Setting amp gains.

[Dan208 23]

I picked up a DMM the other day and have been reading up on setting my amp gains. I came across this the other day:

Using a DMM to set gains based off AC voltage on the outputs of the amplifier is a poor way to set your gains to begin with. It assumes you know both the impedance of the driver at the specified frequency you are testing and the maximum unclipped voltage that the amplifier can produce at that frequency into that load. Both are just assumptions. You only know the nominal impedance of the driver (it's average across the frequency range it is designed to play) and the average power output of the amplifier across all frequencies at a set 4 ohm load. Speakers are reactive. Thier impedance changes depending on frequency. What if the speaker is really presenting a 3 ohm load at 50hz or a 6 ohm load? How does that affect your measurement?

With all that said, people put too much effort into setting gains for a subwoofer system in a car anyways. Even a modest subwoofer system in a car is capable of overpowering the main speakers by at least 10-20 db. Setting the subwoofer system up so that it plays at 100% capacity will result in a boomy overpowering sound. Subwoofers play two octaves. Set the gains on your main speakers and blend the subwoofer system in to match that for a sound that suits your taste. There is no need to put anymore effort than that into it.

So my question is, is it still safer to set your gains with a DMM than by ear?

[jcarter1885 995]

If you cant tell signs of stress or distortion in your sub by ear then use a DMM, but if you feel your confident enough to set it by ear then go by ear. Just my .02

[CrazyKenKid 64]

YES it is safer to set gains via DMM over ears! seriously go set your gain via ears then plug in the dmm and see how close you came! also the SAFEST way to set the gains is via a oscope!!

with the oscope you cna see the output wave form and when it starts to clip (look like a square wave) you have tuned the gain to high lol!

[CrazyKenKid 64]

If you cant tell signs of stress or distortion in your sub by ear then use a DMM, but if you feel your confident enough to set it by ear then go by ear. Just my .02

funny storey though every time i set my gains via ear i am always off by a good -5-10volts! kind a odd and it happens like that ALL the time nominally 5-10volts lol!

[Dan208 23]

Thanks for the quick replies. I don't have access to an o-scope, so that's not an option. I think I'll see what happens when I set it by ear then use the DMM to test how close I got it. I don't fully trust myself to just set it by ear.

[jcarter1885 995]

If you cant tell signs of stress or distortion in your sub by ear then use a DMM, but if you feel your confident enough to set it by ear then go by ear. Just my .02

funny storey though every time i set my gains via ear i am always off by a good -5-10volts! kind a odd and it happens like that ALL the time nominally 5-10volts lol!

5-10 volts to low or high?

[CrazyKenKid 64]

normally 5-10v to low lol! i guess my ears are shot lol!

[CrazyKenKid 64]

Thanks for the quick replies. I don't have access to an o-scope, so that's not an option. I think I'll see what happens when I set it by ear then use the DMM to test how close I got it. I don't fully trust myself to just set it by ear.

ALSO make sure it is on A/C volts not D/C lol i have done that a few times! if it is on D/C and yet set it to say 30volts d/c you are actually seting it lower then when you pop it into a/c you will read like 25volt ac those numbers are just for reference sake! but A/C no D/C

[jcarter1885 995]

Thanks for the quick replies. I don't have access to an o-scope, so that's not an option. I think I'll see what happens when I set it by ear then use the DMM to test how close I got it. I don't fully trust myself to just set it by ear.

ALSO make sure it is on A/C volts not D/C lol i have done that a few times! if it is on D/C and yet set it to say 30volts d/c you are actually seting it lower then when you pop it into a/c you will read like 25volt ac those numbers are just for reference sake! but A/C no D/C

Yes make sure you do that, I had somebody trying to set there gains the other day doing the same thing since his DMM didnt label which was which.

[Dan208 23]

Thanks for the quick replies. I don't have access to an o-scope, so that's not an option. I think I'll see what happens when I set it by ear then use the DMM to test how close I got it. I don't fully trust myself to just set it by ear.

ALSO make sure it is on A/C volts not D/C lol i have done that a few times! if it is on D/C and yet set it to say 30volts d/c you are actually seting it lower then when you pop it into a/c you will read like 25volt ac those numbers are just for reference sake! but A/C no D/C

Will do. Thanks. I'll probably end up printing out some instructions so I have them right there with me.

[NB50 206]

Thanks for the quick replies. I don't have access to an o-scope, so that's not an option. I think I'll see what happens when I set it by ear then use the DMM to test how close I got it. I don't fully trust myself to just set it by ear.

ALSO make sure it is on A/C volts not D/C lol i have done that a few times! if it is on D/C and yet set it to say 30volts d/c you are actually seting it lower then when you pop it into a/c you will read like 25volt ac those numbers are just for reference sake! but A/C no D/C

If your measuring DC on the amplifiers outputs you got problems.

[Dan208 23]

Ok, another possibly dumb question.

My amp has 2 sets of speaker terminals and one gain knob. I'm only using one terminal. Since there's only a single gain knob, do I have to test it on the terminal that I'm running the sub? Or can I use the unoccupied terminal (but still disconnect the sub at the box)? The amp is under the seat and it's a little tough to get at the screws on the terminals.

[jcarter1885 995]

Ok, another possibly dumb question.

My amp has 2 sets of speaker terminals and one gain knob. I'm only using one terminal. Since there's only a single gain knob, do I have to test it on the terminal that I'm running the sub? Or can I use the unoccupied terminal (but still disconnect the sub at the box)? The amp is under the seat and it's a little tough to get at the screws on the terminals.

Im pretty sure it wont matter which set of terminals you use since both sets should read the same voltage.

[Kanaworm$ 292]

Ok, another possibly dumb question.

My amp has 2 sets of speaker terminals and one gain knob. I'm only using one terminal. Since there's only a single gain knob, do I have to test it on the terminal that I'm running the sub? Or can I use the unoccupied terminal (but still disconnect the sub at the box)? The amp is under the seat and it's a little tough to get at the screws on the terminals.

Most class D amps have 2 sets of speaker terminals to make installation easier on multiple subwoofer applications. Shouldn't matter as long as one is + and the other is - . If in doubt refer to the owners manual or tell us what amp you have and we should be able to help you out.

[Dan208 23]

It's an MTX RFL600D. I wouldn't think that it would matter since there's only one gain knob, but I want to make sure just in case.