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    The WHY of gain setting

    Car audio gain architecture: the better you understand it, the better you can use it.

    What is Gain?

    The answer to that simple question will shed a lot of light on how your audio system works and how all the individual components interact. In the terms of a signal, which is what we are concerned with, gain is simply an increase in the voltage of the signal. The entire purpose of an audio reproduction system is to take a very low voltage (a few milli-volts) source signal and convert it to a mechanical signal that can be converted back to an electrical signal by your ears. The speakers do the actual energy form conversion, but they require a much higher voltage to operate than is present directly from the source converter. This article will be concerned with what occurs between the source converter and the speaker: Gain.

    What gives Gain?

    We now know what gain is but where do we get it? Every active component in the audio signal chain is a gain device of some sort. They all incorporate an amplifier of some sort and an amplifier is nothing more than a gain stage by another name. In the simplest of terms an amplifier takes a small voltage signal and uses it to modulate a constant voltage source to duplicate the source signal at a voltage different from the source. It does this with a silicon device known as a transistor. A transistor is nothing more than an analog switch. When you think of a switch, the picture that normally comes to mind is a digital switch. It has two positions: fully on or fully off. An analog switch differs from this in that in addition to all on and all off, there are an infinite number of partially on positions in between. Since an audio signal is by definition an analog signal, this is a requirement for a device that can accurately reproduce the signal it is given.

    Every signal must have a source. In the case of you car audio system, the source could be the D/A converter in your CD player, your radio tuner, your satellite tuner, etc… In every case, this source signal is initially of a very low voltage. The first thing it encounters is a fixed gain stage of some sort to bring the signal up to a level usable by the next portion of the signal chain. The quality of the source signal and the quality of this initial gain stage are of utmost importance because any noise or distortion at this point is going to be further amplified by every subsequent gain stage in the system.

    In the most basic of setups, the next stage in the signal chain is the source unit preamp. The preamp is little more than a gain stage with a variable gain. The variability gives you control of the volume. Without it you would have one volume setting on your system and you final signal voltage going to you speakers would be a fixed multiple of the voltage coming from the source. This wouldn’t be terribly useful so some form of variable gain needs to be readily accessible. Enter the preamp volume control.

    In a perfect world, every preamp component would have the same range of output voltage and every power amp would have the same range of voltages needed to get full power out of the amp. The world is far from perfect, as you well know though, so something has to be done to make various pieces of equipment compatible with each other. For just this reason automotive power amps consist of two separate gain stages. The main section is the fixed gain block that takes the signal that it is given and increases it by a fixed amount up to a certain max value. The second is a level matching preamp stage that takes the signal from the previous component and matches it to the range of input voltages that will keep the fixed gain stage within its limits.



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    Re: The WHY of gain setting

    To see why this is needed let’s look at an example…

    The limiting factor on the output stage of a power amp is the voltage of the fixed power supply rails. These are the constant voltage sources that the transistors modulate to create the final output signal. The transistors can’t make power only manipulate it so the max output voltage is the rail voltage. For the sake of argument, we’ll say this is 20V. This means that the peak of the signal cannot exceed 20V (forget about RMS at this point, it really isn’t relevant to this discussion). The output stage is a fixed gain device. This means that it multiplies the signal that it is fed by a fixed amount. For our example we’ll use a multiplier of 10x. What this means is that as long as the peak signal voltage is less than 2v, we don’t have a problem exceeding the capability of the output stage. At the same time, however, if the peak input signal doesn’t approach 2V, there is wasted potential output that we can never tap into. Also if the input voltage exceeds 2V, the output is maxed out at 20V for the portion of the signal where the input equals or exceeds 2V. Where the output stage reaches its voltage limit the signal is said to be “clipped” because the portion of the signal that is beyond the limits of the system to reproduce is clipped off leaving a flat-topped waveform and a lot of distortion. Now insert a preamp stage into the amp between the source and the output. This preamp is meant to take a wide range of input voltages and bring them into the range of voltages that will allow the output to produce full power without clipping.

    The next thing to understand is that any stage of gain can be clipped. It is possible to overdrive a preamp by mismatching settings and clipping the signal before it ever gets to the output stage of the power amp. Once the signal is clipped by any component in the chain, it is going to be clipped through the rest of the chain. The distortion is going to be amplified by each gain stage on down the line. Even you headunit is capable of clipping its output. This is the reason that it is advised that you set the volume on the HU to something other than the max and turn off all boost circuits and loudness circuits before you set the gains for the rest of the system.

    Final output voltage controls the volume of the sound we hear. The peak voltage of the output is the limiting factor in our max system volume. From that point we can turn it down (reduce voltage) but we can’t turn it up any more without problems. Max volume is now your 0dB reference and everything else is a relative decrease from there.



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    Re: The WHY of gain setting

    How does this relate to your audio system?

    You’ve got the theory, now I’ll explain why it matters and what it means in relation to component specs that you see thrown around.

    The over-riding principle that everything hinges on is that the “gain” knob on your amp is there for the sole reason of matching the output voltage of the previous component in the signal chain to the required input voltage of the output stage allowing it to reach full power without clipping. Set it too high, and the output stage will clip. Set it lower than optimal, and the amp will not make full power even at max volume (this can be used to limit the power produced by an amp to match levels between speakers or keep from sending too much power to a speaker).

    Does HU preamp voltage matter? This question gets thrown around a lot and all too often, the answer given is “more is better.” Without any other information, this can’t be categorically stated as fact. A lot of the tests that I have seen for HUs with rated 4V outputs found that the HU simply didn’t make its rated voltage without unacceptable levels of distortion (10% in many cases) and/or without the bass or treble levels maxed out and then it only mad ethe rated voltage at the center freq of the boost. A HU with higher pre-out voltage won’t necessarily sound any better or allow a higher output from the amps. The fact is that the max pre-out voltage is only ever seen with the volume at max with a 0dB sine wave. If the volume is less than max or the level of the recording is less than 0dB (pretty much all music is well below that level) then the spec is meaningless. Combine that with the fact that most all amps will make their rated power with as little as 250mV and you can see that this spec isn’t that important. The only real benefit to a higher voltage signal is allowing the amp gain to be set to a lower setting and increasing the ratio of the signal to the inherent noise floor of the system. With good components, this noise floor is inaudible (the S/N spec is cited referenced to 1W, it increases from there) even with the amp gains maxed. HU preamp voltage is pretty much the last thing I look for in a good source unit. Outputs that don’t clip at max volume are much more important, and much harder to come by. For the purposes of being noise free, low output impedance is much more important than the rated voltage (which you’ll never see anyway).

    Will component X work with amp Y? The whole point behind including an adjustable preamp on the amp in the first place is to allow almost universal compatibility between brands. There are amps out there that cannot handle high input signal voltages, but they are few and far between. There are also HUs out there that boast up to 8V pre-outs. AudioControl processors claim 9V outputs. The fact of the matter is that you have control over all of this. If your HU is really capable of really high output voltage, keep the volume below max. It’s really that simple. Turning down the volume reduces the output voltage. Also, just because your processor is capable of sending 9V through the outputs, doesn’t mean that you need to set it to do so. The key to getting all your components working together is correctly matching levels. At every stage of the signal chain you have control over the voltage it sends to the next component and you also have control over the level of input required to drive that signal gain stage to clipping.

    Do I need a Line-driver? Again a common question. 99% of the time the answer is “no.” A line driver is more likely to introduce noise than eliminate it. It is another component in the signal chain and as such, it will add to the noise floor (noise is additive and what’s the point of trying to overcome noise with a component that is going to add more?), will increase the chances of creating a ground loop (introducing more noise), and won’t allow your amp to make more power than you could by properly matching the gain setting on the amp to the signal from the HU. A line driver is nothing more than a preamp with an adjustable output voltage. It also has an adjustable front end gain to match it to the input from the HU. Another thing to consider when debating the need for a line driver is that most every outboard processor (EQ, crossover, etc…) has a line driver built into it. Adding another is pointless. The only times I would consider adding a dedicated line driver is if I needed to literally drive a line, i.e. the signal cable is unusually long (greater than 20’) and the HU has really weak preouts. In this case I would be more likely to go with a balanced line system than a conventional line driver or install a component with an integrated line driver (EQ) near the HU that would normally be installed closer to the amps. This kind of install very rarely comes along. Another situation is where the signal is going to be split several times via Y-cords. This setup is prone to noise because the ratio of the amp input impedance to the source output impedance is reduced by the Y-cords. The lowered output impedance of the typical line driver can help shift this back in the right direction. Again in this case I would be more likely to use a processor that I need anyway with a built in line driver rather than a standalone unit.

    Hopefully I’ve answered a lot of your questions regarding how gain controls work and why setting correctly is important. This is a living document. I ask for your input to make it better. Any questions?



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    Re: The WHY of gain setting

    bump for educational reasons...

    And now I find that squeak has added it to the sticky, once again, "you da man!"
    Last edited by helotaxi; 01-15-2007 at 01:07 PM.



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    Re: The WHY of gain setting

    When you think all hope is lost for a forum something like this comes up and renews your faith.

    Good read. Good to understand and not just do because your told to.



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    Re: The WHY of gain setting

    What is Gain?
    gain is simply an increase in the voltage of the signal.


    Maybe better wording is;
    In electronics, gain is usually taken as the mean ratio of the signal output of a system to the signal input of the system. A gain of five would imply that either the voltage or power is increased by a factor of five. It has wide application in amplifiers.

    http://en.wikipedia.org/wiki/Gain

    or

    http://www.allaboutcircuits.com/vol_3/chpt_1/4.html
    Because amplifiers have the ability to increase the magnitude of an input signal, it is useful to be able to rate an amplifier's amplifying ability in terms of an output/input ratio.

    It's better to say that gain is a ratio vs. "gain is simply an increase in the voltage of the signal".

    You can have negative gain so it's not always an increase in voltage. There
    are circuits designed to attentuate signals too strong.

    You can delete this post as needed.




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    Re: The WHY of gain setting

    What gives Gain?

    Optimize and trim some fat to make the paragraph lean.

    * You can delete the word 'constant' from "constant voltage source" because
    voltage doesn't need to be contant.

    * You can delete the word 'silicon device' from silicon device known as a transistor.', because there are non silicon transistors.

    * Rephrase this -> "A transistor is nothing more than an analog switch."
    By traditional sense, a switch is a mechanical device for on/off function.
    An analog switch is the semiconductor version of the mechanical device for
    on/off function. A transistor is much more than an analog switch.

    A transistor is a semiconductor device that uses a small amount of voltage or electrical current to control a larger change in voltage or current.

    http://en.wikipedia.org/wiki/Transistor

    * You can delete the word digital to remove confusion. "When you think of a switch, the picture that normally comes to mind is a digital switch. It has two positions: fully on or fully off. " .... we don't call ordinary mechanical switches
    digital switches, we just call them switches.

    * re: "The preamp is little more than a gain stage with a variable gain".
    Another design is where the preamp is fixed in gain and the volume knob is
    the attenuator {could be input or output side} giving the illusion that it's a gain control.




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    Re: The WHY of gain setting

    The sad thing is that the people who need to read it, won't, and the people that don't need to read it will nit-pick it to death.



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    Re: The WHY of gain setting

    Quote Originally Posted by Immacomputer View Post
    The sad thing is that the people who need to read it, won't, and the people that don't need to read it will nit-pick it to death.


    Repost - page 3

    "This is a living document. I ask for your input to make it better. Any questions?"

    The document has potential, but some fat needs to be trimmed to make
    the document leaner to keep the reader interested. Maybe some p0rn
    pics in between paragraphs /jk

    I didn't analyze the whole doc either. Lettuce see where this goes.




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    Re: The WHY of gain setting

    That wasn't an attack on you nor was it really even directed at you. Either way, my comment is true and I really think it needs to lose a little weight or maybe have some **** added to keep the reader's attention.



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    Re: The WHY of gain setting

    Sorry I write big...

    Thanks for the inputs. I'll go back and edit it now. I wrote it over a week ago and the exact verbiage is no longer fresh in my mind.

    thylantyr-while everything you've mentioned in relation to actual content is correct, the article is meant to give a basic primer. Whole textbooks are written on the topic that I'm just trying to give a basic intro to for the newbies. Covering every possible circuit desgn is beyond the scope of the article and would add only to confuse the intended audience.



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    Re: The WHY of gain setting

    wow, very nice write-up

    thanks alot for it



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    Re: The WHY of gain setting

    IMO the easiest way to understand why Gains are important to adjust correctly is probally a video example/analogy, it will actually show whats the difference:

    If you are into video production(like myself) you will probally understand this a lot better.

    When using a camera, ex. at night, and have the gains set properly on it you will notice a nice sharp image, you can tell even better after you capture it to a computer and look on it in fullscreen, although it will be a bit dark since you are filming at night. Now take the same camera and same time of night and turn the gain up so you can see its brighter now. You probally wont be able to tell on the display but when you capture it you will notice how much "Grainier" it is. You loose that quality, in result a crappy video quality that no one would ever really want.

    Now there are ways to use the proper gains and still get great brighter(like audio quality) video. First thing to do would be add more lights if possible, that would be like adding/setting crossovers & eqs. More light at night will have better image because the camera can take in more light to see whats going on without boosting the gains.

    so having a grainy video is like having a clipped audio signal.



    ~if that example/analogy is confusing then im sorry.~



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    Re: The WHY of gain setting

    im sorry, what is Gain again??































































    just kiddin



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    Re: The WHY of gain setting

    I don't mean this the way it sounds...

    Can you have cliffnotes? I read through that, but honestly, most people who need to know why you set gains properly and the repurcussions that can happen if you don't won't read this.

    Just a simple:
    Why: To match signals
    What it does if set too high: Shorten amp's life.

    ^^not spot on to what you would write, but just an example.

    Other than that, great write up. I suggest this be moved to the rest of the stickied Amp FAQ thread.



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