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Old 1st November 2011, 05:14 AM   #11
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Well, I've done the mod; swapped out the Adcom opamps for sockets, and then bunged in the OPA637's. Any difference? Hard to tell. There is an element of my wanting there to be a difference of course but I can say that there seems to be a little more mid range detail and bass has a little more authority. Nothing huge mind, but I believe there is some sonic difference. Enough to justify the change? Dunno, but it really doesn't matter either way. I enjoyed the small challenge it presented and the sound is certainly no worse, so at the very least I didn't end up with a dud player.

I'm tempted, after owning the player for nearly 20 years, to purchase a NAD, a brand I've always liked, or maybe even a Chinese player. I'm hardly rolling in cash so a budget of $500-600 is all I'll be able to get past the missus probably. I definitely don't have the budget, or the desire to own really high end gear...there is a fair bit of snake oil in hifi I feel, and hifi is a fine example of the law of diminishing returns.

Many years ago I assisted a chap starting up a high-end hifi shop here in Perth, and was exposed to some highly-desirable gear. Unfortunately I also saw many examples of smoke and mirror antics which folks with lots of cash readily swallowed, presumably for the bragging rights rather than sonic appeal. Any protests from me soon had me booted out of the inner sanctum and out the door for being a non-believer and clearly not worthy, and for having the temerity to question the dubious science behind some of the hocus pocus presented to folks with more money than sense. Anyway, that's another story, and I certainly don't want to start a religious war with the true believers....

Moving along, I think I'll hang on to the Adcom a little longer and see how it fairs when partnered up to my new SET tube amp, but I'm sure it will be replaced soon.

All in all, good fun... something gained, and nothing lost

Cheers, Martin
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Old 1st November 2011, 04:36 PM   #12
pchw is offline pchw  United States
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Thanks for the report, Martin.
Quite frankly, we all hearing sensitivity, memory, and preference.
I have heard some mod perceived as day and night by others that I embarrassingly couldn't tell much of a difference :-)

I will recap and socket my GCD-575 soon. Let see I will hear anything :-)
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Old 2nd November 2011, 11:40 AM   #13
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Now, it all depends also of the rest of audio chain. If you amplifyer is tube based, you might not hear a difference - it will "cover" the resulting change with the lower signal/noise ratio, higher distortion levels, reduced bandwith (depending of the output transformer).
Try a good pair of headphones directly on player output, to see what I mean.
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Old 24th November 2011, 09:49 PM   #14
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Question Change Op amps in Adcom GCD-575?

I recently purchased an Adcom GCD-575 and are interested in the Burr Brown OP amp upgrade - Unit shows to have dual opamps (IC 203,204)

They are labeled: "Adcom 6A 8905", and "Adcom 6A 8903" respectfully.


Cost of Op amps?

Thanks,

Steve
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Old 25th November 2011, 01:37 AM   #15
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Adcom 6A = LT1056. The rest of the numbers are the fabrication date (year+week).
I would try the LT1057 instead. Or if you want BB, OPA2134.

PS: Other Adcom OpAmps:
1A = LT1010
6A = LT1056
7A = LT1057
8A = LT1028C

Last edited by SoNic_real_one; 25th November 2011 at 01:52 AM.
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Old 28th November 2011, 08:59 AM   #16
pchw is offline pchw  United States
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Quote:
Originally Posted by pchw View Post
I will recap and socket my GCD-575 soon. Let see I will hear anything :-)
Finally, I am done with recapping my GCD-575 which has passed 22 years old (was born in Sept '89). The result is very satisfactory, and I am still using the same Adcom 6A opamps (LT1056 rebadge). Here are the mods that made the major improvement:
1. Replaced the caps in the power supply chain with the higher capacitance.
2200uf/25V x 2 => 3300uF/35V x 2
2200uF/16V => 4700uF/16V
3300uF/16V => 4700uF/16V
470uF/16V x 2 => 1000uF/16V x 2
220uF/16V x 2 => 470uF/16V x 2
2. All caps are Panasonic FC, except C223 and C224.
These 2 x 22uF/63V are the output caps after the active LPF.
I replaced them with Silmic II 10uF/35V.
3. At each power pin of the opamp, I added a 120uF/25V cap.
I also added a 0.1uF across pins 4 and 7 to anticipate of dropping
in the opa627 and LME49990.

I also unplugged the head amp and the variable output as I don't use either.

The most noticeable improvement is the mid and the bass. The bass slam is tight and authorative. For about $30 of parts, I will say this is a well worth effort if you one of these player sitting in the closet :-)
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Old 28th November 2011, 11:37 AM   #17
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Quote:
Originally Posted by pchw View Post
3. At each power pin of the opamp, I added a 120uF/25V cap.
I also added a 0.1uF across pins 4 and 7 to anticipate of dropping
in the opa627 and LME49990.
Wrong.
You need one between EACH pin and ground. Otherwise is not efficient, you are pushing HF noise from one rail to another (because transitors in the final stage work alternatelly).

Decoupling 101 - by Analog Devices.

OpAmps proper decoupling - by Analog Devices.
Look at figure 3 on this above document.

Last edited by SoNic_real_one; 28th November 2011 at 11:42 AM.
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Old 28th November 2011, 08:58 PM   #18
pchw is offline pchw  United States
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Quote:
Originally Posted by SoNic_real_one View Post
Wrong.
You need one between EACH pin and ground. Otherwise is not efficient, you are pushing HF noise from one rail to another (because transitors in the final stage work alternatelly).

Decoupling 101 - by Analog Devices.

OpAmps proper decoupling - by Analog Devices.
Look at figure 3 on this above document.
I actually searched this topic quite a bit. My understanding is that the film cap across pin 4 and 7 is to prevent oscillation. The 0.1uF bypass from the power supply pins to ground are not needed if decent caps (especially low ESR caps) are used and soldered near the pins. I will find it out myself whether I will need the bypass to ground.

I can't quantify which change provides the best bang for the bucks. Even with the stock LT1056 opamp, it sounds very good. When I posted last night, it was only 3 hours after I finished the work. If caps break-in does exist, then there should be more improvement to come .....

To honest, the newer and faster opamp will sound different for sure, but I don't have high hope as the current circuit really isn't optimized for these opamps, e.g. the feedbacks network are not optimized.
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Old 28th November 2011, 11:15 PM   #19
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There is no need for "feedback optimization". Just better power supply rails (since the faster opamps tend to draw more curent).
And no, a cap between 4 and 7 (when the opamp load is connected to the ground) will just increase the garbage on the power rails.
The engineers that wrote those App Notes are those that designed the OpAmps and definitely are not stupid (like some people on this site tend to belive).
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