Re: Picture of the modification
You took all your stickied pieces of aluminum tape and the like off for your photo. It's OK. I wouldn't ridicule you.
*rimshot*
In all seriousness, since Ben Franklin got it wrong and textbooks are offered the in two different versions (trad. and electron flow), have you considered that the ground is actually the source...and that buffering the grounding scheme will result in far better currrent flow and a slight reduction in haze, within the circuit? It is a compact design and you should be rewarded when ground plane buffering, by a decrease in 'ground' noise, and a increase in dynamic correctness, due to better current flow. It may not need to be said to many/some folks here, but unimpeded ground current flow--is vital when it comes to high fidelity. Same rules as speaker wire-keep the diameter of the wire or the width of the traces below the skin effect point of the highest frequencies invovled. In the case of audio for humans, that is ~19 guage-no larger. No skin either, if possible. Complex transients and harmonics will still smear at that diameter, but the fundamentals will be relatively unharmed.
It's a bit more effort, but the effect is like that of going from an top flite electolytic to a film cap, overall. It's all cummulative, as we all know (I hope). Kitchen sinks and all that.
markk02474 said:
You took all your stickied pieces of aluminum tape and the like off for your photo. It's OK. I wouldn't ridicule you.
*rimshot*
In all seriousness, since Ben Franklin got it wrong and textbooks are offered the in two different versions (trad. and electron flow), have you considered that the ground is actually the source...and that buffering the grounding scheme will result in far better currrent flow and a slight reduction in haze, within the circuit? It is a compact design and you should be rewarded when ground plane buffering, by a decrease in 'ground'
noise, and a increase in dynamic correctness, due to better current flow. It may not need to be said to many/some folks here, but unimpeded ground current flow--is vital when it comes to high fidelity. Same rules as speaker wire-keep the diameter of the wire or the width of the traces below the skin effect point of the highest frequencies invovled. In the case of audio for humans, that is ~19 guage-no larger. No skin either, if possible. Complex transients and harmonics will still smear at that diameter, but the fundamentals will be relatively unharmed. It's a bit more effort, but the effect is like that of going from an top flite electolytic to a film cap, overall. It's all cummulative, as we all know (I hope). Kitchen sinks and all that.
kbk,
You said "...and that buffering the grounding scheme will result in far better currrent flow and a slight reduction in haze, within the circuit? It is a compact design and you should be rewarded when ground plane buffering, by a decrease in 'ground' noise, and a increase in dynamic correctness, due to better current flow... "
So, exactly what changes are you suggesting? What components/values etc?
You said "...and that buffering the grounding scheme will result in far better currrent flow and a slight reduction in haze, within the circuit? It is a compact design and you should be rewarded when ground plane buffering, by a decrease in 'ground' noise, and a increase in dynamic correctness, due to better current flow... "
So, exactly what changes are you suggesting? What components/values etc?
Re: Re: Picture of the modification
I agree that signal returns are very important and voltage variations in what should be "ground" are all too common and ignored. So, yes, compact design is good and thus surface mount components. Here, though, I wished for more room for film caps like there is on the commercial Aleph P and the popular version here. I'm not complaining loudly because the GFP-750 is a great cost-reduced Aleph P.
Huh?KBK said:
I agree that signal returns are very important and voltage variations in what should be "ground" are all too common and ignored. So, yes, compact design is good and thus surface mount components. Here, though, I wished for more room for film caps like there is on the commercial Aleph P and the popular version here. I'm not complaining loudly because the GFP-750 is a great cost-reduced Aleph P.
Re: Re: Re: Picture of the modification
Do a google search for "Peter Belt" Or a search here on the forum should suffice, it will make your eyes go crossed.
All I'm suggesting is to clear off ground traces and solder a solid copper OFC or better wire (mebe silver, if so inclined) of 19ga. wire to the ground traces. Tack at the trace corners and then proceed to solder the wire to the ground trace- in totality. It is a workable way to effectively add ground plane mass (and lower impedance in a Hz related neutral way. AC Hz - kHz skin considerations, etc) to the tracing, without imparing speed and creating eddy currents, skin effect issues, etc. I won't detail the expected sonic improvements, as that will color your expectations. Psychology 101 and all that. Other variables may swamp the noticed improvements, as all of these slight changes are barely, if at all, measurable. I do them all..as they are cummulative. One minor change toward clarity, creates a situation that makes other efforts more audible. You would not think that this would do much, but the ground does microscopically 'bounce' along with the signal imparted on the rest of the chain. Since these things affect transient signal considerations by the far greater degree, and the human ear hears via acting like a diode and listens only for transients and levels (time, level and shape of signal peaks)....these changes are well heard by the ear, even though they constitute a 0.01- 0.001% measurement change when measured the traditional way. It's a case of logical deduction applied to the aspects of circuit design and the human hearing function.
if you think of the esoteric aspects of audiophile phenomenon in that manner, the whole seeming 'circle jerk' of audiophile logic becomes abundantly clear in a scientifically and engineering way-as clarity.
Microscopics involving transients (within all aspects of a signal's handling, including physical motion) suddenly become brutally important. The light bulb goes on and light shines itself into the mystery. Suddenly audiophiles with golden ears aren't idiots, and neither are engineers whom the audiophiles berate as being deaf. What was missing was the connective point of why- for both parties. The answer is the analysis of the actual human hearing function.
I've been working on such aspects (and many others) for over 20 years now (over 10 years on this mentioned aspect), and those who hear our systems (mine and that of my business partner) are always completely blown away as they feel these systems are the most startlingly real sounding audio systems they have ever heard.
markk02474 said:
Do a google search for "Peter Belt" Or a search here on the forum should suffice, it will make your eyes go crossed.
All I'm suggesting is to clear off ground traces and solder a solid copper OFC or better wire (mebe silver, if so inclined) of 19ga. wire to the ground traces. Tack at the trace corners and then proceed to solder the wire to the ground trace- in totality. It is a workable way to effectively add ground plane mass (and lower impedance in a Hz related neutral way. AC Hz - kHz skin considerations, etc) to the tracing, without imparing speed and creating eddy currents, skin effect issues, etc. I won't detail the expected sonic improvements, as that will color your expectations. Psychology 101 and all that. Other variables may swamp the noticed improvements, as all of these slight changes are barely, if at all, measurable. I do them all..as they are cummulative. One minor change toward clarity, creates a situation that makes other efforts more audible. You would not think that this would do much, but the ground does microscopically 'bounce' along with the signal imparted on the rest of the chain. Since these things affect transient signal considerations by the far greater degree, and the human ear hears via acting like a diode and listens only for transients and levels (time, level and shape of signal peaks)....these changes are well heard by the ear, even though they constitute a 0.01- 0.001% measurement change when measured the traditional way. It's a case of logical deduction applied to the aspects of circuit design and the human hearing function.
if you think of the esoteric aspects of audiophile phenomenon in that manner, the whole seeming 'circle jerk' of audiophile logic becomes abundantly clear in a scientifically and engineering way-as clarity.
Microscopics involving transients (within all aspects of a signal's handling, including physical motion) suddenly become brutally important. The light bulb goes on and light shines itself into the mystery. Suddenly audiophiles with golden ears aren't idiots, and neither are engineers whom the audiophiles berate as being deaf. What was missing was the connective point of why- for both parties. The answer is the analysis of the actual human hearing function.
I've been working on such aspects (and many others) for over 20 years now (over 10 years on this mentioned aspect), and those who hear our systems (mine and that of my business partner) are always completely blown away as they feel these systems are the most startlingly real sounding audio systems they have ever heard.
anatech said:Hi Ken,
No such thing if you can solder to it.
-Chris
Yes..one must be careful what they say. more correctly....' Clear off ground traces to the bare copper, and then solder the wire on top of them.'
~~~~~~~~~~~~~~~
It reminds me of the literal take by the bounty hunters in the film "Domino". They were getting out of cell phone range, and the call from the (Delroy Lindro) boss, was 'spotty'..words missing.
Delroy wanted them to 'take the guy's arm, and read the tattoo'. He said it a few times.
What Domino heard, (she had been trained to NOT question orders and to carry them out-or people die) was....'take the arm'. She asked for clarity. She heard that twice (second time 'take his arm'), as the cell call scrambled in almost exactly the same way again.
So her compatriot..went to the back of the van (mobile home-headquarters)..Put his foot on the guy's sholder, stretched the guy's arm out..and used has 12 guage shotgun...and 'took the guy's arm'. Just below the shoulder.
Ouch.
~~~~~~~~
So be careful what you say out there.
it wasn't so bad. In the end, they were kind enough to give the guy his arm back.
I'm tempted to try building up the traces on power and ground.
Compared to no preamp or passive mode, I think slightly reduced soundstage width and focus is the biggest difference. I'm guessing its due to the shared transformer and regulator. Another modification thought I had was to add a capacitor between the regulator and the two resistors going to the cap for each channel for an extra RC filter stage. Perhaps I'd need an extra resistor between the regulator pass transistor and the added capacitor to not draw too much current at turn-on. Perhaps it should just be a small value or else have high ESR.
reg - 3R3 - C L ch.... to....reg- C - 3R3 - C L ch.
...+- 3R3 - C R ch......................+3R3 - C R ch.
Any thoughts?
Compared to no preamp or passive mode, I think slightly reduced soundstage width and focus is the biggest difference. I'm guessing its due to the shared transformer and regulator. Another modification thought I had was to add a capacitor between the regulator and the two resistors going to the cap for each channel for an extra RC filter stage. Perhaps I'd need an extra resistor between the regulator pass transistor and the added capacitor to not draw too much current at turn-on. Perhaps it should just be a small value or else have high ESR.
reg - 3R3 - C L ch.... to....reg- C - 3R3 - C L ch.
...+- 3R3 - C R ch......................+3R3 - C R ch.
Any thoughts?
Are your GFP-750 Mods worthwhile??
I realize it's been a couple of years since this thread's been active, but I just acquired a working GFP-750 dirt cheap ($20.00) at a garage sale. I've looked at the capacitor mods performed by markk02474 and others, but I'm not sure of the benefits in perceived performance these changes brought. Any body care to comment, in retrospect, if the mods you performed were worthwhile? Thanks, Denny
I realize it's been a couple of years since this thread's been active, but I just acquired a working GFP-750 dirt cheap ($20.00) at a garage sale. I've looked at the capacitor mods performed by markk02474 and others, but I'm not sure of the benefits in perceived performance these changes brought. Any body care to comment, in retrospect, if the mods you performed were worthwhile? Thanks, Denny
WoW! Great deal!!!
I guessing the input selector on your preamp has failed. Digikey is where I got mine, for around $50. It failed because the design runs the voltage to that part right around its limit. Its a common failure. You still made a great find.
Years after doing my mod, and about to do another unit soon, the most important change is less harshness and the place to start is with the no-cost wire replacement. There is a back to back pair of caps (to make it non-polar) that doesn't exist in the Aleph P. Its not needed, but probably for safety, limiting low frequency gain.
Do the other electrolytic caps at the same time as reassembling the unit is tedious with lining up the front panel LEDs.
Replace the power supply diodes with low-noise ones also. I've not done it in mine yet, but have in other gear and it reduces harshness and noise. If you don't hear any difference between active and passive mode, don't bother with mods - work on your other components first.
These preamps are getting old and so all the electrolytic caps have degraded in performance. Consider replacing all of them with new and better ones.
I'm not sure what electrolytics to use myself as Black Gates are getting more scarce and I've not done my research yet on alternatives.
I guessing the input selector on your preamp has failed. Digikey is where I got mine, for around $50. It failed because the design runs the voltage to that part right around its limit. Its a common failure. You still made a great find.
Years after doing my mod, and about to do another unit soon, the most important change is less harshness and the place to start is with the no-cost wire replacement. There is a back to back pair of caps (to make it non-polar) that doesn't exist in the Aleph P. Its not needed, but probably for safety, limiting low frequency gain.
Do the other electrolytic caps at the same time as reassembling the unit is tedious with lining up the front panel LEDs.
Replace the power supply diodes with low-noise ones also. I've not done it in mine yet, but have in other gear and it reduces harshness and noise. If you don't hear any difference between active and passive mode, don't bother with mods - work on your other components first.
These preamps are getting old and so all the electrolytic caps have degraded in performance. Consider replacing all of them with new and better ones.
I'm not sure what electrolytics to use myself as Black Gates are getting more scarce and I've not done my research yet on alternatives.
GFP-750 Service Manual Scans
Some of the Diodes (D32-D38)on the GFP-750 went up in smoke a few days ago and really made a stink . Also a black looking resistor named R30 next to the diode family smoked as well. I'm trying to figure out those part numbers so i can solder new ones in. Also I jumpered on a switch because the original one failed, and I was wondering that part number as well.
Basically could you email me a scan of the service manual. That would be just splendid
Cheers
Todd
Some of the Diodes (D32-D38)on the GFP-750 went up in smoke a few days ago and really made a stink . Also a black looking resistor named R30 next to the diode family smoked as well. I'm trying to figure out those part numbers so i can solder new ones in. Also I jumpered on a switch because the original one failed, and I was wondering that part number as well.
Basically could you email me a scan of the service manual. That would be just splendid
Cheers
Todd
Hello,
I just bought Adcom GFP-750 in great cosmetic condition. It works well exept one thing - right channel is noticably loudly than left channel, especially on low volume level. The same effect is on the passive mode. I must set up balance pot in 11 or 10-30 position. What may be the reason of this? May it be defect of volume pot? Is it interchangeable?
Sorry for my poor english.
DL
I just bought Adcom GFP-750 in great cosmetic condition. It works well exept one thing - right channel is noticably loudly than left channel, especially on low volume level. The same effect is on the passive mode. I must set up balance pot in 11 or 10-30 position. What may be the reason of this? May it be defect of volume pot? Is it interchangeable?
Sorry for my poor english.
DL
I have heard good things about the Elna Silmic II electrrolytics. These are available in 100V values for the Adcom GFA-750. I have heard that they compare favorably to Blackgates at a tenth of the price. They are available from Digi-key and Mouser.
As always with good parts, they are physically larger than the stock caps in my GFP-750. Mine are still breaking in but my first impression was they had deeper bass than the stock caps.
I am planning to change out the mylar bypass caps on the electrolytics with good polypropylene caps. So far I have put some 0.1 uF Panasonic P-series polys across the mylars on the underside of the board. This helps alleviate a rather dark,distant closed-in sound quality that I disliked in the stock GFP.
The next step I plan is to upgrade the stock audio rectifiers to shottkys, increase the C23 1st filter cap to 2200uf Nichicon, Replace the mylar bypass caps with 1uf Panasonic polypropylenes with 0.1uf bypasses, upgrade the output bypass to a Solen cap if it will fit. The good thing about this preamp is that there is plenty of space under the mounted pcb to add parts to the underside of the PCB. I am also hoping to replace the 20 odd ceramic shunts on all the inputs/outputs with 100pF Wima polypropylenes. I have my fingers crossed that they will all fit without crosstalk.
I found a PDF service manual online using Google. Diodes D19-D22 are plain 1N4004 rectifiers. Diodes D32-D38 are 9.1V Zener diodes. It is common for the Q28 pass transistor to fail. It originally was an IRF610 mosfet, and Adcom changed it to a 2SC4793 NPN transistor. The adjoining resistor values are changed to properly bias the pass transistor. The original R32 and R33 were 2.21K ohms. If your resistor is R30, then you have the NPN replacement. R30 is 221 ohms and R31 is 2.21K ohms with a 1N4004 in parallel. its possible the pass transistor has probably failed as well.
The schematic also shows that the 10uF electrolytics C163 and C171 in the back right of the board are only in circuit when the mono switch is engaged. it should have no effect in normal operation.
Thanks Mr Pass for such an excellent design.
As always with good parts, they are physically larger than the stock caps in my GFP-750. Mine are still breaking in but my first impression was they had deeper bass than the stock caps.
I am planning to change out the mylar bypass caps on the electrolytics with good polypropylene caps. So far I have put some 0.1 uF Panasonic P-series polys across the mylars on the underside of the board. This helps alleviate a rather dark,distant closed-in sound quality that I disliked in the stock GFP.
The next step I plan is to upgrade the stock audio rectifiers to shottkys, increase the C23 1st filter cap to 2200uf Nichicon, Replace the mylar bypass caps with 1uf Panasonic polypropylenes with 0.1uf bypasses, upgrade the output bypass to a Solen cap if it will fit. The good thing about this preamp is that there is plenty of space under the mounted pcb to add parts to the underside of the PCB. I am also hoping to replace the 20 odd ceramic shunts on all the inputs/outputs with 100pF Wima polypropylenes. I have my fingers crossed that they will all fit without crosstalk.
I found a PDF service manual online using Google. Diodes D19-D22 are plain 1N4004 rectifiers. Diodes D32-D38 are 9.1V Zener diodes. It is common for the Q28 pass transistor to fail. It originally was an IRF610 mosfet, and Adcom changed it to a 2SC4793 NPN transistor. The adjoining resistor values are changed to properly bias the pass transistor. The original R32 and R33 were 2.21K ohms. If your resistor is R30, then you have the NPN replacement. R30 is 221 ohms and R31 is 2.21K ohms with a 1N4004 in parallel. its possible the pass transistor has probably failed as well.
The schematic also shows that the 10uF electrolytics C163 and C171 in the back right of the board are only in circuit when the mono switch is engaged. it should have no effect in normal operation.
Thanks Mr Pass for such an excellent design.
Last edited:
GFP750 upgrades
I think that Adhocdiy should look at the 470uF 10V electrolytic capacitors used in the balance circuit first. I replaced mine with Elna Silmic IIs. I hope his problem is that simple.
I've done all the modifications to the Adcom that I mentioned in my last post. The Coupling caps are all Panasonic 250V metallized Polyproplylene types bypassed with 0.01 RelCap thetas.
The preamp sounds more open and transparent now. It has good depth and space to the soundstage. It is exceeded only by the Pass B1 line stage I built recently.
The treble is a little on the bright side for my tastes. If I could, I would try to replace all the Coupling caps with RelCap thetas, but I would have to hang them from the underside of the pc board.
I still enjoy the Adcom for the ease of its remote control.
I think that Adhocdiy should look at the 470uF 10V electrolytic capacitors used in the balance circuit first. I replaced mine with Elna Silmic IIs. I hope his problem is that simple.
I've done all the modifications to the Adcom that I mentioned in my last post. The Coupling caps are all Panasonic 250V metallized Polyproplylene types bypassed with 0.01 RelCap thetas.
The preamp sounds more open and transparent now. It has good depth and space to the soundstage. It is exceeded only by the Pass B1 line stage I built recently.
The treble is a little on the bright side for my tastes. If I could, I would try to replace all the Coupling caps with RelCap thetas, but I would have to hang them from the underside of the pc board.
I still enjoy the Adcom for the ease of its remote control.
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