~ Classic CD Player Tweaks ~

Philips CD104 ~ Repair ~ Tweaks ~ NonOversampling

My CD players like the rest of my audio equipment are either the very first of their type or made by QUAD before 1995 ~ I purchased the first tray loading CD player from Philips ~ the CD104 ~ when they were introduced in the early 80's but within a year it failed so was returned to Philips who "fixed it" and within a few months it failed again

Philips then sent me a new one which lasted less than a year and this continued until they gave me a CD 104B which was either B because it was Black not Silver/Grey like the previous models or maybe they had ironed out the all faults and this was the improved B version

As it happened the Philips CD104B was an improved version which lasted about 2 years before finally going wrong and then I had to either get it fixed or fix it ~ By this time the Philips stock faults were becoming well known ~ Some problems with my first CD104 could be attributed in part to the mechanism but there is one design fault that plagued all the models which was through PCB rivets or bushes that did not solder well

The solder bushes are used to connect from the ground–plane of the PCBs to the other side of the boards and depending on which ones fail the player does some strange things like spinning fast and continuously or appearing to play okay but drops off the end of long CDs

There are 11 bushes on the Philips CD 104 Decoder PCB and 2 on the Servo PCB ~ The Servo PCB (the one you see when you lift the lid) also uses the leads of electrolytic capacitors to link both sides of the board and these connections are normally fine ~ The main problem area is the PCB underneath it which is the Decoder PCB

Tracks skipping may be caused by a faulty capacitor C2519 shown bottom of picture

The Control and Display PCB mounted on the front panel uses component leads to link both sides of the board and I have seen faults where the soldering has cracked causing the display not to light ~ A problem where the display does not light may be caused by C2059 on the display PCB ~ This provides the reset at switch on for the Display processor and Servo PCB

There were several versions of the C.D.M.–1 as noted in the service manual those marked with yellow labels B C H or G as pictured could have the motor PCB problem ~ Those labelled A B E or F were based on the C.D.M.–0 deck with a traditional motor driven from the main servo PCB plug 23 pins 5 & 6

Note I removed this deck completely as other parts were to be aligned but the motor PCB can be removed simply by removing the 2 torx bolts that hold the part (1) and undoing the nut that holds part (2) so this can be "screwed" off by holding the CD centre clamp from the other side after removing the white top CD clamp

With the PCB (3) removed a small amount of light oil like 3in1 can be applied to the recess marked (4) ~ Leave the player or deck upside down for a long time so the oil soaks into both bearing bushes ~ A small amount of oil can be applied to the swing arm ball bearings at (5) and at head of the nut on the top side now visible with the white plastic CD top clamp removed

If impatient for a result you can remove the circlip from the spindle and with the deck upside down remove the spindle just enough so that the oil runs into the space between the top and bottom bearing bushes ~ Replace the spindle and circlip and when the player is back together working but not in use place it on one side to allow the oil the better reach the top bearing ~ Repeat this process every 20-30 years as applicable

Note there are holes (A) in the clamp arm parts to adjust the tension and level of the clamp ~ if you have good copy of Philips info on these adjustments please let me know

So the PSU tested good but you decide to change the capacitors anyway so while the PCB and Heatsink are out why not fit some better output connectors by removing the fixed audio output cable and drilling another hole above the original one ~ You will need to fit some large flanged type or insulated Phono connectors to fill the original hole

New isolated phono sockets with twisted wires are prepared ready to connect to the Decoder PCB

Screened cable is not required as there is no measurable crosstalk or pick~up due to the twisted wire and twisted ~ PTFE wire in this case is so much easier and neater to work with but does it improve the sound ?

As nice and practical as they are ~ Gold plated phono sockets with PTFE insulation WILL NOT reduce or introduce any distortion ~ They WILL NOT make the Philips CD104 or ANY CD player sound cleaner or more transparent etc. ~ Those ridiculously overpriced audiophool grade cables that you bought from Russ Andrews or some other flim flam firm could now connect your CD 104 via your passive pre amp volume control but even this WILL NOT improve the CD104 sound

But This May .  .  .

The TDA1540 data sheet states the accuracy of the current division is dependant only on the equal mark/space ratio of a series of binary divider and this process requires "filter" capacitors which do not need to be close tolerance

The ceramic capacitors fitted to the CD104 DAC do appear to affect "the sound" of it's output ~ This is possibly due to losses and or voltage coefficient of the capacitors but whatever it is ~ It is not good ~

The original surface mount components were glued to the PCB underside to enable both through hole and surface mount devices ~ SMD ~ to be soldered at the same time ~ Take care removing the ceramic SMD capacitors ~ Use hot tweezers if you can to melt the solder both ends at once and melt the glue at the same time

For years I owned a CD84 and I still own an original CD104 plus my modified CD 104 which is in my opinion the most natural sounding of them all ~ The CD84 also has copper / polystyrene capacitors for the output and de–emphasis filters and these could also have an influence on its natural sound as polystyrene is an ideal dielectric for audio

Some later Marantz models used ceramic DAC capacitors as dictated by Philips but eventually they realised ~ when the 16bit TDA1541 was introduced and was used in many more CD players like the Revox 226 ~ B&O CD 5500 and Marantz 94

Maybe there was something about poly caps that mattered for the DAC analogue stage and the Philips CD880 and DAC960 are proof that Philips also believed so ~ The DAC960 was made in Japan by Marantz and very similar to their CD94 using dipped Mylar capacitors

Unfortunately the analogue stage coupling and de–coupling capacitors tend to let the Marantz CD84 down and this was also a problem with the Philips CD104 which had 100Ω resistors in series with the op–amp supplies and ceramic 22nF de–coupling capacitors except for the +Ve supply to the Right channel NE5532N op–amp ?

Picture shows the analogue section of my Philips CD 104B Decoder PCB in 2002 ~ Bottom centre is the Left channel NE5532N op–amp the two 100Ω through hole resistors either side are feeding its +Ve and –Ve supplies ~ SMD 22nF ceramic de–coupling capacitors are on the other side of the PCB

In the picture centre is the Right channel op–amp which has SMD 100Ω resistors in series with its supplies but only one 22nF capacitor on its –Ve supply ?

Here 1µF 63V polyester are added across the 4 new op–amp decoupling tantalum and the solid aluminium output coupling capacitors

The sound improvement with poly bypass caps is subtle but noticeable with the DAC capacitors also been changed for poly caps

Non Over–Sampling "downgrade"

Such a sharp filter is needed to remove the energy from the products around 44.1kHz ~ 88.2kHz ~ 132.3kHz etc. which make THD measurements difficult using notch filter analysers ~ The original Philips Signetics data sheet recommends a 9th order Cauer "reconstruction filter" probably to better remove the large 44.1kHz sidebands when not using oversampling

I use both notch filter and FFT analysers to measure audio equipment and each has its merits ~ The Tektronix AA501 is a good a.c. meter with a tracking notch filter that enables viewing the analogue input signal both before and after various filters ~ Measurements of a standard CD104 with the maximum audio band passband filters are not good

Looking at the harmonics and various 44.1/n kHz frequencies from a home made "audio master" test CD on a HP35670A FFT analyser showed the harmonic distortion in the audio band was very low ~ better than –80dB for full amplitude signals but even with the 4x oversampling there was some "noise" at 44.1kHz and significant "noise" at higher frequency

In the picture you can see 4 brown wire links between pins 3 & 20 Left Data ~ 10 & 17 Right Data ~ 7 & 21 Strobe ~ 6 & 18 Data Clock ~ where the SAA7030 has been removed and the TDA1450 DACs are now connected directly to the SAA7000 timing IC

The final part of the non oversampling change is to remove the ground 0V from pin 16 of the SAA7000 and connect it to +5V to make it output 14bit clock bursts

Also in the picture above ~ Green dots indicate where the ground "buss" for the TDA1540s filter capacitors are now connected directly to the top ground plane at pin 6 and C2631 and C2581 ~ The yellow bars are where wire links forming part of the track between the DAC outputs and transimpedance amplifier (TIA) inputs have been removed and replaced with the red and white wires directly across the PCB

The inductors and series capacitors form notches which can be varied in the region 25kHz to 35kHz thus reducing the "energy" in this region which should help with the reconstruction of audio frequencies around 10kHz to 20kHz ~ That was the plan!

The double notch filter switching involved several multipole changeover switches and at one point I was even considering using the switches to change between oversampling and non but I decided against that idea ~ The changes were compared against two other standard CD104s which did not have their DAC or other caps changed and the auditions often involved several bottles (shared) of red wine so not a very scientific test but I was becoming convinced that non–oversampling with a passive LC re–construction filter was the way to go

If I were to fit an LC cauer or elliptic filter it would need to have a controlled input and load impedance to work correctly ~ The dual notches gave good measured and subjective results so a lower order filter could be the way to go and it could be placed between the the NE5532 op–amp stages which are already part of the output chain ~ The required source and load impedances could be accurately set by series resistors

One section of each NE5532 after the TDA1540 is used as a transimpedance amplifier (TIA) which converts the DAC current into a voltage and adjusts the output high frequency response to correct for the digital fitering and provides switched de–emphasis if it were ever required

The second section of each NE5532 is a Sallen & Key style low pass filter which together with the response of the TIA form a Bessel filter to maintain a linear phase response ~ It is d.c. coupled to the TIA output and is non–inverting so both op–amp outputs only swing between 0V and about +6.8V

Rather than build an elliptic filter I pulled a pair of pilot tone filters from an old 1980s Kenwood stereo receiver ~ These filters are 5th order with notches at 19kHz (stereo pilot tone) and 38kHz (decoder switching frequency) ~ due to the resistance of the coils giving lower than perfect Q the filters could be adjusted with notches at about –50dB at 23kHz and –60dB at 38kHz while still having a smooth frequency (but not phase) response to about 17kHz

The filters were removed on a section of the original PCB which had ground–plane copper making it easy to solder this sub PCB to the CD104 ground–plane as marked F1–F2 in the picture above ~ The Sallen & Key components were removed and the second section of each NE5532 was used as a buffer with input resistors to ground to match the filter

The coupling was kept d.c. so the voltage from each output op–amp was about +1.7V which ensures the only series capacitors ~ new 68µF OScon ~ remain biased correctly and never get reverse current through them ~ the TIA op–amps now see a 6kΩ d.c. load and so drive a small current which can be better than driving a high impedance load or a.c. coupling especially when the output never crosses zero

TDA1540 outputs (like many a DAC) are a zero–order hold staircase signal where the level of each sample is held until the next sample period ~ This gives a low pass filter effect to the output with a normalised response of sin(πx)/(πx) which has zero output at the sample frequency and multiples thereof ~ With a 4x oversampling frequency of 176.4kHz the response at 20kHz would be 20log(Sin(20π/176.4)/(20π/176.4)) = –0.18dB and even this small loss is compensated for in the CD104 by the SAA7030 digital filter and TIA / Sallen & Key filter responses

With 44.1kHz sampling frequency the loss at 20kHz is now 20log(Sin(20π/44.1)/(20π/44.1)) = –3.17dB ~ This coupled with the elliptic filter gives a response about -3dB at 16kHz which falls off rapidly whereas the original oversampled design with digital filtering is flat to 20kHz with very little phase shift ~ One slight advantage of the zero–order hold response is that the energy near the sampling frequency and its multiples is very low hence the sencond notch of the filter was kept at 38kHz and not moved closer to 44.1kHz

The CD104 as now configured sounds very good considering it is only 14bit NOS and has and elliptic low pass filter with less than the ideal phase response in series with the output ~ It is like listening to BBC FM radio here in the UK which has since the 1980s been distributed via 14bits (32kHz sampled) digitally companded down to a 10bits via a BBC designed system known as NICAM3 yet many listeners to the classical music station BBC Radio3 claim it is better than CD quality and live concerts are so realistic !

Quad 66CD ~ TDA1541A

The QUAD 66CD player is based on a Philips transport and PCB which also has both through hole and surface mount components and again has surface mount ceramic capacitors for DAC filtering glued on the solder side of the PCB

Unlike the CD104 there is little room between the underside of the PCB and the base of the player so surface mount capacitors ~ shown circled in yellow ~ are required

14 x 100nF 16V 1210 size PPS capacitors were fitted on their sides in place of the 1206 ceramic capacitors on the TDA1541A ~ 3 other 100nF PPS capacitors were fitted in place of the 22nF supply bypass capacitors

It appears that filter capacitors as fitted to DACs like the TDA1540 and the TDA1541 do have "an effect" on the perceived sound and this has been noted by many others and some better manufacturers fitted them as standard

Better coupling and de–coupling of the post DAC audio stages of the CD 104 and CD66 also gave an improvement in my opinion but the DAC output current in each case is still converted to an output voltage via a transimpedance amplifier ~ could this be changed .  .  .