summing crossover (one woofer from two stereo amplifiers)

[Basil]

I want to add a woofer to a stereo amplifier. I want to sum both channels, L&R, at low frecuencies.
Medium and high frecuency speakers are the original ones drived by the highpass LC filter.
Low frecuency (woofer) is tricky. I just can't connect both low pass output together to the woofer. Amplifiers are based on a **broken link removed**

Can this problem be solved with pasive (L, C and perhaps a transformer) components?

 

[FvM]

I don't see that the IC power amp differs from a regular discrete amplifier. Connecting amplifier output through low-passes generally involves the problem of DC voltage differences and behaviour with low frequency differential signals that would be shorted by the low-pass. Typical audio signals have negligible low frequency L-R component, but you shouldn't rely on.

Truely adding the output signals by using one or two transformers is basically possible, but the amplifiers must be suited to drive a low DC resistance, or additional capacitor coupling is needed. And of course low frequency transformers are rather bulky.

All-in-all, there are good reasons why sub-woofers are usually implemented with a mono amplifier and active crossover and summing stage in front of it.

 

[kam1787]

Are you talking about a woofer or sub-woofer?

I suppose you could use a transformer but we are talking about low frequency signals which is not the forte of your usual audio transformer.

You have to make sure the transformer can handle the lowest frequency you hope to reproduce.

Perhaps something like this would work well http://www.transformer8.com/product...formers-Ultra_Low-Frequency-JT_112P_2HPC.html

 

[ahsan_i_h]

Alternatively, using a dual voice coil (DVC) subwoofer may solve the problem.

ebay link: https://www.ebay.com/itm/NEW-8-DVC-...t-inch-Sub-Dual-Voice-Coil-bass-/181150365903

 

[kam1787]

So you want to sum the outputs of the stereo amplifiers to feed a woofer or sub-woofer?

It is still not clear which it is.

I had assumed you would sum first, then feed a separate amplifier.

ahsan_i_h made a good suggestion.

Otherwise, the only way I can think of is to use a transformer. power audio transformers are used in tube amps but in your case it is more tricky, as [I guess] you want to drive the existing speakers and the extra speaker. As the Zin drops you will load the amplifier more, but as Zin increases you will have less power going to the additional speaker.

 

[crutschow]

I doubt there are any standard transformers that will handle speaker level signals and impedances at both input and output.

You can use a dual voice coil woofer as ahsan_i_h suggested but you will still need a crossover network for each channel to the woofer.

The better way is to use a subwoofer amp which will have the crossover networks built-in, such as one of these.

 

[Basil]

So you want to sum the outputs of the stereo amplifiers to feed a woofer or sub-woofer?

It is still not clear which it is.

Yes, from two stereo analog (not Class D) amplifiers I want to sum both output below 100 Hz.
Using passive LC network (i.e. not summing at low level and then amplify sum signal)

I think the problem is not the new loudspeaker lowering the impedance, because when amplifier sees the woofer it doesn´t see medium and tweeter.
I think the problem is to short circuit both outputs, LC filter doesnt's avoid "shortcircuit at low frecuencies".

 

[BradtheRad]

I see no one suggesting (so there must be a good reason not to do this), that you:
(a) invert the polarity of the signal to the L channel input, and
(b) bridge the woofer across the R and L channels.

On paper it would appear to be a solution. However internal circuitry may suffer a short circuit, if you are not careful.

Nevertheless there is (or used to be) the 'derived-four-channel' effect, where you connect a speaker across the L and R speaker outputs only. A difference signal comes through. It's the 'poor man's' version of four channels.

I have not tried inverting one channel before doing this. It could have unpredictable results.

 

[FvM]

(a) invert the polarity of the signal to the L channel input, and
(b) bridge the woofer across the R and L channels.

and (c) reverse the regular left speaker, too. This can actually work and is at least much simpler than a transformer solution. Works best if the subwoofer has double impedance.

Inversion of input signal would usually require an inverting preamplifier, unless the power amplifier has differential inputs (unlikely with home audio).

 

[BradtheRad]

and (c) reverse the regular left speaker, too.

Yes, if the amplifier is already being used normally.

In that case it is possible to overload an amplifier with additional speakers.

For some reason I thought the OP wanted to use an unused amplifier to drive the woofer.