- Posted by Lynn Olson (M) on October 19, 2005 at 23:42:21
Hi folks, my first appearance in this forum, and many thanks to Robert and Bill for introducing me to the Apollo at the RMAF - my favorite room at the show, no matter whether it was the Allen Wright modified SACD player or the Stellavox tape machine.I've been listening to a Prometheus going through its burn-in process at a friend's house, and I've had some thoughts on ways to improve the break-in process for the unique Bastanis drivers. I understand the break-in of the spider, as typical for most drivers, takes only a day or two. This is typical for most dynamic drivers, where the cone material is stable and doesn't change with time or use - it's the spider that changes, and results in the well-known lowering of Fs, as well as improvement on dynamics and tonal qualities.
What's different with the Bastanis drivers is the *cone* requires a break-in as well - and a much longer break-in than the Lowther or Fostex. This can try the patience of Americans - especially the folks around here in Colorado, who are a more go-go, hurry-up kind of crowd than the people I knew in the Pacific Northwest. So I've been thinking about not only a quicker break-in, but also better, more thorough, one that would result in symmetric properties and a quicker arrival at the chemical/mechanical steady-state condition (the tail end of the asymptotic break-in process).
My thoughts so far - and I would welcome comments from anyone who knows more than I do, especially Robert - are this:
1) Get an analog signal source that puts out very fast rise-time square waves, 1 microsecond or less rise-time (not a sound-card or CD player). There are plenty of old analog sig generators that put out very fast square waves. Get one that runs as low as 20 Hz - you'll see why later.
2) Get a decent-quality transistor amp with a bandwidth of at least 100kHz and anywhere from 20 to 50 watts output power.
3) Face the two Bastanis widerange drivers towards each other - maybe an inch apart or less. Connect one to each channel of the amplifier, but connect one driver out of phase. Get a "Y" splitter for the signal source so both channels of the amplifier are driven together.
4) Set the generator anywhere from 20 to 30 Hz, adjusting the frequency for maximum displacement of the cone - the third harmonic should land on the resonant frequency of the driver, and it should be obvious when it does.
5) Increase the power level until the cone is moving about 3 to 4mm - Robert would know what a safe long-term level would be, but these are basically PA drivers, so they should absorb a few watts with no problem.
6) Leave it on for 24 hours, with a blanket over it if the sound is annoying (and it will be). I would recheck every few hours, adjusting the frequency downward as the spider and cone break-in together, which will gradually lower the resonant frequency of the driver. Sweep the frequency up and down to find the new resonant frequency, and leave it there for a few more hours, then repeat until the 24 hours are over.
7) my best guess is 24 hours of this treatment will be equivalent to hundreds of hours of music player at a fairly high level. Why? Loudspeaker drivers are constant-acceleration devices - a theoretically perfect driver with a flat acoustic frequency response also has a flat acceleration vs frequency curve.
This implies that the cone and voice coil former, and most of all the cone/voice-coil-former junction, experiences extremely high G levels in normal use. It is this area that is changing in sound as the driver breaks in - the spider is what responds to the large excursions of the bass (displacement), but the cone and junction area that experience the high G levels in music playback.
If we want to encourage the process of re-forming the fibers of the cone, and remove small stress fractures in the cone/VC glue region, we want to give it lots of acceleration, not displacement. We don't want to destroy it - obviously 200 watts of square wave power is too much - but do want to take the cone assembly to a reasonable fraction of its maximum acceleration. If max acceleration is 100%, we'd like maybe 10% to 20% of that for 24 hours, so the driver is well within its limits, but working at a reasonable fraction of its potential.
The problem with music is the duty cycle - 95% of the time far below peak modulation - and rather low peak acceleration, except for very loud percussion, or maybe trumpet sounds. There's lots of variety, but the peak energy, especially peak acceleration, is only happening a very small percentage of the time, even if you are feeding the driver with a 200-watt amp at full power.
Anyway, that's where we come to the test generator, which can be an old surplus instrument. The high rise time generate the maximum possible G-force, as well as maximizing the fourth derivative of motion (jerk), which should relieve stresses in the fibers of the cone and the materials used to assemble the driver. The key concept is to carry it out at levels that are well within the thermal and mechanical design limits of the driver, but using a signal that efficiently - and completely - removes the artifacts of manufacture.
Any thoughts?
