Calibration of the pre-amplifier

This was the procedure used to check the calibration of the home-made pre-amplifier described on the previous page. It could be modified for use as the basis of testing any pre-amplifier, provided that you have available a sound editor software package such as Cool Edit or Adobe Audition that can generate tones, record sounds and analyse them for decibel levels.

• A series of two-channel tones was generated using Cool Edit and saved. The characteristics chosen for the tones were: one second duration each; amplitude around -3 dB and all the same; pure sine waves; frequencies range encompassed all those in the equalisation curve specification.

• A lead was taken from the speaker output on the sound card to the phono input on the pre-amp.

• Two instances of Cool Edit were started and the pre-recorded tones were loaded into one. On the other a new stereo file on Cool Edit was prepared having the same sample rate as the tones. Both Cool Edit windows were open simultaneously.

• Using Windows volume setting control panel, the input to sound card was on Line In and was set about half way up.

• The output from card was on wave device and its level was set to around first notch up. No other outputs were checked. The level was monitored in Cool Edit when replaying tones and was set to maximum without clipping.

• Recording was started in a new file and replay was started for recorded test signal. When finished, it was checked that recorded wave was not distorted by zooming in on the wave form and re-adjusting levels in (4) and (5) if necessary.

• When recorded, sections at each frequency were selected and the statistics option of Cool Edit was to measure decibel level at each frequency.

• Results were plotted to show response characteristics. The graph shown is for the Decca feedback loop of the pre-amplifier, with the specified equalisation curve in red, the calculated one in yellow, and the measured one in blue. The maximum difference between these three curves is about 1 decibel. There is no difference greater than about 2½ dB between the specification and the performance of the pre-amplifier throughout the frequency range of any of the curves given in the circuit design.