New Turntable Advice

@lewm

What is “slam”? I mean to ask what can a TT do to create slam? It might be the capacity to maintain constant rotational speed despite stylus drag. But that is a strength of dd as well. Otherwise, I’m stumped. I think slam is also an often repeated adjective associated with idler drive

Don’t direct drive turntables rely on feedback mechanisms to continually adjust the speed?  If so, there first needs to be a speed discrepancy, no matter how slight.

By contrast my Garrard 301 idler drive has no feedback mechanism at all.  A stonking big motor, which has been likened to a washing machine motor, spins at about 1,000 rpm.  Like some direct drives, it can spin the platter up in under a second, and in my estimation is unlikely to be over-bothered by stylus drag. 

It is not a synchronous motor either, so it does not rely on mains frequency, or any electronically generated frequency.  Fine speed is controlled by varying the drag created from eddy currents induced in an aluminium disk attached to the motor shaft, by a movable permanent magnet.

It takes about 10 minutes to warm up to final speed.  Not sure if the motor windings or the bearing lubricant, or both, are responsible?

The motor of the Garrard 301 is indeed physically large, but if I recall correctly, the torque is less than one might expect based on size alone.  And as you adjust V to regulate speed, torque goes down, until at some point torque disappears and the platter coasts to a stop unless braked. But I admit I don’t know how important that phenomenon might be, because torque comes into play when the platter is accelerating.  I do know there is a consensus of opinion that 301s benefit greatly from a motor controller. I was thinking earlier of the Supreme Court judge who said he could not define "pornography", but he knew it when he saw it.  The same may apply to slam. I certainly would not deny that idler drive TTs have a quality one might call slam but so also do DD TTs when you compare either type to a belt drive that is not well engineered.

By the way, DD TTs are not constantly adjusting speed based on feedback. Different designs from different companies use different circuits that have varying sensitivity to speed. For example, my Kenwood L07D has a relatively loose feedback mechanism but it uses a heavy platter with peripheral mass that tends to keep speed constant without much feedback,  Technics TTs, on the other hand, tend to have rather tight control on platter speed. I think Richard Krebs can comment, because he has looked extensively at the audibility of feedback affecting speed.

@lewm 

Yep feedback is a can of worms, or a Pandora's box...name your analogy. But done right it works wonders. 

I have never understood the naysayers criticizing DD TTs and their use of feedback yet they happily listen to amps with local and global feed back...its the same stuff..almost. 

The key difference in a TT is the drive's frequency response and how the feedback wrapped around the drive interacts with this. Typically the FR would be quite low, well below 100hz. The key is to get the inertia of the paltter and the feedback working in harmony with the drive's capability. This to make the drive responsive enough but not overly tight ,resonant such that the platter is excited or too soft such that stylus drag speed changes are audible. 

An aside. we read much about massive belt driven platters with high inertia being immune to stylus drag speed variations. so lets look at a thought experiment.

A compliant belt is driving a platter playing a record which has a constant low level 1000hz tone. The belt is stretched such that it delivers enough torque to the paltter to maintain a close to constant speed. (No platter runs at an absolutely constant speed.) The motor's rotor has moved back in phase a little with respect to the rotating field in the stator such that it can deliver the required torque to the belt. We have a stasis situation where the torque required to overcome the stylus drag is being supplied by the drive

The amount of energy stored in the platter is a function of its moment of inertia and its rotational speed. Its moment of inertia is fixed..

Now the track being played suddenly goes into a very loud low frequency tone. This increases the stylus drag considerably, taking energy out of the system. Since the moment of inertia is fixed, the only way to balance the energy equation is for. the platter to slow down. There is no way around this. 

We now trigger a chain of events. the tension in the belt increases but since it is stretchy, this takes time. This increase in tension demands more torque from the motor. The motor responds to this increased torque demand by the rotor moving further back in phase wrt to the stator's rotating field, This takes time.....eventually the drive catches up and the platter is brought back up to the correct speed. 

Now the track reverts back to the low level 1000hz tone so the stylus drag drops. The drive is preconditioned for a higher torque demand, so the belt dissipates its excess tension into the platter by speeding it up. Compounding this the motor's rotor moves forward in phase, also speeding up. The result is a temporary acceleration situation. None of this happens rapidly and the higher the inertia vs the drive torque capability, the slower this all happens. Obviously we do not spend our music sessions listening to test tones but I think you get it when it comes to real music. But the point is that it does happen, it is absolutely unavoidable. Some would say that this isn't audible.. I strongly disagree.

It is possible to run a synchronous DD motor open loop and on average it will rotate nicely on speed. But stylus drag will cause small changes. This causes to rotor to move about relative to the rotating field, which to my ears is annoying. 

Now back to feedback... is it audible? when done poorly, yes absolutely but when the effort is put into tuning the drive well, any deleterious problems virtually disappear. note I didn't say it is gone, it just becomes innocuous. that said not all DDs are created equal and some definitely have a "sound" that points to poorly turned feedback inaccurate speed measurement or simply a poor quality motor. There is nowhere to hide this in a DD. In contrast a BD can paper over poor build and implementation as it can be argued that the negative's are more sonically benign.

The trick in a DD is to use as little feedback as necessary. 

There are also ways to reduce the activity of the feed back loop. This, by preloading drag on the motor to drive its rotor back in phase more than would be the normal case. . This tightens the motor allowing it to ride over stylus drag changes far better than a drive which has a low friction bearing. the result is that the feedback loop has much less work to do. 

We do this in my designs, Perhaps there are other DD designs doing this but I'm not aware of any. 

We should touch on feed forward as well as it applies to DD's. This is where the controller is put into calibrate mode and it looks at the BEMF coming out of the motor. If the  motor was physically perfect we would expect to see 3 perfect sine waves. But of course we don't. This imperfection results in torque ripple. Note cogging is different and is zero in a well designed motor, The magnitude of this ripple is a function of the motors physical imperfections and the current it is drawing. 

the controller "remembers' where within the 360 degree circle these imperfections occur. When placed back into run mode the controller corrects for these imperfections by adjusting the drive current at the point(s) where the imperfection resides, in real time. It is a very cool feature. I don't think that a lot of drives have this feature as its is expensive.  It appears that it is in the technics 10R and of course its in my designs, recently updated to a more granular version. 

All my opinion of course and YMMV

Cheers.

First, I sincerely thank the forum members who have contributed thoughtful ideas and practical recommendations about turntable (TT) topics.

That said, some voices here seem consistently critical, often assuming newcomers or quieter members lack understanding. Before offering further criticism, it might be useful to bring fresh perspective into the conversation. At times, this forum resembles a morning routine—a “breakfast club” of the same few individuals discussing TTs daily but seldom introducing new insights, approaches, or measurable results.

The passion for vinyl is undeniable, but the question remains: what measurements set your experience apart? Have you quantified the noise levels in your signal chain? Have you measured cartridge cantilever and motor bias beyond just inspecting the housing? Have these parameters been recorded electrically? What signal-to-noise ratio do your TT and phono stage achieve—60, 70, 80 dB? Do you know where your system’s first harmonic appears? Are your measurements made in millivolts or microvolts? These are challenging measurements to make at home, no doubt.

The absence of responses regarding wow & flutter or signal-to-noise ratio (S/N) suggests that some self-proclaimed “experts” may not fully understand their systems or possess setups capable of detecting such subtle changes. Often, when someone says “you should hear this,” suggestibility overrides actual listening—a dynamic frequently exploited by salespeople promising you’ll hear “slam” like never before.

Album eccentricity was mentioned but left uncommented on. Have you heard it? Categorically yes—it’s a real, audible phenomenon. If not, perhaps your system or listening acuity is not sufficient to discern it.

Regarding credibility: I’ve designed sub-arcsecond pointing and slewing systems, many of which operate in orbit on long-term missions. I understand bearing systems and the engineering required to design inertial measurement units spinning at 11,567 rpm continuously for years on sleeve bearings. So while these conversations are engaging, not every topic discussed here is directly relevant to TT, vinyl reproduction, or even human hearing limits.

To progress the discussion, if you consider yourself a TT expert, let’s focus on measured data supporting your claims. Help those eager neophytes to learn by sharing scope traces or other objective evidence of what “slam” looks like in your system. Then we will be sure to publish more in the forum.
 

RIP Dad, miss you (D-Day Veteran) pass away on 9/11

@lewm 

And as you adjust V to regulate speed, torque goes down, until at some point torque disappears and the platter coasts to a stop unless braked

The Garrard 301 does not adjust speed by changing V (Voltage?).  Instead, drag from eddy currents induced in an alumium disk are adjusted by moving a permanent magnet.

A separate mechanical brake which pushes a felt pad into the rim can stop the platter about as quicky as the motor can start it - in under a second.

While playing, the motor keeps the platter spinning by supplying enough torque to overcome main bearing friction, idler wheel friction and that eddy current drag operating on a disk spinning at 1,000-rpm.  Oh, there is the friction of the stylus which might be around a gram or less?  Vinyl is pretty slippery.

You can adjust the drag and immediately see the effect on the strobe markings on the rim of the platter (extra cost when new) but I for one can see no slowdown effect when the stylus is lowered, nor speedup as it traverses to the record centre.

Similarly, I can’t see extra stylus drag when playing highly modulated music even doubling the normal friction drag without ejecting the stylus from the groove!

For @badbruno the as-new specifications are

• Wow: less than 0.2%
• Flutter: less than 0.05%
• Rumble: almost non existent

These are far worse than for example the direct drive $300,000 Wilson Benesch GMT One table where they are unmeasurable!

I am just curious as to why the Garrard 301 seems to be so highly rated!  The fast start and stop times may be one reason, and the lack of any (need for) feedback for speed control may be another