I have a 50-250w RMS sub and a 300w RMS amp. The instruction manual for the sub says it recommends amplifiers whose RMS output is lower than the RMS of the speaker. So my question is if I turn the gain down, will that be OK? Or what exactly does the gain control?

 

Gain decreases voltage thus lowering the power. To adjust gain you would need to listen to a bass track and put the gain in the position where your sub does not distort. If you turn it all the way up, it can blow the sub (clipping) so set it about half way and adjust it up or down from there.

 

Disc108,
The vendor usually means: look at the max rated power on the speakers. Then look at the max rated power on the amp. Now, choose the speaker that is rated higher than the amp.

In the real world, this means some day your little brother is going to spin the vol **** on your stereo to see what it does. If the 300w amplifier then puts 300w into your 100 w speakers, your speakers are now dead. But really, the amp vendor watt rating (with some spec on distortion like 0.5% THD) is not really the max out rating (unlimited distortion). So put a little margin for error in there and get slightly larger rated speakers.

Just turning the gain down is not OK. Sooner or later someone will turn it back up.

BTW, the distortion does not change the total power level that is coming out of the amp, it just spreads the power over a wider frequency band. Usually it adds more power up at the higher freqs (hiss, pops, clicks and clips are all high freq sounds). That just changes what part of your speaker rips out first. More highs kills the tweeters, low freqs rips the bass cones.

Tom

 

While true...I think this is too much of a generalization which could lead to too much fear.

Turning your volume up will not garauntee a destroyed sub. Music is too dynamic to say that too much volume will destroy a sub.

There are other factors which I think could destroy a sub easier (assuming your speakers are closely matched to the amp).

The general rule of thumb (at least, as I've always heard it...) without an O-Scope is that you turn your HU a bit more than 3/4 of the way up with the gain all the way down. Start turning the gain up until you start hearing a bit of distortion, then back the gain off just a touch. This is considered set...(I think )

Tim

 

So does turning down the gain affect the wattage output? I just realized the amp is 400w x 1 (4 ohm) and the sub is 250w. Both numbers are RMS. Since the amp is 2 channel, can i just hook up the woofer to only 1 channel and not bridge it? It's 150 x 2 (RMS).

 

Ted, usually the more power the better. You can have a 1000WRMS amp and adjust the gain to play with a 100wrms sub. Bridge the amp, put gain at 4/6 and see how it sounds. Turn it up a little and see how sub is behaving. Don't turn it up all the way but get to the point where your sub would distort a little (running out of excursion, sounding bad). From that point turn the gain down a little where sub sounds fine. Remember to do that with full volume set on your headunit. This way if someone plays with your stereo, they won't blow the sub.

 

Sweet. Thanks Alex.

 

Good explanation Retro. I'm not sure about subs, but more multi-range speakers are blown from "too small" an amp rather that "too big" an amp. Although that sounds odd; the user of an underpowered amp will always be driving around with the volume turned up because it's not loud enough. He'll keeping turning it higher and higher, getting more and more distortion. Since that distortion takes the power out of the low /mid freqs and puts it in the high freqs - pop goes the tweeter dome or even the midrange cone.
In that same setup, a higher powered amp would be left at 50%, be plenty loud and produce minimal distortion. Much less power will go to the tweeters.

Tom