switching subwoofer coils and back emf/voltage spikes?

[AmmoJammo]

I'm considering wiring up the following:



The idea is to be able to run just one coil per sub most of the time, or switch the second coils in when needed.

I'm concerned that the inductance of the coils will cause voltage spikes if the switch is triggered while the amplifier is on, or playing music.

I'm wondering if this issue could be negated, if I switch the "positive" output of the amplifier, as the "negative" side is in fact tied to ground, and therefore, shouldn't damage the amplifier in the event of a spike.

As I'm wanting to do this with a relay, I'm also concerned that the relay could open due to vibration and voltage drop (this is for use in a car) so am especially worried about damaging the amplifier from voltage spikes!

Any info, input, or ideas would be great thanks!

[jahonen]

You shouldn't leave DVC driver coils open. If you absolutely insist, short the other coil when not used. You would need a SPDT type relay.  Doing that will degrade the sensitivity figure. Otherwise the driver will not work as expected (driver Qts will rise, thus the frequency response is not what you expect).

But as your question, turn the volume down when switching coils, and you shouldn't have any issues.

Regards,
Janne

[T3sl4co1l]

Would they not have the same EMF by motion, and if the windings are on top of each other, then by leakage (to a pretty fair extent) as well?

Tim

[AmmoJammo]

I'm beginning to think that it shouldn't be an issue...

The amplifier already uses relays to disconnect the subs in the event of a fault, such as over temperature, low impedance, or low voltage I believe... If disconnecting the speaker(s) while the amp was running flat out was going to be an issue, then the amplifier would kill itself every time it tried to protect itself :p

[Yago]

Would 6 coils in parallel reduce the impedance to a level that might cause the amp problems?

[AmmoJammo]

Would 6 coils in parallel reduce the impedance to a level that might cause the amp problems?

0.33ohm nominal.... she'll be right 

[AmmoJammo]

Wired it up..
There's a pop through the subs when I disconnect the second coils, but not when I connect them, she'll be right!

[qno]

Do not short the second coil when not in use.
They will act as an electromechanical brake on speaker movement when not in use and shorted.

I assume you have 2 coils per speaker. Both are 8 ohms.
When switched parallel your speaker becomes 4 ohms.

You must have an amplifier with a very high current drive capacity to drive 3 voicecoils of 8 ohm in parallel.
3 times 4 ohms is almost unimaginable for a normal commercial audio amp.

Remember you have to have very thick copper wires to drive 3 speakers of 4 ohms in parallel.

Maybe you should consider a different connection scheme of placing 3 speakers of 4 ohms in series or 3 8 ohm speakers in parallel.

[AmmoJammo]

No, they're dual 2ohm... amp is at 0.66ohm daily, with the switch to run it at 0.33ohm.

I wasn't going to short the second coil, I've run these subs using just a single coil in the past.

[Anks]

At 0.66ohm a lot of the energy will be wasted in the wires and in this case that's a good thing or the amp would blow. You using a crown macro tech or something?

[jahonen]

Both coils need to be connected to a low impedance source in some way or otherwise the Qts (or Qes exactly) will change. It doesn't matter Qes-wise whether it is connected to an amplifier or just shorted. If shorted, it acts as an electrical brake but that braking action is necessary to keep Qes constant. Power handling might need to be derated. Sensitivity will degrade by 3 dB.

Now consider the following:

Qes = 2*pi*fs*Mms*Re/(Bl^2)

Where Qes is the electrical damping factor of the driver, fs is the free-air resonance frequency of the driver, Mms is the moving mass including air load, Re is the resistance of the voice coils and Bl is the driver motor force factor, N/A or T*m.

Namely, the Re/Bl^2 part should remain constant. If the other coil is not connected, Bl will be equal to the case where coils are connected in parallel but resistance is doubled. That means the Qes will also be doubled. If both coils would be connected in series, the resistance will be 4x to the parallel case but force factor Bl will be also doubled, since there is now double length of wire in the motor air gap, thus the Re/Bl^2 part remains the same.

Now since Qts is dominated by the electrical quality factor, it will be usually also increased by similar factor, because mechanical quality factor is usually much higher than electrical one, (something like factor of 10).

Approximate efficiency for typical driver can be calculated by

no = rho*Sd^2*Bl^2/(Mms^2*2*pi*c*Re)

Sd is the effective cone area and rho is the density of air, c is the speed of the sound.

So compared to parallel connected case, doubling Re will halve the efficiency and reduce the power sensitivity by 3 dB.

Regards,
Janne

[AmmoJammo]

At 0.66ohm a lot of the energy will be wasted in the wires and in this case that's a good thing or the amp would blow. You using a crown macro tech or something?

It's in a car, so its a class D monoblock.

Rated at 1500watts into 1ohm, and realistically, after impedance rise, it's probably never seeing below 1ohm anyway.

~40amps maximum, into three drivers, each of which have their own 12 gauge speaker cable going to it