Gain/Bass Boost

I've searched the topics here and have found numerous posts on setting my gain.  However, there are some threads that say that I shouldn't use my amp's Base Boost feature, but I've found that it sounds better when I turn that up.  I'm going to play around with the whole system tomorrow (again) and try to set it correctly, but I'm pretty happy with the way things sound right now.  However, I don't want to blow up my new amp or subs.  What am I going to hurt by running the Base Boost, or if the gain is up too high?

If it matters, I'm running a factory Bose system with a Bazooka BA1500 amp (500W x 1 channel at 2 ohms) and 2 10" Orion XTR SVC subs.

You can use the bass boost, as amplifiers wouldn't add them if there weren't some benefit from using them on some systems.  The general rule is that IF the system were set up with the proper quality/quantity gear for what you expect to achieve....and if the vehicle is amply damped for best quality/quantiy sound....you wouldn't need to use the boost feature.  But if using it makes things sound better on YOUR system, don't feel bad for doing so.

But understand what the bass boost does, and what you might be able to do to the system to correct the faults that are making you rely on the boost feature in the first place.  The amplifier is using it's available power to concentrate into a bandpass region, generally at about the frequency that most cars will vibrate most at, about 45 or 50 Hz. 

This exertion of power into the boost frequency starves the rest of the range of frequencies you need for a full sub sound, so in most cases one would try to compensate by turning up gain screws or turning up the bass response on the deck.  This quickly, and I mean very quickly, causes the amplifier to clip the signal (and the deck can do the same thing) which will cause the attached load of speakers to live a very short life.

Setting the gain must be done after you adjust the bass boost, even to a slight degree.  As you saw above, just a slight turn would net 3 db...and that's a doubling of power output to that boost frequency.  You can achieve good results by using the feature, but you must also take considerable care to set the amp's gain.  Problem is this:  most people who would use the boost feature are not the type who take great care in gain setting.

The gain must not be too high.  Period.  Best way to do it with your system is to use your ears, and take your time.  You may need to do it a few times, as ear fatigue can play a role, as well as the audio source you are testing with.  And if you turn the boost up or down, do it all over again.  Try to use no more than 3 db gain with the boost, as anything more than that points to inefficiencies in the system that should be corrected by other means.

Here is an exerpt from one of DYohn's posts about setting gain.  The subsonic filter mentioned in it would not be applicable to your system:

"Clipping is what happens when an amplifier is driven too hard and involves compressing the peaks of the output sine wave (it shows up as a near-square wave on an ascilloscope, and is called clipping since it appears that the sine waves have been clipped off.)  It can be detected by ear as a "shrillness" or "distortion" of a pure tone.

Here's what I suggest for your setup.  You will need 1) hearing protection, and 2) a CD with a test tone on it at about 60 Hz.  Also, I suggest setting the low-pass crossover on your amp up to about 80 Hz.

First, turn the bass boost off (and leave it off) and turn the gain on your amp all the way down (counter clockwise.)  Next, set your head unit volume down and play the test tone.  Wearing your hearing protection, slowly turn the head unit volume up and listen to your sub output until you can detect that the tone seems to have changed (it should seem to get a little "sharp" or shift upward in frequency.)  This is being caused by clipping in your head unit or your factory sub amp.  Turn the volume down until the tone returns to normal.  NOTE THIS SETTING.  This is the HIGHEST you can ever turn your head unit volume without introducing clipping.  Next, leaving the head unit playing the tone at the just-below-clipping point, slowly increase the gain on your sub amp until you again hear the tone change, then back it off.  Leave the gain set here.  You have just matched the input sensitivity of your amp to the maximum non-clipped signal from your system.  Turn the volume down and remove the test CD (and your ear plugs) and you shuld be good to go.  But get that subsonic-filter if you want to keep that woofer for a while."

That's about how gain is set using any system.  If you're using the boost, try it at about 3 db and set the gain after the adjustment.

Build the box so that it performs well in the worst case scenario and, in return, it will reward you at all times.