Recording Front End for PCs (September 1998): It would appear that the input resistance of the line inputs on some sound cards is much lower than the typical figure of 40k ohms, for which the RFE was designed. We have found one example measuring 11k (in parallel with 2.2nF). To prevent the low input resistance of such cards from causing bass attenuation, the values of output coupling capacitors C33 and C34 could be increased, from 0.47uF to 2.2uF. However if your sound card has sufficient input sensitivity to allow a reduction in effective RFE gain, another approach would be to increase the values of output series resistors R51 and R52, to increase the effective load resistance presented to the output amplifiers of U4 -- and hence minimise distortion as well as restoring bass response. If you increase the value of R51-52, they should also be shunted with small capacitors to compensate the resulting divider and prevent treble attenuation. For example if you have a sound card with inputs of say 20k//2.2nF, you could increase R51-52 to 20k and shunt them with 2.2nF capacitors. This will result in a 6dB drop, but maintain a flat response.