Stupid Question: Sub Amp Out to Front Speakers?

[BlueWizard]

I'm almost embarrassed to ask such a simple question, but it is something I've never considered before.

First, I have a straight stereo system, meaning I've never had a Subwoofer. So, that is our starting point.

I was asked a question in another group that got me thinking about Subwoofers.

If I recall correctly, one of the ways of connecting a Subwoofer is to take the Speaker outputs from your Stereo Amp and feed the High Level Inputs of the Subwoofer.

Next, you can take your front speaker input from the Sub amp which will filter all the low frequencies that the Sub is going to take care of. Though of course when is say 'input from the sub amp' I don't mean from the sub amp power section.

In short you feed the sub amp with the front speaker outputs, then feed the speakers themselves with the filtered output from the sub amp.

At least, this is what I've alway assumed was one possibility for connecting a Sub. If that assumption is completely wrong then stop reading now and just tell me so.

However, if this assumption is correct, then I'm slightly confused.

It would seem that even though you are feeding your front speakers through the sub amps low filter section, it must still be the outputs of the main amps that are feeding the filtered front channels?

Again, if that is a wrong assumption, tell me now.

Now to the actual point, if the sub is filtering low frequencies from the front speakers, then it can't be using a active filter or it would interfere with the power distribution from the main amp channels.

So, the conclusion is that it must be using some type of variable passive filter (variable crossover) to the front channels. But, that doesn't quite seem right. With all the complaints I hear in this group about the complexity of implementing a passive 4-way crossover, it seems odd that using a sub as I have described, is really nothing but a glorified 4-way crossover (assuming the front speakers are 3-way).

So, what am I missing here? What don't I understand?

Are front speaker not fed by passing through the sub amp high pass filter?

What is befuddling me so, is the idea that if you can feed front speaker by passing through the sub amp high pass filter, then the only possible nature of that high pass filter is that it is a passive filter?

Can anyone enlighten me?

Now I know most people with an AV amp, let the amp sort things out and connect the fronts directly to the amp itself. But if you are implementing 2.1 with a stereo amp, it would seem to be a different approach.

So, I'm confused and need enlightenment.

Steve/bluewizard

[gedlee]

You are correct, any filtering of the mains would have to be passive. Personally, I would never do what you are saying for any "good" speakers. It's fine to feed the sub with the mains signals, but it's NOT OK to HP filter the mains with some cheap passive filter in the sub. Just forgo the HP function for the mains and feed them ful range. If they are ported just close off the ports. If they are not ported then they should work fine with the sub left as full range. In fact, what you really want to do is add two or more subs with full range mains. Thats the best that you can do in all circumstances.

[DcibeL]

Using the high level speaker output from a subwoofer plate amp is only a good idea if you don't care how it sounds.

It is very easy to see what is going on by simply looking at the PCB where the high level inputs and outputs are connected. On the plate amp I used to own there was a big 100uF bipolar electrolytic in series with the high level speaker out.

Why is this such a bad thing? Well have a look at the impedance plot of your average "real" speaker. There is usually an impedance peak somewhere in the bass region because of the woofer resonant frequency in your speakers. There may be a second peak for ported speakers. When you place that filter cap in series, it absolutely destroys the frequency response in the bass region of the speakers.

This 100uF cap is a joke to hifi-audio, as it is calculated from an 8 ohm perfect resistance, not specifically designed to work with any speaker. And no it does not change the crossover freq based on the selected crossover freq for the subwoofer.

If you really want to have an accurate crossover between loudspeaker and subwoofer, you could design a line level active crossover for this specific application.

[BlueWizard]

Excellent, that is what I needed to know.

I'm not a real big on subwoofers, I would rather have a nice pair of multi-driver tower speakers that dig down to about 30 to 35hz.

Though at some point in the future, I might break down and buy a surround system, though it only has modest appeal. So, this is good information to know.

So, if I understand correctly, the external speaker outs of the sub amp are not a variable crossover. You can adjust your sub crossover all over the place, and the pass through to the front speakers remains fixed? Well that sucks.

I'm also concerned about resistor noise if you use the high level inputs, it has to be stepping that high voltage down to something the Sub pre-amps can use. Isn't just a high drop across a resistive voltage divider going to be noisy?

Though one would hope they would drop the signal in stages across several voltage dividers and op-amps.

Again, not having a sub, and without thinking, I assumed there was some active circuitry that worked with the variable crossover of the sub itself, and coordinated that crossover with the front speakers.

Only today did it occur to me that the Subs crossover circuits CAN'T be involved if you are passing power from the main amp through the sub box and on to the front speakers.

I mean, NOW it seem obvious, but I had never given it much thought before.

As to using some kind of active filter, isn't that built into most AV amps? Most people say to set the Sub crossover high, and let the AV amps sort out the various speakers.

Thanks for the responses.

Steve/bluewizard

[Drew Eckhardt]

Originally posted by BlueWizard

I'm also concerned about resistor noise if you use the high level inputs, it has to be stepping that high voltage down to something the Sub pre-amps can use. Isn't just a high drop across a resistive voltage divider going to be noisy?

The values can be smaller than for a line level input. Ex 1K and 100 Ohms for 20.8dB of attenuation versus 10K-50K for the line level input.

As to using some kind of active filter, isn't that built into most AV amps?

Yes. The better and/or more modern ones will even do some equalization.