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Old 30th March 2007, 08:32 AM   #1
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Default Increasing CD platter inertia

I am thinking about increasing the inertia of the CD platter, thus decreasing mechanical jitter. It would be analog to a high-mass LP turntable, which reduces wow and flutter. I experimented with a Marantz CD-74 (having a Philips CDM-1 transport with Hall spindle motor). In my first experiment I put a brass disk on top of the CD. With this combination the rotation started slowly, and after a few seconds the servo speed regulator gave up (the servo PLL never locked), so the rotation stopped. Then I came to the idea that a capacitor in the servo feedback circuit is equivalent with a mechanical mass after electrical/mechanical transformation. So I connected a capacitor in the servo feedback, forming an integrator. The effect with this arrangement was the same as before: the motor just started rotating, but the servo fell out of sync immediately.
My next step will be to leave the speed stabilize (servo PLL locked) and only then connecting a large integrating capacitor in the servo feedback circuit with a relay. The control of the audio output muting relay might be used for this purpose.
Any comment welcome.
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Old 30th March 2007, 11:59 AM   #2
phn is offline phn  Sweden
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Who says there is any mechanical jitter to begin with?

To quote James Randi: "how can a fat guy in a red suit get down a chimney? Whoa! Let's find out is there really is a fat guy, first, before measuring chimneys."

A CD has no relationship with vinyl. The information on a CD is interleaved.
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Old 30th March 2007, 12:32 PM   #3
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I remember about 10 years ago I glued a 1 coin to the CD platter on a Teac CD3. I worked in a hifi shop at the time and customers were always suggesting tweaks. I tried quite a few of them.

My recollection at the time was that it did make a difference to the sound quality, more detail in the sound at mid and top ranges.

However that CD player motor died a year later. I was never quite sure wether my mod had hastened its departure, my suspician was it did.
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Old 30th March 2007, 12:48 PM   #4
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I once had a Marantz CD63 with a CDM12, which I quite heavilly modded.
One mod I did was to stuff blu-tak into the magnetic clamp, and adhere a very old, 1.5 inch diameter bronze penny coin on top.

I think it improved things. I also believe it helped the CDP read some CDRs that it previously had trouble with.
I think there is a recommended maximum clamp weight for each CDM (if I recall Rowemeister quoted for the CDM12 on the huge CD63/67 mods thread - if you're brave enough to search it).
The Marantz died - but not the transport, and I've moved onto a CDM9 based player which I've stripped down to be a transport.

Anyway, I am tempted to do the same to my CDM9, but am a little hesitant as its harder to find a replacement CDM if I have any issues.

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Old 30th March 2007, 01:05 PM   #5
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No need to search...

I think increasing the clamping weight is more to do with reducing vibrations than increasing inertia.

According to the link in the thread, 200g is the maximum weight for a CDMpro2. I don't know what it is for other CDMs and what their weight is already (but it should be a minimum of 135g for CDMpro2, so you might be able to add 65g).

Maybe I could try with my CDM9, just a bit timid.

I never did post a photo. I'll see if I can find one.
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Old 30th March 2007, 01:52 PM   #6
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I have just fitted a new CDM12/ Vam 12XX to my player and it keeps slowing down and distorting I'm going to change the disc motor but I'll try some extra weight first. I've nothing to lose.

Do you think that the magnetic strength of the disc clamps could be important at all? I personally doubt it but welcome any opinions. I guess it is more likely to matter when weights are added to the clamp.
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Old 30th March 2007, 03:14 PM   #7
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Phn is spot-on here.

Don't forget that the servo response is trimmed around the expected rotational inertia of the cd and little more. There's absolutely no technical merit in increasing this - the only place timing jitter matters is at the dac at the moment of conversion, and the transport doesn't affect this at all (and no, I can't see the pint of belt-drive cd players, either...)

Providing separate power supplies to the disc servo(s) on the other hand, can be helpful by reducing crosstalk when this supply is all too often shared with the other digital and critically, analogue stages.
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Old 31st March 2007, 04:35 AM   #8
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Default Iso-Drive

Here's a pic of an ISO-DRIVE disc damper. It's a machined nylon(?) disc , the same size of a cd , which is placed on top before closing the tray. It's fairly high in mass , so I guess there is some flywheel effect. It's also suppose to reduce disc flutter(vibration) so there is less error correction necessary , thereby less servo induced noise. These were sold in the late 80's.
Attached Images
File Type: jpg iso drive disc stabilizer.jpg (34.0 KB, 820 views)
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Old 31st March 2007, 04:52 AM   #9
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Increasing the rotating mass makes it much harder for the motor to change rotation speed which it must do continually as it plays the disc. You'd have to size-up the motor and control electronics along with the mass, but other posters have already pointed out that there is absolutely no point to doing either.

CD is a dead format, just like vinyl, shellac, and stearic wax. If you really want the best possible playback, stream the digital signal off an HDD to a squeezebox.

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Old 31st March 2007, 06:33 AM   #10
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I always thought that the main reason for doing this is not to increase inertia (as I_forgot mentioned, you'll be straining the motor), but more to reduce vibration in the CD itself.

Or, maybe its to slow the CD down a bit, making it easier for the laser to read the disc? Perhaps not.

CD isn't a dead format. I still get a lot of enjoyment from playing with my CDP.
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