Plz help, hpw amplifier work???

[DIGIDIGIBOMBO 10+ year member]

Since the supply voltage is 12V DC, when using a test tone cd etc...on the DMM i can cranck the gain and get like 30-40V AC and the DMM converts them into rms accurately if its a pure sine wave, so 30V AC rms is similar to 30 V DC ( they will give same power...) => i am having an output voltage higher than the supply voltage, how is that???? i know from physics, that energy in= energy out, so how is this? from where this excess power? thx

is it like adding in series the battery voltage supply+ preout voltage of head unit or what, dammmm i can understand

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[JimJ 5,000+ posts]

http://www.bcae1.com

30VAC on the output terminals is no way comparative to the DC input. The amp is taking the low level signal from the head unit (which is AC) and amplifying it, not the DC voltage...

 

[thch 10+ year member]

car audio amps, 2 inputs, 1 output -- 1 DC voltage input, 1 AC voltage input, 1 AC Voltage output.

DC voltage in -- SMPS converts the DC into AC then back to DC (at a higher voltage) -- Amplifier itself takes this high voltage and the input from the RCAs and boosts the signal's voltage.

so amps in car's are DC-AC-DC-AC converters...

in you case, its this Switch Mode Power Supply, or DC-DC converter in the amp that allows the amp to put out higher voltages then the 12V DC input. energy is conserved though. if the output is 120V, 1A, then the input will be at least 12V, 10A*.

If you are very interested in this you can read about how these things work. typically you will store energy in either a capacitor or inductor, then release the energy quickly. an example which is easy understand, but not really useful, is charging 10 capacitors to 12V. now wire all 12 in series. you now have 120V from 12V. all that needs to be done is build electrical switches to alternate the capacitors from the charging (parallel) and discharging (series) cycles. typically these cycles happen 30,000 times or more per second.

*this is conservation of power, which isn't a law, but if power is conserved energy will be conserved. an example of how they are different would be thowing a ball at a window. energy is put into the ball then released at a later time. as the ball transfers energy to the window you are not transfering any energy to the ball.

 

[DIGIDIGIBOMBO 10+ year member]

So basically power P=V*I is conserved, i can increase V but i will loose some current capabilities I or vis versa, but either way the product V*I will remain a constant true? and this is done when i turn the gain button which will decrease the resistance valus inside the amp, allowing more current to be drawn from the power supply, sice the voltage of the power supply will always be the same. I imagine many resistances in series, one of them is attached to the gain button, once button is turned, the resistance decreases, more current will flow, since the remaining resistances are still the same=> more voltage flows thrue them but less voltage thrue the resistance which was decreased, and the sum of all the voltage thru the resistances will always be the same= voltage supply, right?:S

 

[maylar 10+ year member]

Uhh.. no.

An audio power amplifier takes the low voltage high impedance signal from the source (HU) and amplifies both voltage and current (power) to run a low impedance load (speaker). The gain control simply sets the amount of signal being fed into the amp's internal input stage.

As Chris pointed out, most amps have internal power supplies that boost the 12vdc battery voltage up to whatever level is necessary for the desired output level. You mentioned 30V RMS on a DMM at the output, which is 42 volts peak. The internal power supply must provide at least +/- 42V DC to the output stage of the amplifier to get that.

The exception is low power amps like HU speaker outputs. They use the battery power only, but they also use a "bridge" type amp circuit that doubles the amount of output voltage from the battery. With a 14.1 V supply a bridged amp can give about 50 watts peak into 4 ohms which is the rating of most HU's today.

 

[thch 10+ year member]

So basically power P=V*I is conserved, i can increase V but i will loose some current capabilities I or vis versa, but either way the product V*I will remain a constant true? and this is done when i turn the gain button which will decrease the resistance valus inside the amp, allowing more current to be drawn from the power supply, sice the voltage of the power supply will always be the same. I imagine many resistances in series, one of them is attached to the gain button, once button is turned, the resistance decreases, more current will flow, since the remaining resistances are still the same=> more voltage flows thrue them but less voltage thrue the resistance which was decreased, and the sum of all the voltage thru the resistances will always be the same= voltage supply, right?:S

pretty much. Vin * Iin = Vout * Iout of the POWER SUPPLY. most amp manufacturer's fix the Vout at a certain value so the amp can run to full power. some amp called "class H" amplifiers change Vout according to the music being played. some amps like JL RIPS and some PG amps will sense the speakers attached and adjust the value of Vout to allow equal power at any speaker impedance.

you're way of invisioning the amp is a bit simplified. in some ways it is correct. as gain increaes and power output increases, the amp draws more current, thus the resistance effectively seen from the battery decreases. but in terms of actual audio amplification, well, not really correct.

for a better discription of an amp, you need to look at transistors and opamps.