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Old 30th June 2010, 08:12 PM   #1
rongon is offline rongon  United States
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Default Old CD player mod to vacuum tube output?

I have an old Magnavox CDB502 CD player I'm thinking I'd like to learn on. It doesn't have a S/PDIF Out, so I was thinking I'd like to do what I can to improve its power supplies, etc. But most of all I'd like to replace the output stage with a vacuum tube.

These old Philips players often use the old TDA1543 or similar DAC chips, right? I've read that these are all the rage these days, so that would be fun to try. I've also read that taking the signal off the current outputs (pre-I/V conversion) is the best place from which to feed the tube.

Now I'm not no expert, see... I just know enough to be dangerous. So go easy on me, OK?

First question --

What's the basic topology for an I/V stage using a triode?

There is an I+ and an I- out for each channel, correct? But the output is balanced, so we need to make it single-ended. So a long-tailed pair to a cathode follower should work. Does this look right?

Click the image to open in full size.

If that's OK, is there a simpler way to do it? Does the first tube stage need to be balanced as well, or can this be done by one single-ended triode stage? Maybe I could take the non-inverting output off the LTP stage, if the output impedance is low enough (this would feed a tube line stage with roughly 100k ohm input impedance).

Last thing... How to figure resistor-to-ground values for the DAC + and - current outputs?

Thanks.
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Old 1st July 2010, 12:33 PM   #2
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The 502 makes a great transport, even if it's quite an odd duck. It uses a Motorola 50423 chip which does have an SPDIF output, it's just not used. If you would like to try it as a transport I can help, I have done two.

Best, Bill
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Old 1st July 2010, 12:43 PM   #3
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The 1543 output is single ended current out. Some like it, I don't, but YMMV.
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Old 6th July 2010, 05:35 PM   #4
rongon is offline rongon  United States
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Quote:
Originally Posted by Bill Fuss View Post
The 502 makes a great transport, even if it's quite an odd duck. It uses a Motorola 50423 chip which does have an SPDIF output, it's just not used. If you would like to try it as a transport I can help, I have done two.

Best, Bill
Thanks Bill, I'd like to add a S/PDIF Out to it. Doesn't hurt anything, I assume. Do you add a transformer, or do you do it more simply? 75-ohm load resistors?

My main focus is to use this player as my first "CD mod". I was thinking of taking the current outputs from the DAC chip and using a tube as the I/V converter and output stage. Some gain will be needed, so should work out.
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Old 6th July 2010, 05:54 PM   #5
rongon is offline rongon  United States
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Quote:
Originally Posted by Bill Fuss View Post
The 1543 output is single ended current out. Some like it, I don't, but YMMV.
I always liked the sound of my old Magnavox CDB-473, which used a TDA1541A. It died a few years ago and I didn't think much of throwing it out. Maybe I should have tried harder to resuscitate it. But who knew it would be considered worth keeping five years later?

I bought this CDB-502 cheap, to keep in reserve. With all this talk about the magical sound of the TDA1541A, I figured I'd like to try doing something to this player and see if I like the sound. Why not, right?

I opened the unit up the other night. Under the top cover, there are two circuit boards screwed together, with their undersides showing. I guess you're supposed to unscrew the long bolts holding the pcb's to the chassis and they'll drop away?

Also, the lampizator dude (Fikus) claims that the biggest improvement to these players is to replace the rows of 100nF ceramic decoupling caps that surround the DAC with MKP 220nF 50V. Maybe I could just start there and see what I think...

[edit] -- Just grokked the fact that the TDA1543 is single-ended current out. That's fine. I'd still give it a try. I could use something like a 6DJ8 common-cathode stage after the current outs.

Any idea what shunt resistor to use from the current outputs? Any harm in using a very small value, maybe 33 ohms? I'm thinking of keeping the output from the DAC low enough that the 6DJ8 won't send too high an output level into the preamp... I could also use a 5687, but that requires a lot of heater and B+ current.

Last edited by rongon; 6th July 2010 at 06:01 PM.
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Old 6th July 2010, 09:04 PM   #6
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I've never messed with tube I/V circuits so I can't help there. I tried the DD1543 parallel dac scheme and was underwhelmed, different strokes for different folks I guess.

Email me and I'll send you what I have for adding an SPDIF output.
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Old 19th April 2011, 11:50 PM   #7
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Default Mods to cdb 502

Could someone send me the information on adding the SPDIF output on a Magnavox cdb 502. Are there other mods that can be done to improve the quality of the audio section of the 502?
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