Hi everybody,
I just checked with Bob, the chief scientist here at Source Audio. He is the "architect" of our effects processing algorithms. He gave me some interesting information about the OFD, which I have included below.
When you select an effect and engage it, and roll the drive knob all the way down to zero, the effect is not fully bypassed. This is actually intentional! The OFD is simulating that circuit type with the drive set to zero, and is it turns out, this is not completely transparent in the real world.
As a simple example to illustrate why this is the case, let's talk about the tube drive effect.
Tube Drive emulates a tube preamp circuit. In reality, vacuum tubes are not at all linear, even when they are not really being pushed into overdrive. They have a characteristic transfer function (effectively, this is the relationship between the output level and the input level) even when they are not pushed into saturation. The OFD's Tube Drive effect was created by measuring this transfer function on a vacuum tube and then applying it to the signal when the effect is engaged - the drive knob simply allows you to push the signal further up the curve in the transfer function. So in fact, you will notice that your bass signal is going to exhibit signs of very mild saturation even with the drive knob set to zero. As the drive knob is turned up, you'll then hear the more pronounced overdrive sounds more commonly associated with a driven tube preamp.
The transfer functions on the OFD are not only nonlinear but also frequency-dependent, like what you'll see in the real world equivalent circuits. This means that the relationship between output and input signal levels depends on their frequency, and the overall result is that different overdrive/distortion types have different characteristic frequency coloration (note that this is independent from the three-band EQ we give you on the OFD - it's an "always-on" coloration of the signal). This matches, in fact, what you'd see with a traditional tube or transistor preamp (or any type of distortion pedal).
Lastly, the user EQ (bass/mid/treble) and the noise gate are active when the effect is engaged, so those might change your tone subtly as well. You can "zero" the EQ by holding down the TONE SHAPE knob until its LED blinks a few times.
So, the good news is that your experimental results match exactly what was intended by the designer! Even with no drive, the OFD will impart a characteristic transfer function (mild saturation, typically) and frequency coloration associated with the chosen effect. The OFD was designed this way because this is what real-world circuits actually do.
I hope this is helpful information!
Brett
[EDIT - Corrected typos!]
