I don't suppose you could apply this program to the BD-2 Blues Driver, perhaps?
Hmmm. Having absolutely no experience with the BD-2, I had to look up the schematic to see what there was under the hood. The schematic is a little blurry, but it looks like C9, C16, R10, and R21 do form a gyrator based around Q7... the values that I'm looking at are 0.056 uF for "C1" and "C2" on my calculator, and 470k for "R1" and 1200 for "R2".
It's a little nutty, and it is enough to make me stop and think... the results that I'm getting are a peak frequency of 120 hz, with a Q of about 20! It is enough to make me wonder if my sources are correct. Jack's calculator does match up with my results, though, so... see my explanation below, that's the best I can come up with.
Is it really shaped like that? Looks like a boost at 100 Hz or so with an amazingly narrowly Q!
Surely that would simply make a couple of notes seem to jump out while doing little to add to the general fatness?
(talking about the Metal Zone) So, umm, yeah. I do believe it is correct... 105 hz, Q of 14 or so...
What I think I'm seeing is that most of the low end is rolled off when the signal hits the pre-emphasis gyrator, ie the range between 800 and 1200 hz is boosted but very little so far below.... Basically, the note contributes harmonic content, but is itself basically filtered out. This matches up with what my ears say, that the low end isn't well-defined (a little boomy/flabby), and with the basic idea that filtering out the low end is exactly fitting the spirit of this pedal - high gain, 80's metal, with lots of articulation and treble response, and no muddy bassiness to pull down headroom. The difference between pre- and post-distortion EQ, right there...
An interesting mod would then be to modify the 'pre' filter... a 0.1uF cap for C1 and a 3nF cap for C2 would keep the resonant frequency in the 900 hz region, but would lower the Q to just under 1, meaning a range of 500-1600 hz. This would indicate to me a fuller tone, albeit one that might not be as trebly and articulated.
Ha. Would never have thought of that one, right? This also makes me think about some of the inter-stage filters, too... hmmm....
It's funny... this is what really interests me these days, trying to understand what the circuit is doing on a fundamental level. I am a little frustrated that I don't have a complete understanding... I can't tell you what the impedances are, or what the gain figures look like, or describe how the curve looks around the corner frequencies (correctly, at least)... but I can at least get us this far. I'm slowly teaching myself the math and electronic theory to go further, but.... well, it's not simple. Well, at least its something to do, right?
Saul T