ams@philabs.Philips.Com (M. Ali Shaik) (03/22/88)
Does any of you know anything about woofers/ sub-woofers incorporating position feedback? (I believe Velodyne & Infinity make them). What are the merits/demerits? Is there any merit at all in doing it? uunet!philabs!ams
torkil@psivax.UUCP (Torkil Hammer) (03/24/88)
In article <3388@briar.Philips.Com> ams@philabs.Philips.Com (M. Ali Shaik) writes:
#
#Does any of you know anything about woofers/ sub-woofers incorporating
#position feedback? (I believe Velodyne & Infinity make them).
#What are the merits/demerits? Is there any merit at all in doing it?
A friend of mine did that kind of job, back in 1970. He used a pickup coil
mounted on the same tube as the voice coil. This measures a velocity, which
was integrated to give the position, so the whole thing had a lower cutoff
frequency, which he managed to get well below 10 Hz.
The most remarkable effect was that the membrane became stiff to touch
when he switched in the feedback. Also, the characteristics measured in an
anechoic chamber became flat within +/- .1 dB, though he was stuck with
small resonances around 80 and 300 Hz, which could be traced to a mechanical
resonance between voice and pickup coil.
Once he moved the contraptions back to his 3 bedroom speaker enclosure :-)
there was no audible and barely a measurable difference between the
listening position frequency response with or without feedback.
Window opening and the general position of pets and kids in the room
had all a much bigger effect.
Yet he claimed two acoustical advantages: He could run the membrane all the
way out and in and not cause speaker damage, because the driving limit
would depend properly on the waweform; and nonlinear distortion was
theoretically less, but for most music not audible and barely measurable
with state of the (then) art equipment.
Back then "high" wattage woofers were expensive, fragile and acoustically
distorting. I think that feedback gave a marginal edge in low frequency
"electronic music" (as it was called back then), and I know the protection
saved him at least one set of speakers. On the other hand,
today's speakers cost less, have higher rating, seem more rugged and
are likely to be less distorting just as they are so I doubt that there is
anything to gain acoustically.
The stiff membrane will still be impressive, though.
And technology as substitute for religion has a potential for pleasure of
ownership.
Good luck and don't mess with those mercury filled cables :-|
torkilmax@trinity.uucp (Max Hauser) (03/24/88)
In article <2143@psivax.UUCP> torkil@psivax.UUCP (Torkil Hammer) writes: [Lots of good stuff about position feedback in a speaker, followed by] >technology as substitute for religion has a potential for pleasure of >ownership. ... How true. On the other hand, religion as a substitute for technology has a potential for appearing on rec.audio ... Max
al@qiclab.UUCP (Al Peterman) (03/26/88)
In article <3388@briar.Philips.Com> ams@philabs.Philips.Com (M. Ali Shaik) writes: > >Does any of you know anything about woofers/ sub-woofers incorporating >position feedback? (I believe Velodyne & Infinity make them). >What are the merits/demerits? Is there any merit at all in doing it? > >uunet!philabs!ams It is interesting to have this question come from Phillips, since one of the best early speakers with feedback was the Phillips RH series in 1974-1981. They incorporated a feedback circuit on the bass cone and were internally biamped. Good speakers. I own 3 sets of speakers with "feedback". I like lots of tight accurate bass, and feel that the extra circuitry is defineatly worth the cost. I have a set of LWE 1's and a set of LWE 6's (I used to have a set of LWE 3's) as well as a set of Infinity RS 2.5's. The Infinity's use the "Watkins dual drive" bass cone, which hooks up a second voice coil that is wired through a massive crossover to passively correct the drive to the first voice coil. The LWE's (Lewis W. Erath) were a very good speaker from the early 70's that actually sampled the signal across the bass cone, compared it with the input signal, and then sent the signal back to the amplifier. At the amp the feedback signal is tied into the amplifiers negative feedback line and corrects for speaker errors. The sound is great, but the problem of hooking up to different amps caused to company to die in 1976. They had engineered many "clip kits" to hook up to the most popular amps/receivers of that era, and there was a "design protocol" to allow engineering a hookup to other amps. CM labs took over their speakers, and continued them, even having "feedback" speakers outputs on their amps. I believe CM labs died in 1982 or so. Despite the difficulties, it is hard to believe how good tight controlled bass sounds. As an added plus, it is very difficult to overdrive feedback controlled speakers, since they tend to control their cone position from excessive motions. I know of an engineer, Robin Wernick, who has designed and uses a system similar to the LWE idea, except it mixes the correction signal into the line inputs to the power amp. Due to time constants, and line and amps speeds he limits the feedback bandwidth to the under 200 Hz range, so effectively it is a bass feedback. BTW - Have you noticed that Phillips is "entering" the US market under their own name? Have they forgotten they made and sold here some nice products in the 70's, including the speakers, receivers, tape decks and one of the nicest simple turntables ever made, the GA-212. "Entering"?? -- Alan L. Peterman Aero Air Hillsboro, OR (503)-640-3711 wk (503)-684-1984 hm Airborne N33291 Cessna Cardinal RG Net !tektronix!(psu-cs,reed)!qiclab!al