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Thread: Experiments of the DAC kind

  1. #1
    Join Date: Feb 2008

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    I'm Neal.

    Default Experiments of the DAC kind

    Back in 1987 I think it was I purchased my first ‘good’ CD player an original Arcam Alpha. A few years later I upgraded it to ‘Plus’ status which involved the swap out of the DAC board for the Alpha Plus version.

    Since then the original board has sat in a draw for over ten years, as it’s based on the TDA1541A I thought it was about time I did something with it! As I’ve been modifying Stan’s excellent 7510 over the last few weeks I thought that the Arcam board would make an interesting comparison and project…if it still worked.



    With the help of Leo (without his valuable advice I’d still be hunting around for solutions) and a rummage through my spares box I drew up an idea to get the thing running. I needed a power supply and a SPDIF receiver chip. The Arcam board accepts I2S directly in the correct format so Leo recommended a CS8412, one of which I already had for an earlier DAC project that never got of the ground.

    I knocked up a stripboard based cct just to see if I could get it to work, sourced a transformer from Farnell and a case for mounting everything in. As the Arcam board had its own PSU I just stole volts of it to power the CS8412.
    You could have knocked me down with a feather. It worked! Music from both channels and nothing obviously wrong, I let it run for about 24hrs before taking a critical listen. It brought back memories, like wearing an old familiar pair of shoes. The sound was warm and cuddly, lacking any real HF extension and having a deep but wooly bass. So I set out on an upgrade program.

    First I upgraded the TDA decoupling caps junking the old Rubicon caps and ceramics for some OSCONs. I replaced the opamp output caps with some FK BlackGates and also replaced the I/V ‘lytics with some Nichicon FG caps. This lifted the sound a little adding some clarity and separation.

    Next I replaced the NE5534 opamps with some LME49710 opamps and the sound took on a whole new dimension, I was getting somewhere. Next up I replaced all the regulator output bypass caps with Starget ROD caps and took a leaf out of Guido Tents decoupling article and used a ferrite bead in the -15v supply line replacing the installed inductor.

    The -5v reg for the TDA had no decoupling cap and yet the ‘Plus’ board did so I added one. All this helped but I still felt the top end was somewhat veiled and I struggled to make out some low level detail that Stan’s DAC clearly highlighted.

    The next change involved supplying the CS8412 with its own regulated digital and analogue +5v supplies. At the same time I changed the diodes on the Alpha board for some Schottkys and replaced the old Rubicon smoothing caps with Panasonic NHG’s. Bingo! These last changes lifted the veil and changed the depth and texture of the sound for the better. The sound has become much more detailed and clear, little things are now easily made out and overall it’s much more engaging.

    Comparing to the modified 7510 is an interesting exercise, the 7510 has a lighter, leaner and more airy balance, the TDA1541 is richer and more textured in its presentation, the bass is a tad deeper as well, the overall impression is of a darker sound and yet with all the detail still apparent.

    It’s really enjoyable to listen to over a long period of time and makes me want to see how much more I can get from it. I can really appreciate the high praise the TDA1541A commands from its aficionados. The next stage would involve high quality super regulators for the TDA1541 or maybe pre-tracking regs ahead of the LM317/337 which would come in at a lower cost. For now I’m just going to enjoy it!

    Pics:


    CS8412 board:


    Modified Arcam board:


    Last edited by NRG; 26-02-2009 at 10:46.

  2. #2
    leo's Avatar
    leo is offline Circuit Junkie & DIY Room Forum Leader
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    Good fun isn't it nicework though Neal, I'm impressed

    Hey your not tempted to try passive I/V into a tubed output stage next then

  3. #3
    Join Date: Apr 2008

    Location: Cheltenham U.K

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    I'm John.

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    Quote Originally Posted by leo View Post
    Good fun isn't it nicework though Neal, I'm impressed

    Hey your not tempted to try passive I/V into a tubed output stage next then
    Or a discrete solid state active I/V?
    Btw, is this running in nos mode?

  4. #4
    Join Date: Feb 2008

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    I'm Neal.

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    Yes NOS...

    Don't start me off Leo! I'm happy as it is.....for now.

  5. #5
    Join Date: Feb 2008

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    I just checked the pics of the 'plus' board currently installed in the player and can see where I made mistakes in modding it many years ago...think I'll apply some of the mods I've recently learnt to it and see how it sounds.

  6. #6
    Join Date: Apr 2008

    Location: New Brighton

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    I'm Tony.

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    Nice work Neal and a good write-up!

  7. #7
    Join Date: Feb 2008

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    I've been tweaking and experimenting and made some improvements to the regulator ccts for the CS8412 and also improved the isolation between supplys for the TDA. I went backwards at one stage but retraced my steps and went in a better direction. The sound has moved ahead, the top end is clearer and more open than previously and the midrange has also improved... the sound is very analogue like and has moved ahead of my modified B7510.

    I'll live with this for a while (like a day or two!) and then start to implement my plan of pretracking regulators for all supplys.

  8. #8
    leo's Avatar
    leo is offline Circuit Junkie & DIY Room Forum Leader
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    Not bad eh Neal, amazing how good commercial units can be improved with a bit of work isn't it

    Anyway I'm pleased to hear yor getting such good results!
    I've still yet to hear ANY Delta Sigma based dac beat a well thought out classic multibit based one like the TDA1541A

  9. #9
    Join Date: Feb 2008

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    I'm Neal.

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    Sound continues to improved. Latest additions are a lower noise configuration for the CS8412 regs with the use of two standard green LEDs for the set resistor (R2). This gives a more presice voltage with lower noise and lower o/p impedence.

    I've also place a simple gyrator or capacitior multiplier ahead of the CS regs, borrowed the idea from the PFM flea cct...not sure about this one though, could be me but I think it has taken some speed and impact from the sound...needs more listening...as I intened to use one ahead of each regulator stage.

    Also added a rough pretracking regulator cct with 'remote sense' for the -15v TDA supply and this looks good on the 'scope compared to before. No gyrator at this time.

    Next will be pretracking regs for the -5 and +5 TDA feeds and also pre regs for the CS supply. This will leave the on-board + and -15v regs to supply the opamp and IV cct only.


    Pics:




  10. #10
    Join Date: Mar 2009

    Location: london

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    have you any links to the circuits for these regulators please?

    I'm proposing using LM317/337 for +/-15V to power 18 OPA134s in an active crossover so any improvements would be interesting, thanks

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