How to Set Your Amplifier Gain to RMS Power Using Ohm's Law

[SSCobaltCrazy]

ok i have 2 tr55 in a ported box and a rt1000d and cant get the gains set right ive hooked up other amps and works great any ideas?

[mtxjt08]

@SSCobaltCrazy What is the impedance of the enclosure? Are they both single 4Ω subwoofers and you have them wired at 2Ω? If that is the case your target voltage should be 44.72V.

[WirelessVault]

i got a piece of paper an quickly calculated the required voltage rating and after I tuned my amp the sound had an amazing 150% improvement over what my buddy ghetto tuned. I have an Alpine CDE-126BT connected to a Kicker DX200.4 hooked up to a Kicker KS650.2 component speakers. I used the 1khz test tone and used 14.14v, just in case someone had the same equipment. I actually have to go back because the actual RMS rating for my amp is tested @ 244watts, so the actual V = 15.62.THANKS

[shaoweed]

thx MTXThunderforce

[shaoweed]

yes i am using a mono and a 4 ch amp...and when i set the volume for the player..the loudness should be off..right? and i dont need the use the loudness function.cause if i use i can break my amps or subs.?( i mean after i set the gains at the level that i normally listen) hope you understand what i meant. TY

[MTXThunderforce]

@shaoweed Yeah you either need to leave it off all of the time or leave it on all of the time. If I were you I would leave it off and yes if you set the amps up with loudness off and then turn it on you will run the risk of damaging your speakers.

[shaoweed]

hi.i have a question.i have a player that has 1 pair of rca's.and i wanna use 2 amplifiers(the rca from the output of the first amp to the input of the second

so how should i set up the gains for the 2 amps.after i connect the 2 together? is there any problem then i would need to know about?

and one more thing..when i setup the gains..the volume of the player how up should it be...it has 60 max .it starts clipping at 50..or 45 i cant remember..but if i enable loudness it starts clipping at 30

[mtxjt08]

@shaoweed Are you using a 4 channel and a mono block amplifier? For the 4 channel you would want to use the 1kHz test tone and you would want to measure for the RMS power of a single channel. You would need to set the front and rear gains independently. For the mono block you would do it just like the video shows with the 50Hz test tone. I would set the volume at the level you normally listen to the radio at when setting it up.

[amitkalsi1]

Can I do this using a mp3 device instead of a cd?

[MTXThunderforce]

@amitkalsi1 Yes you could download the test tone to an MP3 device and play it through your head unit to set it up if you have a USB or Apple connection on your head unit.

[cassiskoolio]

hi i have 2 jvc arsenal subs at 600 rms 4/2 ohm subs and a jl jx1000/1 amp mono block. i cant seem to set the amp correct, and the bass sounds muffled (weak). im running them at a 4 ohm load at 500 watss rms so 250 ea. amps only 2 ohm stable. Any tips how to get more power out of it or set it right?

[MTXThunderforce]

@cassiskoolio Did you follow the steps to use the multimeter to match the voltage? You should be showing 44.72 volts if it is doing 500W RMS @ 4Ω. Is it set like this? Do you have it in a sealed or vented enclosure? What do you have the low pass filter set at?

[subadubawoofer]

hey im tuning my amplifier this way i got the hifonics hfi1500d i have it at 1 ohm on two alpine type rs...during the procces i relized that wont the ac voltage change when there is no ohm load on the amp?? thanks alot

[MTXThunderforce]

@subadubawoofer Sorry for the late reply. If you are playing the 50Hz sine wave through your source unit while you are setting the gain it will not matter that the subs are not connected as long as you know what impedance they are going to be connected at, 4Ω. 2Ω, or 1Ω, and you know what the amplifier's power rating is at that impedance. Voltage equals the square root of RMS power * impedance so the voltage can be calculated even with nothing hooked up.

[texo0borsky]

Output Power is a scalar quantity!

[texo0borsky]

There is no such thing as RMS power in audio output.

[texo0borsky]

The output from the head unit is a dc voltage with a superimposed audio signal. Setting your multimeter to ACv does not give you the true output voltage.

[RoyalBaller9]

If I have a 2 channel amp is there any difference in this process? My Amp's overall RMS is about 640 watts @ 2 ohms, 320 per channel.

[MTXThunderforce]

@RoyalBaller9 The process would be the same, but you would only use the RMS for one channel. What model amplifier are you using that is a lot of power for a two channel? Are you connecting it to full range speakers or are you running subwoofers on each channel or are you bridging it to mono?

[RoyalBaller9]

@MTXThunderforce

I have a Fusion pp-am 12002 amp. I'm running two Audiobahn 12" Natural Sound AW1251T subwoofers (essentially the the current AW1251Js) @ 2 ohms. My amp specs state that @ 4 ohms it's 215 Watts RMS per channel, at 2 ohms it's about 320 Watts RMS per channel, and bridged @ 4 ohms it says it's about 640 Watts.

[MTXThunderforce]

@RoyalBaller9 Ok, it looks like those subs are dual 4Ω so you have one connected to each channel at 2Ω? You would use the same process in the video but you would look for the voltage based on the 320W RMS rating.

[RoyalBaller9]

@MTXThunderforce

Yes exactly. Thanks

[0robsatx0]

@MTXThunderforce not trying to be difficult. Your video states that you need to know the resistance. I am aware of the resistance on my sub. It shows 3.8 Ohms when I put the meter on it. On your video, you have NO load other than DMM when you are checking the AC voltage. With no sub in place, how are you sure you are putting a 4 ohm load on the amp? I mean there is NO sub/load?

[0robsatx0]

When you remove the driver (s), you are no longer presenting a "load" to the amp. With that said, how do you know the amp is still delivering 4 ohms to your MM - for your calculation?

[MTXThunderforce]

@0robsatx0 The impedance is based on the subwoofer's voice coil and its connection to the amplifier. You can figure out what the target voltage would be without having the speaker connected but you would have to connect the speaker to adjust the gain to that target voltage point.

[0robsatx0]

@MTXThunderforce @ 2:16 you state you are going to remove the subs. At this point there is no longer a specified load on the amp. So how can you calculate without knowing the resistance (ohms)?

[MTXThunderforce]

@0robsatx0 It is still early here on the west coast so I missed the 2:16 time stamp in your comment, sorry about that. Since we already know the enclosure was 2Ω we can do the math without having the enclosure connected. The voltage would be the same at any impedance ie. if it was 250W @ 4Ω voltage equals 31.6V or if 500W @ 2Ω voltage equals 31.6V

[0robsatx0]

@MTXThunderforce according to your math: 500 watts @ 2 Ohms derives - 31.62.V while 500 watts @ 4 ohm derives 44.72. I understand the math makes sense. What I still can't gather is how you are setting the gain on the amp and NOT know the resistance on the load. When the sub is removed there is no load, so how does the amp know? Sorry! Just trying to be sure I understand. But with no sub in place, I dont see how the load is "assumed".

[MTXThunderforce]

@0robsatx0 The amplifier that he is using in the video has a power rating of 250W @ 4Ω and 500W @ 2Ω it does not do 500W @ 4Ω. So either pay Voltage = the square root of 1000 which is 31.6V. We know the enclosure in the video is 2Ω, the amplifier doesn't have to see the load to set the gain since we already know the final impedance. It sounds like you have a 4Ω subwoofer if it is measuring 3.8Ω. What is the 4Ω RMS power rating of your amplifier?

[0robsatx0]

@MTXThunderforce That makes sense. The limiting factor here is the amp is rated @ 500 watts @ 2ohms. or 250 @ 4 ohms. So regardless at the amps max RMS output whether 2 or 4 OHM, the math works out to 31.62. :-) I needed to look at the max output on the amp by load presented. I was assuming that amp could do 500 watts @ 4 ohms. thx!

[0robsatx0]

@0robsatx0 Well maybe I am not as crazy as I thought! :-) I am going to power two MTX thunders, with a PDXM12, Alpine states it will do 1,200 watts RMS @ 4 or 2 Ohm load . . . I think that is where I was "confused" So, I am back to square one! How will this amp know if I have a 2 or 4 ohm load? In this case the 2 or 4 ohm load would make a pretty big difference based on final load. Correct?

[MTXThunderforce]

@0robsatx0 That PDX amp has the ability to do the same amount of power whether you have the subs connected at 2 or 4 ohms. The wiring and the subwoofer voice coils will "show" the amplifier what the impedance is. All that matters is that you hook up the subs at either 2 or 4 ohms which you can measure with a multimeter and the amp will do the rest.

[christopher12344321]

Aaannnd what if the amplifier doesn't do rated power like 70% of the amps out there?

[MTXThunderforce]

@christopher12344321 Well in that case you are most likely going to cook either the subwoofer or the amplifier. Unfortunately bad data leads to bad results.

[13youngj]

Will this be the same if i have a epicenter

[WilkinsChunilall]

Would this also apply to tuning a 4channel? i have my H/u hooked up to a EQs746

[MTXThunderforce]

@WilkinsChunilall Yes the same principle would apply to a four channel. There is usually a gain for the front channels and one for the rear channels. You only need to use the RMS power of a single channel though to set each gain. So if the amp is 60W RMS x 4 @ 4Ω and your speakers are 4Ω you would do the square root of 60(W) x 4(Ω) which would equal 15.5 Volts.

[titone14]

Can i use any multimeter? Someone answer me!!LOL

[MTXThunderforce]

@titone14 As long as the multimeter can measure voltage than yes any multimeter will work. You can get the $10 version at Walmart and do exactly what he is showing in the video.

[titone14]

@MTXThunderforce Perfect, thanks! So i have 2 Sundown Audio SA 12's hook up with a MTX TE1501 at 1 ohm load, so my target will be 38.72 am i right? Hope you can answer.. again!haha

[MTXThunderforce]

@titone14 Yes that is correct, the square root of 1500*1=38.72.

[snipwow]

I Think "Meade916" won't be happy after this, cause we now know how to set our gain correctely without buying the expensive "SMD DISTORTION DETECTOR DD-1"

Thumb-UP :D

[1millionscovilleclub]

@snipwow I agree. How can anyone do anything without "Free" Steve Meade not holding their hand.

[blooddog54]

i just got the 12in mtx terminator package that has the subs and amp what should my gain be set at?

[MTXThunderforce]

@blooddog54 The amplifier that comes with that package is rated at 250W RMS and the box is 2Ω so using Ohm's law, Voltage = the square root of power times resistance, or the square root of 250*2 which equals 22.36 volts which is your target. If you use a multimeter to measure this based on the output voltage of your headunit, the amp will be set correctly.

[MTXThunderforce]

The frequency does not affect the voltage. 50Hz is a good frequency to use because it is within the bandwidth of the amplifier, for a class D subwoofer amplifier usually 20Hz-200Hz, and it is above where the subsonic filter could be set. Additionally you will most likely have 50Hz notes in most music so it is a good frequency to test at.

[JohnnyBlaze6954]

When using a test tone, does the frequency of the test tone effect the voltage +/-? Lets say 30hz vs 50hz?

[mcs71]

I disagree that the head unit's preamp out is clean over the whole volume range. I've been doing this since 1992 and have never seen a head unit that had clean, undistorted output at full volume. Typically, the preouts will show some clipping at 80-90% of full volume. The only TRUE way to properly set gain levels is with an Oscilloscope. I agree that this is a compromise method for consumers who might not own an Oscope and it will get them in the ballpark much better than doing it by ear!

[goatskindreams]

someone introduce this guy to a dd-1

[Yuneitee]

@goatskindreams someone needs to introduce you to DD-eeznuts! MTX ruuules!! ......snoogins