Adcom GFP-555 preamp. Upgrade It or Replace It? - diyAudio
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Old 4th November 2006, 05:37 PM   #1
Ron101 is offline Ron101  Canada
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Default Adcom GFP-555 preamp. Upgrade It or Replace It?

Need some opinions from the experts.

I would like to upgrade my system a little; was looking at just buying a replacement preamp but then I started reading DIY and saw that some simply modified existing units. So I was wondering about upgrading the components in this gfp-555 instead of buying a replacement. Are the returns usually worth it? I'm no EE but I have some unviersity electrical circuit education; can't design much but can re and re ok and of course, I love DIY projects .

I can't find any info specific to this model. If I throw up some pics of the internals and list some components would it help to make a recommendation?

Thx in advance
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Old 5th November 2006, 12:37 AM   #2
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Get the schematic from Adcom and look it over. I'm sure there's lots that can be done to improve it.
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Old 25th October 2007, 09:29 PM   #3
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Default GFP 555-II MODS

Hello, FYI - have tried several mods to this unit and have it sounding quite a bit improved (IMHO). Not even expensive to do.

1. Added filter caps to the p/s filtration. Mostly panasonic V series multilayer 60v (low inductance) caps with extremly short leads on the bottom of the pcb. Used mostly 1uf caps accross the main filter caps and accross the +/- supply lines close to the opamps. Mostly made a sutle more "impact" change.

2. Changed most of the opamps out for much newer ones. Used the LM6172 for the headphone out - drives my 300ohm headphones MUCH better that the stock chip.

Changed out the single opamps with the LM49710 and use the LM4562 for any dual opamps. (Love these opamps!)

Also when changing out the opamps, I put a Panasonic Vseries 1uf 60v cap soldered directly across the opamps power supply leads (over the top of the opamp). To make sure any noise is filtered close to the amp. (No ground pin goes into the chip!)

All I can say it this seems to work just fine everywhere I have done it. Added up, definately updated the refinement of the sound. Details are much clearer. Nice. (I do listen closely as not all the mods I have done have resulted in the improvement desired.)
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Old 25th October 2007, 10:23 PM   #4
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Some time ago there were some articles in Audioxpress, by Gary Galo, modifying an Adcom preamp. I am not sure if it was that specific model or not. But I believe they might be a way to go.

A major change were the supplies, which I believe were changed for super-regulators.

Beside that, I don't think you might get too much by replacing it, except if you pay a lot more.
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Old 27th October 2007, 02:45 AM   #5
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Default ADCOM GFP 555II PREAMP MODS

Hello Carlmart,

Yes, I read the Gary Galo piece several times. Did not apply much to this model of pre as it was the next one up. Also a couple of points as to why I went this way instead of following his super-regulator boards with compound custom opamps method.

1. The two op amps I used for everything other than the headphone driver are at least a generation newer than what was available at the time several years ago when Gary wrote his article.

2. The power supply rejection is so decent on these new chips that even the added filtration I used probably was not needed.

3. The distortion measurements on these chips are as good as anything you can make discretely or purchase currently.

4. These chips will easily drive any reasonable cable and if you are like me you build your own cables - as short as possible - which keeps the inductance/capacitance/resistance as low as humanly possible, you should have no problems with cables at all.

Note: Several people have stated in various forums that these two chips seem to sound much better after a hundred hours or so of burn in. I have used them now in several projects and agree with that assesment. Put a radio or CD player (on repeat) going into the pre and let it run - you don't need to turn the main amp on at all to do this.

Anyway, it is relatively cheap to try this and see what you think. I think you will be pleasently surprised at the change.
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Old 12th November 2007, 06:16 AM   #6
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Default Try rewiring

In my GFP555, I found that the connections from the switches in the back to the front were made up of long PCB traces and multiple jumpers. I replaced them with shielded teflon audio cables and the audio quality improved a lot.

You can also put another stage of regulation in-and while you're at it, use individual regulators for the two channels' opamps (the two single opamps)..

By the way, the single opamps originally used were OP-37's.
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