Gain formula?

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97Pathy

Member - Posts: 46

Joined: December 10, 2004

Location: Canada

Posted: December 16, 2004 at 8:43 PM / IP Logged

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

Poormanq45

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Joined: October 27, 2004

Location: United States

Posted: December 16, 2004 at 8:52 PM / IP Logged

First, put in a CD with music that you usually listen to, or turn on your favorite radio station(it's always best to tune to your own preferences)

I usually turn the HU to 75%~85%(never any higher the HU may start clipping), with the gain on the amp turned all the way down.

And then you turn the gain up slowly, until you start to hear distortion.

Then you turn the gain down slowly until the distortion goes away.

That's about it.

Oh, and if you are tuning the gain on an amp for a sub, I would suggest playing a CD with alot of bass in it after you finish tuning the gain. If you notice any distortion, turn the gain down until it goes away.

97Pathy

Member - Posts: 46

Joined: December 10, 2004

Location: Canada

Posted: December 16, 2004 at 9:20 PM / IP Logged

Thats all fine and dandy if you like hunting in the dark, , but theres an actual formula i have seen on here a while back. Im looking for that. It has something do with measureing the voltage on the speaker outputs.

Poormanq45

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Joined: October 27, 2004

Location: United States

Posted: December 16, 2004 at 9:40 PM / IP Logged

Oh, lol. I thought you just wanted to know how to set the gain Gain formula? -- posted image.

I havn't really heard of a specific formula for setting the gain. The problem is that each individual amplifier may put out different voltages at equal power.

Well, I do know one thing that measuring the Voltage of the speaker outputs will tell you. You can find out when/if the amplifier starts/is clipping, as there will be a large voltage spike/drop Gain formula? -- posted image.

haemphyst

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Posted: December 17, 2004 at 12:40 AM / IP Logged

No such thing as a "formula"... 'cause if you set an output voltage for the RMS power, it may actually be overpowering the driver attatched to the amp outputs... example - say you have an amp rated 100 watts, which is 20 volts RMS. Let's say you have set the output voltage to 20 volts RMS, but you now attach a speaker rated only 50 watts... what's gonna happen? You're gonna BLOW THAT SPEAKER, if you try to run even CLOSE to the rated output of the amp - leaving you with the question - "But why did it blow? My DVM said it was set correctly..."

Take the above method - you'll be safer in the long run, because you will also be able to hear the speaker complaining, in case that might happen first... If you do not have an oscilloscope to actually VIEW when the gain stages are clipping, don't try using a DVM... you will NEVER be able to set the gain correctly with a DVM. Granted, you can set the output for the maximum RMS output of the amplifier, but that tells you nothing.

Using the front-to-back method of setting gain will also allow you the best possible signal to noise ratio, and maintain the minimum distortion through the system's entire signal path.

It all reminds me of something that Molière once said to Guy de Maupassant at a café in Vienna: "That's nice. You should write it down."

Lizardking

Copper - Posts: 184

Joined: August 23, 2004

Posted: December 17, 2004 at 12:54 AM / IP Logged

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.

kklagge

Member - Posts: 41

Joined: November 19, 2004

Posted: December 17, 2004 at 7:48 AM / IP Logged

So, if I understand what you just posted Lizardking (do you post on Ecoustics under a different name...this post looks somehwat familiar)...

If my amp will do 150 per channel RMS and I'm using 4ohm components I want to set the gains so that EACH speaker output will read 24.5 volts (will there be any slight difference between the 2 speaker outputs?)

Also, my sub amp will do 450 per channel @1ohm so I would need to do the same routine for that but I'll want to read 21.2 volts AC from those speaker outputs.

Is this correct?

stevdart

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Posted: December 17, 2004 at 7:52 AM / IP Logged

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).

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

stevdart

Platinum - Posts: 5,816

Joined: January 24, 2004

Location: Pennsylvania, United States

Posted: December 17, 2004 at 7:56 AM / IP Logged

kklagge: Yes, you are correct: each set of speaker terminals. And yes there usually is a slight difference between the two. For sub amp, the set of speaker terminals that the sub is connected to.

kklagge

Member - Posts: 41

Joined: November 19, 2004

Posted: December 17, 2004 at 8:01 AM / IP Logged

So if there is a slight difference in the 2 speaker terminals then set the gain so that neither one goes over that 24.5 volts?

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