The Adcom GDA 600 modification thread

SCompRacer · August 2012

All hail Jay! All hail Jay! Thanks for the kind words but you did great! I've read about them Hakko's, but using it makes me want one to replace my spring operated solder sucker. One handed, trigger control action. Love the tone of the pump changing when you have a clean, cleared out hole in the PCB.

Some of them solder joints reminded me of working with the RoHS lead free solder. The stuff just didn't want to flow completely out of the hole in the PCB. We ended up adding more solder and having another go with the Hakko 808.

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I didn't hear it before but it was a great sounding modded DAC in the retro rig downstairs and in the main rig upstairs!

sda2mike · August 2012

well, as i posted, it's back:cheesygrin: i had some lengthy sessions friday and saturday..it's a huge improvement! like going from a redbook cd to a well-done re-master! it is not sutble! i was pushing some serious current into my 2btl's and i think my ears were the weak link;) the bass now has incredible slam and depth!that's on all recordings...not just when it feels like showing up, like before;) the drum sounds are great! snares sound like snares....you can almost se the wires in the snare drum...pianoes sound better than ever...and the SDA???? holy schnikeys! active alive and all over!! this is a must do upgrade! thanks to george daniel for this thread and for doing the surgery! i couldn't have done it! the parts he sent back are TINY:)

mike

heiney9 · August 2012

Pretty serious upgrade, eh. It's been so long since I did mine that obviously it still sounds fabulous but I forget what the stock unit sounded like by now. It would be nice to compare the stock unit again.

All I know is I grin from ear to ear with my rig. Finally have things dialed in where I like them as far as cables, tubes and gear, etc.

H9

sda2mike · August 2012

with the stock unit, i thought i'd reached the pinnacle of what my system could do...it was depressing when i took it out of service to send to george...veiled, digital and lifeless..now, there's things jumping all around the room!

heiney9 · August 2012

The Adcom dac's were very, very good for their day especially the BB PCM-63 chipset. Add a modern, better Op-amp and better caps in the signal path and beef up the PS section with new caps and Viola', you have a superb sounding updated DAC that sounds excellent, for a fraction of the cost of something comparable, new.

H9

FTGV · August 2012

Great job Rich and Jay.

george daniel · August 2012

sda2mike wrote: »

well, as i posted, it's back:cheesygrin: i had some lengthy sessions friday and saturday..it's a huge improvement! like going from a redbook cd to a well-done re-master! it is not sutble! i was pushing some serious current into my 2btl's and i think my ears were the weak link;) the bass now has incredible slam and depth!that's on all recordings...not just when it feels like showing up, like before;) the drum sounds are great! snares sound like snares....you can almost se the wires in the snare drum...pianoes sound better than ever...and the SDA???? holy schnikeys! active alive and all over!! this is a must do upgrade! thanks to george daniel for this thread and for doing the surgery! i couldn't have done it! the parts he sent back are TINY:)

mike

Took me long enough Eh Mike?-- I'm glad that you are happy with it.

sda2mike · August 2012

well worth it George! Thank you!

heiney9 · March 2013

Dangerboy (AL) just bought a GDA 600 so I bumped this for him.

Still interested in Jay's and Mike's thoughts now that you've had some serious time with it.

H9

sda2mike · March 2013

H9, it's still a must-do upgrade...it hasn't 'worn on me', the effect of the upgrade, that is...see post #484, that sums it all up..thanks

mike

agfrost · March 2013

I'm still exceptionally happy I both a) got the DAC (thanks, Dave!) and b) did the George/Fred modding (thanks, Rich!). When I first got it, I could toggle between my CD player's analog outs and the Adcom via my preamp's remote, and the sound via the modded DAC was clearly better--quieter background, more spacious & airy soundstage, and more... Now that I've upgraded my preamp to the Manley Shrimp, I've lost my ability to toggle back and forth, but I'm still offering the Adcom pride of place in my rack. Super happy with my current sound.

I promised some months ago I'd add a shot of my power supply board once I got the Nichicon FGs soldered in, so I'll take the opportunity to do so now...

SCompRacer · March 2013

agfrost wrote: »

I promised some months ago I'd add a shot of my power supply board once I got the Nichicon FGs soldered in, so I'll take the opportunity to do so now...

Kewl! Good job Jay! It looks and sounds great!

george daniel · March 2013

congrats!

kind · July 2014

Hi guys!
Sorry for wakening this thread up :-)
Life goes on and I've just got my GDA-600. Actually I like it's sound, but I'm mad about mods.

I've ordered OPA627 and going to order caps for replacement.

Read all 17 pages carefully. Everything is clear except .1 bypass film caps.
May be there is name for it? I can't get which one to order.

Appreciate for your help.

FTGV · July 2014

They are Panasonic V series stacked films. Digi Key part# http://www.digikey.ca/product-detail/en/ECQ-V1H104JL/P4525-ND/13790

When/if installing try to keep lead lengths as short as poissible.

kind · July 2014

Thanks!

I've unsoldered original OpAmps.
Installed Dip8 socket and have tried 2 OpAmps: LM4562 and AD8599.

LM4562 sounds too sharp and flat, but very dynamic. Didn't like it actually.
AD8599 is very good, but it sounds just like original Adcom 7AA OpAmp (which I like a lot BTW).

I also biased OpAmps into class A.
Just soldered 3,3 kOhm resistor to 1 and 4 leg of socket, and another 3.3 kOhm resistor to 7 and 4 leg.
Seems like it made OpAmps sound more tight.

soundfreak1 · July 2014

Gl with the mod, did my 700 and have been thrilled with the results, my tech did a 600 also for a friend and is was fantastic. Big diff. Enjoy

FTGV · July 2014

kind wrote: »

I also biased OpAmps into class A.
Just soldered 3,3 kOhm resistor to 1 and 4 leg of socket, and another 3.3 kOhm resistor to 7 and 4 leg.
Seems like it made OpAmps sound more tight.

There is already provision for a resistor or optionally a fet for biasing the stock op-amp into class A.I don't have a schem handy so not sure of the value.3.3 k seems low and may be pulling excessive current through the opamp's output stage.

The OPA1602 also looks like an interesting option.It's an SMD part so an adapter would be required.

kind · July 2014

I don't have a schem handy so not sure of the value.3.3 k seems low and may be pulling excessive current through the opamp's output stage.

Neither do I. :-)
OpAmps are warm but not hot. So current seems to be ok.
15V / 3300 Ohm = 0,0045A.
Diyers experiment with lower and higher values.

The OPA1602 also looks like an interesting option.It's an SMD part so an adapter would be required.

I'll keep that in mind, thanks.
AD8599 is SMD as well. I soldered it to extra dip8 socket. In fact I don't have skills in that, so that was funny :-)

Sev1 · July 2016

Hi all, I know this is a super old thread, but I'm currently modding my GDA 600. I had a question regarding the output stage. Every inverting Op-Amp Diagram I have seen shows a resistor prior to the - Input on the Op-Amp. In the GDA 600 schematic, I don't see a resistor in line between the PCM63 output and the Op-Amp Input. I do see 2 resistors, but they appear to be tied to the +/- power supplies.

Also, if the output stage is an inverting Op-Amp followed by a non-inverting buffer, does that mean the output of the DAC is inverted?

Thanks!

FTGV · July 2016

Since the PCM63 dac chip has a current output the opamp directly following it is configured as an I-V (current-voltage)convertor or transimpedance amp,not as a standard inverting opamp.While the inverted opamp connection does invert phase other phase shifts in the circuit result in the actual output phase being non inverted.

lightman1 · July 2016

What????

Sev1 · July 2016

Thanks FGTV! Regarding the 2 resistors after the PCM63. Area they in the signal path?

george daniel · February 2017

Wow,,,it's been 7 years,,, mine is still purring like a kitten,,time flies,,many thanks to Fred for his comments and advice!

heiney9 · February 2017

This was a fun project. About a year ago I moved mine on to someone here. I currently am running a slightly modded Eastern Electric DAC Plus w/discrete DEXA op-amps and I have to say as good as the Adcom was, the EE is a LOT better. But then it should be.

Getting ready to swap the tube output caps from what looks like Mulitcaps w/EE script to Clarity Cap ESA's and I am replacing (3) I/C voltage regs with DEXA discrete voltage regs. Should improve it even more.

Doing the Adcom mod has given me the confidence to do this mod. The Op-amps were plug-n-play since they were socketed....

agfrost · February 2017

Still running mine; still happy with it. Thanks George, I believe some of the caps I installed were extras you passed along from your original purchase.

keck · March 2017

Good day to all,
I am new to the forum. I have just purchase the Adcom GDA600, still waiting for it to arrive. Did some search and found out this thread discussion on the its mod. I am very interested to unlock its potential

.
I have already ordered all the components needed for the PS and Analog section mod. It actually cost nearly twice than the $ i pay for the GDA600

. as mention previously by annihilatorg. So glad that this thread is still active and thank you so much for all the members contribution to make it happen

Am so excited for the big project, but will not start in this few weeks cos i would like to hook up to listen the original DAC potential first.

My current setup:
1) Azur 651C
2) MSB link dac
3) B & K Reference 5 S2
4) Adcom GFA535II
5) Polk Audio SDA 2B
6) Infinity Kappa Series 6.2 (sometimes only)
7) Klipsch RF25 (sometimes only)
8) Azur 350A (sometimes only)

keck · March 2017

Hi all, i have received my GDA600 a week ago. Hook it up and it sounds really good.
Today i just open up and realized that the a lot of the caps seems like not the original. Is it being modded by someone else previously or is there any other version that anyone known of?
The red caps are all Rubycon Black Gate , The caps at power section is Nichikon Muse

heiney9 · March 2017

Looks like some caps were replaced, but it's not a full mod as the op-amps and the caps in the signal path of the op-amps look stock. That's the part of the mod that really gives most performance boost. It's also the more expensive part of the mod

FTGV · March 2017

And hey they installed a $50 "audiophile" fuse. That alone should improve it an order of magnitude.

At some point you might want to install a pair of quality 8 pin DIP sockets and try rolling some different opamps. There are several dual opamps that are worth trying and are relatively inexpensive.Some of the better opamps in standard DIP packages are getting harder to find as they are being discontinued in favour of SMD devices.The latter would require a special SMD to DIP adaptor.

The Adcom GDA 600 modification thread

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