See the Jim Strickland design of the Acoustat TNP, for some possible clues. The schematic is out there.
In that unit, Jim used very lage value BP (220uf 35V )caps in the feedback circuit..which also involved or encompassed the balance control. The caps have to stay in circuit, removing them is out of the question, unless one wants to embark on a new design. since it's feedback (possibly) then caps are critcal components, when it comes to the SQ of the unit. Perhaps I'm an idiot when it comes to understanding teh schemactic. If I'm in error, I wouldn't be suprised. All I know, is the location of the caps in the Strickland design makes them critical.
What I did, is I put two static value resistors in there, and then put a 'shunt' volume type in there. I eliminated the balance control, and used a stepped attenuator for the shunt volume.
Just my guess.
In that unit, Jim used very lage value BP (220uf 35V )caps in the feedback circuit..which also involved or encompassed the balance control. The caps have to stay in circuit, removing them is out of the question, unless one wants to embark on a new design. since it's feedback (possibly) then caps are critcal components, when it comes to the SQ of the unit. Perhaps I'm an idiot when it comes to understanding teh schemactic. If I'm in error, I wouldn't be suprised. All I know, is the location of the caps in the Strickland design makes them critical.
What I did, is I put two static value resistors in there, and then put a 'shunt' volume type in there. I eliminated the balance control, and used a stepped attenuator for the shunt volume.
Just my guess.
steenoe said:
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 solutionStrip the thing for its guts, and pop back in an X'ed BosoZ
Whatever the case, you will win, bigtime!
Steen
I believe the GFP-750's circuit is very closely similar to or pretty much is the Aleph P pre-amp. I'll invest in one if I get a chance.
How similar is the GFP750 to the Aleph P?
Hi markk02474,
If you get a hold of the GFP750 schematic, please post!
I just got my hands on a GFP750 and I am very interestd in seeing what else it can do. As for it's stock form, I think it sounds great. No real critical listening yet.
I always wondered why Sterophile gave it Class A status and no one else ever really reviewed it.
I got it very cheaply because it has an issue with the input selector - but that is resolved by using the remote. I'll post more about that in a separate topic.
BTW, the guy who had the Stan Warren modified unit on Audiogon told me that all that was done was 10 caps were replaced with Blackgates. He did not have any more specific details:-(
Hi markk02474,
If you get a hold of the GFP750 schematic, please post!
I just got my hands on a GFP750 and I am very interestd in seeing what else it can do. As for it's stock form, I think it sounds great. No real critical listening yet.
I always wondered why Sterophile gave it Class A status and no one else ever really reviewed it.
I got it very cheaply because it has an issue with the input selector - but that is resolved by using the remote. I'll post more about that in a separate topic.
BTW, the guy who had the Stan Warren modified unit on Audiogon told me that all that was done was 10 caps were replaced with Blackgates. He did not have any more specific details:-(
Re: How similar is the GFP750 to the Aleph P?
Some people were making and selling products based on Pass designs posted on his site and his lawyers encouraged him to take down the schematics. With a little looking, I was able to find the Aleph P in hobbiest places, but don't think I should link here. Sorry.
I got my GFP-750 and a bunch of other goodies at the bankruptsy auction for Platinum Audio, a speaker company. My input selector was also broken, and the remote was missing. I got both parts from Adcom along with the service manual and a box. I paid $200 for the 750 and was going to sell it on audiogon until hearing it blow away my Audible Illusions Modulus 3. My low investment, its sound quality, and remote control all made me keep it. I since modified and re-tubed the AI and use it as a phono preamp. By the way, you can read the part number off the input selector and get it from Digikey.
Stan must be replacing (on each channel) the two input caps, the two output caps, and replacing two polar with one NP in the balance/gain network. On the Aleph P, the input caps are the same value, 10uF, but the outputs differ. Aleph P can be 3x10uF film caps or a single BG N 47uF/50v. That's probably going to be the replacements I use - 10uF and 47uF Black Gate N-types, since piles of film caps don't fit cleanly.
On further study, the input capacitors I jumpered out on the input (C163,164,171,172), should only be in the right channel signal path when MONO is selected. But, I swear the sound is better without them.
I have the schematic, its just a matter of scanning it. Its a little more complicated than the Aleph P 1.7. Constant current source (css) or X'd that people have been talking about, perhaps. I think this preamp is very much worth modifying due to its advanced design, and nice: relay switching, case, remote, power supply, fiberglass PC board, and clean lay-out. Part upgrades on a good design is pretty straight forward and should bring this up to Aleph P performance. Mr. Pass and Adcom built a nice, affordable (1/3 the price) Aleph P remote so many more consumers could get a taste of the high end.BlownRx7 said:
Some people were making and selling products based on Pass designs posted on his site and his lawyers encouraged him to take down the schematics. With a little looking, I was able to find the Aleph P in hobbiest places, but don't think I should link here. Sorry.
I got my GFP-750 and a bunch of other goodies at the bankruptsy auction for Platinum Audio, a speaker company. My input selector was also broken, and the remote was missing. I got both parts from Adcom along with the service manual and a box. I paid $200 for the 750 and was going to sell it on audiogon until hearing it blow away my Audible Illusions Modulus 3. My low investment, its sound quality, and remote control all made me keep it. I since modified and re-tubed the AI and use it as a phono preamp. By the way, you can read the part number off the input selector and get it from Digikey.
Stan must be replacing (on each channel) the two input caps, the two output caps, and replacing two polar with one NP in the balance/gain network. On the Aleph P, the input caps are the same value, 10uF, but the outputs differ. Aleph P can be 3x10uF film caps or a single BG N 47uF/50v. That's probably going to be the replacements I use - 10uF and 47uF Black Gate N-types, since piles of film caps don't fit cleanly.
On further study, the input capacitors I jumpered out on the input (C163,164,171,172), should only be in the right channel signal path when MONO is selected. But, I swear the sound is better without them.
After studying the various Aleph P schematics, I think the Adcom is virtually the same with just a few value changes. The Adcom has a potentiometer for volume instead of stepped attenuator, and has a balance control which is an elegent modifier of gain. The electrolytics in the gain network are omitted in the Aleph P. That's about it other than the cheap DC blocking caps on input and output.
I've attached a scan I made of the right channel so people can compare to the Aleph P 1.7. Part numbers differ for the left channel, so I suppose I can email a limited number of service manual scans out if you need them. My Black Gates should come in the next day or two, so I'll have a report soon on how the changes went. Of course, Black Gates won't achieve their ultimate sound quality 'till months of break in so my initial expectations are humble...
Later on I may try bypassing the BG's with old MIT multicaps I have lying around - 5uf PPMFX and .1uf RTX. My preference is to use just the big ones as the RTX tend to sound really nice up top and the sound below sounds like the bypassed cap. I rather have the coherency and the 5uf should reach down to about 40Hz, I think.
Later on I may try bypassing the BG's with old MIT multicaps I have lying around - 5uf PPMFX and .1uf RTX. My preference is to use just the big ones as the RTX tend to sound really nice up top and the sound below sounds like the bypassed cap. I rather have the coherency and the 5uf should reach down to about 40Hz, I think.
Mod completed
Tonight I replaced the electrolytic coupling caps with Black Gate N types and removed the polyester bypasses. It may sound a little better, but I have to give the caps time to break in. Passive mode is still more open and detailed. I'm somewhat jaded because I recently changed my CD63's op amps to $2.50 LM4562's and the improvement was huge. The time and expense here has not really paid off like that (yet?). This is an excellent preamp to begin with and the weaknesses I hear are only in relation to no preamp at all!
Changes:
Output coupling caps C189, C229, C269, C309: 100uf/100v replaced with Black Gate N 47uf/50v.
Input coupling caps C191, C231, C271, C311: 10uf/50v replaced with Black Gate N 10uf/50v.
Jumpered out: C211/C212, C291/C292
Polyester bypass caps removed: C192, C199, C232, C239, C272, C279, C312, C319
Power supply bypass caps added to C25, C27: 2.2uf/200v metalized polypropelene.
Notes:
1. 100uf replaced with 47uf because they are the same diameter and 47 is used in Aleph P. There might be a slight loss of bass.
2. Black Gates not bypassed by polyesters - supposedly Black Gates are so good that most any bypass is inferior. High quality bypasses will likely need pigtails because the board holes are small.
3. I added the power supply bypasses on general principal.
4. This was about 2 hours work - half was getting the board out and in the chassis. Hence, I didn't alternate small changes with listening tests.
I'll post a picture of the modified board along with additional listening reactions as the caps break in soon.
Tonight I replaced the electrolytic coupling caps with Black Gate N types and removed the polyester bypasses. It may sound a little better, but I have to give the caps time to break in. Passive mode is still more open and detailed. I'm somewhat jaded because I recently changed my CD63's op amps to $2.50 LM4562's and the improvement was huge. The time and expense here has not really paid off like that (yet?). This is an excellent preamp to begin with and the weaknesses I hear are only in relation to no preamp at all!
Changes:
Output coupling caps C189, C229, C269, C309: 100uf/100v replaced with Black Gate N 47uf/50v.
Input coupling caps C191, C231, C271, C311: 10uf/50v replaced with Black Gate N 10uf/50v.
Jumpered out: C211/C212, C291/C292
Polyester bypass caps removed: C192, C199, C232, C239, C272, C279, C312, C319
Power supply bypass caps added to C25, C27: 2.2uf/200v metalized polypropelene.
Notes:
1. 100uf replaced with 47uf because they are the same diameter and 47 is used in Aleph P. There might be a slight loss of bass.
2. Black Gates not bypassed by polyesters - supposedly Black Gates are so good that most any bypass is inferior. High quality bypasses will likely need pigtails because the board holes are small.
3. I added the power supply bypasses on general principal.
4. This was about 2 hours work - half was getting the board out and in the chassis. Hence, I didn't alternate small changes with listening tests.
I'll post a picture of the modified board along with additional listening reactions as the caps break in soon.
I really do not know much about all the part changes. I can tell you that the Adcom 750 in passive is pretty damn good. I base this on the following. Up until I got the 750 I ran my Wadia CD player straight into a Pass X-250. This sounded and sounds better than any preamp that I have ever used. But I wanted to run phono and fm tuner. So I neede a preamp. I borrowed a Pass X-1 for a couple of weeks. This is a great preamp. The CD player still sounded better straight through the power amp ( not a big difference at all through ). I picked up a Adcom 750. I run the CD player through the 750 this way. The 750 with the volume all the way up/open ( least resistants on the signal), then use the volume control on the Wadia just as I would running it straight through the power amp. I here some, but very, very, little influnence on the sound. I like the fact that I can still run the Wadia balanced. I think that the fact the there is very little influnce by the 750 on the CD player this way shows how good the stock 750 is. I feel there is less influnence on the signal running through the 750 passive than going through the Pass X-1. Active to active I would pick the Pass X preamps, but I use passive.
GE
GE
I've always thought of the idea of cranking up a set of balck gates to their maximum potential, and then physically 'whacking' them, to see if this alters their break in periods. Also, run spiked current through them (peak ripple and voltage). THEN install in the circuit.
If you've ever taken apart a black gate, and looked at them under mid power scopes or the like, you might get what I mean.
If you've ever taken apart a black gate, and looked at them under mid power scopes or the like, you might get what I mean.
Years ago there was a suggested break-in procedure for BGs. I assume the marketing people figured out that it scared off potential buyers as too complicated and it went away.
Similarly, I worry that I'm putting in non-polar caps backwards. I don't know which is the inner conductor, which lead its attached to, and if it should connect to the source or higher potential. I may have my BGs in backwards! What are the guidelines, anybody?
On active vs passive, the passive does have a wider soundstage, and for me too, passive is very close to no preamp at all. So, for soundstage width, a dual mono power supply would probably help. There are only separate RC sections now - not a bad compromise.
Compared to passive, detail is also slightly reduced, probably from the coupling caps, and the depth is a little less. I also noticed that a choke on the mains supply is not fitted, just jumpers. This is part L17 on the schemo and must have been changed in production. There are a few parts on the underside too, mostly diodes, probably to protect the constant current sources. I can also see the documented service note implemented where the voltage regulator was changed from a FET to a bipolar. The manual has these updates, but Adcom must never have re-spun the board (dual-layer fiberglass made in Taiwan, BTW).
The switching relays are labeled Siemens, and the volume and balance pots look like nice Alps ones. Still, there aren't switched attenuators and fancy RCA (or XLR) connectors so there are limits. Upgrading some of the connectors does seem practical, however...
Note, on the attached schematic, you can see C163/C164, C171/C172 that I previously jumpered out. They should only play a part during mono operation, but I think the sound is better without them in stereo too. One oops on my part is I forgot to upgrade C29 which filters zener noise. Oh, well, the unit isn't noisy anyway.
Similarly, I worry that I'm putting in non-polar caps backwards. I don't know which is the inner conductor, which lead its attached to, and if it should connect to the source or higher potential. I may have my BGs in backwards! What are the guidelines, anybody?
On active vs passive, the passive does have a wider soundstage, and for me too, passive is very close to no preamp at all. So, for soundstage width, a dual mono power supply would probably help. There are only separate RC sections now - not a bad compromise.
Compared to passive, detail is also slightly reduced, probably from the coupling caps, and the depth is a little less. I also noticed that a choke on the mains supply is not fitted, just jumpers. This is part L17 on the schemo and must have been changed in production. There are a few parts on the underside too, mostly diodes, probably to protect the constant current sources. I can also see the documented service note implemented where the voltage regulator was changed from a FET to a bipolar. The manual has these updates, but Adcom must never have re-spun the board (dual-layer fiberglass made in Taiwan, BTW).
The switching relays are labeled Siemens, and the volume and balance pots look like nice Alps ones. Still, there aren't switched attenuators and fancy RCA (or XLR) connectors so there are limits. Upgrading some of the connectors does seem practical, however...
Note, on the attached schematic, you can see C163/C164, C171/C172 that I previously jumpered out. They should only play a part during mono operation, but I think the sound is better without them in stereo too. One oops on my part is I forgot to upgrade C29 which filters zener noise. Oh, well, the unit isn't noisy anyway.
markk02474 said:I've attached a scan I made of the right channel so people can compare to the Aleph P 1.7. Part numbers differ for the left channel, so I suppose I can email a limited number of service manual scans out if you need them. My Black Gates should come in the next day or two, so I'll have a report soon on how the changes went. Of course, Black Gates won't achieve their ultimate sound quality 'till months of break in so my initial expectations are humble...
Later on I may try bypassing the BG's with old MIT multicaps I have lying around - 5uf PPMFX and .1uf RTX. My preference is to use just the big ones as the RTX tend to sound really nice up top and the sound below sounds like the bypassed cap. I rather have the coherency and the 5uf should reach down to about 40Hz, I think.
Adcom's implementation of a balance control is interesting! It looks like they are adjusting the gain of the mosfets.
This would seem to increase distortion near the extremes of the settings, but cleaver since balance is usually adjusted only slightly.
-David
Picture of the modification
Here is a picture showing the changes I made. That section is seven inches wide, showing the active circuit for both channels. Very compact, not allowing convenient use of large capacitors.
The red caps are Black Gate N types. The larger one is 47u/50v replaces a 100u/100v output cap. The empty space in front of it was its 2.2uf polyester bypass. The smaller BG is 10u/50v and the empty space in front of it and to the right of the larger BG is where its 1uf polyester bypass had been.
To the right of the first and third FETs on the bottom row are two circles where electrolytic caps in the gain setting network were. I put jumpers across where the negative leads were to maintain continuity.
In the top, right there are two sets of a circle and a square. This is where the DC blocking caps on the input for mono mode were. I cut out the electrolytics, leaving the circles, and also the polyester bypasses, leaving the squares. Then I soldered jumpers across what leads I left sticking up from the bypasses. This again takes capacitors out of the circuit and signal flows through jumpers instead. The black boxes across the top are the input relays.
The remaining (blue) polyester capacitors behind the larger BGs are 1uf/100v polyester power supply bypasses - one near each symmetrical half of each channel. I may upgrade these another time.
Here is a picture showing the changes I made. That section is seven inches wide, showing the active circuit for both channels. Very compact, not allowing convenient use of large capacitors.
The red caps are Black Gate N types. The larger one is 47u/50v replaces a 100u/100v output cap. The empty space in front of it was its 2.2uf polyester bypass. The smaller BG is 10u/50v and the empty space in front of it and to the right of the larger BG is where its 1uf polyester bypass had been.
To the right of the first and third FETs on the bottom row are two circles where electrolytic caps in the gain setting network were. I put jumpers across where the negative leads were to maintain continuity.
In the top, right there are two sets of a circle and a square. This is where the DC blocking caps on the input for mono mode were. I cut out the electrolytics, leaving the circles, and also the polyester bypasses, leaving the squares. Then I soldered jumpers across what leads I left sticking up from the bypasses. This again takes capacitors out of the circuit and signal flows through jumpers instead. The black boxes across the top are the input relays.
The remaining (blue) polyester capacitors behind the larger BGs are 1uf/100v polyester power supply bypasses - one near each symmetrical half of each channel. I may upgrade these another time.
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