Let's talk Boss BD-2 Blues Driver for a moment

[ayayay!]

I've been developing a mod for this guy.  One can do a bazillion things with it.  But there's been a couple things that have jumped out at me that have made me scratch my head.  (And no, I have no interest in the other mods floating around out there.  I don't like any of them.)

http://www.indyguitarist.com/schematics1/boss/BOSS%20BD-2_001.gif

What I don't like is C27.  Why such a tiny cap there?  There are such massive gains to be made by increasing that little sucker.  It seems they've put all this effort into, frankly, a pretty good gain stage, then chopped it down like Dorf. 

C20 and C21.  Are they really even necessary?  Besides maybe a Miller cap affect maybe?

C15 and C7.  Why so huge?  I don't see what 10uF accomplishes there that a .22uF of so wouldn't.  (I know when these were created, cost had to be part of it.  Electros are/were cheap, but I just wonder if these values are that important.)

I'm not wild about the gyrator at the end either, but whatever.  It's there. 


There's a lot of other room to mess with things too, and believe me I've been horsing around with all kinds of fun ones, but these things always kinda bothered me.  Just thinking out loud I guess...

[BubbaFet]

Have you searched the 'historical documents' yet?
JDoyle posted a bunch of good info on the BD-2.

Those cap values are all about frequency coupling and shaping,
but I'm sure you know that.

[ayayay!]

Oh yeah I'm all over that.  I'm specifically wondering about those components I listed. 

[JDoyle]

C27 (0.0022uF) is actually not that small at all. With R35 (1 Meg) it creates a high pass filter with a -3dB frequency of 72Hz - which is below the lowest note on a standard guitar.

C20 and C21 are necessary, in my opinion. They are compensation caps for the discrete opamp stages. The unity gain frequency which they set will be well above audible range. Leaving these out runs the risk of stability issues and I would advise against it. The circuit may still work (sort of) if they are left out.

C15/C7 are almost certainly that big due to cost savings, although making them that big also makes certain that no audio frequencies are attenuated at all, that any phase changes associated are also minimal, and that the C15/C7 corner frequencies don't overlap/alter the filtering characteristics (such as C27/R35, above) of the 'actual' circuit when activated.

Regards,

Jay Doyle

[ayayay!]

C27 (0.0022uF) is actually not that small at all. With R35 (1 Meg) it creates a high pass filter with a -3dB frequency of 72Hz - which is below the lowest note on a standard guitar.  

Ahh I blindly missed r35!  How silly.  I was only viewing that as Vref to Q14.  Derp.

C20 and C21 are necessary, in my opinion. They are compensation caps for the discrete opamp stages.

I was only looking at it from the other side.  I was thinking they were there for purposes of Q9 and Q12, but you're saying they're for the discrete opamp stages.  Then I need to read up on them more as I'm not as informed on them as I thought.  I do see what you're saying though.

Very good, answered my questions.  Thanks JDoyle.  

Hey since I've got your attention, what would one call the section between the first gain stage and the clipping?  The proper name?  

[Paul Marossy]

I like Wampler's "Brent Mason" mod on that pedal. It's a little monster unleashed after you do that mod. 

[ayayay!]

Hey since I've got your attention, what would one call the section between the first gain stage and the clipping?  The proper name? 

Wait a minute, isn't that just a basic Fender Tonestack?  Everything wide open?  I think I answered my own question.

I like Wampler's "Brent Mason" mod on that pedal. It's a little monster unleashed after you do that mod. 

Thanks Paul but I'm not crazy about that one either.  Just me I guess.