Stylus Rake Angle

Doug and Csontos seem to have the way of it.

The cutting stylus on an LP cutterhead only does about 10 hours before it needs to be replaced. You'd think they were all exactly the same but after replacement you wind up resetting a lot of parameters on the cutterhead. Tiny little adjustments can have a huge effect on the groove you cut, so its anything but cut and dried. I find the talk about SRA a bit amusing as a result.

As the stylus wears, sometimes you have to make little adjustments, like the stylus temperature. Funny thing- it cuts a slightly different angle depending on the temperature. Some LPs don't have very much in the way of dynamics so you can change groove depth a bit to allow for more time on the LP, conversely if something has a lot of dynamics or out of phase bass, you might cut a little deeper. So groove depth affects stylus angle too.

Bottom line: don't get too upset about it. Its more important for the mastering engineer to cut a good groove than it is to get it exactly at 92 degrees. You are never going to be too far off either- the stylus won't cut right if its a few degrees off... trust me on this one- there are far more important things to worry about :)

From what I understand, the cutter head angle may be a particular value, but the resulting profile on the record is different due to the springiness of the lacquer material which varies with temperature, such that the resulting optimum SRA is not that at which the record was cut. Variations in modulation mean that the cutter has to dig more or less deep, thus varying temperature and hence the profile, so that an optimum SRA at one point on the side may not be so at another. Would that be correct?

No- but that is a common myth to which I subscribed before I started working with the cutting lathe. There really isn't 'springiness' in the lacquer surface. When modulation varies, the groove depth stays the same and the two can be set independently. The heated stylus cuts through the lacquer like butter if its set correctly - if not you get surface noise.

Csontos, I have been flying kites for some time :) seriously! But you are correct- if you can set SRA/VTA on the fly. LPs thicknesses vary as do the cutter angles. There is no other way to do it IMO. I don't think the thing you mentioned about standard cutting angles is correct though. What I have seen is that the cutter stylus needs a slight amount of rake (varies with each one as I said) and its really in the 1-4 degree range off of exactly perpendicular to the LP surface, so about 91-94 degrees. What we really don't know because no-one has really studied it is whether or not the best SRA position in playback is actually the same as the cutterhead was set to... I suspect that that varies with the individual cartridge.

Surely the groove depth changes with signal? A right channel signal for example must cut into and out of the lacquer at 45 degrees. I can see that a heated cutting stylus will help the process, but then does that affect the final profile as the material cools?

I used to hear about this thing called rebound associated with lacquers but yet to experience it. Yes, the groove wall is cut at a 45 degree angle, so if you were to cut an out-of-phase bass signal, the groove would indeed modulate up and down. Of course, the needle would not be able to track it, and such a signal fortunately does not exist in nature. If we encounter studio productions where out-of-phase information is a problem, we have a simple passive processor that takes care of it. So groove depth can be considered a constant.

Another way to look at this is if the cartridge body rises higher, that is a change in groove depth, if the needle simply traces a signal, the groove depth has not changed.

Sarcher30, it might, but if so it would be very very slight! If you look at any lathe you will see at once that they are built to be quite sturdy and machined to exact tolerances. Thus the cutterhead mount is exactly perpendicular to the platter surface; one of the better examples of how excellent machining really looks, such that the azimuth error cannot be measured. So in essence, I am saying 'no, azimuth does not vary from lathe to lathe'.

I wasn't talking about out of phase signals, but left or right signals. I can see that a mono signal varying only in amplitude, side to side, will maintain a constant groove depth which doesn't vary, but stereo?

I imagine my response was not entirely clear so I will try to restate in a less ambiguous way. There is no variation in groove depth due to signal. There is only signal modulation that varies, whether stereo or mono. It sounds to me like you are confusing the two- and I can see why.

Look at it this way- groove depth is something you can set on the cutterhead. Take it as a definition unique to the technology that 'groove depth' means how deep the groove is with no signal. The signal is a function of the electronics. Think of modulation in an radio signal and its the same idea- if you want to reproduce the signal, the groove depth is a constant, just as the frequency of the radio station is a constant. If you want to look at the groove as being variable depth according to the signal, you won't be able to reproduce it.

If the cutterhead were to rise and fall with respect to the lacquer surface, then we would see a varying groove depth. In theory, during playback the cartridge body is held in constant locus by the mass of the arm so that the cartridge can only respond by the motion of the needle in the groove, not anything else.

But when the cutting stylus records any signal other than a pure mono one it must move more and less deep into the lacquer, otherwise how can the signal be cut? That was what I was getting at and confused by your answer. I didn't mean the whole mechanism moved, solely the cutter itself

John, you might want to re-read my prior posts as my answer here will be very similar. But to be clear, part of the groove does show variable depth with the signal.But regardless of what that signal is doing, if you look at the center of the groove it will be the depth described by the adjustments on the cutterhead, and not related to the signal, and that is what is called 'groove depth' in the parlance of the field. The only exception is if out-of-phase information is being recorded, which I covered previously- that will cause the center of the groove to seem to change in depth.

Which type of lathe do you have?

Its an old Scully (well, *all* Scully lathes are old...) equipped with a Westerex 3D cutterhead. The cutterhead does not need a lot of power and has a moderate impedance, we are easily able to drive it with our M-60 amplifiers.

Kiddman, it helps to take a look at what is happening in the groove with a microscope. Of course every lathe has one, you can't do the work without it! We have a multi-color LED light source that we installed, so we get different colors depending on what we are seeing- the lands, the groove wall, or the bottom of the groove. As I said before, the defining quality of groove depth is the center of the groove.

If you have high frequency modulation, that shows up under the 'scope like a groove with modulation on the walls. If you have bass information, the *entire* groove moves from side to side with the bass modulation. If the bass is out-of-phase, the groove will stay in one spot and seem to vary in width and depth. This sort of condition will knock the stylus right out of the groove and so has to be avoided. In natural 2-mic recordings it does not show up- only if the recording is multi-tracked to start with.

I Googled for images of this stuff. Without a multi-color light source it can be a little hard to see, but there is a photo that appears at this link that is not bad for what I am talking about http://en.wikipedia.org/wiki/Gramophone_record

Scroll down aways, the photo appears on the right hand side about half way down, just above the heading 'vinyl quality'. If you look closely at the grooves, you will see that they squiggle a bit. Those larger squiggles are bass notes. This is why I say the groove depth does not vary all that much, as the *entire* groove has to move to make the in-phase bass note.

I'm still wondering why, if slant angle varies not only from record to record but by the signal itself, by a cutter who's VTA is for all intents and purposes static, does cartridge VTA/SRA need to match VMA?

The SRA of the cutterhead does not vary with signal. It is a constant.