passive subs

[Too Tall]

Passive subs
I want to know if this is possible.

Start out with stereo 2-way speakers with passive xovers.
For power a single stereo power amp.
Now we want to add a single 12” sub.
This sub must get its power from that same power amp.

No self powered sub or dual voice coil woofer.
No adding another woofer.
Thanks

[PeteMcK]

depends on whether your amp can handle the load, I've seen such a connection suggested many years ago - an inductor from each side of the stereo amp to the single woofer.
I'm guessing that BTL amps need not apply, & if any strange earth loops occur, you may see the magic smoke escaping...

[Drew Eckhardt]

Originally Posted by Too Tall

Passive subs
I want to know if this is possible.

Sure. You can high and low pass the two channels and feed the low pass outputs to a transformer which sums them to mono for a sub-woofer.

[wrankin]

Originally Posted by Drew Eckhardt

Sure. You can high and low pass the two channels and feed the low pass outputs to a transformer which sums them to mono for a sub-woofer.

As a mental exercise this is fine. But you are probably going to spend more on the tranny than you would on a decent DVC woofer. I guess if you have an old power transformer with dual secondaries laying around it could work. Level matching to the mains becomes the issue at that point.

If this is a "scrounge from the parts bin" type of project, then you are better off going out and finding an old receiver or integrated amp to drive the sub. A $20 special off of Craiglist or your local Goodwill could work.

-bill

[Too Tall]

Originally Posted by Drew Eckhardt

Sure. You can high and low pass the two channels and feed the low pass outputs to a transformer which sums them to mono for a sub-woofer.

I thought a transformer would be suggested.
The center back to the sub?

What kind of transformer and where to find it?

Thanks

[DrDyna]

I've had passive subs for many years and they work fine. Used them with a few different configurations of gear. I have left and right though. Passive crossovers are about 80hz.

[wrankin]

Also, if the amp is capable of bridged operation then the following *may* work:

1) invert one of the input channels.
2) invert the corresponding main speaker hookup for that channel (so that the phase is correct)
3) run the sub in parallel across the two positive outputs from the amp.

Caution should be taken - this is a good way to smoke the amp and maybe your input source.

-bill

[DrDyna]

Originally Posted by wrankin

Also, if the amp is capable of bridged operation then the following *may* work:

1) invert one of the input channels.
2) invert the corresponding main speaker hookup for that channel (so that the phase is correct)
3) run the sub in parallel across the two positive outputs from the amp.

Caution should be taken - this is a good way to smoke the amp and maybe your input source.

-bill

Depending on amplifier topology, I'd be very cautious trying something like this.

[sreten]

Originally Posted by wrankin

Also, if the amp is capable of bridged operation then the following *may* work:

1) invert one of the input channels.
2) invert the corresponding main speaker hookup for that channel (so that the phase is correct)
3) run the sub in parallel across the two positive outputs from the amp.

Caution should be taken - this is a good way to smoke the amp and maybe your input source.

-bill

Hi,

It will work if the amplifier channels are 4 ohm capable, the subwoofer
is 8ohms, the speakers 8 ohms or the speakers are 4ohms with a series
capacitor applied to each, but might mess up the bass alignment.
The sub would need a passive filter, big inductor + capacitor.

Or possibly get something like this :
TDA7294 2.1ch amplifier board 80W*2+160W SUB + heatsink | eBay



rgds, sreten.

[Too Tall]

Originally Posted by wrankin

As a mental exercise this is fine. But you are probably going to spend more on the tranny than you would on a decent DVC woofer. I guess if you have an old power transformer with dual secondaries laying around it could work. Level matching to the mains becomes the issue at that point.

If this is a "scrounge from the parts bin" type of project, then you are better off going out and finding an old receiver or integrated amp to drive the sub. A $20 special off of Craiglist or your local Goodwill could work.

-bill

I guess it will help if I explain the project.

This project is for retail sale. This is not a one-off for my garage,
We need to keep an eye on cost and price point.

The top speaker is a 1” compression driver and a 5-1/4” woofer.
Coaxial and no horn.

With this top a single 12” woofer for a sub is more than enough.
To use two 12” drivers would be a waste of money.

The customer wants to buy all drivers from a single manufacturer.
The manufacturer does not offer anything smaller than 12” driver for a sub.
It does not offer a dual voice coil driver (My first pick).

For keeping the cost down we are restricting the design to a single stereo power amp. No additional power amp such as a plate amp which again moves the price point up.

We do have an example building a proper Y-Cord for line level.
It is purely resistive, but it would not see more than a couple of volts
Trying to use it for this purpose would turn it into a radiant heater.
It would take some HUGE resistors.
What I expected to hear was using a transformer.
Top and bottom to the power amp outputs and the center tap to the sub driver.
The trouble is I don’t know much about transformers. I would not know which one to use or even if I was correct to use it at all.

I hope this clears things up.
Thanks

I am doing this for a friend. I have already surmised that a transformer capable of handling the juice involved might cost a great deal.
OTOH One of these pro drivers also cost a great deal.

On the subject of inverting the output and looking at 2 ohms, we are not selling the power amp. The customer has to source it elsewhere so there can be no restrictions. I don't see our customers buying the latest Crown or QSC power amp. More likely a Behringer or worse.