = (12.6+12.6)^2 / 4 = 159 W.
Per speaker.
My question then becomes: how come car stereos don't include bridged amplifiers as a standard? 159W per channel, x 4 channels, becomes
636W - more than enough for everybody - no need to buy a separate amplifier. The most I've seen on typical stereos is 50W per channel - and I'm guessing they're really stretching that 50W.
When a single ended amplifier is powered from 12.6 volts, it can apply half of that across the load at a time + or - 6.3 volts, max. That would produce 6.3^2/4=9.9 peak watts (5 watts, average, for a sine wave). Actually, a couple volts are usually lost in the output stage, so the maximum undistorted swing may be as low as + or - 4.3 volts, for a peak power of 4.3^2/4=4.6 peak watts.
Sorry, no. A bridged amplifier puts up to the supply voltage across the load, in either direction, so, ideally your original 39.7 peak watts, or 19.8 watts, average, for a sine wave. Only, now you have two amplifiers wasting a couple volts in series, so you actually may get only (12.6-4)^2/4=18.5 watts peak from the bridged pair.
Many do and provide about 20 peak watts into a 4 ohm load with an automotive DC volt supply. Higher power amplifiers usually include step up supply voltage boosters.
However, "honest" audio power is based on RMS voltage, since nobody much listens to square waves. So if we accept that music is closer to a sine wave, then if the peak is 4.3, the RMS will be 0.707 of that or only 3.04 VRMS, and the power will be 2.31 watts into 4 ohms.
As for the question about why manufacturers don't use bridge mode, I thought they had been doing just that for many years. At least, back in the mid-70s when I was an engineer at GM, Delco was looking into that. Part of the reason is that up until then they had been using germanium output devices in order to get more power from a single-ended output stage, and germanium devices were getting more expensive (and I doubt they were second-sourced). So twice as much cheap silicon was starting to look better than expensive germanium.
I assume everyone is using bridges by now... heck, they are available in all-in-one chips. The only reason I can think that they would *not* use bridges is if they want to use a chassis ground as the return wire from the speaker.
Best regards,
Bob Masta DAQARTA v3.50 Data AcQuisition And Real-Time Analysis
formatting link
Scope, Spectrum, Spectrogram, FREE Signal Generator Science with your sound card!
I imagine there may be exceptions, but the car radios I've looked inside of all used bridged amplifiers, and as you say, they can get it into one package so it might not be immediately obvious.
The giveaway would be how there are two leads coming out of the radio for each speaker. If one side of the speaker was going to ground, then there's far less reason to dedicate a connector pin to it.
ElectronDepot website is not affiliated with any of the manufacturers or service providers discussed here.
All logos and trade names are the property of their respective owners.