Hi,
I'm trying to mod my old and first cd player, a Philips CD 371 (1986).
I have already replaced the OPA.
Now I would like to improve the PSU by changing the regulators and the diodes.
The regulators are : 78M15CT , 79M15CT, 78M05CT and 78M06CT
Is there any drop-in replacement for these regulators ?
The diodes are BAX18, can I replace them with something better ?
I also plan to replace my 6800uF filtering cap by a 47000uF.
Regards
I'm trying to mod my old and first cd player, a Philips CD 371 (1986).
I have already replaced the OPA.
Now I would like to improve the PSU by changing the regulators and the diodes.
The regulators are : 78M15CT , 79M15CT, 78M05CT and 78M06CT
Is there any drop-in replacement for these regulators ?
The diodes are BAX18, can I replace them with something better ?
I also plan to replace my 6800uF filtering cap by a 47000uF.
Regards
Attachments
The best mod you can do to improve the supply, is to feed the SAA7220 with its own 5v via a new regulator placed very close by and fed from the unregulated 5v (C2515)
Kevin
Kevin
Thanks Kevin for your reply.
With what model of regulator ?
Don't you think that the OPA also will need PSU improvement ?
With what model of regulator ?
Don't you think that the OPA also will need PSU improvement ?
How will that improve things? It could make things worse, by narrowing the charging pulse and so increasing induction.ads2 said:
Indeed, this is not only an overkill, but will make your PSU slow, inert and high output impedance. Going over 10.000uF will bring no benefits but on the other hand may kill the rectifier bridge or even the transformer.
AFAIK, there is no direct replacement for those regulators which have internal adjustment circuit. You could try with LM3xx regulators or their modern counterparts, the LT1xxx family, but with the addition of resistors and capacitors in FB circuit. Or, you could just leave those old regulators in place if you can live with 40uV ripple and 80dB noise rejection, which is fair.
The regulators have their best noise suppression below 100Khz or a bit lower. (On-semi is a bit better than most other Mfgs)
So all HF (created by eg SAA7220) will cause problems. KevinKevin's suggestion to give the SAA a regulator of it's own is a very good plan!
Recently I did some investigation to the noise injected by the SAA by simulating it's load in LTspice. The decoupling circuit (RC) proved to be not too effective in suppressing the noise (2.8Mhz and 11.2896Mhz fundamentals and their harmonics).. only about 20db (max.).
So I'd suggest to exchange the resistor (1 Ohm) by a small choke (from 2uH upward and Ri less than 1 ohm is fine) (or add a ferrite bead to one of the wires of the resistors) and use a capacitor with better HF characteristics (os-con??) than the standard electrolytic and increase the value to about 220uF. The lower ESR will also contribute to better noise-suppression.
The 6800uf caps... 47000uF could be too hefty for the diodes and the transformer. I would advise you not to go beyond 15000uF.
So all HF (created by eg SAA7220) will cause problems. KevinKevin's suggestion to give the SAA a regulator of it's own is a very good plan!
Recently I did some investigation to the noise injected by the SAA by simulating it's load in LTspice. The decoupling circuit (RC) proved to be not too effective in suppressing the noise (2.8Mhz and 11.2896Mhz fundamentals and their harmonics).. only about 20db (max.).
So I'd suggest to exchange the resistor (1 Ohm) by a small choke (from 2uH upward and Ri less than 1 ohm is fine) (or add a ferrite bead to one of the wires of the resistors) and use a capacitor with better HF characteristics (os-con??) than the standard electrolytic and increase the value to about 220uF. The lower ESR will also contribute to better noise-suppression.
The 6800uf caps... 47000uF could be too hefty for the diodes and the transformer. I would advise you not to go beyond 15000uF.
The opamps will certainly run from lower than 15V supplies. So you can install series inductors or resistors to give passive filtering prior to the replacement regs at, say 12V. Then use TL431 shunts on the opamps down to 7V or so. LC filter after the shunts. Simply adding capacitance is less than optimal for reasons already given.
Yes... the regulator has proper cooling (SAA7220 consumes about 180mA)
10000uF is a good value, but check for the ESR value!
The board could be used - however, I would not use the screw termials provided rather soldering leads to the board.
Also remember to keep the ground wire SHORT and to keep the board very close to the chip (preferably on top).
Also use ferrite beads on the supply in/out lines to stop any interference getting back to the supply. The SAA7220 chip feeds back shedloads of hash back into the power lines.
Kevin
Also remember to keep the ground wire SHORT and to keep the board very close to the chip (preferably on top).
Also use ferrite beads on the supply in/out lines to stop any interference getting back to the supply. The SAA7220 chip feeds back shedloads of hash back into the power lines.
Kevin
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Almost true ...
although QUAD used a variant of this board for the QUAD CD66
Kevin
These performed extremely well and I think, being a former ASD for Philips and Marantz before I retired, that there is little to gain from modifying the power supply.
Hi ads2,
go to; SKA sponsored Audio Forum - Products and go to minireg. These things work extremely well.
HTH,
Pete
go to; SKA sponsored Audio Forum - Products and go to minireg. These things work extremely well.
HTH,
Pete
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