Question on FR 66s

Wrm57

If you use an integrated headshell cartridge like the SPU then you can't adjust the offset angle.

The SME arm you have is designed for LofgrenA (Baerwald). It is also designed such that that alignment can be set by adjusting the base for whatever cartridge is mounted.

In other words the P2S distance is variable, the effective length is variable and the offset angle is fixed. If you used a headshell and separate cartridge with sufficient clearance in the holes or slots, then the cartridge can be twisted to allow other alignments to be achieved (although Lofgren B doesn't require a twist, merely an adjustment of the sliding base.)

You can use any standard two point protractor to set up, or a the Dennison which works for any unknown mounting distance. (The Uni protractor uses the same principle, and doesn't require you to know or set the P2S, but it has to be Lofgren A/Baerwald.)

Unless you know the exact actual effective length, an arc protractor is not going to help, and if you did know it, it would only apply with LofgrenA/Baerwald.

If you have the wrong E.L, then the offset angle will be wrong for whatever P2S distance that enables the arc to be tracked.

John

Chris,
If you change the mounting distance to 297mm it will be fine if this gives a 12mm overhang. If you are using an SPU the headshell offset is fixed, so the only variable is P2S.

If the arm was designed for a 50mm as opposed to 52mm pickup, then the real effective offset will be slightly less as the effective length increases, but the basic arm geometry remains the same, it just needs more P2S distance. You can use the original protractor to check, as the nulls will be the same.

John

Bill,

With the UNI-Pro, if I set the tonearm to SME's prescribed P2S and check alignment with the Baerwald template, which Daniel says is the one to use with the SME, it's way off, as one might expect. So, I can slide the SME along its base, changing P2S until the stylus drops in the template's hole. But then offset angle is wrong because the orientation of the template itself is no longer proper, seeing as how has been situated in relation to the prescribed P2S.

The Uni Protractor functions in the same way as the Dennesen - the principle does not require the actual dimension in millimeters of P2S distance to be known. It is irrelevant to the principle, and for some arms with sliding or movable bases like the SME, saying that it is necessary to measure P2S is just plain wrong. It is not my opinion, just a fact.

The micrometer function adjusts for the different alignment options and, if it is set for LofgrenA/Baerwald IEC, then, just like the Dennesen, when the movable arm is correctly positioned over the tonearm pivot point, the null is correctly orientated, and the cartridge should line up with the grid lines.

SME give an effective length dimension, but it is nominal, based on a notional standard cartridge mounting hole to stylus dimension. And the related P2S refers to this. The SME sliding base accommodates variations in cartridge dimensions (which alter the effective length and cartridge offset), thus allowing your SPU to be set up correctly even though is has no adjustment.

However, I assume that the setting on the Uni-Pro's micrometer has been determined according to the manufacturer-specified P2S.

The micrometer setting relates only to the particular alignment (LofgrenA, B Stevenson) which is initially derived from the inner and outer recorded radii (IEC, DIN etc), which gives rise to fixed numbers for the linear offset and the nulls, which in the case of your arm (and any other using that alignment of whatever effective length), LofgrenA/Baerwald IEC.

It is easy to get lost in all this stuff. And I know there are many people confused by it, which is why I try to comment when I see misunderstandings.

Regarding the SPUs, the fact of differing compliances might cause slight variations as VTF and VTA change when changing from one to another. How much of a variation is there in effective length?

John

Halcro,
Yes you are correct: the only way to adjust cartridges like the SPU on arms like the FR64 and 66 is at the base, as the headshell offset is fixed. Which is why on a specific arm you have to use the existing specific arm alignment which uses that headshell offset. In other words, the given nulls. Only by doing this will the cartridge line up correctly.

John appears to be offering no solution at all......and if he thinks the UNI-protractor uses Baerwald alignment.....I believe he is mistaken?

The solution I gave for SPUs is straightforward: adjust the base to fit the original nulls. Don't use an arc.

If you have two SPUs or FR7s or whatever, and they don't have the same dimensions, then the base has to move or you accept a compromise.

For removable headshells with slots, you can use whatever alignment you like, but as I have told you in a past post, if you want to swap with an SPU you have to set it up first, and only then use a slotted headshell to mount and adjust another cartridge, without moving the P2S.

and if he thinks the UNI-protractor uses Baerwald alignment.....I believe he is mistaken?
Dertonarm has developed his own alignment called UNI-Din which places the null points differently to the others.

You are right to question your belief ;-)
As it is the Uni (for universal) protractor, DT has incorporated an adjustment to allow different alignments, one of which is LofgrenA/Baerwald IEC, and another is his version of LofgrenA which is derived from inner and outer recorded radii parameters which differ from IEC and DIN, thus giving different nulls. In the case of the SPU, unless the arm has an offset which corresponds to that alignment, then the alignment can never be achieved, and the cartridge will only ever align at one null. That is the way it is. All else is compromise. Whether the compromise is to accept a misalignment of a couple of degrees is up to you.

In the case of the FR66 the original P2S and overhang seem fine to me. A change to 296 makes little difference. Nulls around 63 and 115 would work ok. The thing is to get the offset as accurate as possible, as this is the parameter most prone to error.

John

Hi Lewm
The Dennesen principle allows any arm to align to LofgrenA/Baerwald.

If the arm is designed to that alignment to begin with and is mounted at the correct P2S the cartridge will align with the grid when square in the headshell.

If the arm is mounted with P2S incorrect or is designed with a different alignment (say, Stevenson) it can still be aligned if it has the facility ( ie has slots) to move the cartridge to the correct offset and effective length to align with the grid.

In the case of Dertonarm's variant, various alignments are possible. The physical structure of the arm's headshell doesn't matter if it allows twisting by a few degrees and some fore and aft movement.

The way it works is that the protractor alters the angle of the grid relative to the arm pivot automatically as the sliding arm is moved back and forth, because their relationship is fixed for whatever effective length. So if the arm pivot is in a fixed position, the only adjustment available is to twist and slide the cart to obtain the correct offset and effective length for the fixed P2S whatever it is.

John

Halcro,
As an aside, if Dertonarm's recommending setting the P2S at 231.5mm for the FR64 with FR7, that presumably means that when using the FR7 pickup, the effective length is changed as is the cartridge offset, and he reckons that is the best way to adjust for that, using the reduced offset and increased overhang to allow alignment at different nulls of around 63 and 115. Simply changing the P2S if the effective length remains the same is not a good idea with an integrated cartridge.

A friend of mine has pointed out that these discussions as to P2S distance for integrated headshell pickups would be moot had the FR arms been designed with sliding bases like the SME. Perhaps someone will put one on the market, like the aftermarket adjustable VTA base which I believe was available, and put FR owners out of their misery...?
John

Chris,
Your new P2S would be fine. To get the new effective length it is more useful to also take 307 * sine 16.8, as this gives the linear offset, which is a constant for any particular alignment, and doesn't change when using integrated headshell cartridges. Not that it makes much difference to the calculation, except to illustrate the change in cartridge offset as the effective length increases. You can get more of a sense of this if you imagine an elongated SPU with stylus at say 200mm. The headshell offset of the arm doesn't change -it is still 16.8 - but the angle between the plane of the cantilever and the arm pivot would be much much reduced as the effective length increases.

In your case, 307* 0.289 = 88.73mm. Together with your new extended third side at 295.9 this gives (by Pythagoras) the new effective length of 308.9mm and an effective offset of 16.69 degrees. A 12mm overhang would be a good choice. To maintain the original alignment it should be trifle less, but it is neither here nor there. As you say this gives a P2S of 296.9mm /297mm.

Halcro,

If one were to do the calculations as you suggest......with and SPU or FR-7 type of fixed headshell......surely the Off-Set angle cannot be changed?
Nor can the Overhang or Effective length.
The only parameter able to be changed is the Pivot to Spindle distance?

The offset changes as the effective length increases, and overhang is simply what is left after P2S is set, or vice versa.

Ideally, you are correct in saying that one should measure the actual effective length if you wish to set an exact P2S for a particular alignment. Alternatively with a sliding base this is not necessary, as the P2S will be correct when the cartridge is aligned with the arm's two original nulls.

One would imagine that the FR-7 series (because they are made by the same manufacturer as the tonearm) would be spot-on the specified 12mm?

One would hope so, though I don't know, in which case altering the P2S would not be a good move.

The problem that Jazzdax poses
The length of the SPU is 52 mm instead of the 50 mm recommended by Ikeda.
does not necessarily indicate a 2mm difference in stylus Overhang?
One would need to accurately measure this as I describe..... before plugging those figures into the Vinyl Engine Calculator to ascertain the new P to S distance and the consequent new Null Points with the accompanying distortion figures.

The thing to realise is that it is effective length that increased by 2mm if that is the case. The overhang is the effective length minus the P2S, whatever it is. With a slotted headshell, if effective length varies, then because P2S is fixed, overhang varies. With a sliding base, if P2S varies, then because effective length is fixed, overhang varies.

Unoear,
Thanks for that.
John