Boss DD-5 Harsh Dry signal - is it the OpAmp (M5218A) or the gain design.

[vondran]

I've recently added true bypass to my Boss DD-5. One thing I noticed was that with the effect level turned to zero (completely cutting the wet signal) the engaged effect had more treble and harsh sound then when switched to true bypass.  The path the dry signal goes through when the effect is engaged is:

Input -> Unity Gain non-inverting OpAmp -> Inverting OpAmp -> Summing Inverting OpAmp with wet -> Out

I had three thoughts of the main source of the stronger treble of the dry path:

1.  The path uses M5218A OpAmps.  How are these (good, bad, ugly)?  Should I try swapping these with something else?

2.  The Inverting OpAmp stage has a 47K in parallel with a series 0.0047uf + 10K resistor connected to the "-" input.  A 47K is then on the feedback path.  What this means is frequencies below 1KHz have unity gain thru the OpAmp and frequencies above ~3KHz have gain of 5.  The Summing Inverting OpAmp (3rd stage in path) has this reversed with the series 0.0047uf + 10K in parallel with the 47K in the feedback of the output OpAmp.  This essentially inverts the gain frequency response of the previous stage.  I see this same design on the Boss CE-2 schematics.

My guess is they're boosting the treble to minimize the loss for the wet signal treble and harmonics as it goes thru the A/D, memory, and D/A (or the BBD chip in the CE-2).  Even though the dry signal doesn't need this, the wet signal has not yet been split off, so the dry goes thru the same gain stages.  So the culprit may be the frequency response between these to stages may not be precisely the inverse of each other.  Or there may be a bit of clipping in the Summing OpAmp stage (3rd stage of the path) as the high frequencies are amplified at this point as it feeds into this stage.

3.  The third cause may be simply poor capacitors since they're all surface mount parts.

So my question is any ideas of the cause for the tone coloring?  Any suggestions of mods.  I was going to first try replacing the 10K's I mentioned in item 2 with 12K's to reduce the treble gain to 4 from 5, and hopefully reduce any clipping.  If that doesn't work, then maybe I'll change the OpAmps.

Any thoughts?