Modding a GDA-700 DAC

ben62670
ben62670 Posts: 15,969
edited May 2010 in DIY, Mods & Tweaks
Well here it is. I am only interested in modding the analog out stage, and the hoards of 10uf/35v caps.
DSCN1152.jpg?t=1218809461
Thanks guys

Ben
Please. Please contact me a ben62670 @ yahoo.com. Make sure to include who you are, and you are from Polk so I don't delete your email. Also I am now physically unable to work on any projects. If you need help let these guys know. There are many people who will help if you let them know where you are.
Thanks
Ben
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  • fbm211
    fbm211 Posts: 1,488
    edited August 2008
    I would go with the rubycon caps.1.50 each...Link>> http://www.referenceaudiomods.com/Merchant2/merchant.mvc?Screen=CTGY&Store_Code=RAM&Category_Code=RUBY
    SDA-2BTL with custom IC
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    4 Furutech FP-314Ag with FI-11cu Plugs/FI-11AG IECs--- Power Cords
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  • Face
    Face Posts: 14,340
    edited August 2008
    Mundorf S/O. :)
    "He who fights with monsters should look to it that he himself does not become a monster. And when you gaze long into an abyss the abyss also gazes into you." Friedrich Nietzsche
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  • ben62670
    ben62670 Posts: 15,969
    edited August 2008
    You see all those little caps like c501-521. I don't get all those being in the signal path? Is it the opamp choice that dictates all those caps?
    Ben
    Please. Please contact me a ben62670 @ yahoo.com. Make sure to include who you are, and you are from Polk so I don't delete your email. Also I am now physically unable to work on any projects. If you need help let these guys know. There are many people who will help if you let them know where you are.
    Thanks
    Ben
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  • zingo
    zingo Posts: 11,258
    edited August 2008
    Face wrote: »
    Mundorf S/O. :)

    I'd like to see you try to fit those. :D Plus, the cost of your DAC would soar astronomically...
    Jake
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  • Systems
    Systems Posts: 14,873
    edited August 2008
    ben62670 wrote: »
    You see all those little caps like c501-521. I don't get all those being in the signal path?
    Its hard to tell from the pics you will have to meter through and see if they are in the signal path.
    C518-521 could be part of the low pass filter section ,C504-505 appear to be feedback caps for the inverters U500-and U501.
    These 2 op amp inverters produce the neg half of the balanced signal.The rest of the little ceramics are likey for bleeding off RF and therefore should not be in the signal path.

    Replacing those 10uf's with a parallel combo of .1uf Panasonic stacked film caps and Panasonic 120uf FC series low impedance electrolyics should work great.
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  • ben62670
    ben62670 Posts: 15,969
    edited August 2008
    Its a pia to remove the board. Not too bad, but still... The OpAmps have Adcom P#ers on them. I would like to know what they are. I should check the rails, and see what kinda voltage there is on the OpAmps. The unit is about 10 years old. C514/515 are in the signal path I am thinking about bypassing these.
    Please. Please contact me a ben62670 @ yahoo.com. Make sure to include who you are, and you are from Polk so I don't delete your email. Also I am now physically unable to work on any projects. If you need help let these guys know. There are many people who will help if you let them know where you are.
    Thanks
    Ben
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  • Systems
    Systems Posts: 14,873
    edited August 2008
    ben62670 wrote: »
    C514/515 are in the signal path...
    Are you certain??If the output where to be coupled through those little ceramics bass response would dearly suffer.
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  • ben62670
    ben62670 Posts: 15,969
    edited August 2008
    It ties directly to the output. I wish I had xray vision to see under the board:)
    Please. Please contact me a ben62670 @ yahoo.com. Make sure to include who you are, and you are from Polk so I don't delete your email. Also I am now physically unable to work on any projects. If you need help let these guys know. There are many people who will help if you let them know where you are.
    Thanks
    Ben
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  • ben62670
    ben62670 Posts: 15,969
    edited August 2008
    I am going to have to operate:(

    Edit
    There are no other caps after the OpAmps. I would asume that there would be some sort of DC blocking cap. No?

    Edit Edit: The only other caps(cans) are all 10uf 25v on the OpAmps which I would asume are just power caps?
    Please. Please contact me a ben62670 @ yahoo.com. Make sure to include who you are, and you are from Polk so I don't delete your email. Also I am now physically unable to work on any projects. If you need help let these guys know. There are many people who will help if you let them know where you are.
    Thanks
    Ben
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  • Systems
    Systems Posts: 14,873
    edited August 2008
    ben62670 wrote: »
    It ties directly to the output.
    meter the other side of it and see if there is continuity to the ground on the RCA connector.Im guessing there will be.
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  • ben62670
    ben62670 Posts: 15,969
    edited August 2008
    GV#27 wrote: »
    meter the other side of it and see if there is continuity to the ground on the RCA connector.Im guessing there will be.

    Gotcha. I should have figured that one out:o
    Ben
    Please. Please contact me a ben62670 @ yahoo.com. Make sure to include who you are, and you are from Polk so I don't delete your email. Also I am now physically unable to work on any projects. If you need help let these guys know. There are many people who will help if you let them know where you are.
    Thanks
    Ben
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  • Systems
    Systems Posts: 14,873
    edited August 2008
    ben62670 wrote: »


    There are no other caps after the OpAmps. I would asume that there would be some sort of DC blocking cap. No?
    Not necessarily they may have directly coupled it which is a good thing or used a servo but I don't see the servo circuit it the pic so I doubt they use one..
    The only other caps(cans) are all 10uf 25v on the OpAmps which I would asume are just power caps?
    Yes they are the local power supply decoupling caps.
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  • ben62670
    ben62670 Posts: 15,969
    edited August 2008
    I am going to check right now! My neighbors can use a break anyway;)
    Please. Please contact me a ben62670 @ yahoo.com. Make sure to include who you are, and you are from Polk so I don't delete your email. Also I am now physically unable to work on any projects. If you need help let these guys know. There are many people who will help if you let them know where you are.
    Thanks
    Ben
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  • ben62670
    ben62670 Posts: 15,969
    edited August 2008
    It has to be servo controlled. The caps after the OpAmps are all tied to ground:)
    DSCN1161.jpg?t=1218837878
    DSCN1158.jpg?t=1218837918
    What U502 is for I have no idea?
    Please. Please contact me a ben62670 @ yahoo.com. Make sure to include who you are, and you are from Polk so I don't delete your email. Also I am now physically unable to work on any projects. If you need help let these guys know. There are many people who will help if you let them know where you are.
    Thanks
    Ben
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  • Systems
    Systems Posts: 14,873
    edited August 2008
    ben62670 wrote: »
    The caps after the OpAmps are all tied to ground:)
    Yep figured as much.
    What U502 is for I have no idea?
    might be for the servo.;)
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  • ben62670
    ben62670 Posts: 15,969
    edited August 2008
    16.5vdc +- rails. Pins 4, and 7. OPA134? I'll have to check:)
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    Thanks
    Ben
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  • ben62670
    ben62670 Posts: 15,969
    edited August 2008
    Just checked, and the BB/TI OPA134 would drop in. I am not sure if this would be the best OpAmp for the job though. AD seems to be pretty popular.
    Ben
    Please. Please contact me a ben62670 @ yahoo.com. Make sure to include who you are, and you are from Polk so I don't delete your email. Also I am now physically unable to work on any projects. If you need help let these guys know. There are many people who will help if you let them know where you are.
    Thanks
    Ben
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  • Systems
    Systems Posts: 14,873
    edited August 2008
    ben62670 wrote: »
    Just checked, and the BB/TI OPA134 would drop in. I am not sure if this would be the best OpAmp for the job though. AD seems to be pretty popular.
    Ben
    Maybe the mucho pricey OPA627?The AD825 is a good'in but needs an adaptor to fit in the DIP footprint as its only available in SOIC.The stock chip might be a re badged OPA604 or 134?
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  • ben62670
    ben62670 Posts: 15,969
    edited August 2008
    Ouch $18 each(OPA627) times many! Does a SOIC have the same dimensions as a Dip. Could it be soldered to the dip holes?
    Please. Please contact me a ben62670 @ yahoo.com. Make sure to include who you are, and you are from Polk so I don't delete your email. Also I am now physically unable to work on any projects. If you need help let these guys know. There are many people who will help if you let them know where you are.
    Thanks
    Ben
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  • Systems
    Systems Posts: 14,873
    edited August 2008
    ben62670 wrote: »
    Could it be soldered to the dip holes?
    It wouldnt be pretty.:eek:You need one of these Aries adaptors to do it right.see Digi key cat for pricing.http://arieselec.com/products/18010.pdf$18 isn't too bad for the OPA627.I can remember them selling for much more than that.
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  • ben62670
    ben62670 Posts: 15,969
    edited August 2008
    Well it would only be a little over $100 to do them all. I wish I could hear the difference ahead of time.
    Please. Please contact me a ben62670 @ yahoo.com. Make sure to include who you are, and you are from Polk so I don't delete your email. Also I am now physically unable to work on any projects. If you need help let these guys know. There are many people who will help if you let them know where you are.
    Thanks
    Ben
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  • ben62670
    ben62670 Posts: 15,969
    edited August 2008
    Dynamic range is only 98db. This is one spec that I do pay attention to.
    http://www.adcom.com/manuals.aspx
    Both the 600, and 700 are direct coupled;)
    Also the thing was designed in 1995. OpAmps have come a long way since then.
    Ben
    Please. Please contact me a ben62670 @ yahoo.com. Make sure to include who you are, and you are from Polk so I don't delete your email. Also I am now physically unable to work on any projects. If you need help let these guys know. There are many people who will help if you let them know where you are.
    Thanks
    Ben
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  • Systems
    Systems Posts: 14,873
    edited August 2008
    I should clarify that my suggestion of using thev 120 uf and .1 uf combo is for use on the power pins of the op amps only.The caps surrounding the PCM1702 DAC chip if replaced should be with ones of the same value.
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  • ben62670
    ben62670 Posts: 15,969
    edited August 2008
    Isn't 120 a bit large?
    Please. Please contact me a ben62670 @ yahoo.com. Make sure to include who you are, and you are from Polk so I don't delete your email. Also I am now physically unable to work on any projects. If you need help let these guys know. There are many people who will help if you let them know where you are.
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    Ben
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  • ben62670
    ben62670 Posts: 15,969
    edited August 2008
    Nevermind 6.3mm x 15mm. Tight, but they will go.
    Please. Please contact me a ben62670 @ yahoo.com. Make sure to include who you are, and you are from Polk so I don't delete your email. Also I am now physically unable to work on any projects. If you need help let these guys know. There are many people who will help if you let them know where you are.
    Thanks
    Ben
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  • Systems
    Systems Posts: 14,873
    edited August 2008
    ben62670 wrote: »
    Isn't 120 a bit large?
    The advantage of having the larger capacitance means there will be a bigger reservoir of juice right close to the chip.Jung and others recommend doing this as it aids in the reproduction of dynamics.


    edit.OOOhh you mean physically large not electrically.The Panasonics are usually fairly compact for there capacitance.
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  • maniac
    maniac Posts: 7
    edited September 2008
    ben62670 wrote: »
    I am going to have to operate:(

    Edit
    There are no other caps after the OpAmps. I would asume that there would be some sort of DC blocking cap. No?

    Edit Edit: The only other caps(cans) are all 10uf 25v on the OpAmps which I would asume are just power caps?

    The 10uF ely caps, if you seem them in the configuration of 2 per OpAmp, they are most likely for power decoupling. (Well, sensibly designed circuit anyways) As this DAC seems to be balanced in design, DC bias is automatically canceled out by simple OP-Amp circuits, no need for the coupling caps to be in the way of analog signals.

    As for modding, personally I'd just dump Silmic or other high quality ely caps on there, with similar capacity. But stay clear of the OS-CONs and other ultra-fast ely caps, they are better for digital sections, and may sound tinny and dry when used in analog sections.

    I personally won't go too large in capacity as they either trade performance for the physical size to fit in the room of the 10uF, or they are just going to be way too big to fit properly. IMHO, when given a choice of make it bigger(capacity wise) or make it better on an cap, make it better usually results in better results.
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  • Systems
    Systems Posts: 14,873
    edited September 2008
    maniac wrote: »
    As this DAC seems to be balanced in design,
    While internally the DAC is in 2 balanced segments the actual current output of the PCM1702 is single ended not balanced (or differential).
    DC bias is automatically canceled out by simple OP-Amp circuits.
    Actually DC is nulled by a capacitor attached to pin 10 of the DAC. There are no op amp based servo circuits.
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  • maniac
    maniac Posts: 7
    edited September 2008
    GV#27 wrote: »
    While internally the DAC is in 2 balanced segments the actual current output of the PCM1702 is single ended not balanced (or differential). Actually DC is nulled by a capacitor attached to pin 10 of the DAC. There are no op amp based servo circuits.

    Ah, I stand corrected, I was thinking about another DAC chip with balanced output.

    thanks.
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  • Systems
    Systems Posts: 14,873
    edited September 2008
    maniac wrote: »
    I was thinking about another DAC chip with balanced output.
    Yeah most of the newer BB DAC's like the PCM1791,1792,1738 do have balanced outputs.:)
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