Subwoofer amp - final resort

[stuee]

Hi.
I have purchased a proper crossover sub filter for my subwoofer.
Would this have enough grunt for my 15" sub mainly very low bass only.

**broken link removed**

also will this powersupply work for it.
https://www.ebay.com/itm/36V-DC-9-7...566?pt=LH_DefaultDomain_0&hash=item25885e555e

i do have a few tordials kicking around, would i be able to hook them straight up if they are the correct voltage.

I know the amp need a heatsink and i have a monster one here off an old surround sound amp.

any input appreciated.

 

[tcmtech]

For the money you would be better off just buying a dedicated single channel mono amp or bridgeable stereo unit. Personally for speakers I prefer to have the amp rated to a similar wattage to what the speaker is rated.

Car audio amplifiers are cheap and pretty easy to hack their power supplies to use external higher voltage power sources with them thus eliminating the taking line voltage down to 12 volts at high current then having the amp boost it back up internally again to whatever rail voltages it uses.

 

[crutschow]

What is the power rating of the subwoofer?

 

[stuee]

this is my speaker but no specs really.
i didnt know you could use car amps. i already have the amp below that ive not used yet as i sold the car. How would i hack it as you mention?
or i also have a power supply which might do the job??
Amp
**broken link removed**

the poweersupply
https://www.aliexpress.com/item/SOP...W-CCTV-power-supply-switching/1653705459.html

 

[audioguru]

That is the stupidest amplifier board I have ever seen. 7 amplifier ICs in parallel!
The datasheet for the TDA7293 says it provides an output of about 61W into 8 ohms just before it clips. 2 in parallel can provide 122W into 4 ohms. 4 in parallel can provide 244W into 2 ohms and 8 in parallel can provide 488W into 1 ohms.
Is your subwoofer only 1 ohm?? I bet it is not even 2 ohms.

The car amplifier is rated at "1400W!" or 70W per channel with no distortion figures and no impedance figure. It might produce 25W at low distortion per channel into a 4 ohm car speaker.

 

[tcmtech]

Hacking automotive amplifiers to run off of external power supplies is rather easy. Just take one apart and find the two higher voltage primary power rails for the amplifier section after the DC/DC power supply and put your external power in there.

Usually all you need to power them with is a good clean dual voltage power supply or a pair of power supplies to make the two + and - rail voltages.

 

[audioguru]

Car amplifier manufactures outright LIE about their output power as I showed. The MAX output power of that car amplifier is 1400W!
But what is MAX? Is it instantaneous peak-to-peak power into a dead short with 100% distortion? Probably.
Its RMS power is 70W per channel but again the speaker impedance and distortion are not mentioned.

 

[crutschow]

You'll likely get a better unit for similar total cost by just buying a subwoofer plate amplifier such as one of these.

 

[stuee]

i so see bridged 2 chanel amps for subs like a 1000w pioneer one which should be better than this crappy one i have already. What could the problem be running a 4ohm output to my 8ohm sub driver?

Thanks

 

[audioguru]

A 4 ohm speaker draws more current from an amplifier than an 8 ohm speaker if the amplifier can provide it. Then maximum RMS continuous power fed to the 4 ohm speaker is higher.
A bridged amplifier is two amplifiers and each amplifier drives one wire of the speaker. The amplifiers are out-of-phase so when the output of one goes positive then the output of the other goes negative then the voltage across the speaker is doubled than a single amplifier if the amplifiers are perfect but amplifiers have losses so the output power is actually about 3.5 times as much as a single amplifier.

Therefore if you connect your 8 ohm speaker to an amplifier with a power rating with a 4 ohm speaker then your maximum RMS continuous power will be a little more than half.

 

[EdG]

We can expose the mythical 1400 watt amp another way. Watts are watts, no matter the voltage. If a lower voltage is used, more current is drawn to make the equation balance.

A common household electric space heater is 1500 watts. At 125 volts from the wall outlet, it will draw 12 amps. If the AC drops a bit to 120v during peak demand or if your home wiring is light, now it's taking a bit over 13 amps. Everything works harder and wiring heats more, because it's the current, not the voltage that has gone up.

Let's examine what's needed to handle 1400 watts in a 12v car electrical system. 1400/12=116.66 amps! Your typical home circuit breaker trips at 15 or so amps and you'll notice extension cords and outlets feeling quite warm when you run that floor heater for awhile. You can pretty much power a light metal-welding job with this much juice.

The tiny hairlike wires in a typical speaker's voice coil would vaporize if actually given this many watts. No benchtop 12v power supply will do, you'll need a bank of batteries and 00-gauge wire.

Even assuming the amp might get close to a truthful 50 wpc rating, it's still a big draw. 100 watts/12=8.333 amps current. Most experimenter's supplies run 2 to 4 amps, you'll need a 10 or 12 amp power supply to run this subamp. On the typical power supply, the subamp would simply flake out when asked to go full output since the ps could not keep up and would trip its breaker. Your car battery wouldn't start your car after an hour or three of this.

As Audioguru surmised, a true 25 watt per channel output is much more honest and realistic. And that's with distortion so high they intentionally leave out that spec and little in the way of protection circuitry for the speaker or ps. For easy math, we're also assuming 100% efficiency, 60% might be closer to actual use. My example is at full output. In use on music, that's a very flucuating number and maybe some big capacitors would help on the peaks to take the instantaneous load off the ps.

Heh, my second post here in 13 years, I'm on a roll!
-Ed

 

[audioguru]

We can expose the mythical 1400 watt amp another way. Watts are watts, no matter the voltage. If a lower voltage is used, more current is drawn to make the equation balance.

A common household electric space heater is 1500 watts. At 125 volts from the wall outlet, it will draw 12 amps. If the AC drops a bit to 120v during peak demand or if your home wiring is light, now it's taking a bit over 13 amps. Everything works harder and wiring heats more, because it's the current, not the voltage that has gone up.

Absolutely not!
A heater is simply a resistor. The resistance barely changes. Ohm's Law says that when the voltage goes down then the current also goes down, not up. Then the power consumption and the heat output both go down, not up.
An audio amplifier is the same. When the supply voltage drops, the current and the power output both drop.

Let's examine what's needed to handle 1400 watts in a 12v car electrical system. 1400/12=116.66 amps!

But if it is class-AB then its efficiency is about 65% so the power fed to it is 1400W for the output and 750W of heat for a total power of 2150W. Then at 12V the current is 2150W/12V= 179A.