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-   -   Grounding dedicated PSU for SAA7220 (http://www.diyaudio.com/forums/digital-source/196373-grounding-dedicated-psu-saa7220.html)

josha 11th September 2011 09:44 AM

Grounding dedicated PSU for SAA7220
 
Hi all,

I'm modding a Philips CD160, so far I've bypassed the output stage and replaced the TDA15141 with a TDA1541A (the board had the required osc cap in place) and the difference is astounding.

Now I want to add a dedicated PSU for the SAA7220 (still the A, I'm waiting for a B). I've trawled through the threads and I understand the concept - I don't want to go over old ground here but I do have a question...

I know which resister to lift for the +5v supply but I don't know where to ground it, do I just ground the PSU anywhere on the board? Or do I lift the ground to the 7220 and ground it direct?

Any help very much appreciated....

Josh

oshifis 11th September 2011 12:12 PM

Simple: cut the track going to +5V (keep the bypass capacitor) and connect the +5V of the separate power supply to the SAA7220. Connect the GND of the separate power supply to the GND of the SAA7220.

josha 11th September 2011 12:16 PM

Excellent, many thanks!

josha 11th September 2011 04:47 PM

That worked, what a mod! This 8 cd player is starting to sound a lot more expensive ;o)

abraxalito 12th September 2011 02:51 AM

You got a CD160 for a mere 8 quid? Wow, I'm impressed. That was my first ever CD player, I think mine cost me around 200 quid in 1986 money. So you've bypassed (or removed) the LM833s and installed passive I/V?

josha 12th September 2011 05:16 AM

I got a bargain, I'd been watching a few on ebay, some of the players from this era are fetching silly money at the moment (at least for my pocket), this one was almost to good to mod, not a scratch on it, no faults and even came with the original guarantee document, user manual and the 'made in Belgium' tag attached!

It has the non-a 1541 as standard but I had a 1541A (R1) lying around from a past project so I put a socket in and tried that.

Next up was to lift the resister after the first op-amp stage and just run a wire through a cap 'donated' by a pair of old Eltex crossovers to some new sockets - a la lampizator.

I then built a 5v psu (only a 7805 at this stage) and fed this to SAA7220/A. I've got a 7220/B on order and this is supposed to bring a big improvement so that should be going in shortly.

The problem is that the 160 is a good 4-5" smaller than 'midi' size players and I can't figure out where to put the extra transformer! (It's outboard at the moment) I think a bigger case is in order!

Still, all in for under 30 including the 1541A and it's a real pleasure to listen too. Next up is some cap replacement and a PSU for the DAC itself.

Cheers

Josh

abraxalito 12th September 2011 05:49 AM

Quote:

Originally Posted by josha (Post 2706473)
Next up was to lift the resister after the first op-amp stage...

So you still have in circuit the original I/V stage built around the LM833? If so that'd be my first thing to chuck out, before exchanging digital filters.

josha 12th September 2011 07:36 PM

I do, in my excitement I almost forgot about that ;o) I'm catching up on another (excellent) thread on here about modding a 650. I have a choice I guess to either improve the op-amp (quick and easy), or go passive I/V, plus there is the option of discreet with/without tubes. I haven't quite figured out this side of things yet - plenty of reading still to do, the mods so far have at least some marked improvement.

abraxalito 13th September 2011 12:05 AM

Quote:

Originally Posted by josha (Post 2707208)
I have a choice I guess to either improve the op-amp (quick and easy), or go passive I/V, plus there is the option of discreet with/without tubes.

I have another suggestion to add to the melting pot of ideas. Leave the opamp in but reduce the value of the feedback cap around it. I think (my memory is poor) that its 2n2? Its a blue polypropylene axial. Anyway, reduce it to 220pF or even if you feel very daring, 47pF. Then see if you notice any difference.

Quote:

I haven't quite figured out this side of things yet - plenty of reading still to do, the mods so far have at least some marked improvement.
Yeah the literature out there is extensive and takes many hours to trawl through. Happy hunting!

AndrewGM 13th September 2011 12:58 AM

What a refreshing thread :)

I remember being at the beginning too and wondering what to do first.
I was unable to help myself and did loads all at once so I never really knew which tweak did the most.
The first mods ( like the ones your doing now ) seemed to have the biggest jumps in SQ.....so don't stop !!

I'm sort of willing you on here.....

Sorry but I've nothing to offer with regards to your machine but I have modded and use daily a phillips based player with 7220 and 1541A.
Your doing all the right things and if you keep going you'll end up with something pretty special.

Just one thing - there is a fantastic value clock from vintage audio lab on e bay ( which is red in colour if you look for it ) it's about 25 and works very well. It'll also give you multiple clock feeds of various useable freqencies ( 2.8,5.6 and 11.289 ) if you want to go mad. If you have a 1541A you can feed pin 2 and 4 with 5.6 and 11.289 for a very noticeable gain. Give it it's own small encapsulated transformer and it's quite excellent.

Good luck


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