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Marantz 7 Tube pre-amplifier PCB Board Design

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MykhailoM

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Joined 2012
#1
Hi Guys!

Those who have technical knowledge or understanding of Schematics and design, can you please have a look at the this Marantz 7 Valve pre-amplifier board clone design if it is ANY GOOD for buying it and changing parts or the design in itself is no good enough or conflicting in itself?!

I can see there are 3 Valves used for stereo signal looks as 1 and 2nd in mono mode and 1 Valve processing stereo signal, is this a good idea or is there a need to use a third Valve for that or any other matter?

Product Link: JBH 12ax7 tube preampliifer 2014 special replica Classic M7 pre amp HIFI EXQUIS amp manufacturer latest master piece-in Amplifier from Consumer Electronics on Aliexpress.com | Alibaba Group

Schematic Link: https://goo.gl/photos/X2MLePfrQFPmQy637

Any inputs highly appreciated.
Mykhailo
 
#3
Hmm--Looks like yet another case of the Chinese not thinking for themselves and churning out Yet Another Rip of an old classic design, trying to gain sales by the use of the old maker's names....

Its really nothing special and can be bettered by a more up to date design, that has a more appropriate gain.
 
#4
Hi Alastair,

Can you please explain what problems you see in gain and what should be best to have it right in it. As I don't have a colleague degree in ellectronics for me is hard to understand what is wrong in the design here and if possible what can be done to better it or wheather it make sanse using it at all.

Thanks
 
#5
Most signal sources these days, CD, DVD players, AV sources etc have a pretty high output in comparison to when that 50's design was made.

The average Stereo Amplifier these days has been designed to accept this higher level level of 'Line-Level' input....

So, if you add this pre-amp you'll be increasing that Line-Level well above what your amplifier was designed for, and attenuating it back down to that usable by your current amp--There's NO point!

I can see NO point in using this device in your system--It would be just another item that will add its own distortions/noise/hum to the sound.

Sometimes, Less IS better.....

Can you tell me--WHY you feel it would be a good addition to your system as it stands....?
 
#6
the problem with the design it self, is not the circuit ,but implementation. The original design used the 12AX7's because of the use of passive tone controls which attenuated the overall gain of the previous stage, this has no tone controls, so there is no need for the use of such a high mu tube for a line stage such as this!!
 
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#11
How good it sounds I don't know, but you must see the original what is it intended for: everything! The pot which adjust the line and CD gain is omitted from your sch, but add tube rectifier, maybe less noisy than original. You sch is the left half, the front is RIAA eq.
 
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#12
Is there a way to lower the gain by changing specific parts inside or capacitors to sort this problem?


The reason I chose this valve pre-amp is its point to point soldering and simplicity to modify parts as attractive solution to improve its sound quality. But if there is a real problem in it as mentioned due to gain being way out of the limit that is acceptable than - it is a problem. Just to be clear we aren’t talking a high gain due to its Valves or Pubes used !? because the tubes can be changed - of course if it solves the problem which unlikely it will..
 
#13
Koonw said:
How good it sounds I don't know, but you must see the original what is it intended for: everything! The pot which adjust the line and CD gain is omitted from your sch, but add tube rectifier, maybe less noisy than original. You sch is the left half, the front is RIAA eq.
Click to expand...

I would think a purist approach to the Marantz 7 would be to keep the original Phono stage, regulated B+, and Heaters, and redesign the line stage from scratch, either with a Mu-Follower, or Common Cathode gain stage w/cathode follower, using a low mu tube, and negative feedback, w/ no tone controls. I'm thinking 12dB gain or lower in the line stage.
 
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#14
Is there a way to lower the gain by changing specific parts inside or capacitors to sort this problem?


The reason I chose this valve pre-amp is its point to point soldering and simplicity to modify parts, as attractive solution to improve its sound quality. But if there is a real problem in it as mentioned due to gain being way out of the limit that is acceptable than - it is a problem. Just to be clear we aren’t talking a high gain due to its Valves or Pubes used !? because the tubes can be changed - of course if it solves the problem which unlikely it will.
 
#15
MykhailoM said:
Is there a way to lower the gain by changing specific parts inside or capacitors to sort this problem?


The reason I chose this valve pre-amp is its point to point soldering and simplicity to modify parts as attractive solution to improve its sound quality. But if there is a real problem in it as mentioned due to gain being way out of the limit that is acceptable than - it is a problem. Just to be clear we aren’t talking a high gain due to its Valves or Pubes used !? because the tubes can be changed - of course if it solves the problem which unlikely it will..
Click to expand...

Yes, the pot. you have is for volume control, the original has another 500K pot before it, so it will cut the input to a lower level better for your volume control. That is solution from original, you can easy add one even fix. You better not touch the rest otherwise it might sound very different.
 
#16
It's an incredibly mediocre sounding design by today's standards IMO, and there are much better designs out there today. Have a look around analog source for phono stages most of which are going to perform better by today's criteria. (Less noise, better RIAA accuracy, appropriate gain levels)

Same story with line stages. Look at the Aikido or any by Frank DeGrove amongst others.
 
#17
Koonw said:
Yes, the pot. you have is for volume control, the original has another 500K pot before it, so it will cut the input to a lower level better for your volume control. That is solution from original, you can easy add one even fix. You better not touch the rest otherwise it might sound very different.
Click to expand...

So that I understand it correctly, the solution is to add another pot (volume control or potentiometer) in addition to what is in there, ONLY to control the Input level, is that right?
 
#18
MykhailoM said:
So that I understand it correctly, the solution is to add another pot (volume control or potentiometer) in addition to what is in there, ONLY to control the Input level, is that right?
Click to expand...

You'r right, just add a 470K resistor on top of volume control, and wire the input on top of resistor and ground will do for CD.
 
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#19
MykhailoM said:
Is there a way to lower the gain by changing specific parts inside or capacitors to sort this problem?


The reason I chose this valve pre-amp is its point to point soldering and simplicity to modify parts, as attractive solution to improve its sound quality. But if there is a real problem in it as mentioned due to gain being way out of the limit that is acceptable than - it is a problem. Just to be clear we aren’t talking a high gain due to its Valves or Pubes used !? because the tubes can be changed - of course if it solves the problem which unlikely it will.
Click to expand...

The design would be fine if you have a the tone controls, then you could call it an original Marantz 7 line stage. Anything else is?? I'm not sure! Listen to what Kevin has said, He wont lead you wrong on this!!
 
#20
kevinkr said:
It's an incredibly mediocre sounding design by today's standards IMO, and there are much better designs out there today. Have a look around analog source for phono stages most of which are going to perform better by today's criteria. (Less noise, better RIAA accuracy, appropriate gain levels)

Same story with line stages. Look at the Aikido or any by Frank DeGrove amongst others.
Click to expand...

I found these thread, An Aikido/Tetra Preamp : http://www.diyaudio.com/forums/tubes-valves/175683-my-first-diy-experience-aikido-tetra-preamp.html
In regards to Frank DeGrove tubes design are there any schematic and photos available of his work for pre-amplifiers?
 
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