Thanks for the help in locating the missing Marantz 7C 1M resistor I inquired about earlier.
Thought I'd discuss a little about what I'm planning to build and also have a question related to using separate RIAA circuit boards since the components can't have a home on the input selector switches as done with the Marantz 7C.
I've been planning and accumulating parts for building a 7C version somewhere between a bare-bones version presented by Preservation Sound a while back and the full Marantz 7C layout. The only thing I'm adding to Chris Ruggierio's stripped down version is the original tone amp section minus all of the filters.
Something I'm hoping someone can lend a hand to me with is determining best, most elegant resistor/cap board layout on a separate small turret board for the RIAA circuits. These will be built on one single turret board around 1.5" x 5" or two separate smaller turret boards. We're only talking 5 components per channel but for the life of me, I can't come up with something that flows logically and elegantly in a small space. Hoping you can post some sketches of what you think would be the best way to go. I will attach the RIAA board/boards on to the extended portion of the turret board shown in the attached images.
Included is an image of another persons build using on-board RIAA circuits attached to the main board instead of the selector switch and one showing my progress so far with the board and tube sockets assembly mounted together.
Any help will be appreciated.
Thought I'd discuss a little about what I'm planning to build and also have a question related to using separate RIAA circuit boards since the components can't have a home on the input selector switches as done with the Marantz 7C.
I've been planning and accumulating parts for building a 7C version somewhere between a bare-bones version presented by Preservation Sound a while back and the full Marantz 7C layout. The only thing I'm adding to Chris Ruggierio's stripped down version is the original tone amp section minus all of the filters.
Something I'm hoping someone can lend a hand to me with is determining best, most elegant resistor/cap board layout on a separate small turret board for the RIAA circuits. These will be built on one single turret board around 1.5" x 5" or two separate smaller turret boards. We're only talking 5 components per channel but for the life of me, I can't come up with something that flows logically and elegantly in a small space. Hoping you can post some sketches of what you think would be the best way to go. I will attach the RIAA board/boards on to the extended portion of the turret board shown in the attached images.
Included is an image of another persons build using on-board RIAA circuits attached to the main board instead of the selector switch and one showing my progress so far with the board and tube sockets assembly mounted together.
Any help will be appreciated.
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I think the 6800pF capacitor in the RIAA feedback network should be 5600pF.
I suppose someone did not read the correct value from the original schematic. I made the exact same mistake 35 years ago when I redrew the schematic from a blurred paper copy.
Torben
I suppose someone did not read the correct value from the original schematic. I made the exact same mistake 35 years ago when I redrew the schematic from a blurred paper copy.
Torben
Yes, you are correct, C15 definitely shows to be .0056 in the original 7C schematic and in looking at later articles, I don't see any improvement mods where a .0068 is suggested. Hard to believe that something as critical as the RIAA components that he would have made a mistake like that but truly did.
Thanks for the catch!
Thanks for the catch!
First off, please forgive the smudged image showing a potential somewhat reduced version of the 7C tone stack side minus the hi and lo filters and output trim pots.
My plan is to utilize a single gang pot per left and right channel for balance control instead of the dual ganged type in the original. The 7C used a dual ganged 1 meg pot for balance control.
Two questions on this: Can someone tell me how many ohms of the 1 meg total the balance pot presented to the circuit when at detent straight up 12 oclock position?
And, noticed Chris Ruggiero used a 500K balance pot instead of a 1 meg for his stripped down version. I happen to have on hand two PEC 500K single gang pots and was hoping to use them in place of the dual ganged 1 meg pot. Any reason this would be a bad idea from a functional or sonic standpoint?
My plan is to utilize a single gang pot per left and right channel for balance control instead of the dual ganged type in the original. The 7C used a dual ganged 1 meg pot for balance control.
Two questions on this: Can someone tell me how many ohms of the 1 meg total the balance pot presented to the circuit when at detent straight up 12 oclock position?
And, noticed Chris Ruggiero used a 500K balance pot instead of a 1 meg for his stripped down version. I happen to have on hand two PEC 500K single gang pots and was hoping to use them in place of the dual ganged 1 meg pot. Any reason this would be a bad idea from a functional or sonic standpoint?
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That would be fine, no problem. The phono stage, or any line input, can easily drive such high resistances.
The normal setting of the Marantz balance control has 500k in series, and 500k in shunt, for each channel.
Removing that control will give you +6dB more gain for all inputs, and 6dB lower noise at a given
playback volume. This is very worthwhile to do.
After replacing the original pot with two single pots, you would normally have both pots at maximum,
and only reduce one if needed for channel balance. This is much better than using the original pot.
The normal setting of the Marantz balance control has 500k in series, and 500k in shunt, for each channel.
Removing that control will give you +6dB more gain for all inputs, and 6dB lower noise at a given
playback volume. This is very worthwhile to do.
After replacing the original pot with two single pots, you would normally have both pots at maximum,
and only reduce one if needed for channel balance. This is much better than using the original pot.
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Marantz 7C has an ill fated phono design as it uses cathode follower on phono and line output. Heard it once some decades ago and forgot about it.
Could be done much smarter, but the Marantz guys don't know how. They use first rate components and build style tough. The asians know that, but at first they buy famous brand names only. Thats why this is so sought after.
Shindo does is better, even EMT did better. Because they have heard and listened to this Marantz design and improved upon.
Good luck with this project!
Could be done much smarter, but the Marantz guys don't know how. They use first rate components and build style tough. The asians know that, but at first they buy famous brand names only. Thats why this is so sought after.
Shindo does is better, even EMT did better. Because they have heard and listened to this Marantz design and improved upon.
Good luck with this project!
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Why do you say that - what's the issue with a cathode follower? The only thing I can see is the number of coupling caps around the RIAA feedback. ARC seem to have a better approach.
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A cathode follower is only used for its low output impedance- that benefit you pay with a fully 100% negative feedback amp stage with gain smaller than one. There is no free lunch in audio.
Musicians use it only for their tone stack stages, because they need it. They don't love it sonically. And this is always the case, nobody loves to have a zero gain stage 100% negative feedback in an amp. If its not avoidable, only then its used. Marantz had two in series with the signal.
What the Marantz guys want is a low output impedance phono circuit, and thats because they knew that high impedance in combination with this kind of active neg. feedback tends to sound cloudy. What they got is a relative lame sounding phono stage. And thats what a trained ear easily can audition in comparison with better phono stages.
So they wanted to achieve one thing- but had to pay with another. Bad deal. They could have done smarter in the phono section, and they could have done better in the line section, but that would have been out of their budget.
I don't liked the 7C. It was not bad sounding, it was just average. But the asians love it, so its up to you to make up your mind.
Musicians use it only for their tone stack stages, because they need it. They don't love it sonically. And this is always the case, nobody loves to have a zero gain stage 100% negative feedback in an amp. If its not avoidable, only then its used. Marantz had two in series with the signal.
What the Marantz guys want is a low output impedance phono circuit, and thats because they knew that high impedance in combination with this kind of active neg. feedback tends to sound cloudy. What they got is a relative lame sounding phono stage. And thats what a trained ear easily can audition in comparison with better phono stages.
So they wanted to achieve one thing- but had to pay with another. Bad deal. They could have done smarter in the phono section, and they could have done better in the line section, but that would have been out of their budget.
I don't liked the 7C. It was not bad sounding, it was just average. But the asians love it, so its up to you to make up your mind.
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But we use unity gain buffers with 100% feedback all over the place. Its an op-amp. It maybe the circuit has some sort of short fall, I don't know without simulation. There are electrolytics in the signal paths could this be what you hear.
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Any chance you could point me in the direction of anything showing the series/shunt layout of this type of balance pot. I guess they were shooting for a constant load impedance?
No success can be derived from an ecc83 cathode follower and here it is done twice or should I say concurrently.
Must have had an over stock of this valve that had to used......
Must have had an over stock of this valve that had to used......
See your diagram in post #4. Center of rotation is the normal setting for that pot.
Marantz used a standard dual pot for the balance control, that's why it is normally at half rotation.
Others (like Dynaco) used a special dual pot with a shorting track for half of the rotation in each section,
so at the center setting there was no series resistance, and no needless level loss happens.
See the schematic here, figure "B". Notice the two pot sections are wired opposite to each other,
so that when one section attenuates, the other pot section output remains constant.
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True. But maybe thats what it sounded like. A little emotionless.
If I hear tube gear, its just that I expected something more than an OP-amp realised with a tube.
I could be wrong, just my 2c.
Good luck restoring it!
Yes, strange indeed. Today its so sought after, but mostly don't be used for cathode follwers...think this design doesn't cut the grade. But great parts nevertheless. Compare with todays Marantz suckers. Thats why they love it so much.
I follow now, makes sense. I had jumped off track and was thinking more along the lines of an Lpad. I had seen the drawing showing the pot Dynaco uses and didn't look long enough to see the symbols representing the shorting tracks. Interesting device.
Thanks for the info!
If you build that circuit, connect the top lead of the 1.5nF RIAA capacitor
to the left side of the output coupling capacitor, instead of to the right side.
You don't want it messing up the RIAA network.
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I think that was done to keep the voltage across the 1n5 lower. 1n5 || 2u2 is still about 1n5. I suppose if the output load is low it could affect the response. I prefer this as it does not have electrolytic in the signal path and has one coupling caps in the FB path to likely to be more stable at LF. Mind would use a ECC88 rather than a 12AU7.
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Patrick Turner's layout goes a long way towards improving the classic (McIntosh, Marantz, etc.) phono equalizer, in the tradition of Audio Research Corp in the late 70's, early 80's.
One possible issue can be raised in the matter of biasing. Even the best valves, including the classic European ECC83's to some extent, is that their grid current isn't really zero, and varies some with voltage. Biasing the valves at significantly less than a Volt makes for a less linear operation than might be expected, so should be avoided as much as possible.
The overall plan of a stout cathode follower (6SN7, 5687, etc. are also great here if you have enough filament current available) and a higher than conventional feedback network impedance are the secret sauce.
YOS,
Chris
One possible issue can be raised in the matter of biasing. Even the best valves, including the classic European ECC83's to some extent, is that their grid current isn't really zero, and varies some with voltage. Biasing the valves at significantly less than a Volt makes for a less linear operation than might be expected, so should be avoided as much as possible.
The overall plan of a stout cathode follower (6SN7, 5687, etc. are also great here if you have enough filament current available) and a higher than conventional feedback network impedance are the secret sauce.
YOS,
Chris