Converting an active subwoofer to passive a good idea?

[arichman]

I have a subwoofer that is working fine in my main system, but is out of warranty (Monolith V12 V2). It has a plate amplifier with DSP. Apparently, if that amp fails out of warranty it is not possible to get a replacement.

This subwoofer weighs more than 100 pounds. Tossing it in the dump for lack of an amplifier is a shocking waste of resources, both in materials and in the energy required to make this thing and ship it halfway round the world. I have seen suggestions that the subwoofer could be converted to a passive device with external amplification. If you read the ad copy from the company, it sounds like the proprietary driver and amp were designed together. Since the specifications of this proprietary system are not public, it seems unlikely that the exact properties of the original system can be duplicated.

Is that right? After all, passive subwoofers do exist. What are the risks of the relatively simple process of bypassing the built-in amplification, destruction of the driver (which is going to the dump anyway in the absence of a solution) or will it just sound bad?

 

[staticV3]

Converting to Passive should be a straightforward and risk-free endeavour (for the sub that is. Be careful when handling exposed electronics!).

You certainly won't destroy the woofer using an external Amp, unless you A: spectacularly mess up the wiring or B: spectacularly overdrive the woofer.

It'll still sound like a sub after conversion, but bass extension and frequency response flatness will likely drop when driven passively.

However, run any kind of room correction (Dirac, Audyssey etc.) and it'll take care of that anyway.

 

[digitalfrost]

The problem with making it passive is the crossover. If you try to build a lowpass with normal components, because of

1) the high power requirements
2) the low frequency

You will need massive components, and since the coils are made from copper they will get expensive very soon. Just look up the price for a 8,2mH coil with 0,5ohm resistance.
Also, if you try a lowpass near the fs of the woofer, you will get more output since the inductivity of the voicecoil will interact with the inductivity of the series coil. So you need an RLC in parallel to linearize the voice coil inductivity. Also, you cannot change crossover frequency that easily so if you want to be able to experiment, you need to buy even more expensive components to do so.

TL;DR: Do not try to filter a subwoofer passive, it's not worth it. You can easily buy a plate amp for what you would pay for passive crossover components.

Now replacing the plate amp with a power amplifier + minidsp or with another plate amp, or some amp with build-in DSP, yes that is a good way to go.

 

[M240i]

Since the specifications of this proprietary system are not public, it seems unlikely that the exact properties of the original system can be duplicated.

Is that right? After all, passive subwoofers do exist. What are the risks of the relatively simple process of bypassing the built-in amplification, destruction of the driver (which is going to the dump anyway in the absence of a solution) or will it just sound bad?

You can create the exact properties easily. You'll need to make a differential measurement between the input signal and the output signal (measured on the driver terminals) of the subwoofer amplifier.
That will give you a baseline you can match with a DSP equalization, or some other type of equalization.

 

[staticV3]

The problem with making it passive is the crossover. If you try to build a lowpass with normal components, because of

1) the high power requirements
2) the low frequency

You will need massive components, and since the coils are made from copper they will get expensive very soon. Just look up the price for a 8,2mH coil with 0,5ohm resistance.
Also, if you try a lowpass near the fs of the woofer, you will get more output since the inductivity of the voicecoil will interact with the inductivity of the series coil. So you need an RLC in parallel to linearize the voice coil inductivity. Also, you cannot change crossover frequency that easily so if you want to be able to experiment, you need to buy even more expensive components to do so.

TL;DR: Do not try to filter a subwoofer passive, it's not worth it. You can easily buy a plate amp for what you would pay for passive crossover components.

Now replacing the plate amp with a power amplifier + minidsp or with another plate amp, or some amp with build-in DSP, yes that is a good way to go.

Just let the AVR handle crossover duties, no?
Then OP can buy any Mono Amp (Fosi, Behringer, Crown etc), without the need for external DSP.

Edit:
Of course, this depends on whether OP has an AVR with Sub output

 

[arichman]

To be clear, yes I have a MiniDSP Flex which currently handles the crossover for the subwoofer. I also have a high quality external subwoofer amplifier which I have been eying for this eventuality. But I'm dimly aware that there are properties of the system such as subwoofer volume and the specifications of the driver which presumably were taken into account by the manufacturer in their "tuning" of the subwoofer using the built in DSP. What I'm understanding from the comments is that these issues can be adequately addressed, at least in principle, using DSP applied either in the external subwoofer amplifier, an AVR, or in my case, the MiniDSP Flex.

 

[Wayne A. Pflughaupt]

I’ve converted two active subs to passive with no problems. You can find a link to a thread in my signature that includes both "builds."

I’d suggest a used, good-quality pro-audio amplifier for a beast like the V12. If you’re not happy with the results, you can re-sell the amp and not be out any appreciable amount of money. In that case, stick with a well-known popular brand like QSC, Crown, Crest, Yamaha, Peavey, etc. that will be easy to move.

Generally, I wouldn’t worry about the proprietary processing. The only thing that might be of concern is built-in limiting, if you routinely run the sub maxed out.

Regards,
Wayne A. Pflughaupt

 

[DVDdoug]

If you read the ad copy from the company, it sounds like the proprietary driver and amp were designed together. Since the specifications of this proprietary system are not public, it seems unlikely that the exact properties of the original system can be duplicated.

That's often true, especially with sealed woofers which normally cut-off* at higher frequency but have a more gentle slope than a ported design. So with EQ (and more power) they can go lower than a similar ported speaker.

EQ doesn't usually work as well ported boxes because the steeper slope means it needs lots of power to overcome the drop-off and the woofer is unloaded (unrestrained) at lower frequencies so you get lots of cone movement with little sound.

Basically, you start with a (hopefully good) driver & cabinet design and then make it better with EQ. A lot of plate amps have DSP/EQ but it's best if you know the characteristics of the driver in the box so you know what to do with the EQ.

You probably won't find the Thiele-Small parameters for your woofer but here is a gizmo for measuring them. You can then enter them along with the box volume (and port dimensions if any) to model the performance with speaker design software. (WinISD is FREE.) Then you can determine what EQ you want/need (if any) and you can try virtually adding or modifying the port, etc.




* Cut-off is usually defined as the -3dB point

 

[digitalfrost]

This might be nit-picky, but if you remove the plate-amp from the sub but still run it with a crossover that is applied before amplification and you have a dedicated amplifier for the subwoofer, I do not consider this "passive".

You simply changed how you're doing active.

 

[Wayne A. Pflughaupt]

This might be nit-picky, but if you remove the plate-amp from the sub but still run it with a crossover that is applied before amplification and you have a dedicated amplifier for the subwoofer, I do not consider this "passive".

You simply changed how you're doing active.

By that definintion, all the main-channel speakers are "active" too, if high passed via the AVR.

Regards,
Wayne A. Pflughaupt

 

[DWPress]

I've converted active plate amp subs to sealed subs with external digital XO - your miniDSP Flex can do the job but is limited in sub EQ ability with limited filters, should be fine. While you're in the guts of the sub you may as well add sealant to all the seems (Great Stuff or similar works really well) and add some bracing to really dampen the thing.

One hidden benefit is getting rid of the internal DSP of the sub which adds delay and other artifacts when you're trying to aline it to the rest of your system that also has DSP.