Obviously I can make all sorts of claims about better performance but naturally I did not arrive at the updated schematic by pure chance. It has been a process of trying out different components or component values after having calculated their effect on the preamplifierís performance. With every change made the result was measured with RightMark Audio Analyzer. This allowed me to gauge exactly if a change was a worthwhile one in terms of total harmonic distortion (THD), the linearity of the preamplifier or the overall noise level. Below youíll see the plots from RightMark Audio Analyzer detailing the performance of the preamplifier after the modifications.
Whatís clear is that total harmonic distortion is in line with what can be expected from a tube preamplifier based on the ECC82 vacuum tube, with the 1st harmonic around -60dB which is about 0.1%. These measurements are taken with an output of 6Vpp into a 22K load. This is far more output than what youíll ever need in practice, most amplifiers need just 1Vpp at the input to deliver their full power at the output. Velleman quotes the preamplifier as 0.04% into a 47K load, but no information is given at what output voltage. The preamplifier however easily reaches that same result at lower output, typically at 2Vpp. The frequency response plot needs some explaining as well, as it just shows the frequency response of the soundcard used in my computer, an Audigy 2 ZS, the preamplifier does nothing to change this, which means it is perfectly linear in that frequency range.
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