Old Boss BD-2 Mods Thread

[soupbone]

I was reading this older thread from Moon Watcher;The low gain/bluesy crunch thing is very doable with the BD-2.

The op amp decay thing is a result of just too much gain and typically too much bass as well, IME. Gain reduction throughout nixes it, and really warms up the pedal. You can end up losing a bit of the cutting trebles that a stock BD-2 is so capable of, but I don't mind the tradeoff, personally.

...The discrete op amps also can sound really nice with some gain reduction and fiddling around with the compensation, particularly on the first one.

Lastly, the "fixed tone stack" can really transform the pedal as well, if you take some time to rework it.

I like to start with the gain reduction, via R31 and R34 (first and second op amps). I've gone as high as 10K for both, but admit that it will lead to a serious gain reduction, and make the pedal very mild. It also typically necessitates other circuit changes to try and recoup a bit of the signal. Possibly 4.7K for both as a starting point might be good. Obviously be aware that C22 and C24 will have to be changed to keep the HP filtering the same, unless you want to change it.

...When you start to really reduce the gain, you will have to address D7 - D10 at some point, typically. Lower gain usually benefits from a larger clipping threshold. Since it's a taste thing, I can't really offer specifics, other than to try for a bit less clipping at those diodes.

I've found that playing with the external compensation on the discrete op amps can work wonders, as well. The most effect can be achieved at R32 and R25, and a little goes a long way. Even a switch from 2.2K up to 3.3K s/b noticeable. I'd only fiddle with one at a time, and I wouldn't alter them by much.

The stock fixed Fender tone stack really wrecks things in this pedal, IMO. It isn't even an accurate simulation. For that, you really need voltage dividers, like pots set at some point partway thru their travel. Have a gander at the schematic for the Fender Champion 600 amp for some ideas. I actually like to mostly nix the tone stack altogether and just replace it with some simpler filtering circuitry, but you could obviously just tweak the stock components for a more subtle effect. Since these tweaks are pretty well documented, I won't bother rehashing them.

...After reducing the gain, you may find that the pedal sounds a bit lifeless, because too much signal is getting attenuated at the tail end. An easy fix is to decrease R26. You can also try removing C19 as well, or altering it's value for less effect (try smaller). You can also remove the input clamps (D1 and D3) as well if the gain reduction is significant. And you can then increase R9 to recover some of it. The cool thing about this is now you can have the op amp contribute a touch of clipping on it's own that doesn't sound like that fizzy crap. Try replacing R9 with a 50K trimmer. I also like to remove C8 as well. And you could also experiment with disabling the gyrator as well.

The BD-2 is a bit tricky because there are 3 amps in it, and it's a balancing act when you redistribute the gain, interstage attenuation, and interstage EQ. I also like fiddling with increaing C23 and C25 to mellow things out just a touch more as a finishing touch, but only after the above are done. He talks about disabling the gyrator.Does anybody know where that is in the pedal?Here's the schematic; http://www.freeinfosociety.com/electron ... php?id=116

[Dirk_Hendrik]

The gyrator is the circuit around Q7. Disable it by removing C9

[soupbone]

The gyrator is the circuit around Q7. Disable it by removing C9

Ahh..Excellent!Thanks Dirk!