DAC modifications

Discussion in 'DIY Discussion' started by elaar, Apr 4, 2006.

  1. elaar

    elaar

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    Hi all,

    I recently bought a Cambridge Audio DacMagic 3 and felt like doing a few mods. The first area i'm attacking is the power supply, rectifying and regulation.

    Basically, there's 3 transformers, each with 15-0-15, all 6 rails go through a discrete diode bridge (crude 1n4004s), have a 100nF decoupling cap between rail and 0v, 3300uF 25cap per rail, and then finally onto positive fixed regulators, all of the 78xx variety. The 6 diode bridges and general seperation show some proper thought into the design of this DAC but i feel it could do alot better.

    Diodes: I was thinking of replacing these with Schottky ones such as 11DQ10's, however i don't know much about diodes, would ultra-fast soft recovery ones be better? or are these schottkys a bit old now? I've also been told to use 0.01mF-0.1mF snubbing ceramic caps across each diode to prevent the hard switching, is this only really for harsher diode bridges?

    Capacitors: Current ones look pretty cheap with no name written on them, I was thinking of replacing them with something like Nichicon HEs 5600mF or United Chemicon KYs, both aren't known for their audiophile quality but are very good caps for the price (I need 10 of them!) and have impedances of 0.012 (at ...) This sound okay?

    Is there anything i should think about with the ceramic decoupling caps? they're just general cheapo ceramic ones at the moment.

    Lastly, not sure about this, but the regulators are all the 4% regulation type, and i could be slightly anal and replace them with the 2% ones, there are great deal of them though


    I'm a beginner when it comes to power supplies, having read relatively few power supply tutorials and would greatly appreciate any advice you could offer.

    Thanks,
    Andy
     
    elaar, Apr 4, 2006
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  2. elaar

    zanash

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    The 11dq10's did a great job in mine......

    Things you need to be careful of

    don't use a a solder sucker use wick ....

    the holes are extremely narrow and some standard components need to have there leads skimmed.

    This was the down fall of my unit ...I broke the pcb tracks in the end.

    Other areas ...opamps there ne5532 if my memory serves these can be a bit pedestrian to say the least ...try opa604 series or the 132 series as a miimum

    The resistors are bog standard and therefore in the output stages use the best you can afford ...vishay bulk foils etc.

    Yes I go for the better components.......

    Don't forget to bypass all the electrolytics even if you have replaced with better units ....

    remember two caps are better than one and four caps are better than two.....keeping the same voltage.

    You can build a vero board module of say 10 x100uf 1 x10uf 1x 1uf to replace a single 1000uf... the extra speed and finesse will be a suprise [well it was for me ]

    if the caps are 200000uf......
    use 10 x 200uf 1 x 10uf 1 x 1uf .

    These gets large but as I recall there's loads of space in the dac 3


    Don't forget to re wire with silver or better on the output sockets...
     
    zanash, Apr 5, 2006
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  3. elaar

    grif

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    Funny old world! I'm also about to mod a Dacmagic3. I bought it on ebay about 6 months ago just out of curiosity. Only paid £25 for it, but it turned out to be way better than I expected so decided to keep it and throw a bit of cash at it.

    I've got all me bits together...just need to get round to doing it now:rolleyes:

    I've gone for 11dq10's and Panasonic FC's for the power caps and Oscons for everything else.

    The output OPamps are AD712's which are suposed to be pretty good, but I've got a few to try in there...OPA2134, OPA2107 and AD825's which I read somewhere are suposed to be excellent here. The 825's are singles though so I've got some excellent adaptor pcb's from David White of WNA....one 825 on top, one underneath and Robert is your Dad's bro :cool:

    I'll be putting sockets in to make it easy to play with various OPamps, including the 5532's.

    zanash...why do you say not to use a sucker, the pads don't come up that easily do they??:eek:
     
    grif, Apr 5, 2006
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  4. elaar

    zanash

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    The word nightmare and oh sh*t not again......

    Its was possibly the most difficult pcb I've worked on ....it could have been just mine but I don't think so.

    The holes are very small so fitting the diodes with fat leads more than once pushed the pads off the pcb. I now use solder wick as it tends to leave a cleaner pad to work on.

    The opamp sockets were good copper solid pin types not the cheap ones and it was as if the holes were not in the eact possitions....a total pain.

    In the end I did one mod to many and it really was not worth the effort to rebuild the damaged tracks by hard wireing.....I'd move on to a better dac by then anyway.

    The sound is very good in deed for the cost.......especially after modding.

    HIFI News did an article about modding this dac ....by ben duncan I think if you can find a copy its well worth the read.
     
    zanash, Apr 6, 2006
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  5. elaar

    MattC

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    see here


    cheers
    Matt
     
    MattC, Apr 6, 2006
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  6. elaar

    grif

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    Oh crud:cry: If it gets too much of a pain I'll get my club hammer out and persuade it :)

    Thanks for posting the link MattC.
     
    grif, Apr 6, 2006
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  7. elaar

    zanash

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    Its not impossible just takes a little thought ...I ended up thinning the leads with a stanley blade...cleaned them up to.
     
    zanash, Apr 6, 2006
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  8. elaar

    elaar

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    Hi Zanash,

    Many thanks for the reply, thanks for the pcb warnings i'll bare them in mind.

    I asked this on another forum with similar replies. I was told to not increase the capacitance of the main reservoir caps due to it slowing things down.
    This makes sense, but you brought up an excellent idea of making a daughter board to take alot of smaller capacitors.
    As the original ones are 3300 uF, I thought a bank of 4 x 1000uF would be best, I don't think i can cope with anymore than this unless you really do think
    some improvement will be heard (48 caps!). So does this
    seem okay on the daughter board: 4x1000uF, 1x10uF and 1x1uF (following your smaller bypass caps advice) all in parallel per rail?

    I definitely will sort out that cheap wire on the output sockets, and although I was going to leave other areas for another time I agree with you that it's worth sorting out those opamps, so I was going put in some 8-way sockets and try some opamps, I have some spare OPA627s which hopefully should sound great in this DAC.

    Did you look into the quality of the transformers when you were modding? People have suggested maybe going for overated toroidals, but from the looks of them alone they look reasonable and a fairly good size considering they're only doing about 20-30VA each.

    Nice price you got that for grif, I paid about £80 i think.
    One other thing, people like to use the Panasonic FCs, I was looking at different caps and on paper the Panasonic FMs looked a bit better, basically the FCs but with a lower impedance, does the FC have a higher slew rate or something else that I haven't noticed? Has anyone got any knowledge of how
    much a good quality 1000uF capacitor slewrate would compare to a 3300uF capacitor of the same make? they never seem to mention any speed type specs.

    Thanks again,
    Andy
     
    elaar, Apr 7, 2006
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  9. elaar

    mosfet

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    From the horses mouth, so to speak. John Westlake, he who designed the DacMagic, commenting on DacMagic modifications. The article referred to is the pdf linked to above on this thread. You could try posting a question on diyAudio.com, Andy, Westlake posts regularly under the pseudonym JohnW.


    In the ending “credits†of the article, Andrew Harrison thanks me – unfortunately, I've never met or spoken with Andrew Harrison – not to mention know anything about the article before it was published.

    Changing the electrolytics to better quality types such as OSCON's will always help, but don't waste money changing C115, the 100n SPDIF input Cap to Polypropylene as recommended in the article.

    Unfortunately, the designs biggest weakness is the TDA1315 – the SPDIF input receiver. The TDA1315 has very poor jitter performance – despite claims otherwise from Philips, by the time this was realised; the front panels and casework had already been manufactured (the front panel has LED's for sampling rate and copy protection). Changing the design to the better performing CS8412 was not an option, as the CS8412 has no direct method to drive the front panel sampling rate LED's – and the budget was already very limited (original retail price was £149).

    If possible, modifying the unit to “clock-lock†with the transport will bring enormous improvements – then the quality of the Dac becomes the limiting factor.

    The first pre-production version of the design used Burr Brown devices (I seem to recall the PCM1710), I much preferred the sound of these DAC's over the TDA1305's, however the Burr brown DAC's had a STUPID design fault that prevented them being used in dual differential mode.

    The Burr Brown devices internal mute circuit was designed to detect all zeros – and not “change of dataâ€Â. During “Mute†in dual differential mode, one Dac receives “all zeros†– and the second Dac “all onesâ€Â. This meant one side would go into Mute – while the other side still operated – resulting in DC offset on the outputs during “Muteâ€Â. As a result was that I was forced into using the TDA1305 as there where no other good performing “budget†DAC's on the market at the time. Still many liked the sound of the TDA1305 – but Naim using it their CD3.5 is really stretching it way too far – these are BUDGET DAC's – limited by there internal digital filtering. They have a very muddled Bass heavy sound, with poor sound stage – possibly, why Naim selected them?

    My best DAC design for Cambridge Audio was the HDCD ISOmagic – if you can get hold of one, then it's a unit worthy of modifications!

    If you have an ISOmagic or CD4SE - don't let go of them - or you will find yourself regretting it later - just ask those who have...

    John Westlake
     
    mosfet, Apr 7, 2006
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  10. elaar

    zanash

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    As to cap types ...I don't like blackgates...they take for ever to burn in and then when left off you need to start again so I'd suggest stargets as an honest alternative. If you build the daughter board you could use loads of cheap 100uf caps....
    you can often pick up bags of caps on ebay cheap !

    You could increase to 25% greater capacitance but no more imo
    better to stick within the 5% tolerance of the caps with the bypassing.

    Not certain I agree with the c115 mod why strangle the signal input with a cheap cap ..its nonsensical.

    But most of the other comments are well founded

    Modding is a very much suck it and see ...loads of people will give advise but its only your ears that are going to listen ....if it sounds right to you go for it !
     
    zanash, Apr 7, 2006
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  11. elaar

    grif

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    Well...having read mosfet's post I wondered if I should bother :D but I've done the diodes, power caps and half a dozen oscons and the difference is HUGE already, even with no burn in, very pleased and well worth the effort!! Looking forward to getting the rest of the mods done now, especially the op amps.

    Locking the clock to the transport, as John Westlake says, is clearly a good idea, but for me its a bridge too far for this unit. Hifi collective are selling the XO3 dac clock for £185..thats about £60 more than I will have spent on the whole dac inc mods...too much.

    I think the answer is to feed it as good a signal as you can in the first place and not worry. I'm feeding it bit perfect flacs from fubar through a reasonable sound card's coax spdif, ripped to HD using EAC, and outputting to Bluesky's. Having compared this to an NS900 which is suposed to have very good jitter specs, the computer was better.

    I can confirm the board is a bit of a pig to work on, but I've not had to do any 'lead shaving' as yet.

    elaar...I think FM's are better than FC's but RS did'nt have any when I was ordering, and I was working off the principle that pretty much anything would be better than the crap thats in there:D Judging by what I'm hearing as I type, I was right:)
     
    grif, Apr 9, 2006
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  12. elaar

    zanash

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    You must have been lucky with your component choice or your pcb may have been made with bigger holes !

    I'm afraid mosfet rarely has anything of a positive nature to impart....and would caution you to trust your own ears rather than someone elses personal opinions, how ever well meant [that includes mine too] !
     
    zanash, Apr 9, 2006
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  13. elaar

    mosfet

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    Reading your audiophool nutbar-isms always keeps me amused zanash. Keep up the good work. :D
     
    mosfet, Apr 9, 2006
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  14. elaar

    wadia-miester Mighty Rearranger

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    Pete,

    Although you may disagree with Mosfet, he's a quite a wisenden old coot. IMO would run rings around about 95% of posters on here IMO
    Oh he things my ramblings are just as whacking too!!!!!
     
    wadia-miester, Apr 9, 2006
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  15. elaar

    zanash

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    If you say so .....evidence is thin on the ground to prove otherwise
     
    zanash, Apr 9, 2006
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  16. elaar

    wadia-miester Mighty Rearranger

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    Pete,

    There are only around 15 posters on this forum that actually Know what they are talking about when it comes to the design, implimenation and construction of circuits,rest just play at it.IMO
     
    wadia-miester, Apr 9, 2006
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  17. elaar

    zanash

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    I agree ....

    So when a question is asked, its down to them to contribute not "naysay" and when they don't contribute anything meaningful its down to the rest of us mere mortals to pick up the batton and run with it.

    If we all had to wait for sage old coots to provide meaningful and usefull answers, we would know doubt be here a long long time.....
     
    zanash, Apr 9, 2006
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  18. elaar

    leo

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    Its actually a shame theres hardly any diy posts on ZG, I must admit its the main reason I don't come on here that much:(
    Its nice to be able to share idea's etc in this fun hobby
     
    leo, Apr 9, 2006
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  19. elaar

    wadia-miester Mighty Rearranger

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    Leo,
    I believe that most guys are willing to share quite a few titbits, though the there are some things that maybe they don't
     
    wadia-miester, Apr 9, 2006
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  20. elaar

    leo

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    Tone,
    Thats to be expected especially from those who do this for a living but I meant it would be nice if there was more posts maybe showing a few pictures and just a few details from the work or projects
     
    leo, Apr 10, 2006
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