Who has done work on needle bar/diaphragm resonance and mass

[PeteLeoni]

If you haven't already, may I suggest reading Wilson & Webb: https://www.gramophonemuseum.com/images ... s-1929.pdf

I haven't read!
Thank you!

[chunnybh]

Very interesting thread.
A while back I was restoring a Meltrope III soundbox. The aluminum diaphragm on these is sandwiched between two domed washers and then punch riveted on, so it's impossible to repair if the center pin from the stylus bar breaks.
I tried solder and glues without any success. I then used some Chinese red sealing wax used for stamps on legal documents and envelopes. It's dried very hard. I used a large ball of it and the soundbox sounded great for a few weeks but then the brittle wax delaminated and bits fell off.
Reading this post makes me think that perhaps the Meltrope III's reputation as a great sounding box is because of the extra mass of the washers right in the center of the diaphragm.

[Inigo]

Could be true...
I once restored an equal soundbox, detaching the needle bar with a Dremel tool, then reattached the diaphragm and cup washers to it using drops of super glue. Works well, but kind of irreversible repair I did. Never tried to disassemble again, but probably will destroy the diaphragm.
Nevertheless you could try using shellac to glue the cups to the diaphragm, this is reversible... Then use the drop of super glue or also shellac to reattach the needlebar.

[PeteLeoni]

Well, this has my interest and attention now, you can bet we will make some progress. Already have incredible audible improvement, want to know why it's working so well. Hard to believe they overlook the math here.

[Lah Ca]

Reading this post makes me think that perhaps the Meltrope III's reputation as a great sounding box is because of the extra mass of the washers right in the center of the diaphragm.

A reproducer has a lot of similarities to a stringed musical instrument: the diaphragm, the needle bar, and the needle are analogous respectively to an instrument's top/body (and to a lesser extent neck), bridge/saddle/nut, and string.

In the 70s, before the advent of mass market effects pedals that could accomplish the same thing, there was a craze with solid body electric guitars for heavy brass nuts, massive blocks of brass set into the body under a massive custom bridge apparatus, and blocks of brass for retaining the ball ends of strings set into the body. The effect of these medications was a marked increase in the acoustic volume of the instrument and a corresponding increase in sustain/note duration, along with an increased clarity (or simplification) of tone.

To a point, increasing resonant (non dampening) mass at the point of contact between the needle bar and diaphragm should accomplish something similar, at least theoretically so.

[OrthoFan]

Hi Pete:

Quick question -- prior to experimenting, did you overhaul the #2 sound box, installing fresh soft diaphragm gaskets (isolaters) as well as a new back flange? That will minimize resonance as well as expand the volume and range.

OrthoFan

[PeteLeoni]

Oh heck yeah, new gaskets careful spacing, correct needle bar position, correct spring tension and also many hours of experimenting with variations of gaskets pressure etc. The truth is there is one hell of a peak and severe ringing in the curve right exactly where our ears are most sensitive. Obviously the bandwidth is restricted, but we are tolerant to that. Our ears "hate" it when there are peaks and ringing in upper mid ranges especially. You can make it a bit better with soft needles but the ringing is still there. As soon as I find a test record I will put one of my calibrated mics on and show you what I know I'm hearing . The putty works.

[PeteLeoni]

I am pretty sure you could also put it on the inside on the screw and make it cone shaped.

[Inigo]

I've been just today trying the blue tack ball trick on the needlebar crook of an exhibition soundbox, and the nasty ringing sound disappears, although it takes with him a large chunk of the treble... will continue tests, anyway. I've tried different sizes of the ball, and different locations too. This idea of the cone is interesting, as it could work as a phase plug at the same time... Watch the similar thread about exhibition soundbox modified as EMG in viewtopic.php?f=11&t=56458&p=335408#p335408

[PeteLeoni]

Yep, phase plug on the inside. This is going to lead to a lot more enjoyment from these old machines. Regarding HF loss, to be honest there really is no high end to speak of in relation to the mid output and I fact, when you tame that high mid peak, the hf is in better curve.