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.
I might try to use it as a headphone amp too.
Hi Ronen!
-Chris
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
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.
I have a much better Conrad Johnson tube preamp that I am using for line level.
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
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
GFP 555-II MODS
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.)
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.)
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
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
LM6172 opamp for headphones
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
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
Supply Voltage
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
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
Hi Mattogan,
The chips I used are rated for 36volts (+/- 18volts in) maximum. According to the ADCOM schmatic, the supply is putting out 36volts, which would be pushing it. However, when actually measured, the supply is putting out +/- 17 volts (34volts), safely within the chips ratings.
The LM6172 was selected because I had heard very good comments about it from a several people I had a good opinion of from reading their comments on different forums for a couple of years. Also, wanted something that could drive headphones, which nowdays most better ones are between 100 & 600 ohms. Most opamps can not handle this heavy of a load. Have been quite happy with it and am glad I tried it. Nice thing about DIY, most of the time you can try several things until you get it to your satisfaction.
The chips I used are rated for 36volts (+/- 18volts in) maximum. According to the ADCOM schmatic, the supply is putting out 36volts, which would be pushing it. However, when actually measured, the supply is putting out +/- 17 volts (34volts), safely within the chips ratings.
The LM6172 was selected because I had heard very good comments about it from a several people I had a good opinion of from reading their comments on different forums for a couple of years. Also, wanted something that could drive headphones, which nowdays most better ones are between 100 & 600 ohms. Most opamps can not handle this heavy of a load. Have been quite happy with it and am glad I tried it. Nice thing about DIY, most of the time you can try several things until you get it to your satisfaction.
Power Supply Upgrades
Hello starkeyg,
Thanks for the quick response.
I have studied the schematic and noted the optional power supply modifications with the increased rail voltage (issue A). In addition to the higher rail voltage they added caps across the rectifier diodes. The circuit board is pre-drilled for two of the caps (across D903 & D904). If you add a jumper between the divided landings for R901 and D901, you can add a cap to replace R901 (parallels D901) and C901 would be in parallel with D902. C901 is a .0047 uF "SPARK KILLER". What type of capacitor best functions as a spark killer?
I have the pre-amp paired with an ADCOM GFA-545II. Has anyone modified this amp? I had to replace a failed bias resistor and put in larger supply capacitors during the repair.
Thanks again,
Matt
Hello starkeyg,
Thanks for the quick response.
I have studied the schematic and noted the optional power supply modifications with the increased rail voltage (issue A). In addition to the higher rail voltage they added caps across the rectifier diodes. The circuit board is pre-drilled for two of the caps (across D903 & D904). If you add a jumper between the divided landings for R901 and D901, you can add a cap to replace R901 (parallels D901) and C901 would be in parallel with D902. C901 is a .0047 uF "SPARK KILLER". What type of capacitor best functions as a spark killer?
I have the pre-amp paired with an ADCOM GFA-545II. Has anyone modified this amp? I had to replace a failed bias resistor and put in larger supply capacitors during the repair.
Thanks again,
Matt
ADCOM GFP 555II
Hello Matt,
Over the years I have done several other mods to this unit such as adding mains filtration (.01 uf 1000v ceramic disks accross the incoming power line) putting caps accross the diodes (.01uf 200volt ceramics) and putting polypropalyne film caps accross the power supply DC filter caps to try to improve the sound. Also increased the power transistors bias current some. Didn't really notice any change with those, but they are still in there.
The SPARK KILLER I believe is accross the power on switch and should not be made much , if any larger as it will let AC thru. It's job is to tame the arcing when the power switch is open and closed.
Also, UL labs and manufacturers would not approve of my ceramics accross the power line as what they would use would be either X1 or X2 rated caps. These caps are much more fireproofed by being incased in epoxy and are designed to fail as a open rather than short. However they cost considerably more and I have used 1 to 5 thousand volt rated ceramics for this use for many years and have never had a failure. If you have strong radio frequency interference problem, adding these helps considerably.
If you search the web for X1, X2 & Y rated caps, you will find more details.
Doubt you would want to change the power supply to put out more voltage. Seventeen volts is about the max for most of the opamps you could consider using.
I have moded an ADCOM 2535 (dual 535's in one chasis) but do not know anything about your model.
Regards,
Greg
Hello Matt,
Over the years I have done several other mods to this unit such as adding mains filtration (.01 uf 1000v ceramic disks accross the incoming power line) putting caps accross the diodes (.01uf 200volt ceramics) and putting polypropalyne film caps accross the power supply DC filter caps to try to improve the sound. Also increased the power transistors bias current some. Didn't really notice any change with those, but they are still in there.
The SPARK KILLER I believe is accross the power on switch and should not be made much , if any larger as it will let AC thru. It's job is to tame the arcing when the power switch is open and closed.
Also, UL labs and manufacturers would not approve of my ceramics accross the power line as what they would use would be either X1 or X2 rated caps. These caps are much more fireproofed by being incased in epoxy and are designed to fail as a open rather than short. However they cost considerably more and I have used 1 to 5 thousand volt rated ceramics for this use for many years and have never had a failure. If you have strong radio frequency interference problem, adding these helps considerably.
If you search the web for X1, X2 & Y rated caps, you will find more details.
Doubt you would want to change the power supply to put out more voltage. Seventeen volts is about the max for most of the opamps you could consider using.
I have moded an ADCOM 2535 (dual 535's in one chasis) but do not know anything about your model.
Regards,
Greg
Above post correction
Hi again,
Correction -
Sorry, the increased bias current was from the 2535 amp not the preamp.
Greg
Hi again,
Correction -
Sorry, the increased bias current was from the 2535 amp not the preamp.
Greg
good day to all... in reply to X-CAPS being expensive, they can be scavenged from old computer p/s. you be amazed at what can be reclaimed from an old discarded atx p/s ...x-caps,IEC recepticals,diodes,regulators, etc...waste not want not.
Elwood
Elwood
GFP-555II Power Supply
Greg,
I was not considering increasing the PS voltage, only adding caps across the diodes. The holes are almost all there for the added caps. The "Spark Killer" is in series with a 3.92K Ohm resistor across the rectifier bridge (step down side of the transformer). The other version of the power supply in the schematic removes the cap and resistor across the bridge and adds the same value cap in parallel with each diode in the bridge. Would I violate code by using ceramics across the rectifier diodes?
I was also thinking of removing the resistor that shunts the op amp output to the negative supply (7.5K Ohm). I have read that they are supposed to force the op amp into class A operation but I don't see how that can be done outside of the op amp. Do they force one of the output transistors to carry all of the load with a constant DC offset?
Thanks for your guidance,
Matt
Greg,
I was not considering increasing the PS voltage, only adding caps across the diodes. The holes are almost all there for the added caps. The "Spark Killer" is in series with a 3.92K Ohm resistor across the rectifier bridge (step down side of the transformer). The other version of the power supply in the schematic removes the cap and resistor across the bridge and adds the same value cap in parallel with each diode in the bridge. Would I violate code by using ceramics across the rectifier diodes?
I was also thinking of removing the resistor that shunts the op amp output to the negative supply (7.5K Ohm). I have read that they are supposed to force the op amp into class A operation but I don't see how that can be done outside of the op amp. Do they force one of the output transistors to carry all of the load with a constant DC offset?
Thanks for your guidance,
Matt
Hi Matt,
Some very smart people went to considerable effort to design that amplifier. Don't second guess them on the electronics. If you wish to add RC components across the rectifiers - have at it. If you want to extend power switch life - go for it.
The amplifier circuits themselves do not have a whole lot wrong with them. Have a good look in there again. How many ceramic caps do you see on the voltage amp board? Do you see the supply bypass caps on the output boards?
Yep, pre-modified at the factory. Just return it to stock condition.
-Chris
Some very smart people went to considerable effort to design that amplifier. Don't second guess them on the electronics. If you wish to add RC components across the rectifiers - have at it. If you want to extend power switch life - go for it.
The amplifier circuits themselves do not have a whole lot wrong with them. Have a good look in there again. How many ceramic caps do you see on the voltage amp board? Do you see the supply bypass caps on the output boards?
Yep, pre-modified at the factory. Just return it to stock condition.
-Chris
Chris,
Your right, I don't want to go overboard but upgrading the op amps is definitely worth the trouble. I read an article in "The Audio Critic" and they mentioned the resistors (between the output and the negative rail) were meant to bias the op amps into class A operation. Perhaps the more recent "audiophile" op amps are internally biased for class A operation and don't need the added load resistor on the output (it is possible to remove parts as part of an upgrade).
The power supply capacitors were considered since they show up on the Adcom schematic for "Issue A" and would be easily added to the board.
Keep me grounded.
Matt
Your right, I don't want to go overboard but upgrading the op amps is definitely worth the trouble. I read an article in "The Audio Critic" and they mentioned the resistors (between the output and the negative rail) were meant to bias the op amps into class A operation. Perhaps the more recent "audiophile" op amps are internally biased for class A operation and don't need the added load resistor on the output (it is possible to remove parts as part of an upgrade).
The power supply capacitors were considered since they show up on the Adcom schematic for "Issue A" and would be easily added to the board.
Keep me grounded.
Matt
ADCOM 555II MODS
Hi Matt,
Yes, design of this unit is pretty good. But like any other commercial unit it was built to a price point. Designer was limited in quantity and quality of the parts he could use and still sell them for a profit at the price targeted.
Doesn't mean you can't subsitute better quality (or larger quantity - within limits) to try to get a better result.
Snubber you described as "spark killer" isn't what I originally thought you were talking about, but is a nice design feature. I do not have schematic where I can find it currently so am doing the best I can from memory.
Unless you go for major mods that require lots of money, parts and effort to fit them in, there are lots of mods you can try that are reasonably easy to undo if they do not give the results desired. Just be sure to listen carefully before and after as it is easy to fool yourself thru expecting a certain result.
Regards,
Greg
Hi Matt,
Yes, design of this unit is pretty good. But like any other commercial unit it was built to a price point. Designer was limited in quantity and quality of the parts he could use and still sell them for a profit at the price targeted.
Doesn't mean you can't subsitute better quality (or larger quantity - within limits) to try to get a better result.
Snubber you described as "spark killer" isn't what I originally thought you were talking about, but is a nice design feature. I do not have schematic where I can find it currently so am doing the best I can from memory.
Unless you go for major mods that require lots of money, parts and effort to fit them in, there are lots of mods you can try that are reasonably easy to undo if they do not give the results desired. Just be sure to listen carefully before and after as it is easy to fool yourself thru expecting a certain result.
Regards,
Greg
Hi Greg,
If you take the time to look at an Adcom of this period, you will find it was intelligently built. The only components you can "upgrade" is really going for a brand name. Not recommended.
Even the zener diodes for the op amp supply are very low noise types. Metal film resistors and film type caps where possible. Power supply decoupling on the output PCBs.
To my way of thinking (and some experience seeing what "tweakers" have done), you can only really damage the quality of these amps.
-Chris
If you take the time to look at an Adcom of this period, you will find it was intelligently built. The only components you can "upgrade" is really going for a brand name. Not recommended.
Even the zener diodes for the op amp supply are very low noise types. Metal film resistors and film type caps where possible. Power supply decoupling on the output PCBs.
To my way of thinking (and some experience seeing what "tweakers" have done), you can only really damage the quality of these amps.
-Chris
- Status
- Not open for further replies.