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Basic car radio amplifier questions #5423752 05/08/20 01:01 AM
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joegreen Offline OP
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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?


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Re: Basic car radio amplifier questions [Re: joegreen] #5423755 05/08/20 01:17 AM
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Char Baby Offline
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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.


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Re: Basic car radio amplifier questions [Re: joegreen] #5423767 05/08/20 02:09 AM
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clinebarger Offline
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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.

[Linked Image from i.imgur.com]


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Re: Basic car radio amplifier questions [Re: joegreen] #5423783 05/08/20 03:51 AM
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bachman Offline
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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.

Last edited by bachman; 05/08/20 03:55 AM.

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Re: Basic car radio amplifier questions [Re: joegreen] #5423808 05/08/20 05:49 AM
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Oildudeny Offline
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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.

Re: Basic car radio amplifier questions [Re: joegreen] #5423856 05/08/20 07:11 AM
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Quattro Pete Offline
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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.

Quote
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.

Quote
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.


2002 530i   2015 Q5 3.0T   2018 Charger SRT
Re: Basic car radio amplifier questions [Re: joegreen] #5423859 05/08/20 07:15 AM
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Oildudeny Offline
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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.

Re: Basic car radio amplifier questions [Re: Oildudeny] #5423870 05/08/20 07:32 AM
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wag123 Online Content
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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.

Last edited by wag123; 05/08/20 07:57 AM.
Re: Basic car radio amplifier questions [Re: joegreen] #5423876 05/08/20 07:39 AM
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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/ [i]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.[/i]





Last edited by Cujet; 05/08/20 07:47 AM.

People who count on their fingers should maintain a discreet silence.
Re: Basic car radio amplifier questions [Re: Cujet] #5423877 05/08/20 07:49 AM
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4WD Online Confused
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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 …

Re: Basic car radio amplifier questions [Re: Cujet] #5423882 05/08/20 07:55 AM
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wag123 Online Content
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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.

Re: Basic car radio amplifier questions [Re: joegreen] #5423898 05/08/20 08:23 AM
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ARCOgraphite Offline
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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 !


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Re: Basic car radio amplifier questions [Re: ARCOgraphite] #5423912 05/08/20 08:36 AM
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Cujet Offline
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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.


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Re: Basic car radio amplifier questions [Re: joegreen] #5423927 05/08/20 08:48 AM
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Electrical Vibrator - DC chopper.

Smooth then transform then tube rectify

[Linked Image]


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Re: Basic car radio amplifier questions [Re: ARCOgraphite] #5423952 05/08/20 09:28 AM
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wag123 Online Content
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Originally Posted by ARCOgraphite
Electrical Vibrator - DC chopper.

Smooth then transform then tube rectify

[Linked Image]


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

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