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Curious subwoofer dilemma, curious solution
Old 20th November 2020
  #1
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๐ŸŽง 5 years
Curious subwoofer dilemma, curious solution

I got a distress call from a friend about his subwoofers. He had a small window to get his hands on a pair of Adamson A218's to replace his EV MTL-1X's and was attracted to the deeper extension they had. Here's a synopsis of how they compare:
- both dual 18" 4 ohm cabs
- EV's are horn loaded, Adamsons are direct radiating
- Adamsons are 1dB less sensitive, but handle 50% more power and reach deeper

So we get to talking about what amps he has. He's been driving the EV's with an RMX2450 per cab bridged. So he's running the amps at 2 ohms, thinking they're getting 2400W each but really more like half that. I suggested he should try driving the EV's with a PL380 to hear what they're like properly driven at 2500W each at 4 ohms, but there isn't time.

Then we talk about what it would take to drive the Adamsons properly, and at 3200W program each it seemed best to get two PL380's and drive each driver at 8 ohms, 1600W each. But that's $5k in amps when he's looking at a net cost of under $200 to swap cabs.

Then it hits me that because this is an install that doesn't have to be hauled, the RMX2450's he already has would be pretty well suited to driving each 1600W driver in the Adamsons at 1300W bridged into 8 ohms, so he would just need another pair of amps ($500-600) to do the same for the other two drivers.

So that's what he's going to do - swap the cabs, drive one side with an RMX per driver, rent/borrow an amp to drive the other side while he's waiting for another pair of RMX's to come along. He's going to go from underpowered horn cabs that do nothing below 40Hz to reasonably powered DR cabs with a -3dB point of 40Hz and extend beyond that. He is in for a surprise!

BTW, this is for his metal band's jam space
Old 20th November 2020
  #2
Gear Maniac
 
Quote:
Originally Posted by AC2SPL View Post
So we get to talking about what amps he has. He's been driving the EV's with an RMX2450 per cab bridged. So he's running the amps at 2 ohms, thinking they're getting 2400W each but really more like half that.
No that is 4ohm bridged which is 2400w so it's not likely the new subs will be louder, but they should certainly notice a difference in the sound of the boxes and an improvement in lowend extension.
Old 21st November 2020 | Show parent
  #3
Lives for gear
 
๐ŸŽง 5 years
Quote:
Originally Posted by paul o View Post
No that is 4ohm bridged which is 2400w so it's not likely the new subs will be louder, but they should certainly notice a difference in the sound of the boxes and an improvement in lowend extension.
4 ohms bridged = 2 ohms per amp, specs claim 1200W x 2. That's 24.5A output. That would require at least 8 gauge wiring. Open up an RMX2450, measure the diameter of the wiring to the speaker terminals. It is not 8 gauge.

So either 2 ohms specs are bullsh|t, like is widely reported of all amps, or the wiring to the output terminals is made of magic copper.
Old 21st November 2020
  #4
Gear Maniac
 
Well if you're using that spec then you're going to be totally dismayed at what you find for wiring inside a subwoofer enclosure.

Facts are music isn't a never ending sine wave like the mains AC supply it has at least some crest factor so the steady state current level is a fraction of what the math suggests.

That said the RMX2450 is well known for being a bit soft at 4ohms bridged, it's designed more to survive it than to accel at it particularly at low frequencies. QSC has HD models that are designed to perform better at 2ohms/ch and 4ohms bridged but even there you won't get perpetual full power sine wave output.
Old 21st November 2020
  #5
Lives for gear
 
๐ŸŽง 5 years
The wiring inside a subwoofer enclosure is for each driver in parallel, not in series, so the amperage is split. In my example two 8 ohm drivers are receiving power from two amps which are only encountering 2 ohms of resistance from the bridged parallel configuration. The 1200W from each amp to the amp's speaker terminals would require 8 gauge wiring, but the 1200W going from the speaker's terminals to each driver is an 8 ohm section of the circuit, so that's only 12.2A, which is safe for 16 gauge cable, literally lamp cord.

And the wire gauge ratings are for AC, so it is an apples-to-apples comparison. Amp manufacturers are claiming RMS output specs that would require heavier gauge output wiring in order to pass basic safety regulations. I honestly don't know how they get away with it. But the fact remains you will not get 1200W continuous RMS in a 2 ohm circuit to render less than 24.5A of current, and therefore passing through any length of cable smaller than 8 gauge will at the very least melt the casing 100 times out of 100. That means either the claimed output specs are a bald-faced lie or every amp manufacturer's insurance company missed the day they taught liability at insurance school.

Last edited by AC2SPL; 21st November 2020 at 07:33 PM..
Old 22nd November 2020 | Show parent
  #6
Gear Maniac
 
Quote:
Originally Posted by AC2SPL View Post
The wiring inside a subwoofer enclosure is for each driver in parallel, not in series, so the amperage is split.
Of course but you know how that is usually done, 16ga from the speaker terminal to the first driver and then a jumper from there to the second so there is a chunk of wire that has to carry everything. Both never mind that, how big is the tinsel lead on the driver itself? Or what gauge wire is the voice coil made from?

Quote:
Originally Posted by AC2SPL View Post
In my example two 8 ohm drivers are receiving power from two amps which are only encountering 2 ohms of resistance from the bridged parallel configuration.
But that's not how it works at all. Two 8ohm drivers in parallel equals 4ohms, period, there is no separating them once they are wired in parallel. A bridged amp pair works as a single amp with each side handling 1/2 of the waveform.. exactly the same way the transistors in a single amp channel handle a waveform, but in this case what was Ch1 now does the positive side of the signal and Ch2 does the negative side for example. The "shared load" analogy is an over simplification that doesn't really occur.

Quote:
Originally Posted by AC2SPL View Post
And the wire gauge ratings are for AC, so it is an apples-to-apples comparison. Amp manufacturers are claiming RMS output specs that would require heavier gauge output wiring in order to pass basic safety regulations.
An RMS measurement is simply a mathematical formula that can be applied to any signal and that does not imply that it is a steady state signal it can be any kind of signal from AC mains to random noise.


Quote:
Originally Posted by AC2SPL View Post
I honestly don't know how they get away with it. But the fact remains you will not get 1200W continuous RMS in a 2 ohm circuit to render less than 24.5A of current, and therefore passing through any length of cable smaller than 8 gauge will at the very least melt the casing 100 times out of 100.
Again.. it depends on the dutycycle, 24amps for a fraction of a second does not require a large conductor. The problem here is not with the claimed specs it is with your interpretation of those specs and the assumptions they are based on.
Old 23rd November 2020 | Show parent
  #7
Lives for gear
 
๐ŸŽง 5 years
Quote:
Originally Posted by paul o View Post
Of course but you know how that is usually done, 16ga from the speaker terminal to the first driver and then a jumper from there to the second so there is a chunk of wire that has to carry everything.
Ok you do have a point there, that it's often done like that, though obviously I wouldn't. But the result is that the wire rating is only exceeded by 50% in a cabinet with LOTS of air movement, which provides lots of heat sink so it doesn't end up taking damage.

Quote:
Both never mind that, how big is the tinsel lead on the driver itself? Or what gauge wire is the voice coil made from?
Those are not insulated (much) and they are exposed to air movement commensurate to the amount of power flowing through them. Sometimes ferrofluid is present to increase the cooling effect.

Quote:
But that's not how it works at all. Two 8ohm drivers in parallel equals 4ohms, period, there is no separating them once they are wired in parallel. A bridged amp pair works as a single amp with each side handling 1/2 of the waveform.. exactly the same way the transistors in a single amp channel handle a waveform, but in this case what was Ch1 now does the positive side of the signal and Ch2 does the negative side for example. The "shared load" analogy is an over simplification that doesn't really occur.
You're thinking Class B amplification, each amp handling a rectified half of the signal. That's not how bridged amps work. Each side operates upon the full sine wave just like when not bridged, and they each encounter half the overall impedance of the speaker network, just like two people lifting a 100lb weight each only carry 50lbs. Read any bridgeable amplifier's manual.

Quote:
An RMS measurement is simply a mathematical formula that can be applied to any signal and that does not imply that it is a steady state signal it can be any kind of signal from AC mains to random noise.

Again.. it depends on the dutycycle, 24amps for a fraction of a second does not require a large conductor. The problem here is not with the claimed specs it is with your interpretation of those specs and the assumptions they are based on.
Wiring ratings are for insulated continuous RMS AC power levels, doesn't matter if they're audio or 50/60Hz sine. When they are exceeded, the damaging side-effect of heat begins quite quickly, but might not be severe depending on the amount and duration of the excess. In moderate cases the casing gets cooked and cracked, or it might melt, or the wire may go full incandescent emitting high heat, eventually melting the copper but that's over 1000F. If an amp capable of continuous 1200W RMS at 2 ohms like the specs say were to do so, that 24.5A would eventually melt a 16 gauge insulated wire in the absence of serious heatsink.

But rest assured, if an amplifier is designed to transmit 24.5A of power through copper wiring, it would be required to be 8 gauge cable or it would not pass the safety standards of all industrialized countries.

Check Ohm's law, 1200W at 2 Ohms is 49 Volts and 24.5 Amps. Those two numbers must multiply to make 1200, and the voltage must be twice the amperage, representing the 2 ohm resistance. 1200W at 8 Ohms is 99V and only 12.2A, much easier to accommodate.

That's why 2 ohm speaker networks are not recommended, because the amperage is unreasonable and amplifiers aren't built to support them except to limp at low power. It can be handy when you need a pile of wedges on the same mix, but it is generally to be avoided.

Last edited by AC2SPL; 23rd November 2020 at 05:56 AM..
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