Monday, June 11, 2018
Miller Effect on Resistive Attenuator
The answer(s) to why transformer / autoformer based volume control is better than resistive volume pots, is quite complex, as there are many factors to it. But, one that stands out, that many may not be aware of, is the Miller capacitance + volume pot effect, that caused the amplifier bandwidth to be reduced significantly when used with a resistive attenuator, or resistive volume pot. This is especially significant if a high value volume pot is used, such as 100K-250K pot.
Everyone knows about Miller capacitance, and why grid stoppers are required, right? But, is he aware that volume pot is part of grid stopper too?
The resistance from the pot has the same effect as a grid stopper (The resistance, starting from input of source, to the volume pot viper section, is part of the grid stopper!), creating a RC low pass filter with the input capacitance of the tube connecting to the volume pot. This RC low pass filter will attenuate any content above the cut-off frequency.
The cut-off will be worsen by higher gain tube as the Miller capacitance is multiplied by the gain, in simple calculation sense, a good example, will be 12AX7. (Could it be the same reason where some prefers lower gain tubes in their amplifier instead of a high gain ones?)
Take 12AX7 as an example, the Cin (Miller capacitance) will be ~200pf, give a take a few or tens of pf.
Say, with 100K Ohm resistive volume pot, the setting now is at mid point of 50K, indicating that there's a 50K+ "effective grid stopper" connected to 12AX7. Ignore the actual grid-stopper connected for the moment. With that, the resultant cut-off shall be:
f cut-off = 1 / (2 * pi * Rg * C) = 1 / (2*3.142*50,000*200pf) = 16kHz (!!!)
That's a freaking 16kHz low pass RC filter in the amplifier without one even realizing it. That is sad...
That's why a lot of folks prefer TVC or AVC versus resistive volume pot. That's why most people will say that the realism is better, the highs are airier and with more body once they migrated to TVC or AVC. Once they tried TVC or AVC, they will never go back to resistive volume pot.
Till then. Go figure.
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