Question, Neumann lathe dashpot

[blacknwhite]

Question - reading through a Neumann VMS-70 operational manual, the portion covering care & operation of the lathe and cutterhead - just curious about principles of quality lathe construction, don't actually have a Neumann... saw this:



Standard definition of dashpot: a mechanical device, a damper which resists motion via viscous friction. But that could be in the cutterhead itself as with Grampians, or somewhere in the cutterhead mounting of the lathe, it doesn't say...

Anyone know where's this dashpot they're talking about physically located on the lathe? I suspect it's in the cutterhead suspension, not sure tho...

[blacknwhite]

Oops, didn't read far enough... found the diagrams section... "4"

[blacknwhite]

No, wrong dashpot... "1" (it IS on the head suspension box)...

[cd4cutter]

Correct. There are two dashpots - the turntable drive is fluid coupled to filter out the power pulses coming from the synchronous motor to keep flutter to a minimum. The dashpot right at the front of the cutter suspension is designed to reduce the bounce of the cutter caused by slight irregularities in the lacquer surface. This damper is adjustable because it has a piston pushing fluid through an adjustable orifice. The cutter heads are massive and their weight is conterbalanced to the required much lower downward cutting force by a combination of a mechanical spring (you can see it and its knurled adjuster in the suspension picture) and the depth control magnetic coil which you see at the back of the suspension. The mass of the cutter ordinarily would cause it to resonate or bounce on the lacquer at a rate of 1 or 2 Hz without the damper. When properly set up, the cutter suspension allows the depth of cut to be finely adjusted by means of the magnetic depth coil which is supplied with an adjustable DC current. The depth of cut must be continuously adjusted while cutting stereo grooves (together with adjusting the cutting pitch) in order to maximize the recording time on the lacquer. The use of electric controls allows this to be automatically accomplished with the "advance" signal which is the actual program content delivered to the automatic pitch and depth controls about one half revolution of the platter before the program is actually delivered to the cutter amp.

[blacknwhite]

Ah, thanks. That all makes sense... except for one thing:

I thought depth of cut was controlled entirely by an advance ball, and the suspension box's only purpose was to provide adjustable spring-loaded control over cutterhead pressure so you can keep the advance ball firmly in contact with the lacquer on test cuts. It's my understanding that there were outfits made where the advance ball could be computer-controlled to increase depth of cut for loud stereo passages with more vertical vibration.

But it seems in the VMS 70 manual, the advance ball is entirely absent. And sure enough, I found in the manual, the exact same same DC magnetic depth coil you described, used in its place.

Why was the advance ball abandoned after the Westrex stereo heads that always seem to be pictured using one? Perhaps with the progression of stereo lathe technology, the advance balls caused too much rumble from riding on the surface of the lacquer, and become "passe" by the time the VMS 70 was popular?

As far as lathe construction goes, it seems an advance ball would be a lot simpler/cheaper to implement, just wondering what the "cons" are that might've made Neumann go extra lengths to avoid it...

[dietrich10]

What does the moiree pattern mean?
Is this the visual effect records sometimes have where a record looks feathered?

[cd4cutter]

Advance balls were anachronisms already by the 1970s, if not earlier. The Neumann suspension box worked so well that it was widely adapted and used on many other lathes, including Scullys. So far as I can remember, none of our lacquer channels at RCA Records still used cutter suspensions with advance balls. The reason probably relates to the ball scuffing the surface of the lacquer and causing an undesirable scuffed or scratched appearance on the finished record surface. Also, it undoubtedly introduced a little vertical rumble component into the cut by virtue of it's mechanical coupling to the cutterhead. This would have been of little consequence with mono lateral cutting, but vertical noise sources were discoverd to be a very real problem with the advent of stereo cutting which has half its signal component in the vertical direction. So I suspect that advance balls were phased out of use after the introduction of stereo cutting in 1958.

[blacknwhite]

Great perspective, thanks. It seems if you dig enough, you can still find a few details on this stuff, but not always as easy to get the big picture.

What does the moiree pattern mean?
Is this the visual effect records sometimes have where a record looks feathered?

That's it.

[Perisphere]

The only Neumann lathe I've ever seen was an AM 32 that had been upgraded to VMS 70 specs (so I was told) that had a Westrex 3D cutter, and it had the advance ball on it. It was in a small studio in Burlington, Iowa; the last time I saw it was in early 1990 when the system was sold as part of the estate of the studio owner's following his death. I don't think it had been used in approximately 10 years at that point. (I wonder where the lathe and the electronics and controls for it all went?)

[Aussie0zborn]

I think the moiree pattern Ditrich asks about is the high freqency content of the groove - usually a high hat.

The advance ball, if not set properly where the groove can cut into the scuff mark it leaves rather than cutting adjacent to it, gives the record a very dull look because it does indeed scuff the record. I had a Westrex 3D-II on a Neumann AM32B lathe and never used the advanced ball.

I also had a SX74 head on a VMS70 and so the idea of using an advance ball seemed very antiquated to me.

BTW... who bought the Neumann suspension box that was on Ebay a month or two ago - anybody here?