Setting my gain
Printed From: the12volt.com
Forum Name: Car Audio
Forum Discription: Car Stereos, Amplifiers, Crossovers, Processors, Speakers, Subwoofers, etc.
URL: https://www.the12volt.com/installbay/forum_posts.asp?tid=34123
Printed Date: May 03, 2024 at 8:29 PM
Topic: Setting my gain
Posted By: chevyman26
Subject: Setting my gain
Date Posted: June 18, 2004 at 11:20 AM
Ok I did the search on settin gain, went through JL's tutorial, but I'm still not sure. I have a Xtant 121m mono running it at 250W at 2 ohms. According to E=(P*R)^.5 the gain should be at 22.36? This is extremely low on the dial, like around 1/4, and I have to really turn up the bass to hear it at this setting. Can't be right can it? Alpine CDA 7893 Xtant 121m IDQ10D.4 V.2 ------------- You'd better get me out of this lord... or else you'll have me to deal with. -- Hunter S. Thompson "F.A.L.I.L.V."
Replies:
Posted By: DYohn
Date Posted: June 18, 2004 at 11:22 AM
That would be correct. ------------- Support the12volt.com
Posted By: chevyman26
Date Posted: June 18, 2004 at 11:28 AM
huh...
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You'd better get me out of this lord... or else you'll have me to deal with. -- Hunter S. Thompson "F.A.L.I.L.V."
Posted By: shaman
Date Posted: June 18, 2004 at 3:25 PM
It is my understanding that the gain is more dependant on the source voltage (your HU), so if you have a high quality HU (you elcipse owners out there know this) your gian will be very low. An amplifier can only boost voltage to the point where it begins to clip/distort/overheat, so if the source is higher voltage like 8V or something your gain wold be half that of a HU with 4V pre-amps and still have the amp output at maximum. (feel free to correct me if I'm wrong here.)
Posted By: DYohn
Date Posted: June 18, 2004 at 3:35 PM
shaman wrote:
It is my understanding that the gain is more dependant on the source voltage (your HU), so if you have a high quality HU (you elcipse owners out there know this) your gian will be very low. An amplifier can only boost voltage to the point where it begins to clip/distort/overheat, so if the source is higher voltage like 8V or something your gain wold be half that of a HU with 4V pre-amps and still have the amp output at maximum. (feel free to correct me if I'm wrong here.)
You're not wrong. The input gain is designed to match the sensitivity of the amp to the voltage output of whatever is driving it (like the head unit.) The way to set it using the voltage method is, basically, to set the head unit to its max non-clipping level and adjust the input gain on the amp until you are measuring the the proper voltage output for the max RMS rating into whatever your speaker load might be. In the example above, 250 watts delivered into 2 ohms requires an output of 22.36 volts. ------------- Support the12volt.com
Posted By: stevdart
Date Posted: June 18, 2004 at 5:37 PM
chevyman26 wrote:
According to E=(P*R)^.5
I understand the formula...the voltage would be the square root of power in watts X resistance in ohms...but what does the .5 in the formula above represent? ------------- Build the box so that it performs well in the worst case scenario and, in return, it will reward you at all times.
Posted By: DYohn
Date Posted: June 18, 2004 at 5:41 PM
it's a way of indicating square root. The ^ means "to the power of" whatever number follows. 0.5 means square root. ------------- Support the12volt.com
Posted By: shaman
Date Posted: June 18, 2004 at 5:57 PM
The power output form an amp is P=I^2*R Were I, the current pushed through the sub is, I = E/R substituting E in the first equ. gives P = (E/R)^2*R So then P = (E^2 )/R and, E = (P*R)^.5 or the square root of the desiered power output ( not always the maximum printed on the amp) times the impedence of load. All terms above are RMS vales For my setup the speaker had a DC resistance of 3.1 Ohm, but an AC impeadence of 6 Ohm, so my 400W 4 channel amp is making the 40V it should but only producing 266W of power.
Posted By: shaman
Date Posted: June 18, 2004 at 5:58 PM
Scratch that ... its a 4 Ohm 2 channel.
Posted By: stevdart
Date Posted: June 18, 2004 at 6:07 PM
lol, quit, dude, you're making me scratch a hole in my head...
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Build the box so that it performs well in the worst case scenario and, in return, it will reward you at all times.
Posted By: DYohn
Date Posted: June 18, 2004 at 6:09 PM
shaman, the DC resistance of the voice coil is N/A to power computations, since musical energy is AC. You have to use the nominal impedence of the driver. If it's 6 ohms in your case, then that's what you plug into the formula. ------------- Support the12volt.com
Posted By: shaman
Date Posted: June 20, 2004 at 12:24 PM
Yup thats what I did, but I had to determine the AC impedence first. I orginally assumed it was 4 Ohms (we all know where assumptions lead) but I was measuring an output of 260 to 270 watts with test tones. MY amp should bridge to 400W on one channel and it should actually make 400W (PPI). When I devided the current into the voltage I got 6 Ohms, problem solved, amp IS working right. (266*6)^.5=40 400W amp at 4 Ohm load should make 40V. The moral of the story, don't assume things. I do have a question though, is 6 Ohms common, and when most speakers say 4 ohm or 8 ohm, is that impedence, or are they cheating by giving DC resisatnce? My sub is an eclipse SW8102.4
Posted By: DYohn
Date Posted: June 20, 2004 at 12:56 PM
6-ohms is very common. Manufacturers list nominal impedence, or in some cases impedence at a fixed frequency. 4-ohm rated woofers actually tend to vary from around 2.5 to around 20 ohms as frequency changes. Here's a good discussion of voice coil impedence behavior if yu're interested. ------------- Support the12volt.com
Posted By: DYohn
Date Posted: June 20, 2004 at 1:47 PM
Posted By: jfk51502003
Date Posted: June 20, 2004 at 4:17 PM
There have been many people who have said that the gain controls were not volume controls and they are right, to a point. Some amplifiers' gain controls are used precisely like a volume control (one end of the potentiometer connected to ground, the other end connected to the pre-amp signal and the wiper connected to the amplifier's front end). This configuration will allow you to reduce the output to nothing at the minimum gain position. These are not very common but they HAVE been used on some amplifiers. I know because I took the cover off of a few amps to see why they had absolutely no output (Doh!). Others are connected similarly but there is a small amount of resistance between ground and the formerly grounded terminal of the potentiometer. This small resistance prevents the gain control from reducing the output to zero output. These are very common on amplifiers made in Korea and China. There are other amps that use the potentiometer to pull the signal toward ground. The pot is the lower half of a voltage divider and may use only 2 legs of the potentiometer. This type of gain control was used on at least one brand of Japanese manufactured amplifier. You can also put the potentiometer in the feedback loop to control the overall gain of the amplifier. The point to all of this is... There are many ways to use a potentiometer to control the output of the amplifier. Some are used precisely as volume controls and others are not. You can not make a blanket statement such as 'gain controls are not volume controls'.
Posted By: DYohn
Date Posted: June 20, 2004 at 5:51 PM
OK, how about this for a blanket statement: input sensitivity controls are not volume controls. And here's blanket statement number two: some sh*tty quality amplifiers are built so poorly that their gain controls do not function properly as input sensitivity controls. And now my blanket stement number three: installers or users who think it is OK to use gain controls as volume controls are setting themselves up for clipping their amplifiers. That better?  ------------- Support the12volt.com
Posted By: shaman
Date Posted: June 21, 2004 at 1:09 AM
DYohn] wrote:
K, how about this for a blanket statement: input sensitivity controls are not volume controls. And here's blanket statement number two: some sh*tty quality amplifiers are built so poorly that their gain controls do not function properly as input sensitivity controls. And now my blanket stement number three: installers or users who think it is OK to use gain controls as volume controls are setting themselves up for clipping their amplifiers. That better?
Amen, brother!
Posted By: jfk51502003
Date Posted: June 22, 2004 at 10:49 AM
Dyohn...I found this on a website...never heard it explained like that ...no impudence intended...
Posted By: DYohn
Date Posted: June 22, 2004 at 11:25 AM
No problem! ------------- Support the12volt.com
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