replacing ADCOM GFP750 caps?

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hi all,

having a little sucess recently in modding my cdp ,
am bout to mod my adcom gfp 750 pre......thinking of upgrading all the caps in the unit........currently its mainly JAMICON in the audio stage and PHILIPS in the power section.............any sugestions......... how much sonic improvment will i be able to get ........say if i change them all to blackgates or nichicon FG series..........


any comment???

thanx
jc
 
ha ..... have just finished the operation while watching COPA america Brazil vs Mexico live.........!!!


the preamp is still alive........... used Nichicon FG instead of Blackgates....... coz i already have them....... replaced with exact same value.........first impression ......everything sounds positive.... the BASS is wonderfull!!!!!!!! it's so tight and with authority....... but the mid high seems a little bit harsh.........maybe it's not burn in yet........... will use a burn in disc for the next 2 or 3 days..........actual result will only be known after it's been burn in i guess......

wonder how the BGs will sound like.........

:D
 
I'm surprised to see no mods for this preamp, so I'll list some!

The major evil in the preamp are the DC-blocking capacitors that the inputs go through. These are 10uf electrolytics bypassed with a 1uf polyester. It doesn't matter if the preamp is in active or passive mode, they are still in the signal path! This explains why many reviewers didn't find any improvement by using passive mode. These parts are in the back, right corner and marked C163, C164, C171, and C172. Jumper wire replacement is my personal preference, but DC offset from sources could reduce the life of the volume control. The volume control appears to be on both the input and output.

In each channel there are four sets of electrolytic caps with mylar bypasses that appear to be in the signal path. Some Black Gate N electrolytics with polypropelene bypasses could be substituted. The layout is compact, so options are limited.

There are two bridge rectifiers made up of 1N4004's, one for the 12V getting regulated to 5V for control circuits, and one for the +82V analog supply that gets regulated down to 60V. Substitue your favorite diodes for each application - I'm not sure which ones people here think are best, and will be reading posts.

Power supply caps for the 60V rails are generic electrolytics with 1uf polyester bypasses. Could be better.

I like having remote control, so I don't plan on using a stepped attenuator for the volume control.

I'll report on the effects of the mods as I do them!
 
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Hi markk02474,
Do they still use the switching IC's for signal switching? If so, they are the worst evil, followed by muting transistors, if used.

Your power supply caps are bypassed, so no gains worrying about caps there. The supplies are regulated, so don't get freaked out about the diodes. I find the 1N400X and 1N54XX series are just fine.

You may want to replace the coupling caps and bypass caps, but most improvements are very, very small with a switching IC in the signal path. Most of these tuner / preamps sound similar due to these signal switching IC's in my experience. I did do warranty service on Adcom, Carver and many others for years, that is where my experience is coming from.

-Chris
 
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Hi markk02474,
I'm liking the sound of this guy all ready.

The coupling caps will be your largest improvement. Supply issues can be seen by shorting your inputs and turning the gain up. Test equipment make this easier. I might just keep my eye's peeled for one of these units. It's sounds like its a vast improvement over what I've seen in the past.

-Chris
 
I just (literally) cut out the input DC blocking caps and put a wire accross what's left of the film caps! I'll take the circuit board out, desolder the leads and put the jumpers in properly when I do the other changes. For now, its fine and a good improvement.

Most noticeable was the increase in soundstage depth and focus. It's more 3d now. Bass got a little deeper and highs smoother. A nice change for the 5 minutes and $0 invested!

A few random notes: Audiogon has this preamp listed fairly often, but still not really cheap. One for sale had mods done by Stan Warren, with no details other than use of Black Gates. I like to get noisy diodes out of everything as they do pollute the mains and then other components, possibly because I'm not using boutique power cables yet.
 
I think I found another upgrade opportunity - a series pair of polarized 470u/10V electrolytics blocking DC to the balance pot, or perhaps limiting DC gain. I don't understand how they function in the circuit, but I don't think there is any voltage accross them, so I plan a single Black Gate N 470u/16v to replace the pair. These are C211, C212 and C291,C292.

Would a "super pair" of BG N 100uf/16v sound better than the single 470u?

If I don't mind DC gain, can I replace with wire? Like the DC blocking caps at the inputs, these are electrolytics without biasing and thus the worst kind - thus my preference for wire.

------ Some other tweeks perhaps-------
C29 is 100uF/100V filtering the zener reference in the power supply. Perhaps a low ESR electrolytic is better than a generic crappicon?

Do RF filtering caps on inputs and outputs make a difference? There are 100pF ceramics from all the signals to ground, and 1uf "layer" from signal ground to chassis ground.

I wish I had an Aleph L schematic to look at, they don't seem to be on Pass web sites.

Comments please?
 
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Hi markk02474,
I don't have a diagram for that one. In general you would want to avoid (at all costs) a DC potential across any pot carrying an audio signal. A good pot will be noisy with DC across it.

You could make DC servo boards and short the caps out. Expensive, but may bring a huge improvement in sound. I wonder why the value is so large?

-Chris
 
I wonder why the value is so large?
I wonder why you go through all those hmmm, problems/thoughts, with a preamp like that. You might have your sentimental reasons or something? I dont mean to sound sharp or anything , but being in the Pass department, here is the easy solution:D Strip the thing for its guts, and pop back in an X'ed BosoZ:D If you go totally wild, make it even a CCS'd one;)
Whatever the case, you will win, bigtime!

Steen :cool:
 
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Hi Steen,
I wonder why you go through all those hmmm, problems/thoughts, with a preamp like that. You might have your sentimental reasons or something? I dont mean to sound sharp or anything
No, it's a fair question.

I think that most designs are valid to some degree. I haven't seen this model, but it interests me. I'd improve it rather than junk it since it still has market value. I also don't believe the product is junk. This is a fair assessment from my point of view.

If you are going to build an X'ed BosoZ, then put it in a case designed for it.

Thinking about things and other designs is always a smart thing to do. Often you will run across other good ideas. "Cross pollination". :cool:

That's just my viewpoint. I suspect that Nelson Pass might agree. I also would agree there were shortcuts made, it's built for general sale.

-Chris
 
Modifying this preamp just seems to be less trouble than building a new one. Getting more from it seems pretty simple and I find it surprizing that hobbists have not done it before. So that makes it more interesting besides the satisfaction from making improvements to anything.

I was able to find aleph p schematics via google and it does indeed look like the balance pot adjusts the gain and the capacitors can be replaced with wire. This was done instead of independant L and R volume controls. Oh and I was wrong earlier, the volume control only acts on the output on the Adcom, as in other Pass designs.

One more upgrade candidate is the 330pF low pass cap near the input. There are four total, generic polypropelene's that could be more exotic like polystyrene or Teflon.

Schematics are available from Adcom, but I may post if I can access a scanner. There doesn't seem to be a copywright notice in the manual.
 
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Hi markk02474,
One more upgrade candidate is the 330pF low pass cap near the input. There are four total, generic polypropelene's that could be more exotic like polystyrene or Teflon.
Polypropelene capacitors are very close to polystyrene types, except they can take higher temperatures. Playing with these might be interesting and they will not cost as much as other things you may try.

I'm really interested in this preamp. It seems to be a departure in what Adcom had been doing when I sold my shop (in the right direction). It also seems as though it was very well made compared to normally available product. That is what brought joy to servicing audio so long ago.

-Chris
 
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