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Re: [TCML] Speakers - was: Terry filters (speaker/motor load modeling)

Hi Phil,

Wow! I didn't actually expect someone to try this out but
I guess the myth is now confirmed! ;^)) Thanks for putting
the myth to the test and reporting the results. Sounds like
a theme for Mythbusters :^)

David Rieben

----- Original Message ----- From: <FIFTYGUY@xxxxxxx>
To: <tesla@xxxxxxxxxx>
Sent: Wednesday, January 02, 2008 5:48 PM
Subject: Re: [TCML] Speakers - was: Terry filters (speaker/motor load modeling)

In a message dated 1/2/08 4:32:35 P.M. Eastern Standard Time,
drieben@xxxxxxxxxxx writes:

BTW, does anyone recall the Bose 901 series speakers? They were  pretty
popular back in the

I currently own two pairs of 901s, one  pair is still in use as the front
stereo channels of our main surround-sound  system. I've experimented with
several 901's of various vintages over the  years.

I assume that they were the standard 8 ohm impedance type

901's are anything but "standard". In later  versions Bose developed their
own unique "woofer" for this application. Like some Tesla Coilers, Bose used
edge-wound ribbon instead of round wire to make  a more compact voice coil
assembly. The ribbon edges are on the inside and outside of the coil, not the top and bottom as we wind it for a TC. Not that strange, other manufacturers have
done the same, and some even use *hexagonal*  cross-section magnet wire to
more efficiently pack the wire into the limited  space in the magnet gap.
   IIRC, all 9 of the identical "woofers" in  the original 901 were all
connected in series. I also remember in later  versions there was a
series/parallel connection. A lot of tweaking and technology went into the later versions.

and it seems like I heard someone say that as part of Bose's advertisement
campaign for these
speakers that they actually connected  the input leads of a Bose 901
directly to

a 120 volt outlet and the speaker did not blow.

   I've seen a lot of the advertising for the  901's, even the original
magazine reviews from the late '60's when they first came out, and never saw
anything like that. Believe me, it would've stuck in  my mind!
   They *did* originally spec them at "unlimited  power handling in
non-commercial applications", but much later changed it to something specific and quite reasonable. The later versions were much tougher, anyhow. I *can* say I've
never seen a blown 901. Like many older speakers,  especially of the
mid-70's, they tended to dry-rot and fall apart (Bose even had a trade-in upgrade
program going for original owners).

Assuming the 8 ohms to be the correct resistance that the 120 volts, 60 Hz
AC saw, that means
that the speaker would have been processing 1875 volt-amps (not  sure what
the power factor

would be in this situation) of sound power!

That's no big deal. I drive my 901's with an older Carver TFM-45, rated
at 375W/channel continuous indefinite, 750/channel  for a few seconds, and
I've measured it as much higher on the peaks. Carver  had some interesting
designs that used power supply transformers with a lot of leakage inductance to store energy, instead of big filter caps on the DC bus (although the TFM-45 has pretty big caps, and Carver amplifiers are another story unto themselves...). I once plugged the speaker leads from my TFM-45 into an uncooked hotdog, and
at full output the hotdog began smoking as it  made music...
   During the heyday of the 901's, there weren't too  many affordable amps
that were capable of putting out enough clean power to  blow them. But the
901's were very inefficient speakers, so it took a lot of power through them to
get them as loud as most other speakers. Another  tradeoff...
   People (myself included) would take 901's, turn  them around, and use
them as PA/sound reinforcement speakers. Bose eventually introduced the 802's,
which were basically 901's turned around without the 9th  driver on the
backside. So they were very tough.

Sorry to get a little OT here, but has anyone else heard this  "story"?

   Well, now that I typed all this, I figured we might  as well find out.
I'm just dumb enough to try something like this, and in a position to do so: I just took a single 901 speaker, Series V, 1983 vintage, downstairs to the garage. Donned safety goggles and *earmuffs*. I plugged it directly into the wall. No ballast, no variac, no switch, no nothing! Twice in a row for five seconds each time. No smoke, no smell, and a good clean 60 Hz sound with no indications of clipping. Lights dimmed very significantly. VERY loud. My
wife upstairs said it vibrated the whole house.

-Phil LaBudde

Center for the Advanced Study of Ballistic  Improbabilities

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