Sub Woofer Design

[Edu Silva]

Tom Priore wrote:
> 
> Hey, I know its a bit off topic, but I'm building my self a subwoofer for my
> stereo.

And how about using it with your synth? Moogs, TB's and 808's would be
very greateful. You have never really heard these beasts until using a
room-shaking sub.

> First I'm building a simple op amp based amplifier. I have some 412 chips
> laying around,...

412? Lots of chips, eh? Didn't you mean LM12?

> ...do you think they would be clean enough to use?

The LM12? It's a power op amp capable of driving ±25V at ±10A while
operating from ±30V supplies, and can deliver 80W of sine wave power
into a 4 Ohm load with 0.01% distortion. It have been used with success
in many sub amp designs. Go for it!

Datasheet here: http://www.national.com/pf/LM/LM12CL.html

This site have a good project based on the chip:
http://icdweb.cc.purdue.edu/~dpaton/audio/lm12amp/lm12.shtml

> I'm also
> running the original signal though a low pass filter. I'm thinking for
> simplicity it will just be a basic first order passive filter,...

Nah, you'll need something better, like a 2nd order one, with variable
fc (cutoff freq). 6dB/oct filters here, simply won't do. And don't
forget to cut the lows on the main speakers - this time, sigle pole
filters are OK, but remember to desinvert the phase of the sub, already
inverted by the filter.

> ...but if I get
> bored I think I might make it a active filter and integrate it into the
> amp's opamp. How does this sound?

Seems to be a nice idea, but you'll get more distortion doing this.
OTOH, distortion isn't really the point in sub amp design, so you'd
better check it out and tell us the results.

> What frequency should I uses for the pole?

It depends on the application (hi-fi, video, musical instruments...) and
the overall design, including the speakers. The best thing to do it's to
use a variable lowpass filter, say, from 50 to 150Hz, and try different
settings for fc. Maybe you'll find more than one "optimum settings",
depending on the kind of music being played. Note that you won't need to
be a perfectionist and match the sub sub LPF fc with the low cut on the
main speakers.


Edu

ES2 Audio  --  Som & Luz
Salvador -- BA -- Brasil