Here's some pics, and a bit of commentary revealing how the Avid Acutus clamping system works. The center spindle is threaded, allowing the clamp to be securely tightened.
The TT's designer seemingly never considered the possibility that anyone would deign to play a 45 on his creation, so a bit of improvising is required, whether the disc has large or small center holes. Each presents its own complications. Firstly the metal plate around the spindle is, for some reason, not level with the TT platter. With an LP it's not a big deal; perhaps it was designed this way on purpose? What it absolutely does serve to do is make the use of the clamp mandatory when playing an LP, otherwise the disc would float above the platter. With a small center 45, this is also the case, so the clamp is essential. A large center 45 will fit over the center plate, which is narrower than the 45 adapter, so it can be played without the clamp, though this would be self-defeating. When tightened a disc with a small spindle will have fair bit of pressure focused on the center of the disc. An unfortunate side effect of this is that it can pop out a tri-center! So when playing a tri-center I place several light pieces of vacuum cleaner filter bags, cut out to fit under the 45 to level the tri-center with the metal plate around the spindle. I suppose I could just as well have cut up a felt or cloth TT mat, but the characteristic of the filter bags seemed ideal for this purpose. I tighten the clamp to a lesser degree, though still securely. When playing a solid center 45 using the filter bag spacers is not necessary, but I generally do it. Having listened carefully to records played back both with & without the filter bag spacers I could discern absolutely no difference.
At the edge an LP record makes extremely close & solid contact with the 20 pound platter, bonding it and giving it great stability, in a sense forging the two into one. With 45's less so, as the disc is more rigid and will not bow to the same degree as an LP. The clamp is 3" wide. The 2" of the 45 protruding from either side does not have the same degree of flexibility as an LP, with 4 1/2" to either side of the clamp, which will tend to bow downwards at the edge owing to the pressure exerted at the perimeter of the clamp. Aside from lessened flexibility 45's almost invariable are thicker at the center than at the edge. On US manufactured 45's this is evident as a little ridge or bump just inside the center hole. On UK/European produced singles there often is a ridge at the outside of the label, or a series of small bumps that I believe are a result of the injection molding process. In either example this serves to make the center of the disc thicker than at the edge. So even with a clamp a 45 does not invariably make solid contact, at the edge, with the platter. It is, however, stabilized to a high degree. So using the spacers to provide a flexible interim medium between the disc & the platter seems the best course. I had considered cork but could not find any thin enough.
The designer of this turntable told me that the material that coats the platter is specially formulated to dampen any vibrations, so he did not recommend using any sort of TT mat. With LP's, which the guy essentially believed to be the only format that would ever be in contact with the TT, this is fine. They connect solidly at the edge owing to the flexibility of the wider expanse of vinyl protruding from either edge of the clamp, with the downwards pressure being channeled outwards, as well as the fact that the labels are generally flatter than that of a 45, resulting in less of a gap between the disc and the platter without the (for lack of a better term, or whatever the correct term is that is slipping my mind might be) ridge.
So a quandary presents itself when playing 7" records. Is it better to use a spacer, that may or may not be sonically benign, as the designer believes, to ensure better contact/stability between the edge of the record and the platter? Or is it preferable to let the edge of the disc be suspended above the platter, while being stabilized at the center by the clamp, as is the case, at least in the instance of the AVID ACUTUS.
I've never read or heard any commentary about this issue, and through my own empirical observations have arrived at a conclusion. Which is to use the spacers for 45's, but not LP's. It's just too freaky to observe the 45 spinning while floating above the platter at the edge of the disc. I have to believe the record would be vibrating from the tumult of the stylus coursing along the grooves, and henceforth adding an element of distortion, which if not provided by the guitarist I would just as soon avoid, to the playback. So the additional contact with the TT platter provided by the spacer/clamp combination provides what I consider to be the best combination to improve playback. Which is immensely influenced, for the better, by the clamp, certainly in this particular TT set up.
I'm not familiar with any other clamping systems other than this one which is, ( with the exception of the "why didn't the manufacturer of the TT take 45's into consideration when designing it" issue), fully integrated into the unit. As Rich mentioned Rega recommends against using a clamp. I previously investigated using one on a Linn TT, but could not find one compatible with their set up. What I can say for certain is that I consider clamping the disc to the platter to be effective, and therefore essential to optimize playback quality. The upside of proper playback procedure will be fully evident to anyone who has ever listened to an accurately tuned in system, as evidenced by, for example, a Boss Hoss transfer.
Here's a few pics hopefully illustrating the points addressed above.
TT platter w/ threaded spindle & clamp.
A high riding 45 viewed from the side. Notice the left side of the disc, yep, that's daylight between the 45 & the platter.
Tightened up but still riding high. The pressure exerted by the clamp might appear to be lifting the edge of the disc; what's really happening is the ridge along the center of the label is in full contact with the platter so the disc is in precisely the same relationship to the TT platter as in the pic above , only it is secured, giving it much greater stability.
The high tech spacers & 45 adapter.
Here's a 45 with a bubble level on the 45 adapter, assuring that the TT is level.
A 45 w/ spacers, prior to clamping.
Centered & ready to be clamped.
Clamped and in solid contact with the platter.
All dialed in. Though by now I'm generally too exhausted to play the record...
For non mission critical playback I simply spin the Linn.
