Alex,
First let me say that I've been following this thread with great interest! This is quite an undertaking.
If I wore a hat..it would go off to you!
So Larry, when you say that changing a BBD can mean you need to re-bias, do you mean that you need to do that because the optimal bias level has changed (i.e. it's not an audible difference, but you can see the difference when you go through the bias procedure with a scope), or do you need to rebias because the new BBD bias point is so far from the old that you get noticeable clipping if you don't rebias?
I have experienced changing a BBD and the bias being so far off for the new device that no signal would pass. This was with different SAD1024s. My experience with the MN series (the pieces that I've tested) is that the bias requirement
does seem to be closer.
I've got around a dozen MN3207 to experiment with and I'm hoping that once I've set a single bias level, changing ICs will still keep things working OK with no clipping.
You may find that this batch is similar in characteristics/bias requirements, but another group manufactured at a different time/place might be different. And let me add, if one were to use the Belling BL3207 or the Cool Audio V3207D, again, you might obtain different results.
I built an AD-3208 (with two 3205s) for a friend's son, Austin. The AD3208 was designed by Scott Swartz and is a mix of an Ibanez CD-10 and a Boss DM-3. The AD-3208 is powered by 9 volts and incorporates a 78L05 regulator to power the BBD. I had previously built a Ibanez AD-80 which is powered by 12 volts for myself.
When I finished the AD-3208 build, I calibrated it and was surprised at the amount of distortion I heard, but chalked it up to a lower headroom effect. I had never played through an original effect using low voltage BBDs, so I had nothing to gauge the distortion amount with except my AD-80.
About a year later, a friend of Austin's wanted me to build him an AD-3208. When I completed the build...I listened to the amount of distortion, and thought....this is not acceptable! this can't be right!
After some research on the net, It seemed that other builds exhibited this issue, so I started a thread on this forum.
Brian from Madbean had been dealing with this issue with members from his site, so with a few suggestions, I started to hack on the board, drilling holes, adding resistors and the like.
It turns out that the V3205Ds I was using output a "hotter" signal than the MN3205! The AD-3208 circuit didn't have any way to accommodate for that. The Ibanez AD-900 has a level adjustment trimmer in between it's two series BBDs.
JD Sleep from General Guitar Gadgets ran across the "fix" thread and PM'd me for the info. I sent him my files so that he could update his AD-3208 project with the "fix"
Sorry for the long story but....I wanted to outline that BBDs can exhibit different characteristics. They do not seem to be cookie cutter replicas of one another.
I've also seen that many pedals don't bother with any sort of BBD output trimpot (Original EM, MXR 117, Morley Flanger, Foxrox Paradox TZF, Boss CE-2, Boss DC-2) while other ones do (Deluxe EM, A/DA Flanger, Flanger Hoax).
I can see how manufacturing differences and ageing affects can change the bias requirements of one IC versus another, but is there really a big difference between the two sets of buckets within the same IC?
I would imagine that production costs, including components and calibration time, has set a standard of what is acceptable.
It could also be that if a BBD device doesn't meet the criteria in a circuit that lacks adjustment, it is "tossed" in a box for a circuit that
does include adjustment.
Example:
MXR M-117 no output trimmer.
MXR M118 (3) SAD1024s in series, output trimmer at last BBD output.
Another theory is: distortion is more acceptable (not as noticeable) in a flanger than in an analog delay. In fact, distortion may add to the "flavor" of a flanger.
