Gain formula?

What is that special math formula for setting gains properly?  And can you describe how everthing should be , like on the HU, and stuff. 

Thanks

So many people have their amp gains set wrong, and to be honest setting them by ear is quite a crapshoot. Doing it properly with a multimeter is actually quite simple.

Here's how you do it:

Set head unit volume to 3/4 of maximum. Turn off all eqs/presets in the head unit.

DISCONNECT SPEAKERS

P = Power in watts
I = Current in amperes
R = Resistance in ohms (effectively the nominal impedance)
V = Potential in volts (Voltage)

Knowns:
Resistance (nominal impedance of your speakers)
Power (desired wattage)

Unknowns:
Voltage (we'll measure this)
Current

Formulas:
P = I*V (formula for power)
V = I*R (Ohm's law)

So after a little substitution to get Voltage in terms of simply power and resistance we get

V = square_root(P*R)

So, for example, say you have a 4 ohm load presented to a 150 watt amp.

V = square_root(150*4) = 24.5 volts

This means you should increase the gain until you read 24.5 volts AC on the speaker outputs of your amplifier.

As a source, use a sine wave recorded at 0db at a frequency within the range you intend to amplify. You can generate tones in cool edit or use a program such as NCH tone generator. (credit for NCH to imtfox, IIRC)

Additionally, you could use a scope to actually check if the signal is clipping, but I'm not going to get into that here as I doubt very few people have access to that type of equipment.

That was very well said, Lizardking.  97Pathy, all the responses above are correct.  In reality, it is a bit of a crapshoot either way.  But it's good to know the theory behind setting gain, and use the tools at your disposal to get the best results.  For one thing, the very first thing you do........setting the head unit volume output at 75%........is always a crapshoot.  There is a fine line there as well as everywhere else as to where clipping begins.  It's hard to hear, and difficult to test for.  In your own car, when you have the time to work on the system from time to time, you may want to go over these steps more than once.  For instance, one head unit may start sounding crappy at 2/3 volume while another is fine to 85%.  Finding that perfect stopping place would result in the best S/N ratio, giving you the best performance from your amplifier.  And keep in mind this is done using the deck's power alone.

Test tones:  a crapshoot if you record them yourself.  Less of a crapshoot if you buy a CD with professionally recorded tones.  A sine wave tone is necessary for a steady reading of volts;  with music sources, you will get every number in the book.

RMS output:  a crapshoot, as well.  Same goes for nominal impedance of the speaker load....close but not exact.

DMM that you are using:  has an internal resistance.  Touch probes to find the reading and add that to the number you are looking for.  My meter reads .8 ohms internally, so I have to consider that when doing precise measurements.

All in all, a good method to use is to use Lizardking's method of using the meter, then... use the 'by ear' method.  You will, remember, have used the 'by ear' method to select the loudest clean volume on the head unit to begin with.   The meter will help you pinpoint a voltage output, and later your ears will tell you if that is correct.  If it doesn't sound clean at the loudest volumes, the amp gain can be turned down (to a less sensitive setting).

https://www.jlaudio.com/tutorials is good visual help for setting gain using the meter.

Poormanq45 wrote:

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"... which is why I always recomend using loudspeakers rated for 2X to 3X the RMS power of the intended amplifier. We call that a safety margin. "

That does not really matter though. If the amplifier starts clipping, I can Guarantee you that you will blow most drivers.

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Amplifier clipping has more to do with the installer and the system setup than anything else (except maybe very poorly made amplifiers)... but if the thermal rating of the loudspeaker voice coil is at least 2X the rated amplifier RMS output, there is no way the amp can "blow" the speaker.  A 100% clipped output (AKA square wave) is 1.414 times the voltage of the full clean RMS output, or approximately 2X the same power.  So, if your loudspeaker is rated for 200 watts and your amp is rated for 100 watts and you bone-headedly overdrive the amp into 100% clipping, the loudspeaker will be driven near its limits and will get hot and will make non-musical noises, but it will not blow.

Now to contradict myself, you could "blow" a poorly made speaker by driving it past its mechanical limits at its max power rating... but that's more a problem with some designs than a clipping issue. 

The most dangerous situation is using an amp with the exact power rating of a loudspeaker (and thinking you are safe) and then not being able to recognize and prevent clipping.  THEN you can toast a loudspeaker pretty quickly.  Of course using an over-powered amp can also destroy a loudspeaker with no clipping at all, but people who intentionally set up systems like this GENERALLY know what they are doing and don't overdrive them.

haemphyst wrote:

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OK, I'll give in...

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Now that'll be the day!   lol   Before I move to strike my testimony from the record, I will state that there are some basic presumptions involved.  Anybody who is so inclined to set his system up accurately (and we know that not everybody is) is presumed to also have first made sure that he isn't setting up a 600 watt amplifier for a 300 watt speaker!  So let's give our setup guy this benefit of the doubt...

...that a lot of people can't tell by ear what they can see on a meter.  Lack of time and experience, if you will...

...and if we also presume that a setup by this RMS voltage output would result in the highest amount of voltage allowed, not a level that can be increased but only decreased if or when clipping or distortion is later detected by ear (as I mentioned earlier...)

.....then if that much can't be allowed in this court, then I reckon I have spent about 14 hours over the past 10 months posting this same information on this forum and nobody told me to quit!

There, that makes me all better.   lol

My specs for those wondering!!

"Eclipse Approaching Perfection"

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The best products for you mobile electronics guys would be the handheld O-scopes. They are comming down in price and easy to use.

The Fluke color triple trace is my favorite, but at $800-$1000 I won't be purchasing that anytime soon.

I have an old school HP dual trace scope and some old school 1960's sound generator. Both of these items put together take up a bit of space, I would need a cart to use them with a car. I use these to test amps inside for now, someday I will upgrade to something more compact and modern.

stevdart wrote:

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haemphyst wrote:

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OK, I'll give in...

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Now that'll be the day!   lol  

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haemphyst wrote:

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...Personally, I use an oscilloscope to discover the clipping point of the headunit, set the gain of the amplifier for the LEAST efficient speaker in the system, running the amp to JUST where the driver starts to complain at the resonant frequency of that driver, then I use an RTA to set the gains on the remaining drivers in the system.

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