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wrote:
BEAR wrote:
BEAR wrote:

wrote:

http://tech.groups.yahoo.com/group/lweusersgroup/

A bunch of us LWE speaker fanatics are collecting and sharing
information about these early (1965-71) servo loudspeakers. These are
the ones that Steve Zipser (RIP) used to rave about all the time on
usenet with the flat frequency response from 20hz-20khz:



I checked the site and the patents...

The trick is that the speakers have some networks worked in, I did not
look carefully at the networks to see their effect beyond that of an
xover. But the intended effect is to create a feedback loop from each
driver that results in a flattened response. In effect a sort of EQ it
would appear... maybe not a bad idea.

Bears some further investigation and maybe reading the patent carefully...

Hi Bear, thanks for stopping by the group and sharing the info about
the CM Labs 911 amplifier.

The best description i've found describing the LWE loudspeakers is
David Winebrenner's review over at audioreview.com:

http://www.audioreview.com/cat/speak...3_1594crx.aspx

or try this tinyurl version: http://tinyurl.com/pedle

Here is a snip:
"Surely there must be some of you out there that have dreamed of using
classic high efficiency large magnet woofers (normally reserved for
bigger reflex designs) in a relatively small bookshelf size box and
"somehow" designing the box or the venting or "tuning" or "something"
so that you could get the extended low end response of an acoustic
suspension system but at a much high efficency and not give up the
transient response. Erath...invented and developed a design that did
cheat and win handsomely. As a point of reference you can see what
happens if you mount old classic high efficiecy large magnet drivers in
a WAY-TOO-SMALL-BOX by referring to the Klipsch model H (Heresy). I
know there are some of you out there who love this little box system,
but frankly the extreme low end down at 32.7 HZ (LOW C on pipe organ)
just isn't there at all. ....The LWE approach to get around this is
based on "negative feedback control". OK....in a typical ampifier
(still mostly vacuum tube back in those days, of course) you have
anywhere from 25-80% of the output of an ampifier used in reverse phase
as negative "feed back" used to minimize distortion and in some gross
cases to make an amplifier stable (not oscillate or 'howl'). The out-of
-phase signal is fed from the output of an amplifier's final output
stage back to the input of that same stage. Actually the same concept
can be used in any intermediate stage of amplification as well. The
concept works well in lowering distortion and, in some cases,
increasing stability. ......Erath's principle was to reduce the
negative feedback selectively only at the extreme low frequencies where
a big 15" woofer in a really tiny box was just not capable of
responding fully. This would leave the feedback and distortion
character untouched at frequencies up above 50-70 HZ...."

If your curious, you can view the patent over at the US Patent and
Trademark Office, it is Publication Number: 03449518:

here is a tinyurl version as the original url is miles long:
http://tinyurl.com/gkc9u

Here is a julian hersch Stereo Review of the LWE-1A:
http://www.classicspeakerpages.net/acoustr/lwe.zip

Here is another review of the LWE-1 in Audio magazine:
http://www.dontgethungup.com/wbcam/E...ew_1968txt.zip

Some details about the LWE negative feedback networks that I should
have mentioned in the previous post are that they electronically model
"ideal" speaker performance, and while connected in series with the
physical speaker, they compare the voltage across the speaker's voice
coil with "ideal" performance voltage. The difference between the two
is sent back to the amplifier as an inverse corrective voltage and fed
into the input of the amplifiers designated negative feedback output
stage creating an immediate, dynamic voltage correction for the
physical speaker at a specified frequency.

Erath told me that the coils he used in the feedback network were
uncommon at the time: a toroid-wound, powdered-permalloy core that was
previously used only by Bell Telephone in their loading coils and in
certain military electronics. The network also used a photo resistor
and light bulb to attenuate the amount of bass boost at higher volume
levels based on Fletcher Munson curves.

Their are some circuit diagrams on the US Patent website link in the
previous post, they are on the 2nd page. The navigation buttons to
select pages is on the left side of the screen (hard to find due to
bad website design) also, your browser will need a TIFF image viewer -
(IE will ask for ActiveX permission)

I have a detailed LWE history posted at the yahoo group that I put
together with notes taken after a few phone calls with Erath as well
as email conversations with people who worked in his shop:

http://tech.groups.yahoo.com/group/l...oup/message/61

It is probably much more than you will ever want to know, but it was
written for LWE fans. (Also, it is a work in progress)

Apparently Erath acted as a mentor and source of inspiration for quite
a few students of electronics and electronic engineering as everyone I
talked to that worked with him had fond memories and spoke/wrote in
reverential tones. The guy has pretty much had a full plate his entire
life, he was recruited by the US govt during WWII to oversee projects
developing depth charge fuses where he rubs elbows with Shockley and
Bardeen, all while pouring over Harold Stephen Black's patent,
utilizing Black's feedback concepts in military devices and later, in
geophysical instruments and of course the LWE loudspeakers.

Even more amazing is that he is still producing hand-built LWE
loudspeaker systems using updated versions of his feedback networks
and modern components:
http://www.lweloudspeakers.com/

I'm curious, does anyone know of an earlier motional feedback/servo
loudspeaker, or is the LWE the first to make it into production?

The Audio magazine review (1968) in the previous post makes it sound
like the LWE is the first to make it to production while there had
been previous prototypes.

thanks
-Sam