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Old 2nd January 2013, 06:44 AM   #21
Mooly is offline Mooly  United Kingdom
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Firstly, test CD's. These are essentially just red book standard (such as a normal CD) discs that are guaranteed to conform to tight tolerances such as thickness, reflectivity, runout etc. Any normal good disc will give identical results. NEVER use a CDR or RW as these are fundamentally different and have poor reflectivity.

What you do need is an oscilloscope. Thats essential for any investigation work.

The adjustments. VCO... DONT TOUCH.

The other adjustments, well you might find some practical hints in this recent thread of mine where I detailed setting up a player with the actual scope shots. The adjustments and procedures apply universally across different makes and models.

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Old 2nd January 2013, 09:46 PM   #22
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I do have oscilloscope...

Why not to touch VCO, what is special about it and what does it do?
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Old 3rd January 2013, 07:22 AM   #23
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Normally (in many players) the VCO (voltage controlled oscillator) is set with a frequency counter. Its used by the servo to enable the data (which varies a little in speed because of mechanical reasons) coming off the disc to be locked to an internal crystal reference.

The VCO has a pull in range. If its a preset then setting to the middle of the two extremes that the player loses lock at is close enough. Some VCO's use an adjustable coil rather than a resistor. Its an adjustment that should never need altering.
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Old 7th January 2013, 09:04 PM   #24
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Is there any benefit in using very high slew rate op amps on the output of DAC (like video op amps)?
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Old 8th January 2013, 06:30 AM   #25
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Quote:
Originally Posted by Tomom View Post
Is there any benefit in using very high slew rate op amps on the output of DAC (like video op amps)?
Definitely not Opamps for video are generally current rather than voltage feedback and would require a total redesign. In any case they are just not really suitable.

Far better to look at what is fitted and look at possible replacement with other audio devices. If the NE5532 or LM833 is used then I would suggest the OPA2134 or OPA2604 .

See also,
Swapping Op-Amps... you have checked to see it's stable haven't you ?
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Old 10th January 2013, 09:55 AM   #26
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Some CD players have paralleled DA converters, in particular those with TDA1541 types. Is the reason for that to get higher current (voltage) out, or is there something more to that?
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Old 10th January 2013, 10:16 AM   #27
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I think the idea behind parallel dacs is to "improve" linearity and resolution and so on at low levels. Dacs that need external I/V conversion nearly always feed into an opamp because that allows the closest approach to what is needed, a true current to voltage conversion. The current levels we are talking about are tiny.
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Old 4th March 2013, 01:17 AM   #28
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Signal from DAC on my CD player is fed into Low Pass Filter, then into a buffer, of course. Some people talk about by-passing that filter. Is that good, can it be done always or only under some circumstances, what are the benefits, if any...
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Old 4th March 2013, 06:16 AM   #29
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The filter removes all the HF noise from the DAC and conversion process. I wouldn't by-pass the filter as that allows all this noise through to the buffer and amp.
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