I just dont understand

[Beandip 3]

Ok guys i might be opening a can of worms here but i dont understand why people do this to set their gain and think its right. to me, it doesnt make sense and i will explain why.

a lot of times i see this chart here and people say set it for xxx voltage and you are set. ignore step 4 since without having a load on the amp is kinda pointless.

if that chart is right, then i am putting 1300 watts to my sub, but with a clamp, DMM, and thanks to ohms law, i found out i am putting ~800 to my sub. i am not saying that chart is mathematically wrong, just cant be used the way its intended.

so whats the deal? i dont understand why you would use that chart (or any formula equivalent of that chart) because the speaker impedance changes while a speaker plays so the amp doesn't see a load of 2 ohms all the time (technically it doesn't even see 2 ohm!). being that, why do you want to set your amp to "xx" voltage and when you turn up the sub to xx voltage the amp sees a different load because the impedance changes with frequency and so does the amount of power the sub is receiving, therfore the set voltage = "this much power" is just BS.

one example, based on that chart, if i follow the instructions 100% and lets say i set my amp to 40 volts since i am running @ 2 ohms so i can feed my sub 800 watts...when i hook up the sub after this procedure, will it see 800 watts? I think not...

Another example... I pasted the chart i made of the actual output of my amp @ 2 ohms (SAZ-1500D 1500 @ 1 ohm, 800 @ 2 ohm...need to recone the sub to dual 2 ohm sometime.)

Volume----Frequency-----AC voltage-----AC amperage----Ohms------------Watts

31----------------20------------49.9------------------14.3----------3.489------------713.57

31----------------25------------48.9------------------12.5----------3.912-----------611.25

31----------------30------------50.1------------------8.7-----------5.758-----------435.87

31----------------35------------50.8------------------4.8-----------10.53-----------243.84

31----------------40------------51.3------------------7.1-----------7.225-----------364.23

31----------------45------------51.2-----------------14.1----------3.631-----------721.92

31----------------50------------50.6-----------------16.4----------3.085-----------829.84

31----------------55------------49.5-----------------17.6----------2.812-----------871.2

31----------------60------------48.1-----------------17.7----------2.717-----------851.37

31----------------65------------49.5-----------------16.7----------2.964-----------826.65

31----------------70------------42.2-----------------15.2----------2.776-----------641.44

31----------------75------------36.4-----------------12.8----------2.843-----------465.92

31----------------80------------31.1-----------------10.7----------2.906-----------332.77

so as you can see, since the impedance changes with frequency, setting a certain voltage is completely useless. also a little bit of the subject is in this thread.

how i got that chart and took the readings explained here from another forum.

This is what we uesd to do for our SPL vehicles.

The first thing we did was use an O-scope to set all of the gains.

Unforfunetly you can't do that unless you have an O-scope.

So just set your stuff up the best you can by ear.

Next take your DMM hook the positive up to the positive speaker wire at the amp so your positive speaker wire and positive wire from the DMM are connected to the positive speaker terminal on the amp.

Now do that with the negative also. Do the negative the same way you just did the positives.

and set the DMM to AC voltage.

Now take your clamp meter and clamp that only around the positive speaker wire comming out of the amp.

Set the clamp meter to AC amperage.

Now the install part is done.

Next you need a test tone disc.

Set the volume on the head unit to were you feel everything is as loud as you think it shoud be.

Or if you have an O-scope set your volume rite at clipping. Just a hair less so your not running a clipped signal and remember that volume number becuase you will need to do all your testing at those same settings to get an accurette Idea of whats going on.

Now with the DMM set to ac voltage and the clamp meter set to ac amperage take the test tone disc and find a frequency to start with, burp your system and write down the results from the DMM and clamp meter.

Both those numbers need to be writen down at the same time or you won't get propper results.

then go up 5 hertz burp again and write it down.

then go up another 5 hertz and do it again.

Example:

start at 30 hertz burp then 35 burp 40 burp and keep doing that until you have gotten to 80 hertz.

Now the physical work is done.

Alright now you need a calculator and a place to sit down and relax and you want to well im going to write it for you the way I've got it wrote down in my notes.

AC voltage from the speaker outputs divided by the amp draw on the positive wire from the amp.

That will give you your impedance at each frequency.

Heres some thing extra for you do you really want to know how much power your amp is really making do this.

Multiply the AC voltage by its self then divide that number by the ohm loads you just came up with.

thats a heck of alot more realistic than just use 4 ohms or 2 ohms becuase you just did the work to find out what your amp is really seeing. so the wattages you come up with are alot more realistic.

I'm going to post this then I'll have another post for you that shows you how I set up my paper.

After my next post if you don't get it PM me and I'll try to help but I would like to have any question you ask to be posted in this thread so it can help others.

Edit 02/09/2010:

To have fun with and to help, Ohm's Law/Watt's Law Calculator:

http://www.crownaudi...ls/ohms-law.htm

Hers an example of a kicker ZR360 running to 2 10 inch solo L7 tens with a nominal 2 ohm load.

Volume-----Frequency-----AC voltage----AC amperage-----ohms------watts

31 --------------20--------- ----41.4------------10.7 -----------3.87------- 443

31---------------25--------------44.1-------------9.6------------4.6---------423

31---------------30--------------44.3-------------9.7------------4.56--------430

31---------------33--------------48.3-------------7.4------------6.53--------357

31---------------40--------------48.9-------------3.75---------13.04--------183

31---------------50--------------38.4-------------3.15---------12.19--------121

31---------------60--------------42.9-------------9.22----------4.65---------396

31---------------70--------------33.5-------------9.16----------3.66---------306

Do you get the idea.

Do you see that even though I am running a nominal 2 ohm load I never hit 2 ohms

but my amp did put out rated power.

Its supposed to put out 400 watts at a nominal 2 ohm load and if you look at the numbers I averaged 400 watts.

Do you see the impedence rise at 40 hertz and 50 hertz and look at the wattage.

IMO i think a combination of this and your hearing will set your gains correctly since you know what EXACTLY your sub is seeing power wise. Ideally i would say set it like this and find out the power you are throwing at the sub and then use a O-scope (if you can get a hold of one) to check for clipping. setting the gain solely by DMM wont help much either since some subwoofers are at more of a risk of mechanical damage than thermal. in the end, doesn't box design play a large role on what your speaker can take? since everyone doesn't have the same box design who is to say this is right? yes a sub might be able to handle XXXX amount of watts but if the box is too large then you are at risk of mechanical damage correct? so again, you can use this method to check for thermal damage, and if your box is larger than the recommended size then use common sense and listen to what the sub is saying for any signs of mechanical stress.

so if the impedance changes though the frequencies so much, why are you setting the gain on that chart, assuming you are at "X" ohms? all i can see is just really under powering your sub....

Also not trying to start shit, just trying to get a good answer on to why...also seems like a good technical discussion

[Robert 9]

Does this mean the whole chart is worthless?

[Beandip 3]

Does this mean the whole chart is worthless?

Since the amp is not truly seeing "X" ohms...i believe so. Since the impedance changes directly effects power output, with this variable you cant say 40 volts = 800 watts @ 2 ohms since you dont know exactly what impedance you are running unless you measure it. But i would like to get the audio guru's opinion on this.

Edited October 10, 2010 by beandip

[Tirefryr 1,742]

It's a simple tool made to help the less-informed enthusiast to set their gain a little better than by ear. Of course it really doesn't work as most will turn it up anyways, so use it how you choose. The ONLY way to get the best performance is with a scope and quality recording.

[Beandip 3]

Ah, i figured it was a "here do it and this will be perfect" way, at least that is how i have been seeing it....but i do see your point a novice would be less likely to overdo the gain doing it this way. especially if its their first system, they would be less likely to know when a sub is distorting or not acting right.... its like a ballpark estimate. I just kept looking this and other posts that recommend doing this it and thought to myself...why, this is wrong and not exactly right.

A scope would be nice....I need to get off my lazy ass and buy one

Edited October 10, 2010 by beandip

[Impious 1,476]

You aren't understanding the purpose behind setting the gain.

When you set the gain, you are setting the input sensitivity of the amp (ideally) so that it operates at maximum rail voltage without exceeding that voltage. You aren't setting the gain based on the impedance of the load, that's simply one means by which to help figure out where to set the input sensitivity control.

Almost every amplifier has a fixed amount of gain. If you drive the amplifier with 1V of input and it outputs 20V, then the amplifier has a 20:1 gain ratio. If the amplifier has a maximum operating rail voltage of 40V (400w @ 4ohm), then it needs 2V of input signal to achieve that level of output. But, almost every amplifier is also designed to be capable of operating at it's full power output with a wide range of input signals....hence, the gain knob. Setting this knob to a certain position manipulates the level of the input signal so that the input signal is at the proper level for the amplifier will operate at it's maximum rail voltage without trying to exceed that voltage level. So if you have a 4V headunit outputting the full 4V, and you set the gain control to the "4V" level, then essentially the amplifier is attenuating that input signal by 2V so that after the 20:1 gain the output voltage will be 40V, right where it needs to be.

But wait.....if the knob doesn't have a "4V" mark on it's dial, how do I know where the 4V setting is? Well, that's where the chart comes in. Since we normally aren't given the rail voltage of the amplifier, we're forced to calculate it back out of the information we are given. So if we know the amplifier is designed to output 400w @ 4ohm, we now know that the amplifier's going to output 40V. So, we now know the target rail voltage to achieve for a "proper" setting of the input sensitivity is 40V. So we put on a test tone and turn the gain knob until we achieve 40V.

Make sense?

Now, I'm not saying this is the ideal method of setting the gain. For starters, it assumes the amplifier is capable of cleanly outputting exactly rated power. On an underrated or overrated amplifier, the gain setting may not be accurate. In addition, there is also an issue with the actual level of the input signal. Using 0db will keep the amplifier from clipping (since HU output will be at it's maximum), but music is almost never at that level so we end up reducing the actual average power we'll receive from the amplifier. Etc, etc, not worth rehashing right now, but you get the point.

The gain setting is really independent of the actual load or impedance rise. If the amplifier is capable of 40V, that's not going to change with impedance (for amplifiers with non-regulated outputs)*. Hence why amplifiers are normally rated at, for example, 400w @ 4ohm, 800w @ 2ohm, 1600w @ 1ohm. If you run the numbers or look on the chart, you'll see that the voltage for each for is the same....40V. If your load rises to 8ohm, you can't try to set it for 400w @ 8ohm because you'll exceed the rail voltage of the amplifier. You would set the gain for 200w @ 8ohm, maintaining that 40V rail voltage.

*EDIT: I should mention that it's possible and not atypical for an amplifier's output to not hold true to the "double each time impedance is halved" rule for several reasons. A weak power supply, internal losses, current capabilities of the outputs, etc will limit the ability of the amplifier to double output as impedance decreases. As a result, the voltage at lower impedances may decrease. Sundown 1200D as an example. sqrt(360*4) = 37.95V, sqrt(720*2) = 37.95V, sqrt(1200*1) = 34.64V.

[Duran 387]

What ^^ he said

[Sir-Lancelot 216]

Use your ears. They are the best tool you have in your arsenal.

[Beandip 3]

^Ever since my first system that is what i have been doing. Kinda want a scope (suggestions?)

You aren't understanding the purpose behind setting the gain.

When you set the gain, you are setting the input sensitivity of the amp (ideally) so that it operates at maximum rail voltage without exceeding that voltage. You aren't setting the gain based on the impedance of the load, that's simply one means by which to help figure out where to set the input sensitivity control.

Almost every amplifier has a fixed amount of gain. If you drive the amplifier with 1V of input and it outputs 20V, then the amplifier has a 20:1 gain ratio. If the amplifier has a maximum operating rail voltage of 40V (400w @ 4ohm), then it needs 2V of input signal to achieve that level of output. But, almost every amplifier is also designed to be capable of operating at it's full power output with a wide range of input signals....hence, the gain knob. Setting this knob to a certain position manipulates the level of the input signal so that the input signal is at the proper level for the amplifier will operate at it's maximum rail voltage without trying to exceed that voltage level. So if you have a 4V headunit outputting the full 4V, and you set the gain control to the "4V" level, then essentially the amplifier is attenuating that input signal by 2V so that after the 20:1 gain the output voltage will be 40V, right where it needs to be.

But wait.....if the knob doesn't have a "4V" mark on it's dial, how do I know where the 4V setting is? Well, that's where the chart comes in. Since we normally aren't given the rail voltage of the amplifier, we're forced to calculate it back out of the information we are given. So if we know the amplifier is designed to output 400w @ 4ohm, we now know that the amplifier's going to output 40V. So, we now know the target rail voltage to achieve for a "proper" setting of the input sensitivity is 40V. So we put on a test tone and turn the gain knob until we achieve 40V.

Make sense?

Now, I'm not saying this is the ideal method of setting the gain. For starters, it assumes the amplifier is capable of cleanly outputting exactly rated power. On an underrated or overrated amplifier, the gain setting may not be accurate. In addition, there is also an issue with the actual level of the input signal. Using 0db will keep the amplifier from clipping (since HU output will be at it's maximum), but music is almost never at that level so we end up reducing the actual average power we'll receive from the amplifier. Etc, etc, not worth rehashing right now, but you get the point.

The gain setting is really independent of the actual load or impedance rise. If the amplifier is capable of 40V, that's not going to change with impedance (for amplifiers with non-regulated outputs)*. Hence why amplifiers are normally rated at, for example, 400w @ 4ohm, 800w @ 2ohm, 1600w @ 1ohm. If you run the numbers or look on the chart, you'll see that the voltage for each for is the same....40V. If your load rises to 8ohm, you can't try to set it for 400w @ 8ohm because you'll exceed the rail voltage of the amplifier. You would set the gain for 200w @ 8ohm, maintaining that 40V rail voltage.

*EDIT: I should mention that it's possible and not atypical for an amplifier's output to not hold true to the "double each time impedance is halved" rule for several reasons. A weak power supply, internal losses, current capabilities of the outputs, etc will limit the ability of the amplifier to double output as impedance decreases. As a result, the voltage at lower impedances may decrease. Sundown 1200D as an example. sqrt(360*4) = 37.95V, sqrt(720*2) = 37.95V, sqrt(1200*1) = 34.64V.

thanks for the detailed answer, appreciate the time you took to write that up. on the gain setting i always knew (well not always) that you adjust it to the output voltage of the HU and that the amp would provide full power at any gain setting (within the amp's input spec), never really read into WHY, and about rail voltage. thanks for clearing that up. So basically while its not ideal to set the gains this way, its a start...

one thing though, why is it the sundown 1200D does not follow the "double each time impedance is halved" rule?

:EDIT: just looked on sundown's site and saw why. i was wondering where you got 360 and 720 from lol.

Edited October 10, 2010 by beandip

[slim2fattycake 85]

You aren't understanding the purpose behind setting the gain.

When you set the gain, you are setting the input sensitivity of the amp (ideally) so that it operates at maximum rail voltage without exceeding that voltage. You aren't setting the gain based on the impedance of the load, that's simply one means by which to help figure out where to set the input sensitivity control.

Almost every amplifier has a fixed amount of gain. If you drive the amplifier with 1V of input and it outputs 20V, then the amplifier has a 20:1 gain ratio. If the amplifier has a maximum operating rail voltage of 40V (400w @ 4ohm), then it needs 2V of input signal to achieve that level of output. But, almost every amplifier is also designed to be capable of operating at it's full power output with a wide range of input signals....hence, the gain knob. Setting this knob to a certain position manipulates the level of the input signal so that the input signal is at the proper level for the amplifier will operate at it's maximum rail voltage without trying to exceed that voltage level. So if you have a 4V headunit outputting the full 4V, and you set the gain control to the "4V" level, then essentially the amplifier is attenuating that input signal by 2V so that after the 20:1 gain the output voltage will be 40V, right where it needs to be.

But wait.....if the knob doesn't have a "4V" mark on it's dial, how do I know where the 4V setting is? Well, that's where the chart comes in. Since we normally aren't given the rail voltage of the amplifier, we're forced to calculate it back out of the information we are given. So if we know the amplifier is designed to output 400w @ 4ohm, we now know that the amplifier's going to output 40V. So, we now know the target rail voltage to achieve for a "proper" setting of the input sensitivity is 40V. So we put on a test tone and turn the gain knob until we achieve 40V.

Make sense?

Now, I'm not saying this is the ideal method of setting the gain. For starters, it assumes the amplifier is capable of cleanly outputting exactly rated power. On an underrated or overrated amplifier, the gain setting may not be accurate. In addition, there is also an issue with the actual level of the input signal. Using 0db will keep the amplifier from clipping (since HU output will be at it's maximum), but music is almost never at that level so we end up reducing the actual average power we'll receive from the amplifier. Etc, etc, not worth rehashing right now, but you get the point.

The gain setting is really independent of the actual load or impedance rise. If the amplifier is capable of 40V, that's not going to change with impedance (for amplifiers with non-regulated outputs)*. Hence why amplifiers are normally rated at, for example, 400w @ 4ohm, 800w @ 2ohm, 1600w @ 1ohm. If you run the numbers or look on the chart, you'll see that the voltage for each for is the same....40V. If your load rises to 8ohm, you can't try to set it for 400w @ 8ohm because you'll exceed the rail voltage of the amplifier. You would set the gain for 200w @ 8ohm, maintaining that 40V rail voltage.

*EDIT: I should mention that it's possible and not atypical for an amplifier's output to not hold true to the "double each time impedance is halved" rule for several reasons. A weak power supply, internal losses, current capabilities of the outputs, etc will limit the ability of the amplifier to double output as impedance decreases. As a result, the voltage at lower impedances may decrease. Sundown 1200D as an example. sqrt(360*4) = 37.95V, sqrt(720*2) = 37.95V, sqrt(1200*1) = 34.64V.

[95Honda 486]

Don't even waste your time with a DMM or O'Scope..

Besides, 99% of the people who think they are doing it right, aren't......

Trust your ears and nose.....

If you can't, buy a smaller amp....

[Robert 9]

Why can't you just stick DMM into amp with the speakers connected?

[ssh 924]

Why can't you just stick DMM into amp with the speakers connected?

If I was to take a guess I would say, With the speakers connected there using power and in turn lowering the voltage reading. Example: check battery voltage at idle with stereo off vs stereo at full tilt (voltage drop).

But this is just a guess

[Robert 9]

makes sense thanks

[Tirefryr 1,742]

the driver would present a reactive load and voltage would vary.

[Duran 387]

But with a set frequency and no reactive load you can get an actuate reading.

[1200 bandit 2]

Use your ears. They are the best tool you have in your arsenal.

x2

[helotaxi 46]

Why can't you just stick DMM into amp with the speakers connected?

If I was to take a guess I would say, With the speakers connected there using power and in turn lowering the voltage reading. Example: check battery voltage at idle with stereo off vs stereo at full tilt (voltage drop).

But this is just a guess

That's part of it but the other part is that you could 1) blow the speakers and 2) blow your ears (actually more a concern with mids and highs).

Also the back EMF from the driver will affect the voltage reading as Tirefryr mentioned.