Line-level preamplifier

Posted on Dec 4, 2012

Since the volume control is after the gain stage, the preamp can be easily overloaded. I tried to limit the input signal by mounting a 470 k resistor in series with the CD input. A voltage divider is formed by this resistor and R107, resulting in a greater than 5-fold attenuation of signal input to the gain stage. Although the distortion had now disappeared, the resistors caused a severe limitation of the bandwidth of the preamp (-3 dB point about 15 kHz !). A lowpass filter was formed by the gate capacitance of the MOSfets in combination with the series resistors.

Line-level preamplifier
Click here to download the full size of the above Circuit.

Help came from an unexpected source. Mr. Colin Rutter, of Spectral Technologies PTY Ltd., Australia, sent me the following e-mail message: "The bad sound you experienced with the BOZ is due to the input signal being clipped asymmetrically as setting the bias current to 40 mA by measuring the voltage drop across the resistor may in fact not set the zero point correctly. A CRO (cathode ray oscilloscope) solves this problem or a bit of maths. My Bride of Zen has its idle current set at 32 mA for symmetrical clipping". Colin was completely right. Watching an amplified sine wave on an oscilloscope, I carefully lowered the bias of my BOZ. In my case, symmetric clipping occurred at a bias current of 28-29 mA. Finally, I removed all series resistors from the audio inputs. Clipping is no longer audible on CD, even during powerful crescendos. Although this is not shown in the schematic, Nelson suggests to include a mains filter in the power supply. I used a shielded filter (Schaffner FN2060-1-06, also marked 9540A, 1 A type) and mounted it between the mains switch and the power transformer. Before the filter was installed, there was a major transient upon turn-on and turn-off (loud thumping noise from the loudspeakers). After installation of the filter, there was no longer any transient. Thus, the filter is a major improvement. The preamp sound seems also to have improved (better resolution of low-level details, superior...

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