MTX sub and amp help (v0rt3x)

[digitalshogun]

Originally posted by v0rt3x
Would it damage anything if the sub was only wired with one coil? My friend accidentally wired it with one coil when we tested to see if it was working. Would this amp handle two of these subs? I will be buying another in the future.

Somewhat brief electrical tutorial.

The sub requires a power source that provides between 175 and 400 watts. Your amp (the subs power source) provides either 150 or 300. 300 is okay but 150 you are underpowering the sub and could run into some issues.

Why the two different wattages? Well it is based on voltage, current and resistance. Voltage is the potential difference between the +ive and -ive on your amp. For all intents and purposes I will lie and say that this is fixed. In your case around 24.5v rms. Now your DUAL voice coils are 4 ohms a piece. Also very fixed (not a lie this time though). The dependent variable in this then is current. Soooo for instance if we were to wire up one voice coil to the amp it would be 4ohms. Calculation for current is voltage divided by resistance -or- 24.5/4. 6.125amps or therabouts. Now the calculation for wattage is either voltage multiplied by current -or- current squared multiplied by resistance. For the purposes of this application lets stick to current and resistance -or- (6.125 to the power of 2) * 4. Around 150watts. Now this is actually fine in your case (only using one of the coils) but you are wasting the potential of the amplifier and the sub.

Hmmm ... so this is a conundrum. How do you increase the amount of calculated current since both voltage and resistance are fixed? Well you can mess around with the "fixed" resistance of the two voice coils. When wiring up loads, resistance does different things. If you were to hook the coils up together in a line aka in series (+ive on amp to +ve on coil 1, -ive on coil 1 to +ve on coil 2 and then -ve on coil 2 to -ve on the amplifier) the resistances add up. In your case this changes the resistance to 8ohms. No good. Apply our previous calculations and this nets you only close to 90watts. So what else is there? Parallel of course. Parellel is a more complicated equation than simple addition but lucky for you when the 2 resistances are equal you half the overall resistance ... or 2ohms. This doubles current and (in this case) wattage. Problem solved. This is because each resistance is "seeing" a potential difference of 24v. Simply hook up both the +ve connections on the voice coil to each other and then to the +ve on the amplifier. So the same to the -ve connections then connect those to the -ve on the amplifier.

Now why did I say that the voltage being fixed was a lie? Well that becomes another tutorial on how to properly adjust the gain knob on your amplifier. Suffice to say for now that the gain knob adjusts the voltage to some degree to offset the change in volume on your headunit, prevent SOME clipping issues and adjust the "loudness" of the speakers of your subwoofer relative to your other speakers.

If you have any questions, better to put them in this thread I suppose.

[rahim]

Just have to point out a few things,
1. Cars only put out 14 volts, not 24
2. If you check the impedence of a sub with a voltmeter it will not say 4, it will be closer to 3 and it changes depending on the frequency the sub is playing.
3. The gain adjusts to match the voltage on the preouts from the deck.

[BumpinTalon]

cars don't even usually put out 14 volts. 14.4 is a standard manufacturer rating because they claim cars will do 14.4 (and they will, sometimes, just not often at all on a stock electrical system). 13.8 volts is a more typical car measurement, but this drops even further and 12 volts is the most accurate measuring tool for your amplifier's power. with large amps on a stock electrical system, this can drop even further, but most amps will go into protection under 10 or 11 volts because underpowering kills amps fast.
most subs do have an impedance closer to 3 ohms when measured with a voltmeter. you can get this spec from the manufacturer, usually, by going into the T/S specs. most 4 ohms subs end up somewhere above 3 ohms.
high gain = more noise
low gain = less noise
Higher gain compensates for a weaker signal from your HU (2 volts, for instance), and in turn this means that any minute differences (say, a spike of .5 volts) will make a very large difference at the outputs because in a 2 volt system, .5 is a difference of 25% so the output will be distorted by 25% which is most definitely noticable.
Lower gain is best for "hot" signals from a high-end deck or line driver (4 volts and up, usually up to 8 volts max). The same spike that would cause so much noise in a high gain system will cause much less in the low gain system.

Summary: your stock electrical system is only good for ~13 volts, if that. you want the highest voltage pre-outs you can find. connect both voice coils for optimum results.

[v0rt3x]

Thanks for the replies. I have now since completed my box (to mtx "sledgehammer" enclosure specs), wired the sub correctly in parallel and the system pounds very nicely. Worth my money IMO. Now since you guys seem like you know a fair bit about electronics, let me pick your brains some more :D Basically I'm building a PC into my car and my question is, would the alternator in my car provide enough juice for the sub + amp + pc. What I know for sure is that the computer will draw roughly 150-175 watts, maybe more depending on device utilization (GPS Unit, 802.11b/g wireless card, DVD Drive, 7" LCD Wired to computer powersupply). I'm wiring in a power inverter for the PC thats rated at 175W RMS and 350W peak. What I also know is that my alternator is rated at 80amps. I was also told by another friend of mine to consider running a capacitor in line with the amp as well. What do you think?

[BumpinTalon]

yeowch, 80 amp alternator is really tiny. you are probably pushing it just with the amp you have, I have a 95 amp alt and my interior lights dim with amps ~600 watts and up (when I had the 1,200 watt amp they dimmed really really bad and RPMs would drop and stuff). ...you could probably do it, just barely. I would recommend upgrading your battery to a Stinger or Optima Yellow Top. They will help a bunch as far as your power goes. You could also look into a smaller computer, maybe a MicroATX, and they will draw less power then a standard ATX-based comp will.

[digitalshogun]

Originally posted by rahim
Just have to point out a few things,
1. Cars only put out 14 volts, not 24
2. If you check the impedence of a sub with a voltmeter it will not say 4, it will be closer to 3 and it changes depending on the frequency the sub is playing.
3. The gain adjusts to match the voltage on the preouts from the deck.

1. Wasn't really talking about the supply voltage of the car.
2. Tried to avoid getting into impedance, and while I do agree ...being that the impedance of the coil being the opposition to current flow ... the whole wave form averages out to a number close to 4. The calculations I was doing were simply to show approx. where the numbers were coming from.
3. Like I said gain is whole 'nother ball of wax lol ... I agree witcha but you can use the "gain knob" for more than that.

[rahim]

1. ok
2. just mentioning that impedance isn't fixed
3. should have said in a system where the speakers and amplifiers are matched, the gain should be set to match the output voltage of the deck