G Bus DRIVE amount
Last modified: 2026-09-02
The DRIVE switch controls two stages at once: the gain of the opamp IC3.2 and an attenuator after the transformer. The gain is set by the resistors R5_CB-R7_CB and R10_CB-R12_CB, while the attenuation is set by R8_CB, R9_CB and R13_CB, R14_CB.
The gain/attenuation resistors correspond to each other as shown below. Note there is no corresponding attenuation resistor for R5_CB, R10_CB.
Tips for modding the DRIVE amounts:
- To increase the gain and therefore the distortion of the DRIVE stage, decrease the value of the gain resistors. To decrease the gain and distortion, increase the value of the gain resistors.
- To increase attenuation (reduce the output level), decrease the value of the attenuation resistors. To decrease attenuation (increase output level), increase the value of the attenuation resistors.
- Because the perceived volume of the output changes with the amount of distortion, the attenuator values must be set by ear. The easiest method is to replace the resistors you plan to change with trim pots, set them by ear, measure the values, then replace with the nearest available resistors.
- To maintain balance between the channels, keep the pairs of resistors the same. That is, if you change R5_CB, also change R10_CB the same value, etc.
- If you plan to decrease any of the gain resistors below 510R, you should increase the value of C1_CB and C2_CB to avoid low-frequency roll off. The gain resistors together with C1_CB and C2_CB create a high-pass filter. You can use an online HPF calculator to check the corner frequency of your resistor/capacitor combinations. To avoid affecting the audio range, aim for a corner frequency of 3Hz or below.
Gain resistors are highlighted green, attenuation resistors are highlighted red.
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