My preout's not 5volt?

[wickedwitt 10+ year member]

Some headunits have a subwoofer output setting (not a bass setting its usually called sub output). This controls the preout voltage of your subwoofer RCA's. You usually have to set it to the max to get your full preout.

I thought that was only on Alpines. Most others 0 is full, anything - is off from full and anything + is boost.

Also, 99.9% of manufacturers say 5v and mean both channels of an output put together. In other words, your "5v" deck is really left channel 2.5v and right channel 2.5v (maximum) combined for 5v. In 90% of its use, you will see closer to 1.8v-2v per channel. In other words, you are probably safe setting the amp gain to 2v input.

 

[goingdef 10+ year member]

I thought that was only on Alpines. Most others 0 is full, anything - is off from full and anything + is boost.
Also, 99.9% of manufacturers say 5v and mean both channels of an output put together. In other words, your "5v" deck is really left channel 2.5v and right channel 2.5v (maximum) combined for 5v. In 90% of its use, you will see closer to 1.8v-2v per channel. In other words, you are probably safe setting the amp gain to 2v input.

you sure because that means my jvc having 5 pre-outs means it would be 12.5v of pre-out and that sounds impressive compared to 5v, allso I have hit a dmm to them and ya there 5v a piece.

 

[TheUnderFighter 10+ year member]

@ OP, just use a DMM and don't worry about what the sticker says.

 

[solidcrowd 10+ year member]

Yea I will... Just made me think why I looked so hard for a headunit with 5volt preouts and then I have this amp telling me it's closer to 1volt then 5.

 

[LosIsATool 5,000+ posts]

why not just use a dmm and not worry about the labeling around the gain adj? All my fosgate amps have that same layout and I have never used it, I think it's decoration since different ohm loads require different setting!

What?

 

[maylar 10+ year member]

Yea I will... Just made me think why I looked so hard for a headunit with 5volt preouts and then I have this amp telling me it's closer to 1volt then 5.

It's 5V at full volume with a 0dB source. In the real world, with real music, you'll never get there. My JVC is rated at 2.5V and I have to set my sub gain at like 0.2V to get rated power from my amp.

Don't worry about it, use the setting that works.

 

[maylar 10+ year member]

Also, 99.9% of manufacturers say 5v and mean both channels of an output put together. In other words, your "5v" deck is really left channel 2.5v and right channel 2.5v (maximum) combined for 5v.

Uhh... no. It's per channel.

Using a MOSFET opamp to drive the preouts, and a 14.4 V power supply, you can get 14.1 volts pk-pk (with distortion) which is 5 vrms.

 

[wickedwitt 10+ year member]

Uhh... no. It's per channel.
Using a MOSFET opamp to drive the preouts, and a 14.4 V power supply, you can get 14.1 volts pk-pk (with distortion) which is 5 vrms.

Why have I tested multiple decks on a tone with a DMM and got exactly 1/2 the "rated" output voltage per channel then?

 

[goingdef 10+ year member]

What?

Have you never went from a 4 ohm load to a 1 ohm load WITHOUT changing the gain setting?

1000 watts @ 4 ohms = 63.25 ac

1000 watts @ 2 ohms = 44.72 ac

1000 watts @ 1 ohms = 31.62 ac

how would you achieve these numbers without changing the gain settings, hence the numbers around the dial are sort of decorative since a 1 ohm load requires a totally different gain setting then a 4 ohm load regardless of the output voltage you receive from a HU.

 

[LosIsATool 5,000+ posts]

Have you never went from a 4 ohm load to a 1 ohm load WITHOUT changing the gain setting?
1000 watts @ 4 ohms = 63.25 ac

1000 watts @ 2 ohms = 44.72 ac

1000 watts @ 1 ohms = 31.62 ac

how would you achieve these numbers without changing the gain settings, hence the numbers around the dial are sort of decorative since a 1 ohm load requires a totally different gain setting then a 4 ohm load regardless of the output voltage you receive from a HU.

Yes I have. Its called using a scope. You dont scope to a certain load. The gain setting does not care what load is on it as long as it is stable.

If you do that formula like this. 1000 @ 1ohm, 500 watts @ 2 ohm... What do you get?

 

[goingdef 10+ year member]

Yes I have. Its called using a scope. You dont scope to a certain load. that is the reason the little formula is not accurate. Its just a safe way of gain setting. The gain setting does not care what load is on it as long as it is stable.

ok your going high-tech, I don't have a scope so I use the chart to be safe, but in the hypothetical sense if I set an amp for a load of 4 ohms I will see a output of 89.44 ac to get 2000 watts rms lets say it blows and I replace it with a sub that only carrys a ohm load of 1 it requires a lot less ac current to achieve the same output so if no adjustment was made to the gain smoke would roll quick since that would be nothing but a clipped signal.

 

[LosIsATool 5,000+ posts]

ok your going high-tech, I don't have a scope so I use the chart to be safe, but in the hypothetical sense if I set an amp for a load of 4 ohms I will see a output of 89.44 ac to get 2000 watts rms lets say it blows and I replace it with a sub that only carrys a ohm load of 1 it requires a lot less ac current to achieve the same output so if no adjustment was made to the gain smoke would roll quick since that would be nothing but a clipped signal.

Wrong. Gain is set to match the RCA preout voltage. NOT THE LOAD.

If your amp makes 2000 watts at 2 ohms it should make 1000 at 4 ohms. The math is the same

 

[goingdef 10+ year member]

Wrong. Gain is set to match the RCA preout voltage. NOT THE LOAD.
If your amp makes 2000 watts at 2 ohms it should make 1000 at 4 ohms. The math is the same

well just to be safe I reset it every time I change subs just to be safe and nothing has died yet, so I guess I would rather play it safe since telling the wife I need more subs has gotten to be uncomfortable. And is always followed by what did you do.