The CD57 is the cheaper brother of the 'regular' CD67. The 57 has no HDAM circuit and no optical digital output, and some electrolytics are smaller. However, the PCB and transport are identical to that of the CD67, apart from the small differences between the components. But those are going to be removed anyway... So a CD57 is an interesting starting point for modifications: in the end, the result will be as good as with a 67, certainly with the discrete output stage that is presented here. With this, it has turned from a simple player into a real CD57-XXL.
In this player the entire analog section after the DAC (the filter and the opamps) has been removed. It's been replaced by a passive 3rd order filter, followed by a discrete output stage*. It consists of a differential stage that sums the two output signals from the DAC and a single-ended buffer (in class-A) at the output. All of this is entirely built on the PCB of the player, by using the original lay-out.
The passive filter is optimized to make the group-delay in the passband as constant as possible (Bessel characteristic). The price for this is an attenuation of the signal of 1dB at 20kHz. Accurate 1% polystyrene capacitors and 0.1% resistors are used. The 10mH inductors are matched by hand with an accurate LCR meter. This insures both filter curves for the left and the right channel are as identical as possible.
In the output stage, either JFETs and transistors can be used, or both. The 2SK170 and BC550 that are used here are pin-compatible: their pins D-G-S and C-B-E match by functionality. Both versions were built and sound very good. Needless to say this also depends on the quality of the components used. The output stage was later upgraded with Holco and PRP resistors, and a Multicap tin-foil output capacitor.
Here only the photos of this specific section are presented, the rest of the modifications are similar to those of the CD67mkII-OSE.
During the modification of the player, the muting transistors are also removed. These are connected between the analog outputs and ground (through a 100Ω resistor) and make sure that the annoying noises that sometimes occur during power-up or -down, or during errors on the CD, are being suppressed. However, the properties of a transistor are voltage and current dependent, and considering the fact the output signal is very dynamic, it's better not to use any semiconductors in this section. In the modification list, the application of a relay is mentioned, to restore the muting function. The contacts of this relay come in place of the transistors, and will short the outputs to ground if no music is played. The power-up and down noise will be a thing of the past with this.
The supply voltage for the relay will be tapped from the collector of QN02. An auxilary voltage that is originally used for muting is present there. It is about 12V, which is perfectly suited for driving a relay. A double-pole/double throw (DPDT) 12V relay with gold-plated contacts is used here. It's best to use one that is specifically suited for switching small signals, for instance the G5A-234P-12VDC from Omron. It is also possible to use two single-pole relays. Two 6V relays can be connected in series, so only one transistor is needed to drive them. If RN30 and RN31 are lowered to 2k2, the original muting traces on the PCB can be used to drive T1. QN24 and QN25 have to be removed, and their base and collector connections have to be jumpered on the PCB. This is needed to connect the muting signal that's coming from the decoder (Q102). RN27 and RN28 have to be removed as well. Connect the base of T1 to the PCB, to the base of either QN07 or QN08.
The two NC (normally-closed) contacts of the relay are connected between GND (the collectors) and the emitters of QN07 and QN08. Make sure the output signal is not shorted to GND directly. A resistor of a few tens of ohms (33...47Ω) has to be connected in series with the output signal, before the relay contacts, in the form of R659/660.
* design based on the SACD Enhancer by UltrAnalog