For the same reasons guitars, amplifiers, radios and speakers don’t all sound the same.
Actually, it’s the same basic set of reasons no two THINGS are ever exactly the same. There are so many elements inside a CD player contributing to the sound that no two CD players could sound the same unless they were the same model. None of these things will ever sound exactly the same because what you hear is the sum of many complex electronic and mechanical elements working together.
Ones and Zeros
A common technical misunderstanding in relation to CDs is that:
It’s all just ones and zeros.
People…
It’s far from that simple. Yes, CDs contain ‘just’ ones and zeros, but this statement is a gross oversimplification of how CD players work. It lacks any consideration of the critical steps after ones and zeros are extracted from the disc and how those ones and zeros are converted into music you can hear.
Even different CDs of the same recordings don’t sound quite the same. Likewise, CD transports only extract the digital data from the disc, yet they sound different. The bottom line is that you can’t listen to ones and zeros and an enormous amount of signal processing has to happen before you can hear music from a CD.
Data has to be read from the disc, error corrected, filtered and anti-aliased, converted from digital into analog, amplified and buffered. There are hundreds of components in the signal path between the disc and your ears and hundreds more contributing to the sound, and that’s just inside the player!
Sum of the Parts
The following elements contribute to the sound of a CD player:
- The CD mechanism or mech, spindle motor and laser
- The hardware carrying the RF signal – shielded coax or unshielded wire, connectors, termination impedances
- DAC type and design – R2R/multibit delta-sigma/chip/discrete – ie the ones and zeros
- Analog and digital filter type and design – HDCD/FPGA/DSP/none
- Device firmware running the digital filters, PICs, FPGAs etc
- Inter-stage analog amplifiers and buffers
- The all-important output buffer – chip/discrete/class-A/tube/transistor/transformer/balanced/singled-ended etc
- Power supply – linear/SMPS, filtering details, regulators
- Clock – frequency/PPM precision/drift
- General layout, board design, wiring, shielding, parts quality
- Condition of the unit, laser power output/health, general state of service
Each of these elements influences the signal that leaves the output connectors of the CD player, as do the discs themselves. As you can see, CD players are actually complex mixed-signal (digital and analog) hardware devices.