Next project: Adcom GDA-600

Well, you can sell them on eBay and recover some of the costs. I know that they will be people interested in them... just allow shipping in US too.
 
Time to raise a thread from the dead...

20160323002700-ec48c6cf.jpg


I just picked up a GDA-600 for a song (ha!) and went through most (but not all) of the modifications that have been outlined here and on the (in)famous Polk Audio Forums thread. Still to do at some point is the CS8414 / DF1704 upgrade, although I have the DIP-28 sockets to do so. For now I'm running pairs of OPA627s per channel in Brown Dog adapters, but I have a sample pair of Burson v5i-duals that I'm going to try out as well that are en route from Australia.

Impressions? Much smoother and more natural (dare I say 'analog') sounding then either my old Meridian 263 or my transformer-coupled Gigawork LM-DAC3 v9. Even though it is limited to Red Book, it is far more enjoyable and less 'hi-fi' sounding.

-D
 
I'm doing a GDA-600 "rebuild" as well and first off wanted to thank everyone on this thread for the detailed comments as they have been a great starting point.
As I've been reading through this thread and the one over at Polk Audio, some of the part replacements seem a bit questionable, while there are other places that could potentially benefit and have not been touched.
I'm by no means an expert so any feedback is welcome. An additional purpose for this post is to provide alternative options for anyone doing these mods in the future.

Power supply:
* Main filter caps, 2200uF 35V -- Nichicon UFG were used in the mod. While these are probably as good (or better) than the original "standard" grade Nichicon VX, a low ESR cap such as Panasonic FC, Nichicon UPW or UCC KYB seem like a better choice.
* Regulator output decoupling caps -- Cap types here varied by mod. 10uF solid tantalum (or 50-100uF electrolytic) is recommended by the datasheets and other papers on LM7805, LM317 family of regulators. It is important to avoid low ESR caps here.
* LM317/LM337 Cadj decoupling cap -- Originally there was an 0.1uF MLCC cap. Increasing this cap to 10uF-20uF (type doesn't matter too much) significantly improves ripple rejection.

2 other changes I made were replacing the LM317/337 with LT317AT/LT337AT which have better specs and are not as expensive as some of the boutique types, and replacing the 75R, 5W resistors with Mills MRA-10.

Analog stage:
* Opamp -- I used OPA2132PA.. OPA627 is 10X more expensive. I might try in the future.
* Analog filter -- Based on the schematic, there is a first-order analog filter (C553- 220pF, R509 1.58K), followed by a third-order Sallen-Key LPF (3* 1.21K resistors, 680pF, 2700pF, 3900pF caps).
** For the first order filter, the cap is a ceramic MLCC and I have not seen any mods replace this. I used a Wima FKP3 PP Film & Foil (7.5mm lead spacing) which fits perfectly.
** For the Sallen-Key LPF, the original IMB LA1 and Wima green cap are actually all Polypropylene film & foils. Sonicaps would theoretically be a downgrade here as they are metallized PP if I'm not mistaken. In addition, the 680pF and 2700pF caps shunt to ground, so may not have major impact. Based on my understanding, the only cap that would be considered an improvement would be a Polystyrene film & foil, such as the Relcap RTE. As such, I've only replaced the 3900pF with an RTE and left the rest as-is.
** Probably not critical, but I've also replaced the 1.21K resistors in the analog filter with Vishay-Dale RN55E 25ppm 0.1% resistors, which are likely lower noise resistors than the originals.

That's about it...
 
I too have come along with a GDA-600, but I am not the tweaker per say, but the tweakeee? Would anyone on this forum be interested in a Mod-Job of this DAC? Send me a PM/DM and we can talk. Many thanks from Detroit.
 
Back
Top Bottom