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Bridging amp vs. dual voice coil subwoofer

swechsler

swechsler

Frog Whisperer
This is more of a hypothetical question, since I don't actually own a subwoofer like this, but it's something I've been wondering for a while.

Suppose you have a single subwoofer with dual 4 ohm voice coils. You have a bridgeable amp that you'd like to use with it, that outputs 2x150 watts into 4 ohms, or 1x300 watts into 8 ohms. You can either connect the voice coils in series and bridge the amp, or keep them separate and drive each coil with a separate channel. Assume that the amp can't handle any impedance lower than stated above.

Which method is preferable, and why?
 
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I'd prefer configuring the amp as series-bridged, and the voice coils in series:

1. Saves a pair of wires between the amp and the sub.

2. A failure of a connection, voice coil, or amp channel will result in immediate silence, not a 3dB drop. OTOH if you're running a show, you might prefer to lose a little volume instead of silence.

3. Better power supply utilization. However, you could get the same result with separate channels by taking one channel and inverting its input signal and its voice coil polarity

4. Easier and better job of making sure the same current flows through both coils at all times. Otherwise the amp channels may not be exactly matched.
 
I would run 300 watts into 8 ohms.

The speakers would increase their BL with the coils in series. A higher BL means the amp had better control over the cone movement.

Also as mentioned above, in parallel, if both channels were not identical, there could be some loss there.
 
toddalin said:
I would run 300 watts into 8 ohms.

The speakers would increase their BL with the coils in series. A higher BL means the amp had better control over the cone movement.

Also as mentioned above, in parallel, if both channels were not identical, there could be some loss there.
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Also, some subs (e.g., my differential drive JBL W15gti and four W10gti's) are speced to run both coils from a single channel, either in parallel or series. I run them in series for 12 ohms (dual 6-ohm coils) and push it with a Crown PSA-2XH that makes ~800 watts in bridged mono at 12 ohms.
 
One thing you might consider doing is checking the DC offset of your amp. Depending on the values of each channel, bridging might help or make it worse. For example, on one of my bridged Yamaha P2200 amps, the DC offset on the left channel is 29.8mV, right channel is 44.3mV, which makes DC offset while bridged 14.5mV. 14.5mV is pretty good and better than what I would have got out of either channel individually.
 
Keep in mind that a bridged amp "sees" the load as one half of it's nominal impedance.

As such, I'd probably go with an eight ohm series VC connection running off the bridged amp. which will be seeing a 4 ohm load.
 
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My subs have DVC drivers. The VC's are wired in series for an 8 Ohm load. Each sub is driven by a Crown XLS-402 in bridged mode (900 W@8 Ohms).
 
swechsler said:
It's my understanding that each channel would see a 4Ω load, but if you consider the two channels as one amp, then that entire amp would still see an 8Ω load.
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That's not what I thought but, hey, I could always be wrong, but you might want to rethink that, particularly when you consider that a bridged amp is essentially one channel.
 
swechsler said:
It's my understanding that each channel would see a 4Ω load, but if you consider the two channels as one amp, then that entire amp would still see an 8Ω load.
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An 8ohm load is still an 8ohm load. What changes is the impedance that each of your amp channels will see, and that change is due to the act of bridging the amp. When a bridged amp is powering an 8ohm load, each channel of that bridged amp will essentially see a 4ohm load. Yes, the "entire amp" will still see an 8ohm load, because it is an 8ohm load, but what determines the impedance that an amp can handle is what each individual channel can handle, and because the amp is bridged, each individual channel will see half the impedance of the attached load. A bridged amp doesn't become a single amp. It may act that way, but it's still two independent channels working together, and it's all about what those individual channels can handle.

Whether you are using a bridged stereo amp to power an 8ohm load, or using each channel independently to power two 4ohm loads, in both cases each channel will see a 4ohm load.
 
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teal'c said:
Bridge and run in parallel 1x600. Why? More bass.
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The OP is talking about a subwoofer with dual 4-ohm voice coils. If those voice coils are run in parallel, that becomes a 2-ohm load. If the op uses a bridged stereo amp to power that 2-ohm load, then each channel of that bridged amp is going to see a 1-ohm load. And keep in mind that the listed impedance is basically just an average. The actual impedance changes depending on the frequency. At some frequencies, the actual impedance can dip pretty far below the "nominal" impedance. So we might even be talking about each amp channel seeing something like 0.5ohms at certain frequencies. There are probably amps out there that can handle that but i'm not sure that would be a useful configuration in the OP's case. He clearly said:
swechsler said:
Assume that the amp can't handle any impedance lower than stated above.
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Which I interpreted to mean no lower than 4-ohms nominal.
 
OK, now let's assume that instead of a dual voice coil driver, you have two 4Ω woofers in a shared cabinet using the same amp. Would the answer remain the same?
 
swechsler said:
OK, now let's assume that instead of a dual voice coil driver, you have two 4Ω woofers in a shared cabinet using the same amp. Would the answer remain the same?
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However you hook them to them to the amp, you don't want to electrically connect woofers in the same cabinet in series. Small deviations between them can prevent them from behaving equally. This is less of a concern the more closely the woofers are matched, and the closer matched their acoustic loading (distance to cabinet walls and ports). But in general it's avoided.

Not a problem with dual voice coils on the same driver, as they are physically linked and prevented from moving differently from each other.

What can really give you options are *quad* voice coil subs. But at 1 ohm per coil, running all the coils in parallel severely limits your amp choices.

MemphisMojo_Q1_15.jpg
 
fill35u said:
However you hook them to them to the amp, you don't want to electrically connect woofers in the same cabinet in series.
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This goes contrary to how most dual-woofer Infinity (and many other brands) speakers are wired, and also to @teal'c who replied before you. In fact I'd venture to guess that a large number of dual-woofer center speakers made over the last 25 or so years have their woofers wired in series.

Now, by "matched" I assume you mean the woofers have actually been measured to have their specs as close as possible, not that they are the same model. But either way, many people on AK have replaced a single woofer in a dual-woofer speaker (with another of the same type) with no reported adverse effects.
 
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