Utilizing one VC in a DVC driver

[sammyxp]

I was reading this thread:
http://www.diyaudio.com/forums/showthread.php?postid=752092#post752092
And was intrigued by Stephan's post about using one voice coil in a DVC driver. I've got a Infinity 10.1D subwoofer in my wife's car that is in a compromised design. I can't wire them in parallel (the amplifier is only rated for a 4ohm load when bridged) so the voice coils are in series, presenting an 8ohm load to the amp, limiting the power output.

What exactly are you doing when you put a resistor across the 2nd VC and only power one of them? Are you trying to dissipate the energy created by the coil moving in the fixed magnetic field? How would varying this resistance affect the subwoofer's performance? Would you really be able to control the damping of the driver with a potentiometer? How much energy are we talking? Would a 1/4 watt pot do? 1 watt? More??

Thanks in advance. I ran a search about this topic and couldn't find any other mention other than Stephan's post in the other thread.

 

[DcibeL]

By placing a resistor across the second coil will allow you to adjust the Q of the driver, so it can be optimised for different enclosure types. I own an Ascendant Audio Atlas subwoofer which is a DVC driver with the second coil designed for the sole purpose of adjustable Q. The coil that gets power from the amp is a 4ohm load, and the coil for adjusting the Q is a 2ohm laod. By shorting the 2ohm coil, a low Q is available which would be best for a sealed box. By leaving the 2ohm coil open, a high Q is available for infinite baffle designs. By placing a resistor of 1.5 ohm (equal to the DCR of the coil), a mid Q is available for ported designs.

Keep in mind that with only utilizing one coil, you decrease BL (motor strength), and decrease power handling. I had originally used my Atlas subwoofer in a mid Q design, but have found that wiring both coils in series for a 6ohm driver works quite a bit better.

Anywhere I have looked recommends a 5watt resistor if you were to go that route.

 

[Collo]

Some time ago, I looked at converting between the different connection possibilities for a DVC driver.

A conversion table, plus notes for resistive damping are at
http://www.users.bigpond.com/bcolliso/driver-conversions.htm

1/4 watt is too small for the resistor. It has to deal with current generated by back-emf in the second coil.

Adire recommends 5w, but that is on a driver where the coils are of different impedance.

regards
Collo

 

[Ron E]

I was reading this thread:

I've got a Infinity 10.1D subwoofer in my wife's car that is in a compromised design. I can't wire them in parallel (the amplifier is only rated for a 4ohm load when bridged) so the voice coils are in series, presenting an 8ohm load to the amp, limiting the power output.

What exactly are you doing when you put a resistor across the 2nd VC and only power one of them? Are you trying to dissipate the energy created by the coil moving in the fixed magnetic field? How would varying this resistance affect the subwoofer's performance? Would you really be able to control the damping of the driver with a potentiometer? How much energy are we talking? Would a 1/4 watt pot do? 1 watt? More??

Good references on the topic would be the original source of info on the topic, from Adire:
http://www.adireaudio.com/Files/TechPapers/RDOOperation.pdf
or perhaps:
http://www.diyaudio.com/forums/showthread/t-56647.html

Your best bet is to use a different amplifier or a different driver if they do not match to make a good system.

You can vary the Q with the RDO setup, which may allow you to tune the system to taste, but no connection should give more sensitivity than you have now, except for parallel with a better amp.

RDO works by dissipating energy. Just as a voltage on a coil free to move in a magnetic field causes motion, Lenz's law states that motion of a coil in a magnetic field causes voltage. This is sometimes called Back EMF since the voltage is of the opposite sense to the voltage that causes the motion. When the coil is open, this voltage is free to be whatever it wants because there is no current flow. No current flow means no damping effect. Imposing a load allows a current to flow, which interacts with the magnetic field, cancelling some of the force created by a signal on the driven coil.

If Adire recommends 5W for the damping resistor, I'd probably use that without batting an eyelash in most cases. If I were worried, I would calculate it.

The magnitude of the voltage created across the coil is EMF=Bl*Velocity, from this you can find the current through the resistor using Ohm's law and you can find the max power dissipated at any frequency through fairly simple analysis.

Some time ago, I looked at converting between the different connection possibilities for a DVC driver.

A conversion table, plus notes for resistive damping are at
http://www.users.bigpond.com/bcolliso/driver-conversions.htm

While things may be different in Oz, most manufactureers in the USA, and most marketing to the USA, give the parameters of DVC drivers as those when the coils are connected in parallel because that gives the highest voltage sensitivity. The arrangement of your table might make one guess at first look that manufacturers usually quote parameters as when driving a single coil...

 

[Collo]

Hi Ron,

things aren't particularly different here. Frustratingly, manufacturers don't give all the parameters for each of the possible connections.

The reason why I chose to list the single coil option first is because I'm coming from a Home Theatre perspective rather than a Car audio one. Most HT plate amps will drive a 8 or 4 ohm loads, but not 2 ohm, so the parallel option is ruled out for any drivers with 4ohm coils.

The car amps are generally happy to drive a 2ohm load, so the parallel option would be the normal choice.

 

[sammyxp]

Thanks guys, it makes sense now. Kind of an interesting prospect, actually - being able to adjust Q with the main compromise being power handling.
Right now, the subwoofer sounds great, but could be louder. It's not a big enough problem warrant a new amplifer capable of a 2ohm bridged load. The Infinity 10.1D is rated for 600W RMS, so my 250w amp shouldn't overtax a single VC. Right now, with an 8 ohm load, it's unlikely the amplifier is generating its rated 250W into the expected 4 ohm load.

Great info!