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    Capacitor on a 300W amp?

    Yet another cap question here, and yes I've already searched, my question is a bit different though:

    I have a Phoenix Gold 300.2 bridged to a pair of JL Audio 10W3 250W subs in my '92 240SX. Not a lot of wattage, but a lot of thump for the car. However, I have to turn the bass boost up on the amp and the head unit to get the sound I'm looking for, and I've already blown one amp this way (luckily it was under warranty). I've been okay for 6 months or so (lower amp setting, higher head unit setting), but I don't want to put unecessary stress on my amp. Plus, it sounds much better if I don't have to give bass with the head unit. I realize that a cap gives power to the amp when it needs it, and probably wouldn't help it from blowing, but could it ease the load on it by giving the power easily? I listen to mostly rap and techno, so very few thunderous bassquakes.

    The other thing that makes me think a cap might be a good investment is that I read on Crutchfield that if the bass is sharp when you turn the car on but turns to mush in a few minutes, a cap could save you. Now, I wouldn't say my bass turns to mush, but it's much sharper and feels stronger when I first turn it on. I always thought this was due to my ears becoming adjusted, but maybe not?

    Thanks for input. I'm not looking for a bunch of "caps ****, buy a new battery" posts either. =)







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    Re: Capacitor on a 300W amp?

    Save your money on the cap and buy a bigger amp. The only fix for the problem you describe is more power than you currently have on tap. Either that or upgrade to bigger subs.

    The bass getting weaker is mostly due to voicecoil heating and loss of motor power, nothing a cap can do about that either.




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    Re: Capacitor on a 300W amp?

    What is the RMS rating of the subs and the amp? If you can't afford a bigger amp try to port your box if its not already. A capacitor is unnecessary either.



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    Re: Capacitor on a 300W amp?

    Thanks for the responses guys. =) Maybe I'll concentrate on sound dampening (specifically that darned hatch) before I try to get more power anyway.

    The amp is 300W, the subs are 250W each. I don't know the RMS ratings... (Edit: I checked the websites, JL Audio doesn't mention it but the amp is 30W/rms x 2 according to Phoenix Gold.)




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    Re: Capacitor on a 300W amp?

    If you want more bump, get better equip instead of dampening.

    Invest in an RE SE and a hifonics 1000d or similar.



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    Re: Capacitor on a 300W amp?

    I would suggest a bigger amp. If you want to stay with PG step up to the 500.2. If you don't care about the brand, shop around. What freq is your crossover set to BTW?




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    Re: Capacitor on a 300W amp?

    If I get a better amp, won't that just cause MORE noise, and more need for a cap or another battery? Thump is fairly important to me, but so is clarity. Lack of rattle. Is that a pipe dream?

    I can't remember what my crossover is at. I played with it for months before I settled on something I liked. It's between 100 and 150. The sensitivity is fairly high as well. And I think the bass boost is at about half. My head unit is at +2 bass (max is +7). I would be interested to hear what you think is the ideal way to get good clear power with this current equipment, actually. Bass at the head unit is bad because it adds it to the midranges too and sounds crappy. I originally thought adding it with the amp's bass boost would be good, but then when it blew and I took it in for warranty, the tech made it sound like like it was my fault for messing with the configuration, and advised me not to put it past half or 3/4 if I had to adjust it at all. So that's why I lowered the sensitivity and raised the crossover. *Shrug*




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    Re: Capacitor on a 300W amp?

    If you want loud and the rattling bothers you, you must address the rattling. Track down the rattles and deaden them. Being afraid to use a bigger amp because you are afraid of more rattles and complaining that your system is not loud enough doesn't make much sense.

    Adding bass boost adds distortion and leads to clipping. A clipped input signal will kill an amp. How are you powering your high range speakers? Bass controls on HUs tend to affect a very broad frequency range and are usually centered at a relatively high frequency. Keeping the bass control from affecting the mids is almost impossible because of this. A larger amp will effectively boost all the frequencies that are played through it.

    Remember also that sensitivity controls are a set and leave device. They are made to match the output voltage of the source unit to the input stage of the amp. They are NOT a volume control. "Turning up" the gain will increase your chances of blowing your amp and subs. As long as you are giving your subs clean power they will soak up a ton of power. Once you ask more of your amp (through excessive bass boost or gain setting) you are no longer working with clean power. Clipped output approximates a square wave or pulsed DC. DC through a speaker is a certain death sentence.




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    Re: Capacitor on a 300W amp?

    Hm. Well, like I said, I haven't had any trouble in the past 6 months or so. But assuming you're right, and it sounds like you are, what should I set the gain and bass boost to? When the guy that installed them originally set it, the bass was pretty weak. So you're saying I shouldn't have touched the gain? What SHOULD I set it to? I want to treat my equipment well. =) The manual fails to mention any danger of blowing anything whatsoever, it just says the knobs are there for configuring to your liking.

    As I understand it, if you cut the gain all the way up, it only picks up the most powerful sounds, which is why I adjusted it to be more sensitive to pick up more drums and stuff. I don't, however, know exactly what it does, or what its purpose is beyond that.




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    Re: Capacitor on a 300W amp?

    The gain is a level matching device, nothing more. There is no standard for line level voltage like there is in home audio. In the home its all plug and play, but in the car you must compensate for all the different possible preamp voltages both to maximize your output and to minimize distortion and noise.

    Say for example, your amp was only designed to accept a 2v RMS input. Anything less than 2 volts and your amp would not make its rated power. Anything more and you would overdrive the input stage which would then overdrive the output stage. End result is clipping, distortion and a dead amp or speaker. The gain allows you to adjust, within a certain range, to compensate for different voltages. A low gain setting is used for a higher voltage input and vice-versa. Setting the gain too high for the voltage you have will exceed the capability of the input stage of the amp and cause it to clip. Feeding a clipped signal to the output stage is asking for disaster.

    I could go on and on about the technical aspects of all of this but the bottom line is when you try to make an amp give more than it is able (excessive gain, bass boost and volume) you will destroy equipment.




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    Re: Capacitor on a 300W amp?

    All very interesting but I still have no clue how to find out where I should set it to, so I guess I'll just leave it and hope it doesn't blow. =) Thanks.

    I don't know exactly what the RMS of the amp is, but even if I did know, I still don't know how much power it's getting or whatever, and therefore what to set the sensitivity to. Will it say on my battery?

    Also, the sensitivity knob goes from like "2" to ".4" and I have no idea what that means. =/ If you have a tutorial place you could point me to or something, I guess that would be cool. Anyway, thanks again.




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    Re: Capacitor on a 300W amp?

    Quote Originally Posted by Darwin
    I have a Phoenix Gold 300.2 bridged to a pair of JL Audio 10W3 250W subs in my '92 240SX. Not a lot of wattage, but a lot of thump for the car. However, I have to turn the bass boost up on the amp and the head unit to get the sound I'm looking for, and I've already blown one amp this way (luckily it was under warranty).
    What you are doing is abusing your amp.
    It's made to make a certain amount of power.
    What that means is, you feed a normal signal in (picture a sine wave), and the amp replicates that wave, of a larger amplitude.
    It can only make a signal of so large of an amplitude. That's an inherent limit in your amp. Always keep that in mind.
    If the highest voltage the amp can output is 35v, that's all you'll get (I'm assuming 300w at 4 ohms here).

    Your gain control is simply a doorway.
    It's a control to tell the amp "expect an RCA signal of ____ volts".
    If your RCA's are 2v, for example, you set your amp to expect a 2v signal.
    If instead you crank your gain control - in doing so you are telling your amp to expect a 1/4v signal.... What does this mean?
    Picture that sine wave again. The middle represents 0v, the high peak represents 2v, and the low dip represents -2v.
    Now, draw a line at +1/4v, and -1/4v.
    All of the wave above the +1/4v line, and below the -1/4v line will be lopped off, before the amp ever sees it, because of your gain settings.

    So what you end up with is more of a square wave... rising sharply, then remaining at the maximum voltage level for a long time, before sharply dipping and repeating on the negative side.
    This is not what the amp was designed for, it's abusive.
    The amp still can only reach 35v peaks on the output end to your subwoofer... but it's holding that 35v level longer, flowing 9a of current longer... which results in more heat, which is not good for your amp.

    Yes, it results in more power to the speaker, but only from an "area under the curve" perspective. Much of the extra power isn't going to be actually powering the speaker to move farther, in fact the farther you turn the gains up, the less of an "increasing loudness" effect it has... but the damaging effect, the heat effect is linear as you turn the gain up further.

    Bottom line is, you need to set your gains correctly, to avoid damaging both your amplifier AND your speakers.
    If you don't have enough output with the gains set that way, turning your gains up is not the answer, getting a larger amplifier is.

    A capacitor won't help this situation, because it can't change your waveform.

    What you are trying to do is not only tell a 7' tall man to walk through a 3' tall doorway... but you are telling him to run towards the doorway, the doorway being invisible, and with a razor-blade upper threshhold.
    He'll get lopped... and more than just in half. Bad.

    I've been okay for 6 months or so (lower amp setting, higher head unit setting), but I don't want to put unecessary stress on my amp.
    The undue stress is from your gain control.
    Turn it down.

    Plus, it sounds much better if I don't have to give bass with the head unit. I realize that a cap gives power to the amp when it needs it,
    A cap can be a nice device to help augment the amp to get over the transition when you are drawing so much current from your car's electrical system that it forces a momentary switch from drawing current purely from the alternator, to augmenting that with some draw from the battery (as batteries respond slower than capacitors), but it's response is instantanious in nature, and disappears as fast as it helps.
    Saying a capacitor supplies power to the amplifier when it needs it is not quite accurate.

    and probably wouldn't help it from blowing, but could it ease the load on it by giving the power easily? I listen to mostly rap and techno, so very few thunderous bassquakes.
    No, turning your gain control down to where it should be, to avoid clipping the waveform on the input (think of that as technically before the actual amplifier stage ever sees the signal), so that the amplifier isn't trying to amplify - and send to your speakers - square waves.

    The other thing that makes me think a cap might be a good investment is that I read on Crutchfield that if the bass is sharp when you turn the car on but turns to mush in a few minutes, a cap could save you. Now, I wouldn't say my bass turns to mush, but it's much sharper and feels stronger when I first turn it on. I always thought this was due to my ears becoming adjusted, but maybe not?

    Thanks for input. I'm not looking for a bunch of "caps ****, buy a new battery" posts either. =)
    Caps don't ****, but they aren't going to do what you are looking for.

    What you need to do, is buy a larger amplifier.
    And when you do, bear in mind upgrading from 300w to 400w isn't going to help you here. Upgrade significantly.
    It takes a 4x increase in power to effect a 2x increase in output. I wouldn't bother with anything less than a 2x increase in power if you are shopping... That will result in a 3dB increase in output. A 4x increase would result in a 6dB increase in output (if your woofers can take that much)

    Ironically, you can be increasing the measured "power" your amplifier is producing by a terrible 2x factor... but it's all because it's producing square wave signals, producing more heat than it can dissipate, due to the longer time that current is flowing during operation. Familiar with the concept of "duty cycle" in tools, or motors? It's similar to that.

    The less expensive option is to downgrade your expectations.
    Normally warranty shouldn't cover this sort of abuse, gain setting is something covered in the manual, and operating the amplifier in the manner you are is simply abusive. It's not a matter of "if" it'll result in damage to your amplifier or speakers, it's "when". It's not designed for that... and shouldn't be (who wants to hear that? Can you imagine the distortion % inherent in that?)




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    Re: Capacitor on a 300W amp?

    Thank you, Geo. THAT'S the kind of response I was looking for. I will check to make sure the settings are safe for now, and when I can justify the cost I'll shop for a stronger amp.

    Two parting questions:

    1. If my subs are 250W each, wouldn't anything over a 500W amp be treading on thin ice? Or is that an overly simplified way of looking at it?

    2. The bass boost on the amp is from +0 to +18 dB. The manual says to use it sparingly because "every 3dB of boost requires double the power at 45Hz"—can you tell me what exactly this means? Does this mean that if I had tiny speakers I could run this boost all the way up, since they wouldn't be drawing hardly any power? The last time it blew, the tech said it was because my bass boost was up too high (it was at least probably +14dB—whoops). He recommended not turning it up past half way. Did he pull that out of his ***? What told him that it shouldn't be higher? What's the limiting factor in my equipment?

    Thanks again for your extensively helpful post.

    Edit: I just checked the manual for my head unit (an Aiwa CDC-X217) and it claims 2.2v preamp output. My amp's sensitivity goes from 2.0v to 0.4v. What do I set it to? If I set it all the way to 2v, won't that still clip? Thanks. =)




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    Re: Capacitor on a 300W amp?

    I have heard of people running 500W a piece to W3s so I wouldn't worry about running considerably more than 500W to the pair. You will almost never actually get that much power to them anyway playing music at any level that wouldn't cause severe long term hearing damage.

    As far as the bass boost goes the tech was correct. Like geolemon said for every increase of 3dB in output you must supply twice the power. Therefore for every 3dB of boost you add you are asking the amp to provide twice as much power at the center frequency of the boost. You can run a lot of boost if you are no running the amp near its maximum output, but turn the volume up and your headroom for boost decreases quickly. Once you hit the amp's limit you are not getting anymore.

    You will only get 2.2V out of the head unit with a 0dB test tone with the volume maxed. The output of most HUs will start to clip before you reach max volume so the 2.2 is just a theoretical value. At music listening levels, you will never get nearly that much voltage out of the head unit just like you will not run the full RMS power of an amp to the speakers. You should be safe as long as you keep the gain below half and keep the HU volume at a reasonable level. As I mentioned above, your HU has limits as well. Turn up the volume too high, especially with the HU bass boost up, and the output will clip. If the HU is clipping, you will damage the speakers and amp, no matter what the gain setting is on the amp.

    From a purist standpoint, you would need a tone generator and an O-scope to properly set the gain on an amp. First you turn up the volume with a test tone while monitoring the output waveform with the scope. Note the volume at which the output starts to clip. This is the max volume for the HU and you will get no benefit from turning it up beyond that point. With the volume set a maximum unclipped and the amp gain at minimum, slowly increase the gain on the amp while monitoring the waveform of the amp output. Once the signal starts to clip you back the gain down to just below clipping and leave it there. With all that set you must remember that any boost you add anywhere will decrease the amount you can turn up the volume.




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    Re: Capacitor on a 300W amp?

    Thanks Helo. =) I was mistaken actually and it goes from 4v to .2v, not 2v to .4v. So it could easily accomade the head unit's max 2.2v.

    I don't know what a tone generator or an o-scope is, but I'm fairly certain I don't have any.

    I checked the settings when I went out today. I generally keep my head unit volume at 75% (60/80), unless there are other people in the car in which case I back it off a bit. I keep the treble boost at +2 and bass at either +2 or +4 (all out of -7/+7 min./max.), which seems to get the best sound without distortion. The amp I set to about half sensitivity (which should be about 1.9v) which, while dramatically reducing bass, at least won't hurt them. I set the bass boost somewhere between 1/4 and 1/2 (+4.5dB and +9dB) and the crossover at about 180. Now it sounds like I have some 6x9's rather than 10" subs. Oh well. ;D Maybe I'll invest in a bigger amp sometime, or these scopes and stuff to properly configure my current setup.

    In the meantime, thanks for the lessons. =D




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