Direct drive/rim drive/idler drive vs. belt drive?

HiHo

Here are a few things which I can prove to be true:

1. Newton's Third Law of motion holds for turntable motors so the reaction torque reflected into the chassis will be the mirror of the forward torque applied to the platter.

2. The variation in reluctance of a "coreless" motor is much smaller than that in a motor using an iron cored stator.

3. As an iron cored DD motor rotates, the servo loop compensates for the variation in reluctance by decreasing torque as the rotor pulls towards the lowest reluctance position and increaes torque as the rotor pulls away from that position. This happens many times per revolution, depending on the slot and pole numbers of the stator and rotor respectively. The exact quantum is the least common multiple of the slot and pole numbers.

4. It follows from 1 that the torque variation reflected into and propagated through the chassis is much smaller with an ironless stator than with an iron cored.

It is my conjecture that this phenomenon explains what you are hearing. Naturally I cannot prove this so I won't say that it is *definitely* the case.

Some support for this idea comes from some engineering work done at Sansui towards the end of the analogue era where they designed a DD with two counter-rotating platters to obviate the problem (called X-99 I believe).

Mark Kelly

Dear Hiho: It is obvious that a TT build material ( any ) can't fix a mediocre motor that ca run on " speed ", but I think that is not the subject on what Ralph want to share with all of you.

I can tell you that through my TT experiences through the years ( dozens of TTs ) almost all main factors on TT performance are already addresses but the TT right build materials.

You can find several threads like this one speaking one and again the same TT topics with no single real advance out there.
I know that everyone has a lot of fun reading and making some TT changes on its systems but that main factor ( TT build material ) remain almost untouchable: I wonder why?. The " sad " issue is that almost all analog people does not care about just like you.
It is obvious that maybe does not care about because don't understand the critical importance on the TT build materials that like I posted the differences in quality performance is: night and day !!

I understand too that because almost no one already experienced what Ralph and I " live " there is not to much of what to talk/share from almost all of you.

IMHO and like Fm_loging posted: we have to evolution if we want to grow up and if we want to improve what we have.

I'm on evolution and that's why I share with all of you my findings. Stay steady where you are is up to you and fine with me.

Regards and enjoy the music,
Raul.

Mark, Just out of curiosity, do you know anything about the build of the motor in a Denon DP80, iron core or coreless? I do know that it is a 3-phase synchronous AC motor the speed of which is controlled via the servo by varying the frequency of the AC. This is different from the SP10, but I had not thought about whether the DP80 motor had an iron core or not. Indeed, I did not appreciate the significance of that fact until Hiho and you had the exchange above. I ask because the DP80 gives me the very same "smooth" feeling that Hiho experiences with his Pioneer. In a heavy slate plinth, the Denon is rather addictive.

Raul and Ralph, I hope some time it will be possible for you to reveal the nature of these especially good tt mats. It would be fascinating to find out whether both mats were made of similar materials.

Lew

The DP80 uses an iron core stator.

The distinction between Denon's "3 phase AC outer rotor motor" and Technics "Brushless DC with integral magnet platter" is largely semantic.

The Technics motor is a 3 phase outer rotor motor which includes a circuit in the motor which generates 3 AC waveforms due to the motion of the rotor. These waveforms are necessarily synchronous with the rotor. The waveforms are then amplified to a level determined by the PLL controlled servo loop and fed back to the motor drive coils. The PLL is fed by Technics famous frequency generator circuit.

Without access to the Denons circuit details I cannot say exactly how the Denon generates the frequency required to run its motor but I can say that it also employs a PLL controlled servo loop to slave the coil drive to a motion dependent signal, this time generated by a magnetic signal recorded on the platter (a primitive version of a rotary encoder). The loop presumably also controls the voltage of the drive amplifiers - if it did not the level of cogging would render the motor useless.

From a practical point of view the only difference would be in the fidelity of the drive waveform. The forward drive voltage in the Techics motors I've seen is fairly ugly, the engineers relied on the high speed of the FG servo to smooth the rotation. Denon's encoder is a lot slower so they would have to have a cleaner waveform to start with. They are both neat solutions to the central problem, neither appears to me to be inherently inferior to the other.

Mark Kelly

Thanks, Mark. That's a lot more information than I can glean from the Denon literature from that era. And the schematic is beyond my comprehension. Others, including Technics as you note, have spoken of the Technics SP10 motor as being of the DC type. Now I see that this is a distinction without a major difference.