Dual Voice Coil Sub Configuration - Home Theater Forum and Systems - HomeTheaterShack.com

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post #1 of 14 Old 10-02-14, 01:24 PM Thread Starter
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Dual Voice Coil Sub Configuration

I spent some time today taking some REW measurements of my DVC sub in different configurations, and am intrigued by the results.

I'm using a Carver a-400x amplifier which is rated at 200wpc @ 8-ohms, 300wpc @ 4-ohms, or 500w bridged with an 8-Ohm load.

The sub is a DIY sonotube sub with a DVC 12" Shiva driver. each coil is an 8 Ohm coil. The enclosure is made from 24" round sonotube, 25" tall, with a single 4" flared port. I built it back in 2001, and don't recall the length of the port, but I did use a program called "WinISD" to design the enclosure using the TS parameters of the driver.

So I ran a sweep from 10hz - 120hz for each configuration, with the measurement mic at the listening position, pointed at the sub.

I took measurements in the following configurations:
  • Both coils connected in stereo to the amp
  • Just coil A connected to the right channel
  • Just coil B connected to the left channel
  • Both coils wired in parallel running off the right channel
  • Just Coil A running off the amp in bridged mode
  • Just Coil B running off the amp in bridged mode

The following 3 tests had the exact same result:
  • Both coils connected in stereo to the amp
  • Just coil A connected to the right channel
  • Just coil B connected to the left channel
So it appears running both coils at 200w each, is no better than just running one coil at 200w.


These 2 tests had higher SPL than the above, but were also exactly the same:
  • Both coils wired in parallel running off the right channel
  • Just Coil A running off the amp in bridged mode
I'm a little surprised at this, as both coils wired together produces a 4-ohm load, which the amp is rated at 300w; and coil A drive in bridged mode is an 8-ohm load, where the amp is rated at 500w.


and this final test had the maximum SPL results:
  • Just Coil B running off the amp in bridged mode
Does this mean coil B is in "better" shape than coil A? I was going to ohm out the coils to see if there a significant difference, but ran out of time.


Here are the plots:



And Here is the mdat file for anyone who is interested.

Is this expected results from a DVC, or do I have an issue with one of the coils? Considering both coils act the same when driven with 200w off each channel, I expected both coils to also act the same when drive in bridged mode; but coil B gives a louder SPL. I ran each coil in bridge mode several times to validate this, and received the same result each time. Coil B always performed louder than A when bridged to the amp.

So, in the end: does a DVC woofer actually provide any gain whatsoever over a SVC woofer?



.

PROCESSOR: Emotiva UMC-200
AMPLIFIERS: JBL Performance AVA7 | Adcom GFA-545 | Carver a-400x
SPEAKERS: NHT vt-1.2a | NHT vs-1.2a | NHT Superones | DIY DVC Shiva Sonotube Subwoofer
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post #2 of 14 Old 10-04-14, 09:59 AM
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Re: Dual Voice Coil Sub Configuration

SPL is most directly correlated to the current passing through the voice coil. Aside from any specs, if you measure higher SPL, you are delivering more current. That happens when impedance is lower AND the amp can deliver the current.

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post #3 of 14 Old 10-06-14, 10:43 AM
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Re: Dual Voice Coil Sub Configuration

Stoopalini, from what I could understand from your post it looks like the two coils are not identical in some respect:

1. One has lower impedance (despite having almost the same DC resistance - note that between any two drivers or coils of the same nominal impedance there can be variations in DC resistance, so DC resistance is usually not a major concern as long as they are within 10%).
2. Their physical construction/relative location might be such that even if they have exactly the same no. of turns and gauge, the magnetising flux through each may be different causing one to have greater sensitivity
3. One of the coils has suffered some damage such as shorted turns, or some turns are wasted/removed.

Was the driver ever repaired? If yes then that could be a probable root cause for what you observed. Ideally, either coil should produce almost the same result individually and the serialed or paralleled combination should simply result in equivalent 3dB increase in SPL when fed with the same power per coil.
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post #4 of 14 Old 10-06-14, 11:14 AM Thread Starter
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Re: Dual Voice Coil Sub Configuration

Quote:
lcaillo wrote: View Post
SPL is most directly correlated to the current passing through the voice coil. Aside from any specs, if you measure higher SPL, you are delivering more current. That happens when impedance is lower AND the amp can deliver the current.
Right, so what confused me is that each coil produces the same SPL when run off a single channel of the amp (200w @ 8-OHM) but produce different SPL readings when run off the amp in bridged mode (500w @ 8-Ohm). There was no distortion created with coil A connected in bridged mode, so I don't think it was the driver breaking up here.

Quote:
rohitbd wrote: View Post
Stoopalini, from what I could understand from your post it looks like the two coils are not identical in some respect:

1. One has lower impedance (despite having almost the same DC resistance - note that between any two drivers or coils of the same nominal impedance there can be variations in DC resistance, so DC resistance is usually not a major concern as long as they are within 10%).
2. Their physical construction/relative location might be such that even if they have exactly the same no. of turns and gauge, the magnetising flux through each may be different causing one to have greater sensitivity
3. One of the coils has suffered some damage such as shorted turns, or some turns are wasted/removed.

Was the driver ever repaired? If yes then that could be a probable root cause for what you observed. Ideally, either coil should produce almost the same result individually and the serialed or paralleled combination should simply result in equivalent 3dB increase in SPL when fed with the same power per coil.
This is my thought, that there is something about the construction of the coils which differs, although I'm still stumped as to why at 200w they act the same, but at 500w, they act differently. I know the amp rating isn't the "delivered wattage", but the load determines the current delivered. So maybe the resistance value is the same with no current, but varies between the two coils as the current rises.

The driver has never been repaired, and has performed flawlessly since I built the sub in 2001.

As far as a single coil connected to one channel measuring the same SPL as both coils connected in stereo mode, I'll bet this is due to the amp design. Upon further investigation, the Carver a-400x channels share the same power supply, so my guess is when one coil is connected, it is getting more current; and when both coils are connected, they are sharing it.


PROCESSOR: Emotiva UMC-200
AMPLIFIERS: JBL Performance AVA7 | Adcom GFA-545 | Carver a-400x
SPEAKERS: NHT vt-1.2a | NHT vs-1.2a | NHT Superones | DIY DVC Shiva Sonotube Subwoofer
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post #5 of 14 Old 10-06-14, 11:50 AM
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Re: Dual Voice Coil Sub Configuration

You say they are the same at 200W but different at 500W - Basically unbridged each performs the same, but when fed power in bridged mode they act different, right?

What happens when you feed each individually in bridged mode but power reduced to 200W? My guess is they would be the same. I think this is coil or cone excursion related - and this test is basically stressing the driver close to its limits, if you are actually feeding that kind of power into the driver.

Generally you should not test with more than 10W of continuous sine-wave input - for accurate results this should be about a watt, so you get the SPL figure in x dB/watt/metre. So you should ideally be measuring with an input power of 1W. With the frequency set to about 200Hz (assuming this is where the coils nominal impedance will be), adjust the output to be about 1Vrms and measure the current through the coil. This will give the nominal impedance of the coil at 200 Hz (formula: Z = V/I) then use Z to calculate what the input voltage to the coil should be for 1W, V = Sqrt(P * Z) = Sqrt (1 / Z).

Last edited by rohitbd; 10-06-14 at 11:58 AM.
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post #6 of 14 Old 10-06-14, 11:56 AM Thread Starter
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Re: Dual Voice Coil Sub Configuration

Quote:
rohitbd wrote: View Post
You say they are the same at 200W but different at 500W - Basically unbridged each performs the same, but when fed power in bridged mode they act different, right?
Yes, this is what I am saying.

Quote:
rohitbd wrote: View Post
What happens when you feed each individually in bridged mode but power reduced to 200W?
Not sure how to accomplish this. When I say "200w" or "500w", I am simply quoting the manufacture's specifications for the amplifier in stereo mode vs. bridged mode.

Quote:
rohitbd wrote: View Post
Generally you should not test with more than 10W of continuous sine-wave input - for accurate results this should be about a watt, so you get the SPL figure in x dB/watt/metre. So you should ideally be measuring with an input power of 1W.
I am simply changing the wiring configuration and sending a test tone to the amp using REW's internal tone generator. The "Level (dB FS)" setting in REW is configured for -12.

PROCESSOR: Emotiva UMC-200
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SPEAKERS: NHT vt-1.2a | NHT vs-1.2a | NHT Superones | DIY DVC Shiva Sonotube Subwoofer
DISPLAYS: Vizio M602i LED TV | Dell 5100MP DLP projector | Favi 120" motorized screen
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post #7 of 14 Old 10-06-14, 12:15 PM
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Re: Dual Voice Coil Sub Configuration

Quote:
Stoopalini wrote: View Post
Yes, this is what I am saying.


Not sure how to accomplish this. When I say "200w" or "500w", I am simply quoting the manufacture's specifications for the amplifier in stereo mode vs. bridged mode.


I am simply changing the wiring configuration and sending a test tone to the amp using REW's internal tone generator. The "Level (dB FS)" setting in REW is configured for -12.
The question that needs to be answered is - when you send the test tone to the amp, how much power is the amp putting out? Measure the voltage at the amp's o/p (with the sub disconnected). Assuming 8 ohms, the voltage at the amp's output (unbridged) should be about 2.83 Vrms. If more then you need to reduce the amp's output either via a volume control or via the REW user-interface. In bridged mode, when each output is delivering 2.83V, the total voltage seen by the driver will be doubled and hence power will be quadrupled - so if 2.83V unbridged is 1W, 2.83V/channel and bridging will be 4W and SPL will increase by 3dB.

The REW's "-12 db FS" setting alone is not of much help - you need to know what the amp is putting out to the driver.

To make it simple - the test you did is not valid because presumably too much power is getting delivered to the driver causing it to reach excursion limits. You should reduce the power input sufficiently enough so that each coil performs identically and only then will you be able to see what its performance in a subwoofer is like.

The numbers 200W, 500W are just specifications (marketing-speak or otherwise) - no amp or speaker or driver is ever expected to work with that kind of power while playing music. Even for a biggest sub the average power hardly ever exceeds 50W (and even this might be an exaggeration). That is not mean that you can get away with a 50W amp for a large auditorium...what it means is that for adequate headroom and minimal distortion for a given target SPL, the amp needs to be capable of handling transients that might need it to put out 200W peak (or momentarily).
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post #8 of 14 Old 10-06-14, 12:32 PM Thread Starter
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Re: Dual Voice Coil Sub Configuration

I understand the 200w and 500w numbers are amp rating numbers and do not relate to the power being output. I also understand the load is variable depending on the frequency being sent; but I would expect both coils to respond the same through the tests.

The sub never reached excursion limits during any of the tests, and it did not experience any breakup or distortion during the tests either. I had the computer connected directly to the sub amplifier when I ran these tests.

With all things being equal, one coil produces higher SPL than the other when connected in bridged mode. I am not looking for 1w/m dB test results for efficiency measurements, but am simply curious as to why the 2 coils behave similar in stereo mode, but one produces higher SPL when both are run in bridged mode.

I would expect increasing or decreasing the level of the test signal being sent to the amp would have an equal impact on both coils, but one would still remain higher. I could try this when I have time to reconnect the test equipment.

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post #9 of 14 Old 10-07-14, 06:28 AM
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Re: Dual Voice Coil Sub Configuration

In stereo mode or in individual mode unbridged, you are feeding the same (lower) power to both coils and are therefore getting similar results. But with bridging you are feeding the same higher amount of power to each coil - this I believe is what is causing the difference.

1) In stereo mode this is the same as feeding both coils simultaneously (or in parallel)
2) Individually unbridged - each coil is fed separately and power is low so both coils show similar behaviour.
3) Individually bridged - each coil is fed separately but this time each coil shows different behaviour due to the higher (almost 4x) power being applied. That is why I suggested reducing the input to the amp in bridged mode - reduce the input voltage by half (I believe that would be 6dB) from that in unbridged mode.

Last edited by rohitbd; 10-07-14 at 06:37 AM.
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post #10 of 14 Old 10-07-14, 08:46 AM
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Re: Dual Voice Coil Sub Configuration

Did you do any critical listening tests during the comparisons? The difference may show up in SQ.

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