Can you underpower a Strong amp?

I'm putting together a system and I'm looking at a PG Xenon 200.4 amp and 1-2 JL Audio 10w1v2 subs. 

https://sonicelectrnix.com/item_2750.html   amp

https://sonicelectrnix.com/item_5161.html  sub

I plan on running the components off the front channels and then either one or two subs off the rear two.  My question is, since the components only need 100 watts and the subs need 150 watts, can I prevent all 200 watts per channel from being used?  Does the input sensitivity regulate this and if so can it be set differently for each channel (lower on the comps and higher on the subs?).   I've read how to set input gain using a volt meter but I'm not sure if that's doing what I think I'm trying to do. 

I could go with the 100.4 amp which I'd have to bridge the two rear channels for enough power on the sub but I figured for $40 more I can upgrade the amp to handle any future system (this is for my system).  Any help would be greatly appeciated. 

That amp will be fine for what you are trying to accomplish.

No, you cannot regulate the power being produced by the amp particularly (although backing down the gain will accomplish this to a degree), but the real question is will it harm any of your speakers in this situation? No. You will simply have some head room and will probably be happier to have the extra power to spare in the long run.

If you haven't read this yet, give it a good read as I think it may explain what you are asking.

You can set the gain using the test tone/mutimeter method.  Do a search here or go to JL Audio's websiste.  Set the gain to the speakers' rated RMS power.  This is assuming that your head unit and amp are a good match (the head unit's outputs don't overdrive the amp's inputs).

To second what geepherder is saying, you would be setting the gain so that the amp reaches - not the highest output it can supply - but the highest input allowed to the driver.  The amp will reach that point when the head unit volume is at the highest designated clean output (designated, of course, by you after testing for that point.)

In this case of setting gains and deck volume max, you would listen for the clipped signal first with the deck to find that high point.  Then, with the deck at the high point and speakers unloaded from the amp, measure the voltage output of the amp through the speaker terminals.  Use Ohm's Law calculations to find the target voltage that corresponds to your speaker's RMS and set the amp's voltage output to that point.  The JL flash tutorial is a good visual of that procedure. 

Hahaha.  Smart aleck, or is that a serious question?  If you use the gain to control the amount of power an amp puts out (due to the power limits of the speakers), you can later reset them when you upgrade your speakers for more power which can net you some more volume.  Here's an example of what DYohn is getting at:

Suppose you have an amp that will accept up to a 4 volt input, but you're using an Eclipse deck with an 8 volt output.  Even if you set the gain to minimum, the amp will still produce all the power it's capable of producing.  The solution to this would be to set the gain with the volume on the headunit at a lower setting, thereby limiting it's output rather than using what the deck is capable of putting out.  Let's say you find the volume level where the deck is putting out 2 volts.  You can use that to set your gain, limiting the power output of the amp.

I'm familiar for the JL flash instructions as I've looked at that a couple of times. 

Another post that addresses this found at https://www.caraudio.com/vb/showthread.php?t=63933

according to it, the formula is Sqrt(Power*Resistance) = Voltage
In my case, I would use the power I NEED not the power the amp can supply right?  So if 150 is the ideal RMS for the sub (4ohms), squareroot (150x4) = 24.49 volts. 

If this is correct, I'd just dial in 24.49 volts with a dvm at 3/4 total headunit volume using the appropriate sine wave for the subs vs components.  I'm not sure how JL audio believes 3/4 volume is a good starting point but it should work I guess. 

After this is set, will turning up my subvolume on the HU to balance the system level affect this calculation?  It seems if I do it will the system flat, I wouldn't want to increase it at all (past 3/4 volume or using any boosts).  I wonder how you then balance the components and the subs?  Any help at this final stage?  Thanks so much, this has helped me tremendously

Dope!  I replied too soon, JL's site says you simply turn down the sensitivity on the AMP after everything is reached to the ideal voltage to balance the system. 

(to geepherder...)

No, I'm being serious (for once tonight).  There was a reason DYohn said that.

...and before I caught the update and was able to delete mine.  Now I have to go back and read everything...

Okay, I gotcha geepherder.  (To see if I got it right..)  It may not always be the case where you can just limit the turn of the gain.  There are times when you also have to limit the volume of the deck.  The test would be, I suppose, if you weren't ABLE to turn the gain down far enough to reach the target voltage (with the deck volume at full).  That's where you have to reset the deck volume to a new, lower high point and then again do the adustment from there.  You would have to strictly control the deck volume level!

bluesdeluxe wrote:

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After this is set, will turning up my subvolume on the HU to balance the system level affect this calculation?  It seems if I do it will the system flat, I wouldn't want to increase it at all (past 3/4 volume or using any boosts).  I wonder how you then balance the components and the subs?  Any help at this final stage?  Thanks so much, this has helped me tremendously

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You can't increase power output by using fades or balances.  Fading one way just takes away from the other side.   But when it comes to a balance of frequencies...if using the sub fade causes the signal to be larger in the sub frequencies....I can see how that would affect the signal that the sub amp receives.  So it the signal is larger than what you set the amp to, the voltage output should be greater than what you tested it at.  My initial take on this is to set that sub fade control to the highest point before setting the sub amp gain.  Or, if it has it, at the point of '0' or 'flat'.

Yeah, it's not a great way to do it, because the problem is then due to mismatched equipment.  That's just an example of what could happen if you're forced to use that equipment.  The best thing to do would be to change some equipment around.  Maybe a customer brought in their own ebay gear and refused to use anything else.  That's when this would come into play- kind of a "Mickey Mouse" way of "fixing" it. 

At this point you must also explain to the customer how critical it is that they do not turn the volume past what you have dictated, or they could easily smoke their speakers due to their poor choice of equipment.  Then when they do, you can tell them "I told you so" and set them up with new gear.

I see what you're saying, but here's my take- your ears won't know whether the speakers are recieving 75 watts a piece as opposed to 100 watts a piece.  It may still be possible to exceed a speaker's thermal limits without exceeding it's mechanical limits.  That means it may still sound good because you're not causing your speaker to "bottom out" yet (and the amp is not clipping), but you could be causing too much heat to build up in the voice coil.  When this happens, you can kiss your speaker good bye.  Just because a speaker has a certain rms rating does not mean that you need to feed it that much power.

So that the amp outputs more than the speaker is capable in RMS terms?  if you use RMS and continuous power handling numbers for the calculations (as in this case), there is room for peaks on both sides (amp and sub).  The amp still peaks at higher voltages and the woofer still handles those higher peaks.  Remember, we are using all-day RMS numbers on both sides. 

So, will the peaked voltages be "cleaner" if the amplifier is stronger than the woofer it feeds?  That, I think, is where a more equal match of equipment makes the most sense...the subwoofer has to produce the sound from that peaked voltage.  It doesn't matter how clean that peak is if the sub can't produce it faithfully.  With this rationale, matching RMS makes the most sense to me.

well it's good to see everyone so interested in a post made by a newbe ; ) 

As for matching systems correctly, I'm going with an entry level system (or a low price system that's been read up on extensively), and I basically want to have some extra amp head room in case I find myself wanting more bass or nicer subs.  Premier-Pioneer head unit/ Phoenix Gold amp/ JL subs can't be a bad first investment!  I guess as everyone has said, it seems if I set everything up correctly I'll simply have more head room for later applications without blowing my subs and I need to set my sub voltage with the HU level up a big or at least in the middle (0). 

As for people having HU that have higher outputs that STILL overload the amp even at lower input gain levels.......is there not something you can put inbetween the two?  Something that would drop the voltage from the HU?   Looks like my HU (P670MP) has a preout of 2.2 volts so I guess I'm not even close to having this problem.  

Two years ago I had the idea of a head unit volume limiter which could be installed and set at the point where the volume shouldn't be turned up past.  Maybe an amber light that switches to red or a real voltage limiter that gives you no choice.  But I'm not an engineer...so 'nuff said 'bout that idea. ( I still don't see one on the market so maybe I should look into this again.)  But what I'm saying, bluesdeluxe, is that the volume knob is the limiter and the person who touches it controls the input voltage.  I cringe when women and girls reach for the volume knob!  "I love that song, TURN IT UP!"

Also this:  yes, set the levels to 0 for the band range of each corresponding amplifier before setting gain with the meter.  Any movement up from the flat 0 point would result only in a reduction to the other outputs, not an increase to that particular output.  0 or "flat" indicates the reference point.

DYohn, good to hear from you on this.  "Thermal ratings" is the part that stupifies me.  When I look at woofer specs I see RMS power, nominal power, max power and of course all the T/S parameters and SPL  How can we look at a woofer specs and see what the thermal power handling capability is?  Is this just one of the power handling specs and I just haven't gotten it yet?  (...which wouldn't be the only thing I haven't gotten yet!...)

tcss] wrote:

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So another question. If thermal limit equates to RMS power handling is an advanced cooling system like JL's "forced air cooling" responsible for the woofer's power handling or is it voice coil, magnet, ecursion ( spell?) or in other words the mechanical limits of the woofer? Good to hear from you again "Ham Fist"

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