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settings gains/overpowering a sub

H

hotdimmes

Hard-core CEG'er
Joined
Dec 26, 2003
Messages
4,112
Location
New Britain, CT
I've read through many sites on how to set your gains properly, using an Oscilloscope, which I have access to. i.e. disconnect speakers from stereo, drive volume to just below clipping, remember position/disconnect amp outputs, drive gains to just below clipping.

That all seems fine, but how do I know what actual rms power delivery my speakers are getting? My concern is that I have a 150W rms sub, hooked up, two two of my channels, bridged, and I want to make sure it never sees more than that, since the amp is rated at 180W bridged.

I think I can figure the math out using my old Electrical Engineering text with some P = I × V = R × I^2 = V^2 ⁄ R action but I'd rather have one of the site's gurus point me in a much easier path, if possible.

I guess the easiest would be V^2/R? See what voltage the amp clips at, and my R is 4ohms?
 
You are better sending more power to the sub than it's rated for and keeping the gain down and it will put less stress on the amp. Ideally you want the gain to match the voltage of the headunit's rca-preouts, which in most cases is 2-2.5v, and found at about the 11 o'clock position on most amp gains. You should never use any bass boost on the amp as this is what clips the signal and kills the life of the sub. The rms rating is really just how well the speaker can dissipate heat, so the bigger the voice coil and more space for air to get in and cool the better. I had a sub that was rated at 150wrms and that seemed pretty conservative since the sub had a 2inch voice coil. I ran 187.5wrms to it no prob. The subs I have for the winter are rated at 300wrms and I have given them full volume with 375wrms and they didn't smell funny or seem to be bothered by the extra power.
 
what kind of amp and what kind of sub? overpowering a sub is not a big deal as long as its a clean signal, cheaper amps clip the signals even if you 'think' the gains set correctly. Oscope is the only way to set a gain IMHO. just make sure the sub can take the power mechanically, thermally it shouldn't be a big deal. but at just 30 watts (doubt its even that) it should be fine.
 
specs on the system are

HU: Kenwood excelon KDC X791 with 4V preouts

Amp: Kenwood KAC-8404 4-channel car amplifier
  • 50 watts RMS x 4 at 4 ohms (75 watts x 4 at 2 ohms)
  • 180 watts RMS x 2 bridged output at 4 ohms (4 ohm stable in bridged mode)
  • 2-, 3-, and 4-channel operation
  • CEA-2006 compliant
  • high-pass (12 dB per octave) and low-pass (24 dB per octave) filters variable from 50 to 200 Hz
  • variable bass boost (0-18 dB at 40 Hz)
  • cooling fan
  • MOSFET power supply
  • speaker-level inputs with signal sensing turn-on
  • preamp-level inputs
Sub: Tang Band W8-740P
*Power Handling: 150 watts RMS/300 watts max
*VCdia: 2"
*Impedance: 4 ohms
*Re: 3.2 ohms
*Frequency range: 28-1,000 Hz
*Fs: 28 Hz
*SPL: 84 dB 2.83V/1m
*Vas: .81 cu. ft.
*Qms: 10.53
*Qes: .30
*Qts: .30
*Xmax: 12mm
*Dimensions: A: 8-1/4", B: 7-1/4", C: 4-3/4"
 
I just installed that same amp in a buddys car last week, the gain setting has the voltage written on it iirc, so just set it to 4v and you'll be good to go. That sub has a 2inch vc on it, so it should be able to handle upwards of 200wrms with still no prob.
 
ah so thats what those numbers mean. I hate when numbers don't have units next to them. I just figured they were an arbitrary gain number scale.

I'm discovering more and more options in my head unit as i investigate further. I think I realized today that the high pass filter options dont work for the front/rear pre-outs.

I was messing with the gains a bit on my lunch break. I was planning on running the LPF at 80Hz, and my HPF at 60Hz.

running the amps with all the gains down, bass boost on, and running the "Loudness" option(compensates for low and high tones during low volume) on the headunit seems to give me the most desirable sound. It didn't seem like the sub was clipping, but I would rather achieve that same sound without the bass boost, as you said.

On the headunit, I've got options for "Bass Center Frequency" "Bass Q Factor" and "Bass Extend" which is pretty much the same as "bass boost" on the amp.

I've also got a "Subwoofer Level" from -15 to +15 on the headunit. I've been playing with this stuff, but I wish kenwood would be more explicit about what they are actually doing behind the scenes when setting this stuff.
 
Don't just set the gain to the 4 v setting I'd say you would be lucky if half that voltage is making it back to the amp. The resistance in the rcas cut it down. And there is no "cea standard" regulation on preout voltage so companies lie to sell product. I have personally tested radios claiming 4-6v that only send 1v to the amp before clipping and these are big name brand radios not junk ones.

Set the head unit flat with all bass boosts off and use your scope to check for signs of clipping if you like how the bass boost sounds turn it on with the scope connected and watch for clipping. If you feel the sub is being overpowered leave the gain at it maximum unclipped and turn the subwoofer level down on the head unit.
The forumla you posted is the easiest way to determine wattage going to the speaker but I really don't think it will matter if your over 150 watts with a clean signal.
Hope this helps
 
The rated power when the amp is bridged is rated at 1khz, so I doubt the sub will see more than 130-150wrms. Also the bass boost on the amp is an on/off +18db, which is huge and uncontrollable. Because of this, I would think the amp would have a greater chance of clipping the signal especially with any sort of bass boost from the headunit. It seems the amp when bridged is more suited to run a strong set of mids/components rather than subwoofers.
 
I appreciate all the info. I guess I just gotta play with the settings and find something that sounds "right"

Right now, I'm running these up front, off the other two channels on the amp:

h130TSD720C-o_mt.jpeg


Pioneer Premier TS-D720C



  • component speaker system includes two 6-3/4" woofers, 2 tweeters, two external crossovers, and two woofer grilles
  • dual layer woofer cone (injection-molded polypropylene with outer basalt fiber/Aramid skin)
  • butyl rubber surrounds
  • 1-1/8" polyester soft dome tweeters
  • 2-way external crossovers (2800 Hz at 12 dB/oct)
  • 4-ohm impedance
  • 2-60 watts RMS power range (260 watts peak)
  • frequency response: 30-33,000 Hz
  • sensitivity: 88 dB

====================================
The trouble I'm noticing is either in my source of music, or my crossover settings, and the possibility that my mids are kinda blown because I definitely had the gain cranked on them for like a month, when I didnt understand gain settings. ugh. They sound a bit muddy and not clear. Tweeters seem fine still.

It seems that on newer rap music, the sub hits hard and sounds great, but any kind of mid 90s music, or rock music, it makes itself much less known. This is with sirius radio and mp3's(what's a compact disc?) Also, I don't have the sub in a box, it is rear deck mounted. Maybe this is causing my frequency response to not be flat and why lower frequencies on new rap sound good, but not so low frequencies get lost?

I think I have my LPF on 90hz, and my HPF's on 80hz at the moment.
 
It could be the source of your music. Mp3s are terrribly compressed and lack the full range quality of a cd. When running quality car audio (aftermarket amped speakers), it is crucial to run high quality files with it or it won't sound as good. This thread explains the effects of compression: http://www.caraudio.com/forum/head-units/114272-effects-compression.html

Also here is a picture similar to what bass boost does to the signal. This is taken from the owner's manual of my Rockford Fosgate monoblock. Depending on the manufacturer the shape of the curve can vary and the hz where the boost occurs in the pic is 45hz, yours is 40hz to your curve would be shifted a little to the left.



Also try lowering your lpf settings. What are they on the headunit? Rock music usually has less bass than rap and 95% of the time won't hit as heavy and hard, this is where good mids come in. Try putting the sub in a sealed enclosure for a flat tight response curve.
 
Clipping doesn't kill subs, over excursion and overpowering kills subs. There is a component to clipping that can cause excess power but most of that extra energy is high frequency content the sub couldn't play even if it wanted to due to the inductance of the driver.
 
dnewma04 said:
Clipping doesn't kill subs, over excursion and overpowering kills subs. There is a component to clipping that can cause excess power but most of that extra energy is high frequency content the sub couldn't play even if it wanted to due to the inductance of the driver.
Click to expand...

You are an idiot. Clipping causes over powering and thus over excursion. Depending on the severity, a clipped signal can contain over 2x the power of the regular sine wave, thus overpowering the sub. The sub's ability to handle rms power is in relation to its ability to cool down. If you are running 500wrms to a sub thats only rated at 600wrms and are clipping the signal, you could be sending a signal thats upwards of 1000wrms to the sub. But you have no way to tell unless you have an o-scope or you will notice when the coils began to stink like electrical. A sub will only play up to its cut off settings on the headunit and amplifier and most clipping in signals occur between 40-50hz, or whereever the bass boost is commonly added, although it will effect other frequencies but far less. When most subs blow, the voice coil STINKS like an electrical fire, because it OVERHEATED from RECIEVING too much power.
 
steelblueSE said:
You are an idiot. Clipping causes over powering and thus over excursion. Depending on the severity, a clipped signal can contain over 2x the power of the regular sine wave, thus overpowering the sub. The sub's ability to handle rms power is in relation to its ability to cool down. If you are running 500wrms to a sub thats only rated at 600wrms and are clipping the signal, you could be sending a signal thats upwards of 1000wrms to the sub. But you have no way to tell unless you have an o-scope or you will notice when the coils began to stink like electrical. A sub will only play up to its cut off settings on the headunit and amplifier and most clipping in signals occur between 40-50hz, or whereever the bass boost is commonly added, although it will effect other frequencies but far less. When most subs blow, the voice coil STINKS like an electrical fire, because it OVERHEATED from RECIEVING too much power.
Click to expand...

I'd advise you to at least attain a basic understanding of clipping before you ever post a turd like this again. I'm embarrased for you.
 
http://www.clubknowledge.com/Car_Audio_FAQ/?t20
So.. what's wrong with this picture? The amp tries to put out the appropriate power, but runs out of voltage from the supply rails and we get a flat spot at the upper and lower peaks of the wave form. In an extreme case, "severe clipping", there is so much additional energy buildup (heat) into the voice coil(s), but the cone does not move (motivate) enough to cool the voice coil and former adequately. Hense, the voice coil over heats and either seizes in the gap or burns the voice coil windings. RESULT: OPEN CIRCUIT and a blown speaker!

I pretty much explained it. Clipping changes the shape of the sine wave, causing excess heat, which kills the voice coil.
 
The clipped wave forum is created by harmonic content, hence the frequency of the power is shifted up. Let's make the erroneous assumption that it doesn't shift up and the inductance of the coil isn't causing the impedance to jump up, and pretend that all the power gets to the woofer. There are still two causes of failure, too much excursion (won't be caused because of clipping) and too much heat. Sending too much power is the cause of both. If you claim that clipping is the cause, I say that any way that you provide too much power (clipping or otherwise) is just a symptom of a stupid user/installer. You do understand that many pros intentionally set amps running to subwoofers into clipping, no? Now, if you had a woofer that had extremely low inductance and therefore could play VHF, you could open yourself up to issues where the extra power under the curve would actually be seen by the subwoofer. However, to get low enough inductance to make that a concern usually involves the use of multiple faraday rings, possibly a big slug of copper in the motor assembly and consequently, the ability to dissipate heat very efficiently. Not that there are many woofers designed that way for car audio subwoofers where the main focus is on massive power handling and excursion at the expense of high frequency extension and sensitivity. Just trying to find a way to make your argument for you, very ineffectively.

And further, when looking at the average power going to a subwoofer, whether or not the signal is clipped is irrelevant, too much average power is too much average power. There are a number of nutcases out there that think as long as your not clipping, you can over power your subwoofer. My goal is to rid the planet of this sort of extraordinarily stupid mind set.
 
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