Setting gain on amp with multimeter


Archer3545

CarAudio.com Enthusiast
Jan 3, 2020
20
0
Oklahoma
TLDR: Today I was setting the gain on my with a multimeter, I calculated that to hit 50w rms I need to tune the amp to 14.1v I played a 200hrz sine wave and set both gains for the front speakers and the back speakers to 14.1v I changed the frequency and noticed that my front speakers seemed pretty normal and close to what I set them too when I was playing the 200hz, but my back speakers if the frequency was higher than 200hz the voltage would read way higher than what the front speakers would read and if the frequency was lower than 200hz the voltage would read would be way lower than the front. I tried tuning at different frequencies and nothing fixed the trend.

I have a stock head unit with the wires spliced to a lc7i LOC (which I have had for quite a few months now). I bought brand new
Rockford Fosgate R300X4 Prime 4-Channel Amplifier https://rockfordfosgate.com/products/details/r300x4/ after my old skar one stopped working. I also replaced the door speakers which I had already replaced once, but one of the speakers were bad and the product was no longer sold. The new speakers are Infinity REF-6522EX Shallow-Mount 6-1/2 Inch Coaxial Car Speakers new. https://www.infinityspeakers.com/REF-6522EX.html
Anyway I hooked them up and I was ready to set the gain on my amp the best way I know how. (With the lc7i you can leave the gain on the amp all the way down and then turn up the nob on the lc7i effectively still the gain, but a light will come up when you start clipping. However using this method was way too loud and I have had a couple of subs blown. I wanted to set it with a multimeter which I have done before a couple times and I know you do volts times the square root of rms watts times ohms. So I want 50w rms, with that equation I should be aiming at about 14.1v. Here's where the weird stuff starts happening. So its a 4ch 2ch bridgable amp, and I have it in 4ch mode. on my head unit I have all of the equalizer settings turned off. I have the hp and lp off on the amp and I have the nobs all the way down on my lc7i. I unload the speakers and hook up my multimeter and start playing a 200hz sine wave with my head unit volume at 25/30. I set both front and back speaker gains to about 14.1 volts. I play a different frequency, at 1000hz I check my voltage and on the front speakers, it read 14-15 volts, a bit higher than what I originally set it too. I check my back speakers and it says it's at 22volts. I check a lower frequency around 75 herts my front speakers were at 13-14 volts a bit under what I originally set it at and my back ones are at like 5-6 volts. A messed with the gains more I tried different frequencies. I tried tunning them at different frequencies and no matter what, frequencies higher than what I tuned it at the back speakers would have a way higher voltage than the front and frequencies lower than what I tuned it at the back speaker voltage would be way lower than the front. Does anyone have any ideas as to what may cause this? Bad amp, bad wiring anything I should check please.
 
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THATpurpleKUSH

Smoke weed everyday
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Sep 30, 2009
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I have a stock head unit with the wirs spliced and wired to a lc7i LOC (which I have had for quite a few months now). I bought brand new
Rockford Fosgate R300X4 Prime 4-Channel Amplifier after my old skar one stopped working. I also replaced the door speakers which I had already replaced once, but one of the speakers were bad and the product was no longer sold. So about right after I got the new amp for my door speakers they are Infinity REF-6522EX Shallow-Mount 6-1/2 Inch Coaxial Car Speakers new. Anyway I hooked them up and I was ready to set the gain on my amp properly. (With the lc7i you can leave the gain on the amp all the way down and then turn up the nob on the lc7i effectively still the gain, but a light will come up when you start clipping. However using this method was way too loud and I have had a couple of subs blown. I want to set it with a multimeter I have done this before a couple times and I know you do volts times the square root of rms watts times ohms. So I want 50w rms, with that equation I should be aiming at about 14.1v. Here's where the weird stuff starts happening. So its a 4ch 2ch bridgable amp, and I have it in 4ch mode. on my head unit I have all of the equalizer settings turned off. I have the hp and lp off on the amp and I have the nobs all the way down on my lc7i. I unload the speakers and hook up my multimeter and start playing a 200hz sine wave with my head unit volume at 25/30. I set both front and back speaker gains to about 14.1 volts. I play a different frequency, at 1000hz I check my voltage and on the front speakers, it read 14-15 volts, a bit higher than what I originally set it too. I check my back speakers and it says it's at 22volts. I check a lower frequency around 75 herts my front speakers were at 13-14 volts a bit under what I originally set it at and my back ones are at like 5-6 volts. A messed with the gains more I tried different frequencies. I tried tunning them at different frequencies and no matter what frequencies higher than what I tuned it at the back speakers channels would have a way higher voltage than the front and frequencies lower than what I tuned it at the back speaker voltage would be way lower than the front. Does anyone have any ideas as to what may cause this? Bad amp, bad wiring anything I should check please.


 
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Archer3545

CarAudio.com Enthusiast
Jan 3, 2020
20
0
Oklahoma
Yeah I didn't read it over to make sure it was clear lol thanks, but seriously I don't know if my amp is bad or what could cause this. Maybe my wiring, but I know that my speakers aren't out of phase or anything but I was thinking that maybe the possitive and negative wires that go into my loc for the back speakers might of been out of phase idk if that makes sense I don't have a bunch of experience with this stuff. I tried switching them around and I was still geting the same result so I don't know
 

jt4x4

CarAudio.com Enthusiast
Jul 22, 2019
78
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Do you have separate crossovers for front and rear, either on your amp or on your deck? If they are set different, that could explain your problem.
 

THATpurpleKUSH

Smoke weed everyday
10+ year member
Sep 30, 2009
7,798
626
Slums of the Shaolin
Every freq will have a diff voltage because of the impedance curve.

Gain setting discussions can really go into splitting hairs but to keep things simple for you use 1khz for your mids and 40hz for subs.

50w 2 ohm mids would be 10v ac at 1khz.
 
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Archer3545

CarAudio.com Enthusiast
Jan 3, 2020
20
0
Oklahoma
Every freq will have a diff voltage because of the impedance curve.

Gain setting discussions can really go into splitting hairs but to keep things simple for you use 1khz for your mids and 40hz for subs.

50w 2 ohm mids would be 10v ac at 1khz.
Ok I will try that my speakers are 4ohm but I will tune it at 1khz and probably leave it. I just don't want to damage my amp because it's play a redardedly high voltage on the back speakers, but it's new so if this issue is the amp I can return it and get my money back still
 
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Archer3545

CarAudio.com Enthusiast
Jan 3, 2020
20
0
Oklahoma
Do you have separate crossovers for front and rear, either on your amp or on your deck? If they are set different, that could explain your problem.
The amp has a two built in crossovers for front and back but I had them off and I had the balance and fade off on my head unit as well
 

THATpurpleKUSH

Smoke weed everyday
10+ year member
Sep 30, 2009
7,798
626
Slums of the Shaolin
Ok I will try that my speakers are 4ohm but I will tune it at 1khz and probably leave it. I just don't want to damage my amp because it's play a redardedly high voltage on the back speakers, but it's new so if this issue is the amp I can return it and get my money back still
Well if they're 4 ohm speakers then 14.1v would be correct. Try the higher freq.

I just noticed you have a factory headunit with loc. That's going to be a huge bottleneck no matter how well the marketing dept of Audiocontrol tries to tell you otherwise. Get an aftermarket headunit or you will forever have signal problems and constantly blow equipment and seek advice.
 
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Archer3545

CarAudio.com Enthusiast
Jan 3, 2020
20
0
Oklahoma
Well if they're 4 ohm speakers then 14.1v would be correct. Try the higher freq.

I just noticed you have a factory headunit with loc. That's going to be a huge bottleneck no matter how well the marketing dept of Audiocontrol tries to tell you otherwise. Get an aftermarket headunit or you will forever have signal problems and constantly blow equipment and seek advice.
Yeah I'll look into that some more the main issue is I have a 2015 mustang base audio system and the head unit is built into the panel with the ac and the start button and a lot of stuff so aftermarket support is kind of shit and expensive
 
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Archer3545

CarAudio.com Enthusiast
Jan 3, 2020
20
0
Oklahoma
Well if they're 4 ohm speakers then 14.1v would be correct. Try the higher freq.

I just noticed you have a factory headunit with loc. That's going to be a huge bottleneck no matter how well the marketing dept of Audiocontrol tries to tell you otherwise. Get an aftermarket headunit or you will forever have signal problems and constantly blow equipment and seek advice.
What if I were to get a DSP do you think that would help?
 

Lasherž

CarAudio.com Veteran
Apr 27, 2020
538
102
United States
Just throwing this out there, when I set gains using a multimeter I choose a number of tones, 40-100hz for the subwoofer and narrow down which uses the most power (has the lowest impedance) then adjust for that. Ohms law is a pretty accurate way to set them if you either do the math right or use a chart available online. Using a true rms multimeter is prefered, but if your signal is a true sine wave it shouldn't make a difference.
 

jt4x4

CarAudio.com Enthusiast
Jul 22, 2019
78
19
ND
Just throwing this out there, when I set gains using a multimeter I choose a number of tones, 40-100hz for the subwoofer and narrow down which uses the most power (has the lowest impedance) then adjust for that. Ohms law is a pretty accurate way to set them if you either do the math right or use a chart available online. Using a true rms multimeter is prefered, but if your signal is a true sine wave it shouldn't make a difference.
I have lost faith in true rms meters from my job. I asked Fluke and they basically told me my true rms meter doesn't read right because it's too accurate. Why do you recommend true rms? My meter is a 325, and my other is a 322.
 

Lasherž

CarAudio.com Veteran
Apr 27, 2020
538
102
United States
I asked Fluke and they basically told me my true rms meter doesn't read right because it's too accurate.
Interesting lol. Mine uses an average based on a large number of readings, I think the only time it gets inaccurate is on peak readings but that's a separate setting and it's mainly for measuring inrush currents and voltage drops/spikes (Fluke 287). I also use it together with an extech DC current clamp with decent results. It's also pretty quick for a true RMS meter, especially the line graph.
Why do you recommend true rms? My meter is a 325, and my other is a 322.
Clipping turns it into a non-sinusoidal wave and the average voltage will deviate from the true voltage in a non-rms meter. It's much more important with music playing since it won't be a clean sine wave and it will much better approximate the power going into the voice coils.

I'm curious what scenario the true RMS meter was the insuperior option, generally they're considered to have no downsides except in some cases relative speed takes a hit.
 

jt4x4

CarAudio.com Enthusiast
Jul 22, 2019
78
19
ND
True RMS doesn't give me desirable results when used to measure the output of variable frequency drives. The current sensors my company uses are not true rms, so they obviously give different readings than a true rms meter. We use current sensors to help troubleshoot pump and fan motors, so I need the numbers to be accurate. For instance: if I see a lower current draw than the motor nameplate states, it could be a broken fan belt or pump coupling. As Fluke explained it to me, it's because variable frequency drives put out a nonlinear load, and the current actually has very high frequency rippling that only a true rms meter can measure, which causes a different measurement between a true rms meter and a non-true rms meter. I responded by asking how that is beneficial when the meter now gives me measurements that don't align with the specs on the motor nameplate, and they said the nameplate is intended for lower non-true rms meters. So for that application, I think true rms is actually a hindrance.
 
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Archer3545

CarAudio.com Enthusiast
Jan 3, 2020
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Oklahoma
Well if they're 4 ohm speakers then 14.1v would be correct. Try the higher freq.

I just noticed you have a factory headunit with loc. That's going to be a huge bottleneck no matter how well the marketing dept of Audiocontrol tries to tell you otherwise. Get an aftermarket headunit or you will forever have signal problems and constantly blow equipment and seek advice.
I looked into some more aftermarket head units for my car and they are really difficult to find and I don't want to spend $500+ on a new head unit that looks janky and has like 1 review. I really want to know your opinion is or anyones on a DSP have you ever used one, do you think that, that's a good alternative to changing out my head unit? I already have a LOC hooked up and everything so if I did get a DSP I would just run RCA's from the LOC to the DSP and then to my amps. I looked at some on Amazon they didn't have a huge selection, but really anything under $400. And it only needs to have 3 left and right outputs.
 
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Archer3545

CarAudio.com Enthusiast
Jan 3, 2020
20
0
Oklahoma
Update: I re tuned my door speaker amp at 1000hz and it helped a lot. There was still some weird differences at some frequencies but for the most part the front and the back channels werent that far off from eachother. That actually helped solve a few other problems, like before, the higher frequencies would just be ear peircing, but that's solved and I moved my 8" sub to a ported box and I have heard a lot that sealed boxes have better sound clarity and sound cleaner, and ported boxes have more rattle and sound deaper. This is just my opinion, but I have run that sub in a sealed box for months and moving it into a ported turned it into a whole new beast this little ass sub hits hard as **** and I'm actually scared to get the two 12's lol.
 

THATpurpleKUSH

Smoke weed everyday
10+ year member
Sep 30, 2009
7,798
626
Slums of the Shaolin
I looked into some more aftermarket head units for my car and they are really difficult to find and I don't want to spend $500+ on a new head unit that looks janky and has like 1 review. I really want to know your opinion is or anyones on a DSP have you ever used one, do you think that, that's a good alternative to changing out my head unit? I already have a LOC hooked up and everything so if I did get a DSP I would just run RCA's from the LOC to the DSP and then to my amps. I looked at some on Amazon they didn't have a huge selection, but really anything under $400. And it only needs to have 3 left and right outputs.
You can bypass using the headunit for audio and just run your audio input into the dsp then to the amps. Prob easiest way to go in your situation.
 

Lasherž

CarAudio.com Veteran
Apr 27, 2020
538
102
United States
True RMS doesn't give me desirable results when used to measure the output of variable frequency drives. The current sensors my company uses are not true rms, so they obviously give different readings than a true rms meter. We use current sensors to help troubleshoot pump and fan motors, so I need the numbers to be accurate. For instance: if I see a lower current draw than the motor nameplate states, it could be a broken fan belt or pump coupling. As Fluke explained it to me, it's because variable frequency drives put out a nonlinear load, and the current actually has very high frequency rippling that only a true rms meter can measure, which causes a different measurement between a true rms meter and a non-true rms meter. I responded by asking how that is beneficial when the meter now gives me measurements that don't align with the specs on the motor nameplate, and they said the nameplate is intended for lower non-true rms meters. So for that application, I think true rms is actually a hindrance.
Ah I see, that's very interesting. I guess what it boils down to though is that the company which made the nameplate isn't using the right tools, no? I mean Fluke is basically saying our tool is more accurate than the tool they used to make the manual so you should use a lesser tool for that. True RMS should give more accurate results for variable frequency drives than without.

I never thought about it that way, with companies actually using non-rms tools and then conflicting with your more accurate one. I guess it makes sense to ditch rms in that regard. I'll say though if you want real values rather than "reference to company values" and you've got RMS vs non-RMS, RMS is always going to be the more 'technically correct' reading.
 

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