kdjupdal wrote:Could you explain how drive would normally be patched in a filter that don´t have this pot?
On the DLF the Drive pot works by increasing the volume of the signal going into the filter, but post CP3 mixer, and decreasing the volume of the output coming out of the filter. The maximum signal gain on the way into the filter core being around six times more than the lowest gain.
Most analogue filters will be non-linear to some degree. This means they behave differently depending the signal level going in. A completely linear filter will behave identically irrespective of the signal level and would normally be described as sounding clean. At points near to self oscillation even a perfectly linear filter will behave differently with different signals levels simply because the amount of self-oscillation is not dependant on the signal level. However, in most analogue filters self-oscillation is affected somewhat by signal level.
A filter will generally behave more linearly with lower signal levels. But lower signal levels also mean more unwanted noise as all filters are noisy. So there is a sweet spot to obtain the best signal to noise ratio.
All these complexities in a real analogue filter mean that selecting overall input level is important to getting the right sound. Simply having level controls on the input signals is enough to do this. This can be done with a mixer on the input, or under voltage control with a VC-Mixer or VCA following a standard mixer.
But changing the input levels will also change the output level. If you want to make up for the gain loss when driving your filter at low levels then you need to amplify the signal up after the filter. This is done in the DLF by the drive pot (which is a two gang pot). However, you can do this with another VCA that works in tandem with the VCA on the input signal. As one VCA increases gain the other reduces it. This is easily done on a dual-VCA module that has an inverting CV input. eg. Oakley Dual VCA.