Hi All,
A friend of mine sent to me a board for this old preamp based on Marantz 7. As I need a pre with a veri high gain I thought to this pcb that lays in a drawer.
I thought to eliminate the NFB to improve gain (and maybe sound), but I need your advice for any other improvement.
A friend of mine sent to me a board for this old preamp based on Marantz 7. As I need a pre with a veri high gain I thought to this pcb that lays in a drawer.
I thought to eliminate the NFB to improve gain (and maybe sound), but I need your advice for any other improvement.
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Gut reaction is to mount the caps under the board, away from the tubes for longevity.
I'd probably look at a choke in the power supply too rather than just relying on an RC. Edit: I see your version has a regulator. I assume that includes the heaters?
I'd probably look at a choke in the power supply too rather than just relying on an RC. Edit: I see your version has a regulator. I assume that includes the heaters?
What are you going to do with all the 12AX7s then? There is no way to keep all the useless high mu tubes and eliminate nfb. As for the gain, it can probably be tweaked into something more usable by the nfb resistors.
what are your source devices and their output voltage levels?
what is your amp's input impedance and input voltage for full output?
what is your amp's input impedance and input voltage for full output?
Ok, that makes a big difference. You can simply adjust the gain to match your requirements. Doubling the 24k resistor will give you ample gain. Another option is to cut down one of the voltage gain stages and compare this to the nfb circuit.
Neither of the schematics has any relation to a Marantz 7, and only one could even possibly apply to the pictures.
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A normal power amp has gain about 20.
The Mofo is about unity.
Therefore you increase the NFB ratio about 20X to get good level matching.
I'm not sure which schematic you have, but roughly like the attached.
This also reduces the NFB from over 30dB to a mere 8dB. It will not be "dead clean" like the Marantz. But maybe that is what you want? (A $5 high-volt opamp would make dead-clean, and a lot cheaper.)
Bypassing the 1k at 2nd stage cathode would slightly increase the effective NFB. Better? Worse? Up to you.
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