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呢部機的問題出在第一級 12AU7,Bias 去到 5.7V, 上 DIYaudio 爬文，
有個人提議左以下幾點，不過看唔明，唔知有無人幫到手？
The gain is set by the feedback loop, so changing/supressing the 12ax7 won't modify it.
the problem is that the voltage accross R3 defines the bias for the 12au7.
because the current in the 12au7 is negligible , you have
Gain = R10/R3= 20 (26dB)
and 12au7 Bias = (output DC) *R3/(R3+R10) = 120V* 1/21 =5,7V
If you increase R3, you need to increase R10 (and vice versa) to maintain the bias voltage for the 12au7.
So if you want to reduce the gain, you can:
-put a divider in the output (this will also reduce the hum!): simple, cheap and very effective!
-change the input valve for another that will accept higher bias voltage (if any) and reduce R10
-bias the 12au7 input (this requires an input cap and a negative supply)
-reduce the output DC, ie the 6922 input voltage either through a divider from the mustage output (decouple the musatge from the 6922 cathode follower using a capacitor , and bias the 6922 grid at 50V with two resistors), you can then halve R10.
-use a combination of the three.
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