7. Centre Detent pots for Line Trims on 8058/68/88 Consoles

Another interesting anomaly has arisen concerning the Line Gain trim pots on the Aux Modules of Neve 8058, 68 and 88 consoles! Whereas the resistor network associated with the original scheme produced a precise +/-10dB range, the actual attenuation required for unity gain (i.e. an output of -15dBu back to the 31099/31102) is -9dB, implying that an error of 1dB will exist if the pot is set at its mechanical centre! This was presumably compensated for by fixing the knob on the pot shaft in such a position that it indicated vertical (i.e. Unity) whilst the pot shaft was set at a 1dB greater rotation. This is especially noticeable if centre detented pots are used (we can supply SFER PA11's designed for this task) and a different configuration can be used to set precisely 0dB at the centre position.

As the 31267 Line Input transformer is sending a -6dBu signal to the Line Trim Pot (ref. 0dBu input), the pot should achieve a -9dB attenuation at the centre to provide a level of -15dBu back to the Channel Amplifier. In order to keep the pot range symmetrical it is suggested that a +/- 9dB range be used to obtain a unity gain position at the mechanical centre position of the potentiometer. The resistors to be used are shown on the illustration below, the two resistors at the bottom of the pot being used to create the 1476 ohm resistance required for the lower -18dB pot position.

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