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DIY Marantz 7 phono preamp
DIY Marantz 7 phono preamp
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Old 3rd January 2019, 07:05 PM   #1
vpapanik is offline vpapanik  Greece
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Default DIY Marantz 7 phono preamp

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).

Click the image to open in full size.

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

Last edited by vpapanik; 3rd January 2019 at 07:10 PM. Reason: change image
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Old 3rd January 2019, 07:58 PM   #2
ejp is offline ejp  Australia
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There are zero volts at one end of those capacitors, and about 2 volts at the other (cathode) end. So you hardly need a 300V or 500V rating. Get the highest precision you can find or afford.
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Old 3rd January 2019, 09:52 PM   #3
Eli Duttman is offline Eli Duttman  United States
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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".
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Old 3rd January 2019, 09:55 PM   #4
vpapanik is offline vpapanik  Greece
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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.
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Old 3rd January 2019, 10:34 PM   #5
cjkpkg is offline cjkpkg  United States
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Why not start with something simpler like this?
RJM Audio - Tube passive phono preamplifier
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Old 4th January 2019, 01:15 AM   #6
TonyTecson is offline TonyTecson  Philippines
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DIY Marantz 7 phono preamp
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...
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Old 4th January 2019, 01:18 AM   #7
TonyTecson is offline TonyTecson  Philippines
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DIY Marantz 7 phono preamp
Quote:
Originally Posted by ejp View Post
There are zero volts at one end of those capacitors, and about 2 volts at the other (cathode) end. So you hardly need a 300V or 500V rating. Get the highest precision you can find or afford.
we have to realise that at the time of the production of this amp, 63v, 100v and maybe 250 volt caps were not yet in full production then, 500/600 volts are plenty and cheap....i am sure designers of that era would be very glad to be living in our today's world...
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Old 4th January 2019, 06:50 AM   #8
Herrmann is offline Herrmann  Denmark
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Quote:

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.
I would say R68=4.7 k is the correct value. It has the same value as the cathode resistor (4.7k) at the input valve. Otherwise the voltages at the cathodes and anodes in first and second stage would differ, which they do not (according to the schematic).
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Old 4th January 2019, 07:39 AM   #9
vpapanik is offline vpapanik  Greece
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Thank you all for your answers ! I will keep the 4.7k value of the schematic.
Do you think that the 1/4W metal film resistors would suffice or I really have to go for 1/2W ?



PSU is the next I have to consider now...
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Old 4th January 2019, 08:47 AM   #10
ejp is offline ejp  Australia
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You probably need ONE watt resistors, not for the wattage rating but for the voltage rating.
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