High-Pass Filters?

So, now I'm going to buy a subwoofer.

My mains are Klipsch Forte II's. They are good down to about 35Hz then drop off quickly. The sub I'm buying should be good down to 20-25Hz.

Actually, I've never been all that impressed with my Klipsch's below about 100Hz. So, does anyone have links to sites that show how to build GOOD High-Pass Filters or crossovers?

I don't want to run my mains through the sub-woofer. My room isn't set up to do that. My current decoder does not filter the mains. It's sub-woofer output has a low-pass at 200Hz. My sub has a variable low pass from 40-140.

So, that leaves me to DIY or DI-ms...


So, I'm guessing either no one knows, or there is some simple solution I'm overlooking.

I'm a computer engineer. So, I know this can be accomplished with simple coils or capacitors. But, wouldn't that introduce noise? Also, wouldn't they have to handle a lot of power?

The other option is a op-amp filter. Again, it would have to handle power, unless I put in my pre-amp connection. But, then the noise could easily become a problem.

Someone help. I would like to hear from a hi-fi audio expert on this subject. My college electronics book won't tell me what I want to know.

You basically have two options here. The first is to use an active crossover between your pre and power amp to divide the signal into a high and low part and then pump the two new signals into separate poweramps for the mains and the sub.

The other is to build a passive crossover that you place after your power amp but before your speakers that splits the signal and routes the highs to the mains and the lows to the sub.

The active solution is usually the prefered one, since it is easier to construct a high quality filter for line level signals than it is for speaker signals. You won't need high voltage components, and you don't need to be as concerned about tricky impedance matching.

The active crossover also has the benefit of allowing you to select the poweramps separately. For the bass, you don't need the ultimate in refinement, but you will need a fair amount of power. For the treble, you won't need power, but you'll want a really good sounding amp.

You could also use a passive crossover with a booster amp for the sub, but I can't imagine how this would be better than an active crossover.

As for introducing noise, any item you stick in the signal path has the potential to introduce noise and distortion. Hence, whichever route you choose, use high quality components and opamps.

Active crossovers (ie biamping) is generally regarded as something that improves the whole system quality, however, because whatever noise and distortion is added by the crossover is more than made up by the improved performance you get from having two speciallized poweramps dividing the load.

Check out the ESP site (www.sound.au.com) for some active crossover plans. You can also get some good info in the last chapter of John Linsley-Hood's book "Audio Electronics." For passive crossovers, most any book on speaker design will include a chapter on them.

Solutions for Active Crossover

I use the Audio Control Richter Scale Series III for my mains to sub.
Perfect solution and cheaper than making your own. Buy used now
since it is discontinued. Simple circuit, discrete parts, easy to make
modules (Eight 1/4W metal films). I used something around 70Hz
for the crossover with a pair of Klipsch KG4s and 12" sub. Worked
well integrating the two. As much as I like DIY, mods are generally
easier. . .just swap parts or bypass others. . .try Ebay.