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Yes, this is somewhat like breaking a hybrid amplifier into two components, line stage and buffer amplifier, which makes sense, as making a typical hybrid amplifier is effectively like placing two line stages in series with each other needlessly. On the other hand, if a passive volume control box is used in the line amplifier's stead, then the unity-gain buffer will not provide full output, only producing a few milliwatts of output power, as the line level voltage are seldom much over one volt. |
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In actual practice, the actual efficiency will probably come in at 40-45% with solid-state devices and low DCR inductors. |
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No-gain triode-based headphone buffer |
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Using a resistive load would quarter the theoretical 50% efficiency figure (12.5%) and thus cannot be universally recommended. The exception might in the case of a pure tube low-power buffer for driving low-ohm headphones (8-32 ohms), as here we only need milliwatts of output. The circuit above shows a 6BX7 triode biased a single cathode resistor. The better approach for driving loudspeakers is to use an active constant current source, as the maximum theoretical efficiency falls to only 25%. |
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This buffer can only be biased in pure Class-A mode, as the inductor must be able to give up same amount of peak current into the load as the MOSFET pulls through the load during peaks. For example, if the idle current equals 2A, then the peak symmetrical output into a load impedance is 2A. Given an 8 ohm load, 2A equals 16 watts of RMS power. (The key word was "symmetrical," as the MOSFET could swing much more positive going current into the load, assuming that power supply can support the needed voltage swing.) Like all inductively loaded Class-A amplifiers, the maximum theoretical efficiency is 50%. |
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