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. . . a specific question regarding the MB and NF. Years ago I built the TS Tube Analyzer and finally got my act together to purchase a Mini Beast . . .I had to pay off all those bloody expensive IT & OPT transformers first! I want to experiment with the two resistors associated with the NF (R4, R11) and hear the changes for myself . . . I've done this with other projects but always need a bit of this kind of guidance from helpful forum members. My Questions: 1 - Do I need to increase or decrease R4 & R11 (3K) to decrease NF?? 2 - What is the approximate NF dB level at 3K? Thanks for your assistance. (using the MB in a second system: Cambridge CXC Transport, DDDAC 1794 NOS kit, DIY/AN combo preamp, and Club-27 Amy speakers kit) Regards, Mike Anchorage, Alaska
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I have not the schematics, but I always think of it so: if you disconnect the resistor from output, the resistance is infinity and feedback zero. Obviously decreasing this resistor increases the feedback.
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Yes, the net gain of this circuit is roughly the ratio of the feedback resistor, R4, to the the cathode resistor,R3, of V1. That is 3k/499 or about 6x or ~15dB. The specs give 15dB. To say what is the dB feedback you need to know the open loop gain. I have not calculated this , but if we take 100X or 40dB, which is probably too low, then the feedback would be ~25dB.
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You should not change the NF. But increasing the resistor decreases the NF. NF stabilizes the amp. and tightens the bass. Unless you only listen to Diana Kroll it will sound sloppy terrible. You will also have way to much gain.
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Thanks for the responses.
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Instead of changing the NF resistor value, try replacing it with a same value naked foil resistor (Vishay, Caddock, Charcroft).
These are expensive but worth every cent. Their inherent noise is much lower than "normal" resistors' noise, this translates to much improved accuracy of the amp.