what exactly do i need to do to set my gains?

[helotaxi 5,000+ posts]

then turn the gain up in your amp until the voltage stops increasing on your dmm.

That doesn't work with an RMS multimeter. The voltage will continue to increase until you have a fully clipped signal. Not good at all.

You use the nominal impedance of the sub for however you have it wired. If it is a nominal 1 ohm load, you use 1 ohm for the calculation.

 

[Twistid 5,000+ posts]

ok so for my sundown 1500 @ 1 ohm, i should set my voltage to 38 volts... but what if the amp is not seeing 1 ohm, what if after imp rise its seeing like 2-3 ohms... then wouldent that drive it into clipping?

 

[DidUHearThat? 10+ year member]

ok so for my sundown 1500 @ 1 ohm, i should set my voltage to 38 volts... but what if the amp is not seeing 1 ohm, what if after imp rise its seeing like 2-3 ohms... then wouldent that drive it into clipping?

You have to go by ratings. If your nominal impedence is 1 ohm, that's what you should use. Actual impedence will vary with frequency, voltage and thermal conditions.

That is why there is a headroom for margin. The head unit is set at 75%, so there is still a 25% margin for head room in the headunit. The amplifier is set based on nominal impdences, and you selected a -3 signal, which gives you another 3db of head room in the amplifier, plus there is at least 3 db above that so you should have 6 db of headroom at the amp. Signals should "clip" at above +3 db. Zero db should be 3 db below clipping.

As long as the entire audio path thru the preamps in the headunit and amplifer are below clipping, which they should be with these settings, and the speaker is within it's power range you should be able to run without any clipping, while still allowing head room for voltage peaks and impedence variations.

That's the point of setting gains. In pro-audio this is called a zero DB referernce. The idea is that a zero DB signal on the CD will be appear as a zero db signal the headunit preamp, zero db at the amplifier input stage preamp (gain), and at the amplifier output and at each speaker. With the same 3 db of headroom throught the entire system.

 

[DidUHearThat? 10+ year member]

If your amp gain input is set too low, you could turn your head unit up to full, it would distort yet still not give the amp enough power to drive it at it's full power. So you would hear distortion, but actually have unused reserve power that's not doing you any good.

If the amp gain is set too high, you could over drive the amp's input with the head unit at only 25% power, and then be driving your amplifer at full power while not achieving the full headroom that the headunit is cable of and you would hear more noise from the head unit than your should. The processing would not work correctly either and would also let you hear more noise from the head unit processing than you should if you were running it closer to 0 DB. For example e.q. phasing, noise and distortion that would be inaudible at 0 db could be very apparent at -25 db, making you think your head unit sucks.

Also it's pretty obvious what would happen if you gave your speakers too much or too little power for what they are designed to handle.

 

[helotaxi 5,000+ posts]

That is why there is a headroom for margin. The head unit is set at 75%, so there is still a 25% margin for head room in the headunit. The amplifier is set based on nominal impdences, and you selected a -3 signal, which gives you another 3db of head room in the amplifier, plus there is at least 3 db above that so you should have 6 db of headroom at the amp. Signals should "clip" at above +3 db. Zero db should be 3 db below clipping.

Nope. If you use a -3dB tone to set the gains, the amp will clip with a sigal above -3dB. A 0dB signal would be well into clipping. People set gains with a -3dB tone to get more power out of the amp at moderate HU volume levels with music. If you give it a 0dB signal at the HU volume that you used to set the gains, though, the amp will be trying to produce double the power that you set it for and will be well beyond clipped. By using as -3dB tone you have effectively removed 3dB of headroom from the amplifier.

 

[DidUHearThat? 10+ year member]

Nope. If you use a -3dB tone to set the gains, the amp will clip with a sigal above -3dB. A 0dB signal would be well into clipping. People set gains with a -3dB tone to get more power out of the amp at moderate HU volume levels with music. If you give it a 0dB signal at the HU volume that you used to set the gains, though, the amp will be trying to produce double the power that you set it for and will be well beyond clipped. By using as -3dB tone you have effectively removed 3dB of headroom from the amplifier.

!!! Duh!

I was smoking crack and got mixed up...

 

[helotaxi 5,000+ posts]

It happens.