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# Car stereo amplifier output ratings

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Hello all,
Looking for a car receiver or amplifier and saw 200W? Well it has 4 channels so each one delivers 50W, right? Maybe, but what kind of watts?
If you measure a car battery to be 14.15V when charging, then use it to power an amp that incorporates the very latest low-saturation FETs, then the power output is 14.15 squared divided by the 4 ohm load = 50W peak-to-peak. But REAL power (light bulbs, toasters and name-brand home amplifiers) is measured using RMS voltage, not peak-to-peak. So the RMS power per channel could be as low as 14.15V peak-to-peak divided by 2 = 7.075V peak, converted to RMS by dividing by 1.414 = 5.0V, squared, then divided by 4 = 6.25W RMS per channel.
The manufacturer does use a good method to boost the power by using 2 amps per channel connected as a bridge which effectively doubles the RMS voltage. So each channel delivers 5V X 2= 10V squared, then divided by 4 = 25W RMS or 50W peak, as the label says.
In the olden days, a store catalog said:" Our 240W stereo console actually provides 6W RMS continuous power per channel, with both channels operating, from 20 to 20,000 cycles per second, with less than 1% distortion, and into 8 ohms" But their marketing people saw: instantaneous (before the power supply sagged) peak-to-peak voltage (phoney double-double) with only 1 channel operating (so that the power supply is extra high), at only 1K cycle (since the amplifier could not provide much power at 20 or 20,000 Hz), with a horrible amount of severe distortion (but extra numbers), and into a very low resistance (lots of current, before it blew-up).
And I see for sale a tiny stereo amp, made in a country that I never heard of, that has printed on it "1000 Watts". But since it has only a 1/4A fuse, I know that 1/4A X 14.15V = 3.5W input, X 0.7 amp efficiency = 2.45W for both channels. So they are using the store's marketing numbers-crunching AND they left-out the decimal point(100.0). Maybe that's why the small print says to use only 32 ohm headphones, not speakers.

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You have to be careful on this type of generic voltage calculation as some high powered amps are using DC to DC converters and are actually running on 35 to 45 VDC. This changes the calculation considerably. Although it still runs from a car battery at 14 VDC, the amp is running on 35 or 45 VDC.

MP

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And I see for sale a tiny stereo amp, made in a country that I never heard of, that has printed on it "1000 Watts". But since it has only a 1/4A fuse, I know that 1/4A X 14.15V = 3.5W input, X 0.7 amp efficiency = 2.45W for both channels. So they are using the store's marketing numbers-crunching AND they left-out the decimal point(100.0). Maybe that's why the small print says to use only 32 ohm headphones, not speakers.

Take a look at:

http://sound.westhost.com/power.htm

for a discussion of the wacky world of power measurements for the purpose of advertizing. :)
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Thanks, Sec, for that excellent link. I've seen the site but not that article before.
Maybe the advertisers should spell Watt as What.
There is another link somewhere that examines the cheap junk that is built into a well-known and expensive home surround-sound sub-woofer and amps with satellite speakers system. Designed by a doctor (with a PHD or is he a Veterinaryist).
You don't always get what you pay for.

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MP, you are absolutely correct. Some of those monster sized and heavy weight car amps produce very much power by drawing up to 100A from the battery, stepping-up the voltage and using high-power circuits with many power transistors. But others cheat the numbers by stating a non-standard 1 or 2 ohm load. They attempt to give tons of power into a dead short but much less into a standard 4 ohm speaker. Those amps are for the boom-boom cars with the trunks (bonnets?) full of woofers.
My post referred to the factory-replacement receivers that are all rated at between 45 to 50 Peak Watts.
Soon some day most amps will be compact class-D (switching) circuits, for those of us with tin ears.

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• 3 weeks later...

I use to sell these Amplifiers at the Flea-markets. It is a subject I hav had many conversations about.
I saw this tiny Equalizer with 400 watts printed on the case.
This thing was only about 1 inch thick and it weighed about 16 ounces.
No way could this thing put out 400 watts. Just for fun I bought it for about \$10.
I took it home and opened it up. I looked up the audio output IC's inside and Guess what?
Each IC was 20 watts Peak Power.
I thought to myself How did they arrive at the 400 Watt Rating Printed on the case?
20 watts + 20 watts = 40 Watts x 10 bands on the Equalizer = 400 watts.
They were wrong of coarse but that is how the CHEEP GUYS do things.
The only way to find out what an Amp really puts out is to look up the parts in the power supply and output also see what size wire and fuses the manufacture uses.
Take the fuse size and use Ohms Law to figure out the rating Watts Used Ect.
Example 12vdc x 20 amps = 240 watts Peak Power Max.
240 watts/ 2 channels = 120 watts Peak Power Per Channel.
120 / 2 = 60 watts per channel Approximate.

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Hotwaterwizard,
Do you remember when transistor AM radios were invented and most had 6 transistors? Well one brand had engraved on its case, "14 TRANSISTORS". I opened one up and counted, 7 for the radio (the 7th was used as a detector diode) and 7 connected together in a neat circle doing absolutely nothing!
Today, many portable boom-box radio and tape or CD units claim 3-way speakers. A woofer, a tweeter and a label instead of a super-tweeter. Sometimes they use a peizo whistle for a tweeter (sounds real bad).
The new class D amp chips put out a lot of power from a surface-mount chip, without a heatsink. They don't sound very good yet so I will use one for DC motor speed control since they use PWM and the high-current Mosfets are built-in.

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Remember the old 150L Zapco Amps?
They had a seperate PWM in a seperate case.
Those Amps sounded Great.
And Guess what?
They didn't use Mosfets.
Why did they stop building Amplifiers out of Discrete Transistirs anyways?
I just don't get it.
Everyone claims that Mosfet is better than Bipolar.
Yea, take a static sesitive device and stick it into a power supply that has stray electro-magnetic fields all around and expect it to work better than something that doesn't need a Static wrist band to replace.
How can it be better if just a touch can fry it in a second?

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The Mosfets works more like a vacuum tube than the bipolars, maybe they want the tube sound. The vacuum tubes are also sensitive devices..... if you drop them! ;D

Ante ::)

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Hotwaterwizard,
Mosfets beat bipolars because they are much more efficient when switching high currents. A mosfet has an extremely low on-resistance so that the load gets the most power, and they switch very quickly so that they spend very little amount of time dissipating power.
There is a very high power (2kW) inverter project (I think for a telco) where the author recommends testing all (there are many) of the mosfets so that the project won't explode if it had a bad one:
1) A 12VDC source and a lightbulb load. Without any connection to the mosfet's gate, the lamp should not light.
2) With one hand, hold the voltage source. With the other hand, touch the gate. The lamp should light. Never mind using a resistor!
Many power mosfets have static protection built-in.

Ante,
You can make a solid-state amp produce "tube sound" by adding the same amount of 2nd harmonic distortion and soft-limiting.

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I just don't like anything that is static sensitive. I tried repairing many Amplifiers with MOSFETs and I think they are junk.

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...almost everything on the motherboard of the computer you typed that on is static sensitive... ;D

Just a little humor there.

MP

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;D I remember once repairing a SONY VCD Component System with a HUGE label outside "4700 Watts (PMPO)"!! ;D
looking at the back saw the label " MAXIMUM POWER CONSUMPTION: 240 WATTS"
I can't figure out how they arrived at that value??
;D

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• 6 months later...

What's even funnier is that it doesn't matter what claims are made as far as amplifiers and electrical watt ratings. When it is musical watts that we hear. And musical watts at normal listening volumes are way low.
P.S. I have one thing to say about discrete transistors vs. mosfets. And that is.......slewrate.

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Hi Matthew,
There is no such thing as a "musical watt". Music varies considerably in volume, frequency and time duration at maximum level. Frequently there are moments in music where many instruments' output phases are all "in-phase" creating a massive output level. Do you want the amp to clip and severely distort during those moments? Or resort to the problems of having a compressor?

Amplifier output power must be rated in volume (output power into a standard load), frequency and time duration (RMS continuous power) so that they can be compared properly and so that you can determine that one will supply enough power to meet your volume/load requirements.

There are many good audio amplifiers using "low slew-rate" bipolar transistors. Mosfet amps are better if properly designed but can you hear the difference?
There is a point for audio amp design where if you double the complexity and price, there is hardly any improvement. But if you half the complexity and price, it degrades a lot. I like to be at that mid-point.

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