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Arcam and the TDA1541

Discussion in 'd.i.y.' started by fatmarley, Dec 18, 2006.

  1. a.palfreyman

    a.palfreyman pfm Member

    Old thread I know but wanted to know how critical the 10Vdc between the -5Vdc and -15Vdc supplies to the TDA1541 is? My Philips CD160 has -5.6Vdc and -14.9Vdc so the difference is only 9.3Vdc. Both are relatively quiet with the -14.9 having 3mV saw-tooth (100Hz, despite a 7915 reg) and 3mm HF noise, whilst the -5.6 has 3mV "hump" (100Hz) and 3mV HF noise. Is the voltage value critical, or the relative noise, or both?
  2. Mike P

    Mike P pfm Member

    TBH I've not tried tuning my regs to achieve exactly 10v between the -5v and -15v supplies.

    Certainly it's worth trying a capacitor between the -5v and -15v pins of the TDA1541 (observe polarity!). The choice of cap used does make a difference here and I know Dowser has experimented with quite a few different caps in the past.

    I've used 10uF Oscons in the past and I'm not sure if I like the result or not, sometimes I prefer it and sometimes I don't. I really ought to try this mod again with a small film cap instead.
  3. martin clark

    martin clark pinko bodger

    The exact voltage isnt significant, it’s about keeping the noise across the nternal current source operating between the two supplies ‘quiet’.
  4. a.palfreyman

    a.palfreyman pfm Member

    Thanks Martin,
    That's what I suspected but wanted to check.
  5. martin clark

    martin clark pinko bodger

    There is one hidden 'gotcha' in trying that idea - you are tying together the outputs of two different regulators with a couplng cap - which brings potential for oscillation.

    Do not try it with anything faster than a standard 3-pin (LM317 or LM7905/7915) and make sure those regs have a fairly large, slightly lossy cap on their output (say 47-100uF electrolytics as suggested by others) and don't go large with the cap tying the pins together - 10uF max - just to keep things stable and quiet. Much better to look at these supply pins before-and-after with an oscilloscope if you have access, to make sure you haven't made things worse.

    One more thing - make sure these regs have a decent size local reservoir cap on their input supply pin and located at the reg, 100uF or more. It helps.
  6. a.palfreyman

    a.palfreyman pfm Member

    -15V has 220uF before and 47uF after, 7906 has 1000uF before and 47uF after. 47uFs are about 1 ohm ESR so should be OK. Both regs are at opposite end of board to TDA1541 so plenty of trace length between. I have some 2u2 film caps and some 4u7 tant beads as well as 4u7 'lytics so will see what I get on the 'scope.
  7. Mike P

    Mike P pfm Member

    Great post Martin, many thanks.

    I've had mixed results with this in the past using 10uF or 15uF Oscons but I hadn't really considered the instability issue and that might explain things.

    Other than capacitance value what are the desirable electrical characteristics for the capacitor used to tie the -5v and -15v supplies? IIRC Dowser prefers a smallish film cap.
  8. martin clark

    martin clark pinko bodger

    TBH I did this to my CD2 ten years ago and have not revisited! A small film cap could be good idea if stable; but the limit is loop area /layout anyway, so no point worrying too much.
  9. Dowser

    Dowser Learning to bodge again..

    Yes, 10uF Wima film here - but to be honest have never tried anything else I think...
  10. a.palfreyman

    a.palfreyman pfm Member

    The TDA1541 -15 supply is from a 7915 so I have changed the 10R+47uF to 22R+220uF low ESR and now have 1mV "hump" (100Hz) and 2mV HF noise.
    The -5 supply is from a 7906 so I have changed the 10R+47uF to 27R+100uF low ESR and now have 1mV "hump" (100Hz) and 2mV HF noise. I now have 9.6V between the two and as there are resistors isolating the 79xx outputs I went for a 10uF tant between the -15 and -5. Bl00dy hell, what a difference. Sounded over-etched at first but has slowly settled in. I now have a much more stable central image (which would move about before) and there is more focus / detail and the odd tonal balance (slightly sucked-out upper mids / prominent lower treble) is now much better. The stereo image was also slightly narrower straight after this mod but this also seems to be getting wider as things settle. Vast improvement! Thanks so much for this, I am very pleased with the results.
    I have a question about the SAA7220 but will do this on my CD160 thread.
    martin clark likes this.
  11. Mike P

    Mike P pfm Member

    Oops, sorry I thought it was you! I definitely remember reading about someone trying around 5 different capacitors types/values for this and giving listening impressions for each type.

    I've only tried 10uF and 15uF Oscons in this position. In my old Arcam Alpha 5 a 15uF Oscon from -15v to -5v was a clear and very worthwhile improvement but in my Sony 337 when I used 10uF Oscons I felt the sound became somehow hard and etched and I took them out.

    I must try experimenting with it again.
  12. martin clark

    martin clark pinko bodger

    @a.palfreyman - glad you find it worthwhile!

    I dare say you could drop the series resistors before the local caps as low as 1ohm without issues. I must dig-out the A5 manual (or the future-spares donor in the cupboard somewhere...) and look at how it is actually laid out inside...
  13. a.palfreyman

    a.palfreyman pfm Member

    I will 'scope both before and after the resistors to see what ripple there is but I chose these to get the 1V drop from -6V supply to give -4.9 and to get as close to 10V differential as possible (actually 9.6V) and to get the ripple / noise figures both lower and almost identical as the -15 had more saw-tooth ripple on it. I do appreciate the input from the knowledgeable people on this forum, as it can save a "bodger" like me countless hours of fiddling and getting nowhere. It also increases my (very limited) understanding. In fact despite having done the decoupling mod (5 off 0.22uF films and 2 off 1uF tants [on MSBs] each side, ALWSR on TDA +5V, decoupling WS and Data with 220R+10pF and clock with 100R) the most significant mod (by a very long chalk indeed) is the -5 and -15 supply mods.
  14. Joe

    Joe pfm Member

    Whats with the missing Fatmarley posts in this thread?
  15. Mike P

    Mike P pfm Member

    The tants on the MSB's will be colouring the sound quite a lot. My experience is that film caps on all 14 positions will sound quite a lot cleaner. If you have more on them in your parts box do try more 220nF films on the MSB's.

    The best I've used are 0.1uF surface mount PPS film.
  16. martin clark

    martin clark pinko bodger

    +1 to that, use film-type caps only.
    Electrolytics, inc tants , have tiny leakage currents, but given how the TDA1541x uses the filter pins, this becomes a source of non-linearity; all the lesser bits are derived-from the MSB internally.

    Even 100nF is enough (Naim used 100nF stacked-film types for all 14 pins, it's still an excellent choice; and I do agree with Mike's suggestion)
  17. martin clark

    martin clark pinko bodger

    @Joe - good point! FM always is worth listening -too :)
  18. a.palfreyman

    a.palfreyman pfm Member

    I will bear this in mind whilst "poking around inside" at the week-end. Only used tants as that is what Arcam use on the Delta70 etc. I did go to the trouble attempting to measure leakage current of the tants I used by charging from a stabilised 12Vdc supply via a 680kR with a high-impedance DVM (>20Mohm) and when the voltage got to 7mV across the 680kR (about an hour later) I gave up because that is 0.007/680,000=10nA and the voltage was still veeery slowly dropping indicating the leakage current must be less than that.
    Did you remove the caps that were there or just parallel them "on top"? Would be much neater to use SMD as I used some 5mm polybox caps which are bulky.
  19. Mike P

    Mike P pfm Member

    I know that many of the cheaper Philips based machines used surface mount ceramic bit decoupling caps on the underside of the PCB but all the machines I've modded have used through-hole parts.

    I've used surface mount PPS caps and soldered them to the underside of the PCB once the original through-hole parts have been removed. The ones I've used are 1913 size (4.8mm length) and so span across the solder pads for 5mm pitch through-hole parts nicely.


    I solder them on their side, partly because they fit more easily this way but also because I read somewhere that they actually perform slightly better that way too.

    This is an old pic of the underside of a Sony CDP-337ESD which shows them.

    [​IMG]20170612_153134 by Michael Pickwell, on Flickr

    I'd be inclined to remove the existing surface mount ceramics first rather than stack on top.
    337alant likes this.
  20. a.palfreyman

    a.palfreyman pfm Member

    Thanks again for your input, it is much appreciated.
    Found these on ebay:
    PANASONIC ECPU1C224MA5 CAP, FILM, PPS, 220NF, 16V, SMD 1206 which are same size as the ceramic chips on the circuit board as I wouldn't get away with anything much larger. Bought 20 off which leaves 3 for TDA bypass duties and 3 spare in case I lose / damage during initial trials. (Maybe I ought to buy an extra 5 on top of that ;))
    Also bought these:
    As i need something slightly smaller to replace one of the the 47pF 1206 caps on the crystal in order to move it as per the CD160 thread.

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