Hello all ! This is my very first post on the forum, so please be kind 
I am planning to build my first tube phono preamp, so I made a rigorous search for available schematics and decided to move on with an easy-to-build clone of the well-known Marantz 7 model. I also read a lot of stuff about this build, so I am feeling a bit more confident in order to write here.
Having looked at a lot of different schematics, trying to spot differences and errors, I am about to make a passive component selection to begin with.
I think that the following is the most 'correct' schematic I've found, compared to the original Marantz 7 design (without tone controls).
I have a couple of specific questions :
1) For the two capacitors of the RIAA equalization feedback (5.6n and 1.6n), what is the appropriate tolerance and voltage ? I read about 1% and 300 or 500V, but I am not sure what to select (and they are pretty expensive).
2) I have compiled a preliminary BOM, which I am attaching here. Do you spot any bad selections in terms of component type, tolerance or voltage ? Of course these are not top-notch components, I am trying to keep the cost to a reasonable level without losing a lot of output quality. 2019-01-03-22-08-08 — imgbb.com
Thank you so much in advance !
Vassilis
I am planning to build my first tube phono preamp, so I made a rigorous search for available schematics and decided to move on with an easy-to-build clone of the well-known Marantz 7 model. I also read a lot of stuff about this build, so I am feeling a bit more confident in order to write here.
Having looked at a lot of different schematics, trying to spot differences and errors, I am about to make a passive component selection to begin with.
I think that the following is the most 'correct' schematic I've found, compared to the original Marantz 7 design (without tone controls).
An externally hosted image should be here but it was not working when we last tested it.
I have a couple of specific questions :
1) For the two capacitors of the RIAA equalization feedback (5.6n and 1.6n), what is the appropriate tolerance and voltage ? I read about 1% and 300 or 500V, but I am not sure what to select (and they are pretty expensive).
2) I have compiled a preliminary BOM, which I am attaching here. Do you spot any bad selections in terms of component type, tolerance or voltage ? Of course these are not top-notch components, I am trying to keep the cost to a reasonable level without losing a lot of output quality. 2019-01-03-22-08-08 — imgbb.com
Thank you so much in advance !
Vassilis
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Vassilis, I suggest you pause and think things over, 1 more time. The 12AX7/ECC83 triode makes a poor cathode follower. That circuitry uses a 'X7 cathode follower to drive both the active RIAA EQ NFB loop and the downstream load. That's most definitely not "the greatest thing since sliced bread".
Thanks a lot ejp. You are right, I was confused because those Mica caps actually start from 300V and up !
An hour ago, I was lucky to find the service manual of Marantz 7C, so I was able to cross check all components. It seems that I have to increase the power of 1/4W resistors to 1/2W.
hfe_marantz_7c_service_en.pdf - Google Drive
And I also found a 50-yr old typo : The actual value of R68 is 6.2K (parts list) instead of 4.7K (schematics). I really don't know if something will change, but just noting this.
An hour ago, I was lucky to find the service manual of Marantz 7C, so I was able to cross check all components. It seems that I have to increase the power of 1/4W resistors to 1/2W.
hfe_marantz_7c_service_en.pdf - Google Drive
And I also found a 50-yr old typo : The actual value of R68 is 6.2K (parts list) instead of 4.7K (schematics). I really don't know if something will change, but just noting this.
actual parts do change in the course for the production life of the unit, i wouldn't consider that as a mistake at all...
6.8k in the cathode position gives lower cathode current and so higher plate voltage than a 4.7k resistor would...this is not surprising as well...
the fact that this preamp used global negative feedback to set the closed loop gain/and response ensured that the freq. response of the amp is maintained from unit to unit and as tubes ages..
i am amazed at how folks still want this after the bloody debate between the feedback and no gnfb camps...
6.8k in the cathode position gives lower cathode current and so higher plate voltage than a 4.7k resistor would...this is not surprising as well...
the fact that this preamp used global negative feedback to set the closed loop gain/and response ensured that the freq. response of the amp is maintained from unit to unit and as tubes ages..
i am amazed at how folks still want this after the bloody debate between the feedback and no gnfb camps...
Hello Vpapanik,
The phono of Marantz 7 is part of me and many others of the Top 5 Preamplifier Phono. So you made an excellent choice.
Personally I know this circuit well, I even realized a specific pcb and optimized certain points such as the decoupling of triodes for example.
You can without going to read and see the article I made about it here: Officina Tron-audio Switzerland: Preamplificateur phono MM Marantz Replica
Note that there are also some other items that could also interest you in the DIY section.
See you soon. Greetings. Tony
The phono of Marantz 7 is part of me and many others of the Top 5 Preamplifier Phono. So you made an excellent choice.
Personally I know this circuit well, I even realized a specific pcb and optimized certain points such as the decoupling of triodes for example.
You can without going to read and see the article I made about it here: Officina Tron-audio Switzerland: Preamplificateur phono MM Marantz Replica
Note that there are also some other items that could also interest you in the DIY section.
See you soon. Greetings. Tony
Thanks very much tron_ic for your very impressive work ! It looks like a top-quality preamp with high-end components, as I can figure out from the photos.
The only difference I spotted compared to other designs is the order of the 1600pf capacitor and the 1K resistor in the RIAA circuit. In some schematics the RIAA starts from the capacitor, but in others (like yours) it starts from the resistor. I don't know if it is the same or not.
Does this 'option' resistor correspond to the 1M in the original schematic ? Why is it optional ?
You also used 220uF for both electolytics instead of 200 and 100.
Is it possible to post a higher resolution image of the schematic and a list of components ? That would be really helpful, thanks a very lot !
The only difference I spotted compared to other designs is the order of the 1600pf capacitor and the 1K resistor in the RIAA circuit. In some schematics the RIAA starts from the capacitor, but in others (like yours) it starts from the resistor. I don't know if it is the same or not.
Does this 'option' resistor correspond to the 1M in the original schematic ? Why is it optional ?
You also used 220uF for both electolytics instead of 200 and 100.
Is it possible to post a higher resolution image of the schematic and a list of components ? That would be really helpful, thanks a very lot !
Came up with a first schematic, can you take a look ?
I assumed a B+ point current of 2mA and the PSU gives 278V.
Any comments are greatly welcome !
I assumed a B+ point current of 2mA and the PSU gives 278V.
An externally hosted image should be here but it was not working when we last tested it.
Any comments are greatly welcome !
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