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DIY Marantz 7 phono preamp

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Really? GNFB in a preamp? With equalisation? How does that work?

look here: cascaded 12ax7 has mu of 100 so open loop gain is 10000 ideally. but in reality much lower than that, perhaps around 2500 tops....furthermore, gain with feedback when the loop is closed is way lower than that..the RCA tube databook gives a design that did not use global feedback, it is also known as passive equalization, in contrast with this active equalization..

 
That's just the phono stage. It is NFB but not GNFB. The original claim was for the Marantz 7, not just the phono stage.

we are talking phono stage here, global as used here means from the output of second stage to the cathode of the first stage...

even the line stage section of the marantz 7 also used global feedback also to take the gain of the 12ax7 to line levels...
 
Now finalized, with a detailed SPICE model. All settled.
Thanks a lot to tron_ic for the valuable info in his site.


An externally hosted image should be here but it was not working when we last tested it.



An externally hosted image should be here but it was not working when we last tested it.

What if Unity Gain Baxandall Circuit is used from Practical Tone Controls. for tone and bass control with this Marantz 7 schematic? What will be the power supply design / current requirement in that case?
 
I'm not sure what you mean, 'cause the topic is the Marantz 7 phono preamp.


Btw, as yet said, half a 12AX7 isn't the best choice for a cathode follower. Replace it by (half a) 12AT7 or a 6AB4/EC92, do some minor tweaks for both cathode resistor values and get much lower output impedance, hence better defined RIAA eq characteristics.


Best regards!
 
Ideally, heaters would sit at several volts positive of the cathodes, but practically, large liberties are taken. Wouldn't at all be a bad idea, if you aren't wedded to making a clone, to elevate heaters maybe 25 Volts above peak cathode voltage. And along those lines, as everyone says, the type 12AX7 isn't ideal as the follower stage. For a DIY design, the type 6SN7 or the even bigger cathode type 5687 has the juice.


All good fortune,
Chris
 
If you are willing to use silicon the cathode follower might be best replaced with a high voltage MOSFET, it will require no elevated heater and it’s high transconductance will guaranteed good performance y’all want to choose the MOSFET with low reverse capacitance, there are plenty available that will meet this requirement
 
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