[net.audio] Homebrew Speakers - It's not only me.

spj@hou3c.UUCP (Stephen P. Johnson) (02/06/85)

My recent article generated a flurry of questions, which may be of general
interest. The address for Speaker Builder magazine is PO Box 494
Peterborough, NH 03458. This is the bible for novice speaker builders. I
recommend buying all the back issues, which are available.
	What kind of equipment is needed to build speakers? That depends.
For closed box systems you might not need any. If you want to do ported
designs you need a good multimeter and an audio signal generator. I also
find a digital capacitance meter (about $50) very helpful for putting
together caps for crossovers ( get to within 1% instead of guessing what
the values really are). I also have a pink noise source and 12 band spectrun
analyser (Technics about $250). A microcomputer or programmable calculator
running one of the Thiele Small design aids is a big help.
	What kind of crossover? Someone suggests using 6db networks because
higher order networks are more succeptible to driver impedance variations.
I suggest that higher order nets are less succeptible because each driver
works over a narrower range. Of course, unless you want to go crazy you
will use drivers with flat impedance curves, or equalize the impedance
with Zobel networks (see Speaker Builder). Otherwise, the perf. of your
speaker is a very chancy proposition. One thing everyone seems to miss
about 6db networks, the effect on the radiation pattern at high freq.
The directivity of a dome radiator at high freq. is primarily determined
by the SIZE of the radiator, that's why tweeters are generally smaller
than about 1", to get good dispersion. So what happens when you use a
6db network? The woofer is radiating substantial energy in the high freq.
range. This increases the apparent size of the tweeter, which causes
beaming in the high freq. response. Note that the "perfect" characteristics
of 6db nets are only present when the two drivers are physically
coincident. Using higher order nets also reduces various forms of
distortion, although there is undoubtedly more group delay with higher
order nets.
	What kind of system do I use? Three way with sixth order Thiele
Small woofer (10" ported in a 4 cu ft box), and a small hexagonal
cylinder on top with a 6 1/2" ferrofluid mid and 1" tweeter). The crossover
is 24db per octave active with tri-amping. It sounds very good.
I've also built several other systems for rich friends that were better,
and more expensive!
	A question about performance changes in an old ADS810 speaker;
the high freqs have gradually disappeared over the years. Two probable
causes: 1) The electrolytic capacitors (if any) in the crossovers have
dried out and changed value. Most electrolytics have a lifetime of only
about 5-10 years. 2) The tweeters have undergone physical changes over
the years. Try testing/replacing the electrolytics with film caps first.

	Good luck.... Maybe someday we'll have net.audio.speakers!

	Steve Johnson