[Vintage-Audio] Stylus Feedback? Why?

[Sheldon Daitch]

Duane,

I think another possibility is feedback via the turntable and record
because the lower frequency energy from the speaker system causes
either the turntable platter itself and/or the record to vibrate and then
that it picked up by the cartridge stylus mechanically, as if it were a
legit groove vibration.

One possibility with the single channel pickup could be the turntable
is located is a point where the low frequency energy from only one
speaker is high enough to send the system into a feedback mode.

It was not unusual in the the broadcast industry to have this feedback
occur, under certain conditions where there was no audio on the
system ( I know, BAD) the turntable pot up a bit too high, the
control room monitor up and then the audio AGC equipment would
raise the gain.

I seem to recall reading about TT cabinet stands specially designed to
allow operators to place bags of weighting material, bagged sand
generally, in those bases, to get the resonance frequency of the
cabinet down low enough so the resonance frequency was below
the frequency response of the station's audio system.  This was
usually no issue with AM stations, but FM broadcast exciters tend
to have excellent low frequency response. 

Oh, in typing this, I just remembered something about low end
response.  Sometimes, we'd have records with a little warping
of the surface, and that would show up at a very low frequency
"sine" wave as the record played.  The AM transmitter never
saw it, but the Harris TE-3 exciter would not only pass this
audio, it would show up as very high level modulation on the
FM mod monitor and under some cases, the AFC on the
exciter would unlock momentarily.

I also had one radio station where the tape cart machines for spot
announcements stopped with a fairly heavy mechanical jolt and
they were mounted on the same cabinet surface where the left turntable
was located.  With heavy bass boost on a receiver and good low end
response on the speakers, I could "hear," on the air, the cart machines
stopping, if the left TT pot was up and there wasn't enough other
program audio to cover up the stop jolt.


73
Sheldon

Duane Fischer, W8DBF wrote:
> Hi All,
>
> Several days ago while I was experimenting with different bass, midrange and 
> treble settings for my B&W 630 three way monitor speakers, that I sometimes 
> blend in with my James B. Lansing Century L-100 ceiling mounted mains, I 
> thought that I heard a rumble coming from a source other then my stomach!
>
> Hark! Was that an electronic Harpsacord or just another harmonic caused by a 
> Gamma Ray rebounding off that atom smashing thing in the Ann Arbor, MI home 
> of the University of Michigan Physics lab where someone got their doctorite 
> by discovering that "Wilson-Phillips" could not sing! No! It was coming from 
> the eight inch active poly ... some kind of thing woof woof wanna be cone 
> three feet to my right.
>
> I got up off the carpeted floor where I had been squatted down doing the IHF 
> +/-2 Db adjustment procedure and switched over to the ceiling mounted JBL 
> Century L-100's. No rumble, Rumba or other residual reverberations. So just 
> what was giving the B&W 630 high frequency bi-wired monitors that rhythmic 
> distress?
>
> The Shure diamond stylus was picking up a rumble from the Sony/Marantz 5520 
> turntable, amplifying it and causing audio distortion. Frankly, it sounded 
> to me as if I had somehow enhanced a turntable drive mechanism rumble 
> feedback loop, but only in the one set of speakers. It seemed to me that I 
> should have had the rhumba rumble in both sets of speakers.
>
> As Maxwell Smart used to say; "Missed it by that much!" Which explains why 
> Secret Agent #99 never became pregnant! (LOL!) It scares me to ponder what 
> might have happened if old Max might have hit the target!
>
> I did a little reading in my previously created audio research note file 
> circa 1999 and found out that the James B. Lansing Century L-100 speakers 
> were low efficiency and that the B&W 630 speakers were high efficiency. A 
> concept that I am still not sure I fully comprehend yet today! One of those 
> "low" is really high and "high" is actually low cosmic time space 
> dimensional shifting event horizon things where one is correct if the 
> velocity of light is positively accelerated by the density of a Black Hole 
> that is not a hole, but a round sphere that is invisible because of the 
> Black On Black optical principal or negatively accelerated if the universe 
> is truly infinite and time never ends! No wonder our children can not tell 
> what time it is on a watch or wrist watch having moving hands!
>
> So more continuous RMS (What the Hell is Root Mean Square all about anyhow?) 
> is going to the B&W 630 speakers and less to the JBL Century L-100's? But if 
> the volume control knob is in the same position for both speakers at the 
> same moment in time, how can that be? Maybe it has something to do with the 
> Metric System! I've always been suspicious about people who called money a 
> "pound", when a pound equals sixteen ounces, unless it is a prescious metal 
> pound and then is has only twelve ounces! Then they call a "meter" 39.2 
> inches and the car windshield a "windscreen"! Someday someone is going to 
> make a lot of Yen, Yuro's, Pesos or Ben Franklin's by announcing that the 
> real secret hidden in the King James Bible, U.S. Declaration Of 
> Independence, ceiling where that famous inventor - painter of nude women - 
> gear reduction thing with a big spring that kept time, the day of the week, 
> the month of whatever year system was then in use, predicted famines and 
> plagues and wars and plagues of Turtles with an image of Susan B. Ugly 
> Anthony on their backs ...Discover that it is really the Metric System that 
> killed off the Dinosaurs, wiped out the Inca's before the Spanish did and 
> tried to cheat the Mayan Indians out of leap year was not the Da Vinci Code, 
> but that damn Metric System!
>
> Sorry about that. I just hate it when the deserving person, be it for good 
> or evil, fails to get his or her dues because some so called Greek Scholar, 
> American archieologist, Soothsayer, cosmic visionary who causes Bald Eagles 
> to grow hair on the sixth Thursday of any month with three full Moons etc. 
> All because they can not spell on a stone tablet, parchment scroll, chalk 
> board, mechanical typewriter or word processing computer program where they 
> click the mouse and it fixes all of their flaws!
>
> So tell me, please! Why do I get a sixty cycle hum from the B&W 630 speakers 
> when the Shure stylus tracking at 1.25 grams is playing what is hiding in 
> those deep dark vinyl grooves and only the sound of silence from the JBL 
> Century L-100's? Is it the Simon and Garfunkle effect?
>
>
>
>
> Duane Fischer, W8DBF - WPE8CXO
> E-Mail: dfischer at usol.com
> Hallicrafters web site: www.w9wze.net
> HHRP web site: hhrp.w9wze.net
>
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