Adcom GFP-555II Schematic

[ronenash]

Hi,

I aam looking for a schematic for my Adcom GFP-555 MkII preamp. Anyone can help.

Thanks

[anatech]

Hi Ronen,
What is the problem you are having with it?

-Chris

[ronenash]

I have an old unit laying around. I want to turn it into a phono stage only amp. I believe that by eliminating the power to all other sections and upgrading the opamps and some caps in the phono stage I can turn it into a decent phono stage.

I might try to use it as a headphone amp too.

[anatech]

Hi Ronen!

I want to turn it into a phono stage only amp.

Seems like a waste of a good pre-tuner. You could build a better phono stage from scratch, and this isn't a bad pre-tuner at all. Why not upgrade the entire thing and use it? Failing that, just use the Tape out and leave the rest alone.

-Chris

[ronenash]

Its a preamp alone and does not include a tuner. Just eliminating the power supplies to all other parts of the preamp (output buffers, line level preamp, etc.) cleans up the sound of the phono section significantly. I assume that replacing some caps in the phono stage and switching the opamps to better Analog Devices ones will create a first rate phono stage.

I have a much better Conrad Johnson tube preamp that I am using for line level.

[anatech]

Hi Ronen,
Interesting that disconnecting the supply from the other sections has any effect at all. I'd be looking at the supplies.

The preamp model was much nicer than the tuner - pre.

-Chris

[starkeyg]

Hello, FYI - have tried several mods to this unit and have it sounding quite a bit improved (IMHO).

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.)

[ronenash]

Starkeyg Hi,

The LM6172 chip you mention for the headphone amp is a 16 pin chip where as the original IC is 8 pin. Did you rewire the 16 pin chip in? How did you achieve this?

Thanks
ronenash

[starkeyg]

Hello,

The LM6172 opamp is available in an 8pin DIP package which plugs directly in as a replacement for most general opamps. See web page below for details:

http://www.national.com/mpf/LM/LM6172.html

As this is a very high speed (bandwidth & slew rate) amp, there is something important you must do to make it work well without oscillating and sounding like crap.

Caps on the V+ and V- supplies should be located right next to the chip (close for the electrolytics) and I usually use a low inductance film cap (Panasonic V series 1Uf is what I use) accross the V+ & V- supplys within a half inch of the chip at most.

Also, I put one of the same caps mounted directly across the top of the chip going to the power supply pins. Some use X7 ceramic caps and swear by them, but what I have been using works well for me. Main point is to use something that has excellent high frequency capability as this chip is fast!

Actually, this cap across the top of the opamp has worked so well for me that I use it for insurance on all opamps now. Keep the leads very short, prebend and cut them so they fit well on the chip, then touch solder the caps lead to the top of the opamps leads. If done carefully you can even use the result in opamp sockets.

Chip was designed to drive several volts at megahertz frequeny range into 75 ohm loads. Is loafing driving headphones above 100 ohms.

I wouldn't use it to drive 4 or 8 ohm headphones, those should probably have a seperate amp, as they need more current.

Search on the web and you will find people who love this chip and those who have hated it. I suspect those that hated it ran into high frequency oscillation problem and did not realize it. Without a scope you would never know it.

Regards,
Greg

[mattogan]

The information provided by starkeyg is very helpful. I used the LM4562 in a Rotel RCD-961 with good results. I had problems using Audioquest Type IV speaker cable before the mod (too harsh) but am able to use the Type IV cable post mod. I also replace the coupling electrolytics with polypropylene capacitors.

I would like to know if the supply voltage was changed (lowered) for the replacement Op Amps. It is easy to change the supply voltage but I would like to minimize the modifications if possible.

I would also like to understand why the LM6172 was selected for the headphone driver. I noticed that the LM6172 has a very high slew rate. Was the slew rate the primary selection factor?

Thanks,
Matt