Increasing CD platter inertia

[lcsaszar]

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.

 

[phn]

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.

 

[justblair]

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.

 

[philpoole]

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.

Cheers,
Phil

 

[philpoole]

No need to search...
http://www.diyaudio.com/forums/showthread.php?postid=858321#post858321

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.

 

[Sonusthree]

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.

 

[martin clark]

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.

 

[homemade]

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.

 

[I_Forgot]

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.

I_F

 

[philpoole]

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.

 

[Boris_The_Blade]

hmmm....

This CD inertia topic is quite silly. Musical data is kept on a CD in frames, and is buffered after read. A CD contains 44.1K samples/second, and as long as the D/A converters receive all of those samples/second, there will be no wow/flutter (This doesn't really exist for digital signals anyway). The ONLY thing that could cause something like that is D/A converter timebase inaccuracy/stability, usually a crystal + frequency divider. They are often quite good nowadays.

 

[poynton]

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.

Cheers,
Phil

http://www.diyaudio.com/forums/showthread.php?s=&threadid=54009&perpage=10&pagenumber=107

Post 1065


The original Marantz mod increases inertia a little as well as damping slight shaft oscillations.

Andy

 

[poynton]

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.

The only worthwhile mods to the cd mechanism must be those that improve the ability of the laser to read the disk.

An extra cd placed on top was often quoted as giving better sound. I assume that it reduced vibration. A similar effect can probably be heard on those cd players that use a 'turntable' to play the cd upside-down.

Rigid mechanisms, dampening material and mechanical isolation from vibration (eg using a belt drive to isolate the motor vibations, if any!) would also help.

By far, a greater improvement is to provide a better power supply and to improve the HF/RF signal from the mechanism to the processor. A cleaner signal reduces errors. (see the super-large CD63/67 thread !)


Andy

 

[tabooes]

The only worthwhile mods to the cd mechanism must be those that improve the ability of the laser to read the disk.

An extra cd placed on top was often quoted as giving better sound. I assume that it reduced vibration. A similar effect can probably be heard on those cd players that use a 'turntable' to play the cd upside-down.

Rigid mechanisms, dampening material and mechanical isolation from vibration (eg using a belt drive to isolate the motor vibations, if any!) would also help.

By far, a greater improvement is to provide a better power supply and to improve the HF/RF signal from the mechanism to the processor. A cleaner signal reduces errors. (see the super-large CD63/67 thread !)


Andy

hi, poynton

You talked about two ways to improve it. I got a question in the following address:

http://www.diyaudio.com/forums/showthread.php?s=&threadid=99197

thanks!

 

[awpagan]

The only worthwhile mods to the cd mechanism must be those that improve the ability of the laser to read the disk.

Andy

Have to agree with andy on this.

If the laser misses any bits of info on the disc, that it, the bits are lost

There is no error correction on data retrieval off the disc, or is there, havn't found any yet.

allan

 

[poynton]

Have to agree with andy on this.

If the laser misses any bits of info on the disc, that it, the bits are lost

There is no error correction on data retrieval off the disc, or is there, havn't found any yet.

allan

As said above, the data is interleaved and there is also redundancy ( I think ). So, it has to be a big 'gap' in the data for the muting to come into play if the interpolation cannot cope.

Early cd players, eg the Phillips CD104, were fitted with an error light. This was dropped from later players, probably as it scared people that their new, perfect discs were not perfect after all !!!!

Andy

 

[phn]

Yes, being able to read the disc is a requirement. But what does that have to do with inertia?

This thread stopped going anywhere a dozen posts ago. Nobody seems to have any interest in hearing what anyone else has to say unless he agrees with your beliefs. That's not discussion. That's religion.

 

[philpoole]

Hi Phn,
I think a few of the last dozen posts have mentioned, perhaps in different ways, that a possible benefit of weighting the spindle or placing a disc of some material on top of the CD is mechanical damping (i.e. reducing linear vibrations) rather than inertia.
This could be helping the laser to read the disc.
I think there is some logic behind it, and I think we've perhaps discussed that the inertia theory is a myth.

Where's the religion in that? I think this is an interesting discussion. Its got me (re)thinking about the subject - which is a good thing.

Anyway, I don't want to ruffle any feathers.

Andy, most Philips decoder chips still have the error output pin on them don't they? I was really tempted to wire one up to an LED at some point, but I never got round to it. I think it needs to be done at some point. I can always disconnect it if I get depressed
My dad had a CD104 when they came out, and the error light rarely came on. I'd dread to see how the error light would perform on the same CDs now (especially after he stupidly let his son near them ).

Cheers,
Phil

 

[tubee]

I used some time a diy dampening disc on the cd, used it with a Philips CD640 with CDM2. I made the dampening mat of soft plastic (PVC?) from Ikea, originally to reduce slipping of kitchenware in the kitchendrawer!

There are also commercially solutions available:

http://www.poortmusic.nl/apparatuur/aanraders/DE MAT/Mat.htm

With some CD's my diy solution sounded better, and other cd's not. Now i'm playing with a CD304mk2 / CDM1, it has allmost no benefit from the dampening disc. In an audio shop they told me not to use the dampening disc anymore because of reduced life span of spindle motor.

Pioneer had a nice player with stable platter mechanism, laser reads disc from above like a TT. People who tried this player, loaded CD often with lable on top as they normal do, having error message: "No disc"

I have been thinking to use a reading error led also Philpoole, but never did so far either.

 

[I_Forgot]

Andy, most Philips decoder chips still have the error output pin on them don't they? I was really tempted to wire one up to an LED at some point, but I never got round to it. I think it needs to be done at some point. I can always disconnect it if I get depressed
My dad had a CD104 when they came out, and the error light rarely came on. I'd dread to see how the error light would perform on the same CDs now (especially after he stupidly let his son near them ).

It depends on what type of error triggers the LED. The vast majority of errors that occur in reading CDs are 100% correctable by the electronics before the data stream gets to the DAC. Such errors don't matter at all. The next level of error is one that can't be corrected, so the data is interpolated. These errors are very brief and your ability to recognize them is directly proportional to the size of your audio ego. The third type of error is what happens when you have a long scratch in the disc or a big glop of peanut butter on the disc- there aren't enough good samples to interpolate so the output is muted.

Which type triggers the LED?

I_F