audioguru Posted September 26, 2004 Report Share Posted September 26, 2004 Hi Guys,In another post we have discussed why this part of a project doesn't work as a low-pass filter that is described in the project's text, and produces severe distortion and noise.The original part of the schematic is on the left side, and I re-drew it on the right side to make it easier to see.This audio amplifier's input is near line level and is DC-coupled from the previous preamp opamp. It doesn't have any feedback, so its voltage gain of about 200,000 will amplify its own and the previous opamp's offset voltage of up to 6mV, so that its output will be resting near a supply voltage, instead of its output being biased near half-supply voltage. Therefore when an input signal is applied, its output will half-wave rectify it, causing severe distortion.The high gain will also amplify the high internal noise level of the 741 and the noise from the previous stage, so that its output will be full of noise.The low-pass single-stage action of R5 plus R6 and C4 is cancelled by the high-pass single-stage action of R5 and C3, so that the output frequency response will be wideband. Quote Link to comment Share on other sites More sharing options...
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