Next project: Adcom GDA-600

[belgianbrain]

Well, as much as I enjoyed my Aqua La Voce S2, it is sold. I retain my Lampizator DACS, which to my ear have a slight edge, and I move on.

I'm done spending big bucks on new gear because I'm not convinced there is much to be gained by doing so.

My next project is an Adcom GDA-600 that I just purchased for $125. I was tipped off to this family of DACS by the thread "sleeper DACS" on this board. I hadn't even realized Adcom made DACs, and yes, they are highly regarded - even without modifications.

I will absolutely go to town on this thing with the mods that are detailed all over the net.

Can is trump the Lampizator? The journey begins... Stay tuned.

 

[SoNic67]

The DAC chips are real multibit ones (PCM63). Refresh the capacitors and maybe moders OpAmps will bring it to the real potential.

"Deeper" mod would be adding the external adjustment pots for the MSB accuracy (to improve slightly the THD). However the exact adjustment procedure requires specialized equipment and is usually not performed.

 

[belgianbrain]

I already have the Browndog OPA627 opamps in a CD player I modded years ago but don't use anymore. So this will cost me almost nothing.

I'm not a fan of opamps, but they're about to take on the tubes in my Lampizator.

 

[SoNic67]

I am not fan of Lampizator. There is no ultrasonic filtering, and a lot more TDH (second harmonic especially).
It might sound "good" to you because your brain got used to that harmonic content, but is not accurate. Give OpAmps some time, to get your brain used to the "correct" sound again.

Those OPA627 are probably the best out there for IV stages, but for the filtering stage personally I prefer LM4562.
In GDA600 the dual channel OpAmps are segregated per channels, not per functions, so one dual will be serving IV and filtering on a channel.
BTW, the OpAmps in the GDA600 are force-biased in class A (with use of some J-FET as sinks), so not knowing the actual bias value, OPA627 is the safest choice (higher current capability).

 

[belgianbrain]

Thanks for the info, Sonic67.

I received the DAC yesterday and I've had a good look inside and at the schematics and ordered my parts.

The GOOD: A very clean path from PCM63 through opamps to output. Once you delete the horrid inductors on the output, you have virtually nothing in the signal path except for the opamps and a few resistors.
The BAD: Op amps. We'll see.
The UGLY: 8X oversampling implementation. Oh man.

 

[SoNic67]

OpAmps are cool in my book. Newer ones that is.
My schematics though has an error, I hope that's not how they build it. The Feedback resistor inside the DAC (that one that is laser trimmed) is not shown connected correctly (terminal 10 - RF2 - is in air, not connected). It should be part of the IV conversion, in the OpAmp feedback loop, like is shown in PCM63 datsheet.

Also, those inductors are there to cut the ultrasonics (low-pass LC filter). If you like the next stage to be overwhelmed by them... go for it.
As for the NPC5813... I never heard one and I don't recall any praise to them (like to PMD 100). So yes, that might be the bad part.

 

[belgianbrain]

Pictures.

 

[belgianbrain]

I followed many of the mods in this thread:

http://forum.polkaudio.com/discussion/97185/the-adcom-gda-600-modification-thread

1. Complete recap - mostly Nichicon on the PS with much larger sizes used for the smaller caps; Elna Silmic II for the DAC board.
2. Brown Dog OPA 627 op amps
3. OP Amp power supply pin coupling caps replaced with Vishay Roderstein 1837 and 220uF Elna Silmic underneath the board (not visible in pictures)
4. Op AMP filter caps replaced with Multicaps
5. Sound destroying inductors and caps eliminated at analog outputs (Sorry, Sonic, sounds much better to me WITHOUT these
6. C122 and C123 on PS board replaced with Mundorf 3.3uF

I also looked at the digital inputs. I have high precision Audiophilleo SPDIF generator supplying high precision, low jitter SPDIF signal to my DACS. Signal arrives at GDA-600 and goes through a lot of BS before arriving at receiver chip, including multiple caps, inductors, 74HCU04 chip, 74HC125 chip, and even a transformer. I hooked DAC up to an oscilloscope and some of these parts were severely degrading a nice looking square wave at the input, especially the completely unnecessary transformer. I eliminated everything and replaced with one OMRON relay just to switch between 1 SPDIF input and Toslink input. Even the balanced digital input on this DAC was BS - balanced input is immediately converted to single ended signal and then converted back to balanced signal via transformer at receiver chip. Can you say WTF 10X really fast? Anyway, end result: digital signal arrives at receiver chip in original form - no BS.

One complaint about this unit: PCB holes for parts are way too small. It is an absolutely PITA to work on this unit.

Will starting listening tomorrow along with my new GFP-750. See the Solid State discussion forum if you're interested in that bad boy that I came upon while working on this one...

 

[sev1]

new to the forum, but very interested. going to pickup a GDA 600 for modding as well.

 

[belgianbrain]

I've done some listening.

It doesn't beat my Lampizator, but I must say this is probably by far the best return on investment I've seen in a DAC. For the ~$300 I have invested in this thing, in my opinion it comes close to my Lampizator and easily matches if not surpasses the Aqua La Voce that sells for $3,000.

Bass is awesome with this modified DAC and midrange and treble are also very good.

Perhaps it will improve as it breaks in.

 

[asilker]

There's a modified 600 dac on CL somewhere. Is it associated with this thread?

 

[belgianbrain]

Nope. Mine is at home in my rack, unless the seller knows something I don't.

After much more listening, I must say this is a fine sounding unit, but on the warm end of the spectrum. It's definitely more bass heavy and less neutral.

 

[asilker]

I would love to hear this unit along with a few other R2R in the sub-$500 price range. This one is a fully new old stock unit

Next time one pops up locally, I'll be trying it

 

[sev1]

So I just scored a nice condition one locally. Was going to tackle the mods myself, but I'm a little concerned as it seems above my level. Anyone interested in performing the mods as a service?

 

[sev1]

Also, those inductors are there to cut the ultrasonics (low-pass LC filter). If you like the next stage to be overwhelmed by them... go for it.
As for the NPC5813... I never heard one and I don't recall any praise to them (like to PMD 100). So yes, that might be the bad part.

You know what's weird. On the schematic it has an NPC5813, but on some pictures i've seen DF1700's installed.

 

[belgianbrain]

Yep, mine has a DF1700. Didn't pay much attention since it is one small area of the unit I didn't mess with!

 

[sev1]

Do you know if any of the resistors are in the IV circuit or is it just the opamps performing that function?

 

[belgianbrain]

Guys, more listening, and this DAC is AWESOME!!

The more I listen, the more I like. I'm not sure which has the edge right now, this or the lampizator. The lampizator might have a slight edge for clarity, but the bottom end is stunning with this modded Adcom. DEEP bass. Ironic because I always thought that was a strength of my lampizator.

 

[sev1]

Been listening to the stock version for a couple days and am really liking it. Can't wait to do the upgrades. Although, I'm also looking heavily at zapfilter 2 instead of the opamp upgrades.

 

[belgianbrain]

Do you know if any of the resistors are in the IV circuit or is it just the opamps performing that function?

Follow the link to the polk audio forum above and you'll find another link to the schematics for this unit.

http://www.audiokarma.org/forums/index.php?attachments/gda-600-schematic-pdf.33267/