Servo noise from heavily (!!) tweaked 16 bit CD player - diyAudio
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Old 17th April 2002, 03:58 PM   #1
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Default Servo noise from heavily (!!) tweaked 16 bit CD player

Hi All,

I am hearing noise from the servo circuits performing focusing and disc spinning tasks during the playing of disks. It is low level compared with the music but it is there. Any ideas on how to get rid of this?

Some detail on the machine:

A Rotel 955 16 bit with the following mods:

1. 2x TDA1541 in parallel
2. Zero-oversampling
3. Passive IV concersion through a 25 ohm resistor. There is no post IV conversion filter and the DAC pushes out about 0.3 V (I think)
4. The DACs are powered by two separate transformers, one for the -15/-5V pins and one for the +5V pin. The powersupplies consist of LM317 pre-reguilatioon followed by TL431 shunt regulation
5. The SAA7210 and processor have their own power supplies and transformers each! These are decoupled by 1000uF low impedance caps (Panasonic). These also use LM317
6. The clock is by Guido Tent and has its own transformer/power supply. The supply is TL431 based with LM317 pre-regulation
7. Everything that sits on the unregualetd 9V line (which includes teh servo circuits) are now powered from a separate transformer and sit at about 12V. These are not regulated...
8. The original transformer is only powering the LED display now!
9. 80% of wire links in teh CD player have been replaced with inductors having DC resistance of about 0.2 ohms and all have been threaded with ferrite beads.
10. Despite all of the above, the CD player still sits in its original box and with its lid on you'd never guess all the nasty stuff happening inside, except for the weight which is now around 15kg!
11. So in total 7 transformers and lots of capacitors and inductors with zero oversampling and no op-amps after the DAC.
12. The Sound? Very good. Tight bass, clear highs and good depth to sound. Just that very low level servo noise that gets to me sometimes. If there is no real solution, I will live with the player as is!!
13. One final thing is that I do not really have any space left for anything else inside the player!!

Thanks for any help
Ryan
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Old 17th April 2002, 04:03 PM   #2
jam is offline jam  United States
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Ryan,

Was the servo noise apparent before the mod ?

Jam
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Old 17th April 2002, 04:28 PM   #3
fcel is offline fcel  United States
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Ryan,
Is it possible that with so much tweaking done to the player, the tracking and focusing of the laser pick up head has been misaligned electrically (too much offset?) to force the servo circuit to work especially hard (& thus the "noise") to try to get back to read the disc properly. Try a badly scratched or warp disc and see if it'll play. If you hear a lot of "noise" from the pick up head, it's most likely that the pick up head needs some major alignment. Get a service manual and hook it up to a scope and just follow the alignment instruction in the manual. Let us know how it turns out.
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Old 17th April 2002, 05:12 PM   #4
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Default Re servo noise

Thanks for your replies.

1. The servo noise was not there before the mods, but i think that it is now audible because the output from the DAC in the CD player is so much lower. Remember that befoire my mods, the CD player was pushing out 2V. Now its just 0.3-0.6V, so any DAC independent noise is going tp appear louder

2. The noise is not like when the servo first searches a disk (i.e. the whining noise that one sometimes hears). It is more like a whirring sound in the background, I'd say around 300-500 Hz.

3. It is in the background and only really audible when the machine is paused. During music, it is too soft to be noticed, except in between tracks. I must say that I'm not sure that other people have noticed it unless I paused teh Cd player and turn the volume up.

4. I was thinking that regulating the +-9V lines to the servo may help. Any thoughts on this?

Thanks for your ideas.
Ryan
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Old 17th April 2002, 05:47 PM   #5
jam is offline jam  United States
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Ryan,

Looks like you have to adjust your tracking and focus gains and focus offset of your player. Obtain a manual and perform the adjustments, you will probably need a signal generator and a scope (60 Mhz min.) This is assuming that your player does not have a digital servo.
Excess servo noise could also be an indication of a weak pickup(laser) assembly.

Jam
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Old 17th April 2002, 07:30 PM   #6
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Let me see if I have this right............

Non o/s with NO post filter?????

Jocko
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Old 18th April 2002, 06:16 AM   #7
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Default Thats right

No oversampling and no post filter!

I may change this to include some inductors, but the sound seems fine to me. You should hear it!
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Old 18th April 2002, 11:10 AM   #8
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At first I thought you were complaining about acoustical noise, but with yesterday's post it becomes clearer that you are talking about servo noise on the analog out signal. My best guess is that you have a botched grounding scheme, the servo circuits and the analog out sharing some common ground impedance. But it may also be radiated interference...

For starters:
1. try to connect some decent low-ESR electolytics (220/16 or up) in parallel with 1F ceramic chip between the +/- supplies of the power servo drivers and the ground return of the servo coils. If those are BTL drivers, there will be no ground return. In this case make sure the electrolytic is connectet between + and - supply of the driver (the latter may actually be ground).

2. Define separate star grounds for servo, digital, analog. Tie them together at one central ground.


Have fun!

Eric
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Old 18th April 2002, 03:06 PM   #9
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Thanks Eric,

I will certainly take a closer look at teh grounding scheme, which admittedly I have paid VERY little attention to.

I must tell you that putting a resistor 10 ohm between chassis ground and circuit ground seemed to lower the noise floor and made the sound more relaxed (sorry no other way of describing it).
But even more was gained by adding a cap (2.2 nF I think or may be it was 22nF) across the resistor (i.e. in parallel with the R). This seemed to have teh effect of increasing transparency in the system and lowering teh noise floor.

I guess that the cap is shunting AC noise on the cirrcuit ground to chassis ground.

I'll let you guys know how it pans out, but am still open to ideas!

Regards
Ryan
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Old 18th April 2002, 03:37 PM   #10
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Post filters are there for a reason.

I do not want, or have to hear it. Haven't you been paying attention to my constant rants against this sort of thing. Nothing personal, I just don't understand why you guys mess with this nonsense.

The problems you describe are a good reason why.

Jocko
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