understanding car amplifiers and bridging

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.

Thanks,

Michael

 

[Phil Allison]

<mrdarrett@gmail.com

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.

** Nonsense.

The AC voltage is 12.6 / 2.82 = 4.5 volts rms.

Makes the max power available only 5 watts.

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.

** Same nonsense.

The real figure is about 20 watts.

My question then becomes:

** Redundant.



....... Phil

 

[John Popelish]

mrdarrett@gmail.com wrote:

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.

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.

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.

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.

My question then becomes: how come car stereos don't include bridged
amplifiers as a standard?

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.

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.

Thanks,

Michael

--
Regards,

John Popelish

 

[Bob Masta]

On Thu, 13 Mar 2008 19:30:22 -0500, John Popelish <jpopelish@rica.net>
wrote:

mrdarrett@gmail.com wrote:
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.

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.

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
www.daqarta.com
Scope, Spectrum, Spectrogram, FREE Signal Generator
Science with your sound card!

 

[Michael Black]

Bob Masta (NoSpam@daqarta.com) writes:

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.

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.

Michael