2 ohms vs 4 ohms

[DSx2]

I think ive got this right

Basically i have a monoblock wired up to a sub in 2 ohms (yes car audio)

The fuse blows on it more often that it should, but as a safety precaution the sub is more powerful than the amp

Theoretically if i wire it up in 4 ohms will it have less chance of blowing the fuse, or am i going about this the wrong way

cheers

[theSeekerr]

OK, let's get this straight - I presume you have a subwoofer with dual voice coils, yeah?

For starters, then, I can't think of a sensible arrangement which would allow you to choose between a 2 ohm or 4 ohm load - dual 2 ohm coils in series will give you 4 ohms, and the same in parallel just a single ohm (similarly, dual 4 ohm coils allows 8 or 2). Hooking up a single coil of a dual is silly and best avoided, throws the parameters of the sub all out of whack.

Next up: It is generally better to have an amp with too much power rather than too little.

Why? Because there are two limits to a speaker driver - the thermal limit, and the excursion limit. This means that there's two possible results of winding up the volume:

- In practice, most speakers reach the excursion limit long before they get hot. This will rapidly lead to the destruction of the driver, but it makes such a horrible noise that you're likely to turn the volume down as soon as you notice.

- Conversely, if the amplifier is too small, you may drive it into clipping, where the amplifier cannot output a high enough voltage to cleanly amplify the input signal. This results in the top and bottom of the AC waveform getting truncated - the further you drive an amplifier into clipping, the more the output resembles DC.

This is bad for thermal performance. Very bad. A DC voltage provides 1.4 times as much power as an AC waveform with the same peak. That's bad enough on it's own, but because the cone won't be moving as much (it will spend some of each cycle "stuck" at each end) the coil doesn't get cooled as well, either. The net result is that a clipping amplifier destroys drivers even more rapidly than an overpowered one, and whilst it sounds horrible, it's possible for it to go unnoticed on a sub.

Anyway, I digress - my point is that there's nothing "safe" about this arrangement.

If you wire up your sub to present a higher impedance to the amplifier, you will limit the maximum power it can supply (because the same power level requires a larger voltage swing for the higher impedance, and the maximum voltage swing is limited by the power supply built in to the amplifier). So yes, your amplifier will be called upon to deliver less current, and will thus draw less current, which may prevent you blowing fuses.

But personally I'd look into it more closely - a properly designed and functioning system shouldn't be blowing fuses in the first place.

[sbadman]

Is the amp 2 ohm stable? Is the fuse the correct rating?

These are the first things to look at IMO

[LostBenji]

Originally Posted by theSeekerr

OK, let's get this straight - I presume you have a subwoofer with dual voice coils, yeah?

My hunch is that he has a DVC sub and wired in 2ohm rather than series 8ohm. A amp bridged with a 2ohm load is going go bang or blow fuses fast unless the amp is a decent amp that can handle stupidly low impedance.

[theSeekerr]

Originally Posted by LostBenji

My hunch is that he has a DVC sub and wired in 2ohm rather than series 8ohm. A amp bridged with a 2ohm load is going go bang or blow fuses fast unless the amp is a decent amp that can handle stupidly low impedance.

A surprising number of modern Class-D car amps are 2 ohm stable, but it is the first thing I'd check. However, I'd expect the amp to be blow internal fuses (or output-stage mosfets) rather than supply-side fuses.

[dakiller]

Originally Posted by theSeekerr

A surprising number of modern Class-D car amps are 2 ohm stable, but it is the first thing I'd check. However, I'd expect the amp to be blow internal fuses (or output-stage mosfets) rather than supply-side fuses.

When you bridge an amp, each channel will see half the impedance of the load. so your 2ohm sub is now 1ohm seen by each channel when bridged.

As others have said, OP, we need more information on your gear, voicecoil configuration and amplifier bridge/channel configurations.

[theSeekerr]

Originally Posted by dakiller

When you bridge an amp, each channel will see half the impedance of the load. so your 2ohm sub is now 1ohm seen by each channel when bridged.

Yeah - I assume that when the OP says "monoblock" that he means "monoblock" and not "Stereo amp in bridge mode". But I could well be wrong...perhaps he'll come back and tell us?

[MetalPhreak]

Actual brand and model numbers would be helpful here.

As mentioned above, clipping is bad. Ideally the amp is rated higher than the sub, and you simply set your gains so you don't overpower the sub.

Clipping is bad because the voicecoil stops moving when the voltage remains constant during the clipping region, so there is no cooling effect. This leads to hotspots within the coil leading to damage. It's technically incorrect to say there's 1.4x more power since it clips at a lower voltage (ie DC at 30v clipped instead of reaching 45v peak AC waveform for example). An amplifier that clips is power limited compared to one that does not

(DC @ 30V is ~1.4x more power than AC @ 30V ie "the same peak" as he said)

Basically subwoofer's rely on constant movement for cooling. Clipped/DC input means periods of no movement = no cooling.

If you are blowing fuses, and its not going into protection mode, and its not sounding rubbish, then you are probably running it with a lower impedance load than it's rated for. If it has decent enough heatsinking, the amp will probably run at a lower impedance load (higher power) than its meant to, but the fuse rating isn't correct (to protect it from long term damage running over 'spec').