Discrete Opamp Open Design

[bcarso]

Originally Posted by Kindhornman

bcarso,
I can't imagine that even if the temperature of the interface does momentarily heat up as was said earlier that the mass of a vinyl record would hold much heat over the time it would take to play an album. The heated area is tiny in comparison to the mass and the dissipation alone would make me think this not likely. Perhaps if you were playing the same few tracks repetitively that would happen but playing an album through I can't see the mechanism for that. If you have ever seen and injection molded part made you would have to question that premise. From injection to ejection in the mold would be in seconds and that would be from a large mass in the injection barrel of an injection machine. Something doesn't add up to what is going on here.

Remember we're talking about the temperature at the stylus tip, not that of a wake of molten vinyl which rapidly cools due to the heat capacity. The stylus will heat up, the diamond will conduct superbly well to the cantilever, and so on. After a while some point not too-far-removed will experience a temperature rise, and we can back out what the temperature had to be at the tip.

If it really is anything like the stated temps, we'll see a measurable rise not too far away, and with a thermal time constant not all that terribly long. There are some very tiny thermistors that could be affixed somewhere nearby that wouldn't drastically interfere with transduction of vibrations.

As far as the LP itself, I seem to recall someone saw deformations that slowly relaxed back into shape given some time. This too could be re-investigated today with some new tools. For example, use a laser table to play and digitize the record, then play with a stylus, then back to the laser table. Compare the digital files before and after.

[Chris Hornbeck]

I think (always a bad sign) that the temperature numbers were calculated from contact pressures required to accelerate the stylus at the hundreds or thousands of G's (not as surprising a number as it may seem!) from a teeninsie contact area. The actual "groove" geometry traced by the stylus is *not* on the surface, but includes a certain amount of the inner vinyl. One reason why the laser scanners are so different - the other is the insane amount of noise reduction needed without a stylus' "averaging". I don't think it's a question of friction, or anything like that. Simply enormous pressure in a tiny space.

When Lord Kelvin (now there's a name to conjure with!) (and his daughter invented computin') calculated the age of the Earth it was by temperature rise caused by gravitational pressure. He was way off because he didn't know about radioactivity yet, but his numbers were right for his day. I'll try to find some numbers to back up the 400 F claim, so I won't be as wrong as The Man ultimately was.

Thanks,
Chris

[Kindhornman]

Lord Hornbeck,
Thanks for the information. I would imagine that some of the heat would go up the cantilever until some equilibrium was reached and that most of the heat would be lost to air as the surface area of the stylus tip must be large compared to the diameter of the cantilever. But I am just guessing on that. Very interesting phenomena that I had never considered before. Still can't see the groove retaining that much heat over time though, the mass and surface area would be so much greater than the small contact area.

[Chris Hornbeck]

Originally Posted by Kindhornman

I would imagine that some of the heat would go up the cantilever until some equilibrium was reached and that most of the heat would be lost to air as the surface area of the stylus tip must be large compared to the diameter of the cantilever. But I am just guessing on that. Very interesting phenomena that I had never considered before. Still can't see the groove retaining that much heat over time though, the mass and surface area would be so much greater than the small contact area.

Absolutely. And the big temperature numbers would be for the vinyl itself, and not necessarily at the surface. This is over my head, but having opened my big mouth I need to find out what truth I can, or at least figure out where the numbers came from.

Has anybody else noticed the long-long term memory that vinyl records seem to have? Could be just *my* poor memory (and likely!) but it seems like records that seemed worn and beat a few decades ago "recover" after a long nap. Or I could be getting older and more forgiving. Thoughts?

Thanks,
Chris

[Kindhornman]

Chris,
The only thing that I could attribute an album recovering over time is the phenomena of memory. The other standard mechanism in plastics is called creep. In one the load over time causes the material to move, just as glass will flow over time because it is not truly a solid. But that type of deformation is usually not reversible. But memory is when the molecules have been created in a specific way, a matrix, and though they have been pushed around they would have a tendency to return to the original shape over time as the stresses are relieved. Since an album is made in a mold and the grooves are set the shape would have that original pattern in the matrix and this could explain the return back to the original shape. I know that there would be more to it than that as dirt and a worn stylus would cut some of the vinyl over time and once material is removed it would be gone for good. And no I am not making this stuff up off the top of my head, been molding plastics for far longer than I care to admit......

[jan.didden]

While Chris his chasing that temp thing (thanks Chris, much appreciated) can I briefly return to the overload electrical damping? Something is bothering me.

It seems most here agree that electrical coil damping does not lead to mechanical cantilever damping; that is, the cantilever resonances are still there but are no longer in the electrical output of the coil, and this leads to a smooth freq response. But this damping system can not differentiate between unwanted mechanical resonances and wanted vibrations from signals on the record. Are we not loosing signal content with this damping method?

jan

[RNMarsh]

Originally Posted by bcarso

Similar to the Marshall Leach design? Actually if GNFB was included it would be a departure from ML. Roughly what year in TAA?

The ML design, which at times was not credited properly, had the feature of a floating power supply, a device to eliminate any net d.c. flowing in the cartridge. It's lived on here and there, sometimes operated at too-low currents for good noise performance. The JFET common-gate versions are in another thread or two in here.

It was TAA 1/82.

[jan.didden]

Dick do you want me to post a scan of the circuit? For study purposes only?

jan

[Kindhornman]

Jan,
Just as you have stated so eloquently the problem with trying to electrically damp a mechanical resonance with the phono cartridge the same principals apply to electrical filters that are placed in electronic crossovers to dampen mechanical resonance with loudspeakers. I just sit there and keep my mouth shut every time I see someone say they are going to use a notch filter to remove the mechanical resonance of a loudspeaker. Isn't going to ever happen, the resonance has nothing to do with the electrical signal, the excitation can be caused by many other physical phenomena and unless you physically change the loudspeaker the original reason for the excitation still exists. Learned this long ago, you can use electrical equivalents to describe the functions but that does not mean they are exactly equivalent. Just a way to understand and study the functions, not something that can overrule physics.

[SY]

If I have two complementary filter functions (say, a peak and a notch) with the same amplitude, phase response, delay, and Q, why would they need the same mechanistic cause in order to cancel? After all, on the other end of the chain, it's very common to use the combination of mechanical and electrical transfer functions to achieve a particular acoustic transfer function (e.g., a Linkwitz transform in an electrical filter to extend the mechanically-limited bass).