Adcom GFP-555II Schematic

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#21
Modifying ADCOM GFP 555II

Hello Chris,

Hate to disagree with your opinion, but have tried a lot of small mods on this particular unit and while some of them have not helped, others have made quite a difference.

In particular the replacement of the opamps made by far the greatest gain. IMHO. I am impressed with these new opamps.

The design was above well above average, which is why I bought it originally. Had polyprop caps at far more places than most and snubber in the p/s and more. But this is a hobby, it is fun trying different things and seeing what if any differences you can make. And when you do succeed, it is satisfying to listen to the results.

As the cost of some opamps at about $5 each is pretty cheap, as mods go, would hope that I could get you to try it and see what you think. I doubt I am that self deluding.

Regards,
Greg
 
#22
Hi Greg,
Oops!
I thought we were talking about the amplifier. Not the tuner - pre!

Yes, op amp upgrades will help with this. Remove the muting transistors and use relays for that function. Now it will sound much better. The muting transistor removal will make a large difference.

Now, if you really want to increase the performance and make a meaningful change, figure out how to route the audio signals through relays rather than the selector ICs. You can run DC through and back out to a relay switching circuit (since these are bidirectional switches).

I apologize for my error. Most times I end up talking about upgrading an amplifier. So it slipped my mind that we were talking about the signal source.

What did you install as a replacement op amp?

-Chris
 
#30
The single opamps in the GFP 555 are OP-37's. Victor Campos (the preamp designer) told me this. They are NOT unity gain stable, so there's a resistor in the phono stage's feedback loop to keep the stage having a minimum gain of 5.
You can jump this resistor out if using unity gain stable amplifiers.

The best thing you can do to this unit is the following:

1. The audio from the swithes in back to the front are long circuit traces with multiple wire jumpers. Replacing these with some good teflon wire makes a big difference.

2. Use individual 3 terminal regulators on each opamp. Makes the channel separation a lot better. Put reverse bias diodes around the regulators and 100 uf caps bypassed with .1's at each opamp power supply pin.

3. Replace the opamps with the new National ones.

Singles: http://www.national.com/pf/LM/LME49710.html

Dual: http://www.national.com/pf/LM/LME49720.html

Alternate dual: http://www.national.com/pf/LM/LM4562.html
 
#31
Hi dpuopolo,
I don't want to be a nay sayer, or kill joy but ............
You can jump this resistor out if using unity gain stable amplifiers.
Click to expand...
Don't mess with the circuit. Just replace the op amp if you want to try that.
1. The audio from the swithes in back to the front are long circuit traces with multiple wire jumpers. Replacing these with some good teflon wire makes a big difference.
Click to expand...
Teflon is only a pain to work with. No magic sound. Any good shielded wire would work well here as long as the quality is good. Copper wire is the best, normal dielectric. You could use 50R test lead coax if you really wanted to.
2. Use individual 3 terminal regulators on each opamp. Makes the channel separation a lot better. Put reverse bias diodes around the regulators and 100 uf caps bypassed with .1's at each opamp power supply pin.
Click to expand...
No! That means routing the noisy supply around inside the pre. Don't do this!! Also, 3 terminal regulators perform poorly at high frequencies and those new National ICs have greatly improved PSSR. If you are going to improve the regulation, try improving the main ones. Careful you don't make things worse. If you want to bypass, try mica and a 1 uF film cap. Tantalums are one part I don't like at all. They can short easily. 100 uF bypass caps are useless for this application!
3. Replace the opamps with the new National ones.
Click to expand...
That might work. I just worked on an old 70's Amcron (Crown for our US members) and it sounded great. It was full of 5534 op amps.

Hi Greg,
I heard that too. Check here.

-Chris
 
#34
Hi guys, two questions:

1. I have Adcom GFP-555, not Adcom GFP-555 II or MKII. What is the difference in schematics? (and I am only interested in the phono stage)

2. You mentioned it couple times, but where I could download the schematics for preamp?

Appreciate your help!
Michael
 
#35
Hi Michael,
It's nothing special. Just an op amp type. At least it uses real switches instead of the silly IC's.

I have not seen the schematics for these. Since I no longer have the service manuals, I wouldn't mind to be able to download them either.

-Chris
 
#36
Chris,

I am asking b/c my 555 is not the top of the line, and in the same time it is pretty unique in phono stage features: has MC and MM switch, capacitance adjustment. low noise (almost dead), and I like it a lot.

So my question was, did Adcom make a better phono stage (not the line -in preamp), and if yes, how better it is and why. And what can I do to improve mine.

Michael
 
#37
Hi Michael,
Sorry for the delay.

I honestly do not recall anything available from Adcom in that time period. You would have to look at other makes and designs.

Remember that any design can be better for a particular application. There is no spec for "the best" design. Even phono cartridges themselves will have different requirements and some designs may work better than others with them.

-Chris
 
#38
Hi Chris,

still looking for some info.

"Remember that any design can be better for a particular application. There is no spec for "the best" design."

despite having some experience and background in that field, I follow the simple rule: to improve one part of the system in a time, and stop when the last improvement did not show any visible (for myself) results.
Like I had Sony -> Yamaha -> Rotel -> Hafler -> Bryston, and I stoped here. I used speaker wire 16GA -> 14 -> 11 -> 8 GA, no improvement, so I'll stay with this cable forever.

So here is the latest dilemma: I need to improve phono stage (between Technics SL-1200MK2 + Shure M97, and Rotel 1068) but the next obvious step up from my Adcom 555 would be something around 2K. Would I hear the difference? Or I could build any of the offered "high end" tube phono stages (I have plenty of hardware and parts) in a few days, but I need to understand which one, I don't have time for experiments.

I know, there is no answers for my questions.
Michael
 
#39
Hi Michael,
I would improve the TT and cartridge if it were me. A bigger bang for the $. With turntables, the platter bearing is all important. Even going from a Thorens TD-160 to a TD 125 MKII is a big step. An Ortofon VMS-30E MKII is much better than an Ortofon VMS-20E MKII. Big difference. Soon I'll hear an Ortofon 540 in my system. 😀

At this point in time, you may hear a difference changing electronics. Those are not the weakest link though.

See, there is an answer to your questions. Differences between preamplifiers and amplifiers can be quite plain, if your speakers are up to snuff. CD players sound different as well.

-Chris
 
#40
Chris,
I agree with every word about better TT, but the signal from cartridge goes through the phono stage. If the phono stage is not up to the task,
you wouldn't hear the improvement.
Now back again to the same question. My TT and Shure are top of the line in their middle range class, when Adcom 555 is on the bottom of the same class.
Or I am wrong?
 
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