MPG losses from car stereo ?

[shovel]

Originally Posted by jamesqf

1 HP = 746 watts. I see typical stereos advertising 200 watts/channel, so if you crank the volume to the max (goodbye, eardrums!) you could theoretically lose about half a horsepower.

Of course in the real world, you'll be using less than that even at full volume, because the sound level will vary...

At this time, NO in-dash car head unit has a DC-DC power supply providing rail voltage to its amplifier IC. (some units, such as a discontinued Panasonic model and some specific Alpine and Kenwood units could be fitted with an outboard switching DC-DC unit to double rail voltage)

What this means is, for all practical purposes 100% of all in-dash car head units you can buy are working with 0-12 (or 14.4 under ideal conditions) volts and the vast majority use BTL configured output transitors with a 0.5/1 bias.

14.4 volts into 4 ohms is 51.84 watts *peak* theoretical power per channel, if there are no losses anywhere else in the circuit. Which means RMS sine wave average output will be roughly half that, again under ideal circumstances. So you have 21 watts times 4 output when all 4 channels are rockin' full boogie 100% of the time, or 84 watts. Figure the head unit's other items (illuminated display, DSP, mechanical cd transport, etc) will use another 20 watts or so, and the whole shebang isn't going to be 100% efficient since BTL A/B type amplifiers - even chip amps like are typical in this application - are at best 75% efficient. So you're around 140 watts absolute maximum continuous draw under the most extreme theoretical conditions at full volume.

I really doubt anyone drives around playing sine waves at 100% volume all the time... so figure the radio at a reasonably loud volume is going to use less than 40 watts average. Not a concern worth even thinking about.


Sadly, no law prohibits manufacturers from making up ridiculous wattage claims for their products except specifically home hi-fi equipment. That is why $15 PC speakers can claim to be 1000 watts, and car stereos can claim to output ridiculous amounts of power. They arrive at those numbers using whatever funny math they want to justify it with, (for example, you can see above in my math how Sony arrives at "52w x 4" from their head units... 14.4x14.4 divided by 4 ohms equals 51.84 watts... rounded up is 52)

[Cd]

Good to know.
I had the impression that these things were power hogs.
I have seen lots of stereo setups where the guys had to have seperate batteries in the trunk just for the stereo.

[shovel]

Originally Posted by Cd

Good to know.
I had the impression that these things were power hogs.
I have seen lots of stereo setups where the guys had to have seperate batteries in the trunk just for the stereo.

Those guys are going for something else entirely.

Basically any time you want to increase output by 3dB you have to double wattage. A normal speaker might produce 90dB at 1 watt, to make it produce 93 dB you need 2 watts, 96dB you need 4 watts, 99dB you need 8 watts, 102dB is 16 watts, and before you see 105dB most regular automotive head units are unable to produce any more wattage.

(ignoring "cabin gain" here)

But you can see how this turns into serious electrical demands when guys are trying to get 140, 150, etc dB out of their system. The same 90dB efficient loudspeaker would need more than 8000 watts to produce 129dB (assuming it could handle the power)

[jamesqf]

Originally Posted by NeilBlanchard

Hi James,

There is probably very little connection between what an amplifier is rated at, and what it actually is able put out into a dynamic load (the loudspeaker and it's crossover). Also, there will be losses since no amp Class A/B amp can be very efficient, by definition.

Sure. I was giving a simple ballpark figure for an upper limit on the power draw. Beyond that, I'll leave it to the EE types, except to remark that the amps & other electronics can't be all that inefficient, since the wasted energy would appear as heat.

[dremd]

Just depends on how crazy you go.
If you go big, then think about efficient amps+ speakers.

The *Last* time I drove a VW it had a monster stereo, you could definitely tell the difference in performance when it was cranked I'd guess that had something to do with the terrible economy.

Obviously you have to look at average current draw (never displayed) to determine how bad you will hurt economy.

[8307c4]

Originally Posted by gascort

Still looking for someone who's replaced their factory 6.5 inch round speakers to replace my fronts.

Any competent stereo shop should have these, now the brand may be an aftermarket such as Bose but by the time the cover goes on it can't be noticed.

[MetroMPG]

GM considers stereo draw significant enough to have given BOSE the task of improving unit efficiency for use in the Volt:

So the Bose folks were challenged to come up with a new sound system that cranks heavy tunes with a light touch, putting out a big sound from a small space. The result? The Volt’s Bose Energy Efficient sound system will be 30% smaller, 40% lighter and, most importantly, use 50% less energy than a comparable existing system. It’s like removing 50 pounds from the car’s weight. When trying to reach 100 mpg, every pound counts.

Source: GM to Volt Suppliers: Get Us 100 MPG | Hybrid Cars

[NiHaoMike]

Why couldn't they use TI's hybrid digital technology to improve efficiency? I have done it for home audio:
Amanda Harris: the Prius of home audio - EcoRenovator

At this time, NO in-dash car head unit has a DC-DC power supply providing rail voltage to its amplifier IC. (some units, such as a discontinued Panasonic model and some specific Alpine and Kenwood units could be fitted with an outboard switching DC-DC unit to double rail voltage)

What this means is, for all practical purposes 100% of all in-dash car head units you can buy are working with 0-12 (or 14.4 under ideal conditions) volts and the vast majority use BTL configured output transitors with a 0.5/1 bias.

Except in hybrid and electric cars that have high voltage electrical systems. No chip amplifier will handle 200+v (discretes can be designed for it - much to the joy of those who build loud car stereos!) so a DC/DC converter will be used to reduce it. Then the design is more like that of a home audio system. In fact, most universal input power supplies for home electronics will run from the high voltage system in a hybrid or electric car without any modifications.

I remember a friend of mine in high school who turned up the (aftermarket) stereo so loud that the lights were dimming... and then complained that gas is too expensive!

[shovel]

Originally Posted by NiHaoMike

Why couldn't they use TI's hybrid digital technology to improve efficiency? I have done it for home audio:
Amanda Harris: the Prius of home audio - EcoRenovator

Except in hybrid and electric cars that have high voltage electrical systems. No chip amplifier will handle 200+v (discretes can be designed for it - much to the joy of those who build loud car stereos!) so a DC/DC converter will be used to reduce it. Then the design is more like that of a home audio system. In fact, most universal input power supplies for home electronics will run from the high voltage system in a hybrid or electric car without any modifications.

I remember a friend of mine in high school who turned up the (aftermarket) stereo so loud that the lights were dimming... and then complained that gas is too expensive!

Panasonic had a few models when they were still serious about making automotive head units, which used Tripath class D amplifiers, that can be typically 90% efficient or better (compared to ~60-75% efficiency typical from A/B or darlington pair outputs) - then built an inverter type DC-DC supply into the head unit which gave the device rails at +12v and -12v (regulated, regardless of alternator voltage) - tremendously powerful as DIN head units go and would avoid the weight penalty of necessitating outboard amplifiers for truly loud sound.

In any case... there's still really nothing to chase with a regular car stereo. Before anything else I'd suggest the added weight of a big subwoofer is the most significant enemy of fuel efficiency in a car stereo because it is rare for anyone with a serious system to just drive around everywhere with it turned up. When they're not turned up loud, they don't draw much current because of the previously mentioned relationship between deciBels and wattage. If you have a "regular" stereo and are a casual efficiency hobbyist, stop worrying about the stereo right now, it's not the droid you're looking for. If you are extreme about efficiency, rip the whole thing out and wear an ipod because the weight of the parts is affecting your efficiency even when it's turned off.

[NiHaoMike]

Here's some information on the stock stereo system in the Prius:
Prius JBL Audio system response, specs, and photos
Halfway down the page, there's a few (not very good) pictures of the amplifier module. From what I could tell, there's two Analog Devices chips (DSPs? Hybrid audio amp controllers?) and an inductor that appears to be part of a DC/DC converter circuit.