Basic car radio amplifier questions

[joegreen]

Let's say an car stereo amplifier is 50w rms. When I turn on the radio does the amplifier always deliver 50w to the speakers or does it deliver 0 to 50w linearly depending on whether the volume knob is at a minimum or maximum?

Also does the amplifier amplify the power it's being fed? An example would be the stock head unit supplying 10w to the amplifier and the amplifier amplifying the voltage an additional 50w for a total of 60w delivered to the speakers. Or does it not matter how much wattage is delivered to the amplifier it will only ever output a max of 50w to the speakers even if say 60w is being inputted into it?

 

[CharBaby]

I used to know this stuff quite well but not anymore. I believe that the amps power will increase with volume. So, NO you don't get all 50w just by turning on the unit. You increase to that max. 2nd question I can't answer accurately.

 

[clinebarger]

Generally....The input to the amplifier is RCA cables/jacks which carry very little amperage/wattage, Not the speaker driver/s out of the head unit which are powered by the amplifier built into the head unit.

The power output out of the Amplifier is linear to volume.

This is the 5 channel Amp in my Camaro, The RCA's on the right are the inputs from the head unit.

 

[bachman]

Yeah, just as the others said, volume knob on the head unit is the accelerator so to speak. Amps are rated to a max power just like home stereo receivers or amps. For a time in my younger years and selling audio systems / equip, I had a car amp as my home set up. Kenwood 70 x 2

As for the head unit, the output to the amp may be configured as RCA or just runs from speaker output to the amp first then to speakers. Note- If you have a so-called high output head unit, the feed may too high for the amp. I believe it's called input impedance. If so, there can be mis match *unless* the amp has a variable level control that allows you to match input. Generally speaking, the power of your system will not be the head unit + the amp. They are designed to be one or the other. The exception could be a unit with both RCA and speaker wire output. In that case, it's usually RCA IF to you go to adding an amp and might be the head unit (speaker wires output) power to the front speakers if your amp is only a two channel amp where it would power a pair of rear only. Noise levels or sources and other types of unwanted distortion are easily magnified by amplifier power so they design these things to be "discrete" and that's is the way you want to set it up for optimal results. BTW -- If you do suffer from noises like whining sounds raising with motor rpm's or ? There are noise suppressors than can be wired in.
Can you detail a bit of what you are using or have ?

In my youngster days, I sold home and car audio equip. Car stuff- Kenwood and Alpine were the easiest to work with and very trouble free in those days.
The last two after-market set ups I had was a Kenwood head unit and a Kenwood amp in my company car late 1980's to early 90's then in my Jeep about 10 years ago.

 

[Oildudeny]

Stereo/car amps only amplifies the sources input via volume and or gain control knob. Amplifiers don't amplify the 12v power supply feeding them.. People do install caps to maintain peak voltage/power to the amp especially on amps driving subwoofers.

 

[Quattro Pete]

Originally Posted by joegreen
Let's say an car stereo amplifier is 50w rms. When I turn on the radio does the amplifier always deliver 50w to the speakers or does it deliver 0 to 50w
It's somewhere between 0 and 50, depending on the position of the volume knob, the voltage of the source signal, and the impedance of your speakers. Power (in Watts) is just a calculated value.

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linearly depending on whether the volume knob is at a minimum or maximum?
Most volume potentiometers are actually logarythmic (not linear) in nature to account for the way human hearing works.

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Also does the amplifier amplify the power it's being fed? An example would be the stock head unit supplying 10w to the amplifier and the amplifier amplifying the voltage an additional 50w for a total of 60w delivered to the speakers.

No. The amp amplifies line level signal (before amplification), assuming you have it connected correctly. You should be using line level (RCA) outputs from your stock head unit, and not amplified speaker level outputs in order to connect to an external amp.

 

[Oildudeny]

Another thing there's a difference between peak and rms wattage. If an amp states 50 watts per channel that's going to be at peak..

Speakers are rated at usually 1watt for the decibels they're capable of producing. So a high end car speaker with a db of 90 plays 90 decibels at 1watt it's how much amplification and gain that makes the speaker play louder.

Long story short the speakers do not really get the amplifiers max wattage.. The amplifiers circuitry would clip out before that.. Also you wouldn't have much for speakers left.. It doesn't Sound like much but 50 watts is really loud.

 

[wag123]

Originally Posted by Oildudeny
Amplifiers don't amplify the 12v power supply feeding them.
Not true. Many amps actually DO amplify the 12v power supply voltage. When using only 13.8v, an amplifier's theoretical upper output limit of what can be delivered into a 4 ohm speaker in a single channel is about 20w RMS at 1%. The 4 ohm speakers limit the amperage of what the amplifier can deliver, so any increase in an amplifier's power output above this level has to come from increasing the available voltage (OHMS law). Higher power amplifiers have a built-in switching power supply that increases (amplifies or steps-up) the 12v input voltage by several times, producing up to 100 volts in some very high power amplifiers.

 

[Cujet]

Some of the above is not entirely accurate.

Automotive 13.8V systems, a conventional amplifier (linear gain device) , coupled with 4 or 8 ohm speakers result in a situation where only about 8 watts (or less) of true RMS (root mean square) power can be "pushed" through a speaker (remember the speaker has resistance) . This is why most older "head units" and "OEM radios" had such poor power output. One reason for the poor sound quality. It's also why your home stereo, powered by 120V, can drive speakers with plenty of power.

Fast forward to the development of the MOSFET (metal oxide semiconductor field effect transistor) and we can now build a simple switching power supply. One that steps UP the cars 13.8V to 28V or more. That higher voltage is needed to "drive" the speakers at higher power. Note: When "cranking it up" that step up in voltage 12 to 28V does put a heavy drain on the automotive electrical system, hence the very large cables required for "big" amps.

Couple that with today's more efficient amplifier designs, and we have a situation where we now have plenty of power to drive speakers.




Quote: https://soundcertified.com/what-does-car-amplifier-do/ A small, very low-level signal is input from a stereo and amplified to a much bigger - but identical - electrical waveform. This way it can offer lots of power and drive speakers with good volume. The +12V supply is "stepped up" (increased) to drive higher voltage to the output transistor stages.

 

[4WD]

And big capacitors and so forth … my son has left behind a battery graveyard despite upgraded alternators, you name it.
Finally convinced him to install an isolation switch for when he's not going to drive for a few days …

 

[wag123]

Originally Posted by Cujet
Automotive 13.8V systems, a conventional amplifier (linear gain device) , coupled with 4 or 8 ohm speakers result in a situation where only about 8 watts (or less) of true RMS (root mean square) power can be "pushed" through a speaker (remember the speaker has resistance).
Correct, when referenced to ground, but the output can be doubled at the same input voltage by bridging the amplifier's output.

 

[ARCOgraphite]

Let's see who remembers how the engineers got 100V + out of a 6V DC system to run tube radios in the 40's and 50's.

I see you are looking it up !

 

[Cujet]

Originally Posted by ARCOgraphite
Let's see who remembers how the engineers got 100V + out of a 6V DC system to run tube radios in the 40's and 50's.

I see you are looking it up !


I'm old, but not that old.... Do tell.

 

[ARCOgraphite]

Electrical Vibrator - DC chopper.

Smooth then transform then tube rectify

 

[wag123]

Originally Posted by ARCOgraphite
Electrical Vibrator - DC chopper.

Smooth then transform then tube rectify

Yep, this, used in conjunction with a step-up transformer.

 

[supton]

Originally Posted by ARCOgraphite
Let's see who remembers how the engineers got 100V + out of a 6V DC system to run tube radios in the 40's and 50's.

I see you are looking it up !

Could also use one of those gensets, 6V DC motor to spin a generator. I think the military did that for some of their radio gear; I recall working at a place that did that for changing power required for some piece of machinery (3 phase to single? opposite way? dunno).

These days you could that in a smaller and more robust manner with an SMPS. Bit of work but it's out there. But I'm not sure why, not sure why I'd want to run tubes in a car (for what good reason?).

 

[Oildudeny]

Maybe regulate the voltage but not amplify

 

[supton]

Originally Posted by joegreen
Let's say an car stereo amplifier is 50w rms. When I turn on the radio does the amplifier always deliver 50w to the speakers or does it deliver 0 to 50w linearly depending on whether the volume knob is at a minimum or maximum?

Also does the amplifier amplify the power it's being fed? An example would be the stock head unit supplying 10w to the amplifier and the amplifier amplifying the voltage an additional 50w for a total of 60w delivered to the speakers. Or does it not matter how much wattage is delivered to the amplifier it will only ever output a max of 50w to the speakers even if say 60w is being inputted into it?

Putting 60W into might cause it to pop... But yeah, there is a point where it simply cannot output more power. You risk overdriving the input and causing distortion (or the output may distort when it attempts to drive beyond what it can do linearly).

The amplifier should be a linear amplifier. Feed it a small signal and it simply gains it, with a fixed gain. Technically low level signals are a power, albeit a low power level. So it's a power gain, or power multiplier, if you'd like.

The amplifier output should not be a fixed power output. Listen to a symphony and you'd hear from milliwatts (during the soft sections) to watts as the music swells. Power drawn by the amplifier will mirror that which is delivered to the speakers, divided by the efficiency of the amplifier (meaning, it will draw more power than it delivers, amplifiers have heatsinking because they are not 100% efficient). During soft music, low power is required--but hit a deep bass note and lots of amps may be required. This is where sub-optimal wiring can be problematic, as the voltage drop from the wiring can modulate the music.

 

[Dave9]

Originally Posted by joegreen
Let's say an car stereo amplifier is 50w rms. When I turn on the radio does the amplifier always deliver 50w to the speakers or does it deliver 0 to 50w linearly depending on whether the volume knob is at a minimum or maximum?


Not quite linearly, but on a curve approximating human hearing sensitivity, and not always delivering 50W, only outputting based on the signal level multiplied by the gain of the amp.

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Also does the amplifier amplify the power it's being fed?


Sort of. Any separate power amp is going to boost the vehicle 12V system to give it enough rail-to-rail headroom to amplify the audio signal by the amount of the gain set. For example if you have a line level input signal of 1V and a gain of 20, it will have to have 20V to work with to get that done or else it'll clip trying to. Really it needs a little more than 20V because of voltage drops in the process. 20V is just an example, most amps boost the power rails to higher voltage than that.

So, it boots the input voltage, then modulates that voltage to produce the AC output mirroring the input signal. Any voltage above and beyond what it needs to reproduce that signal is not used. In a class A amp that is very lossy, somewhat lossy at lower volume on a class A/B but for a typical class D amp it is pretty efficient.

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An example would be the stock head unit supplying 10w to the amplifier and the amplifier amplifying the voltage an additional 50w for a total of 60w delivered to the speakers.


What a stock head unit can produce is only relevant powering a load, the speaker(s). The amp uses a low level input whether it is already low level at the input to the amp or whether it uses a method of reducing the input signal to that level (often just a resistive voltage divider). So, the head unit may be able to supply 10W but the amp is not consuming that wattage, rather a small fraction of a watt.

No, these two values, input wattage and amp wattage do not add together. The amp wattage alone is the maximum output and even that, typically at a very high THD distortion that you won't want to be anywhere near for how bad it sounds.


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Or does it not matter how much wattage is delivered to the amplifier it will only ever output a max of 50w to the speakers even if say 60w is being inputted into it?
See above, there is never 60W truly input and consumed even if the head unit would've been capable of and been delivering that if it were a speaker attached instead of the amp.

It might have been easier to get to the result you want by stating what parts you are using (or still need to select) and what the intended result is. I am not a fan of deafeningly loud music, but I also wouldn't bother putting a mere 50W amp into service unless it is just to power a small sub or to repair a failed factory audio system that had adequate loudness already. Unless it's some boutique audiophile brand, low powered amps tend to have more corners cut than just their output wattage in order to arrive at a low price point, although if it is rated at only 50W, at least it is probably closer to honestly rated instead of the fictional high wattage claims that some dubious manufacturers make.

 

[danez_yoda]

OP,
What exactly are you trying to do? Then you can get some very pointed advice on the proper way to do it. There are generally more factors than just add an amp and crank it up?

For modern factory stereos, the amp in the head unit will generally be ~ 15W "RMS"

< EE sidebar: RMS applied to volts and amps NOT POWER. V(RMS) times I(RMS) does not equal Watts (RMS) The proper term is continuous power. For for those who can't accept that because the industry says otherwise...OK RMS it is. The industry uses RMS power as an amount of power that is easily measured over several seconds. Peak power is generally madeup for marketing claims and should have Zero impact on your buying decision on amps. >

The speakers that come with factory card are only designed to handle the amount of power the head unit puts out. This keeps the cost down. if you drive them with more power, they generally will distort and eventually blow.

Let us know what you are thinking about doing and we can give good guidance.