Here are Mojo's favorite BSIAB2 mid tricks....

[mojotron]

I've been wanting to write these mods up and get them out there for a while now.. Here you go...

I actually posted these in another thread, forgive me for the 2nd post, but these maybe of more general interest for BSIAB enthusiasts.

if you want more upper-mids... but in this circuit I like to add them actively rather than messing with the passive tone control - there's not enough done in the tone control to really make a set of changes there that adds mids that sound more like an amp adds mids (vs. how a speaker compresses the sonic range - nipping highs and lows like a passive tone control does to give you a kind of speaker simulation)....

So, thinking in terms of how an amp adds mids - staying faithful to the "Super Lead" sound - here are changes I added to the BSIAB2 circuit in my quest to match more of a "Mullard 12AX7 equipped" pre-amp sound...

1) What I found was that I really liked a 1M pot in place of R6 in:

http://www.generalguitargadgets.com/diagrams/bsiab_2_sc.gif

And a polystyrene 470pF cap for C5, using the specified J201/2n5451 fets - this adds a low-mid boost.

With the R6 1M pot turned to low resistance - you get a lot more mids and low mids - without sacrificing the hi-mids/highs: At high resistance you get a bit less low-mids and more hi-mids.. kind of an in-line treb-boost sound. With a ceramic cap in C5 this still works, but sounds a little grainy and weak - that is - with a ceramic cap the signal seems to loose power as R6 approaches 1M.... The polystyrene cap seems to give you less loss of signal... but all of this is fairly controlled as you're not dumping signal to ground at this point in the circuit.

So, #1 only affects the amount of low mids you get... adding or removing the lower mids - you need to do #2 to boost upper mids.

2) Either put a pot (10 to 100k..) in front of C2 and make C2 like .2uF... or just play with values of C2 from .1uF to .2uF to tune the upper mid boost you want. The larger the cap, the more high-mids you add. Likewise, using a bigger cap, a resistance via a pot will allow you to tweak the upper-mid boost.

So, with #1 and #2 the combo will give you full control of the mids - boosting from the bottom and top to get the full mid-range boosted.

If you use an RC combo (using a pot for "R") in place of C2 - you get something that is very tunable. However, I simply use .1uF for C2.

3) Try #2 with C7.

When I played with these, it seemed to me that I liked the sound of adding mids prior to #1, but the last "boost" mod you have before Q5 is going to be the dominate one - thus I mention playing with higher values of C7.

4) Now,  some times what you really need is a serious kick in the mid-section (figuratively) while keeping the gain at 1/3 to 1/2 of full rotation. This mod opens a whole can of whoop-**s in the upper-mids.. 

Ok, if you put a 5k pot with a 1uF to 2.2uF cap in series to bypass R15  - making an AC bypass from Q5 source to ground - you get a really great VH1 crunch that is quite addicting. 

If I was to make like a BSIAB v3 - I would add all these mods together. It's a few extra knobs and it's a slight change to the original sound, the control you get is a fantastic change. I've been using these mods for about 6 months now, and what I did was to make #4 switchable for a lead boost kind of like is used on the Fulldrive - kind of a "crunch" control.

Give these a shot - let me know what you think....

God Bless,
--
Mike

[powerplayj]

Awesome!

Thanks.....

[Herr Masel]

Mmm if my BSIAB2 will ever work I will definately try some of those. Thanks.

[John Lyons]

I finally got around to building the BSIAB2 circuit. It sounds great. I used a ceramic bead cap for the 470pf and no suprise...it sounded to grainy. I usually use silver mica or film...polystyrene is a good one as well.

At any rate, I tried the mods outlined above. Here are my thoughts.
Mod #1: The 470pf cap across the 470K resistor is a treble peaking circuit as used in some marshall amps. The 470pf cap bypasses the resistor letting highs through as a sort or brightness control. By making the 470K resistance larger you are letting the highs through the cap while stopping everything else with the resistor.
By making the resistance lower you are taking the cap out of the circuit all the way to 0 which is a total bypass of the cap and treble peaking circuit. So basically, less highs (and more low mids and bass.) The effect is pretty subtle. Nothing is added, just taken away (treble)
I went back to the stock schematic for this one.

I haven't tried #2 or #3 yet. I'll get to those soon as it would be cool to have a mid boost as an option.

Mod #4 is adding gain and some highs and mids to the last gain stage. This is mostly noticable when the gain pot is turned less than half (as noted above)  I like the lower gain settings with the original schenatic so I didn't keep this mod in. I didn't notice much other than the gain increase, if you turn up the gain pot you get pretty much the same thing. So unless you used the mod on a footswitch as a gain boost,  for the lower settings of the drive control there isn't much point.

Anyone have thoughts on this?  Good stuff mojotron, I apreciate the effort but so far I like the stock version.

John

[ubersam]

I just finished this circuit on perfboard, waiting to finish the offboard comp wiring. I decided to swap out the tone control for a 3 pot tone stack. I should have breadboarded it first though.

[John Lyons]

Let us know how that goes ubersam. Honestly I'd like less knobs than the 3 knob stack but if it works well then it's justifiable.

John

[Steben]

at AMZ ( www.muzique.com ) there is an article about tweaking the big muff style tone control, including an extra mid control. This would be nice for a 4-knobbie BSIAB.

Check the lab notebook and look for the presence control.

[mojotron]

Mod #1: The 470pf cap across the 470K resistor is a treble peaking circuit as used in some marshall amps. The 470pf cap bypasses the resistor letting highs through as a sort or brightness control. By making the 470K resistance larger you are letting the highs through the cap while stopping everything else with the resistor.
By making the resistance lower you are taking the cap out of the circuit all the way to 0 which is a total bypass of the cap and treble peaking circuit. So basically, less highs (and more low mids and bass.) The effect is pretty subtle. Nothing is added, just taken away (treble)
I went back to the stock schematic for this one.

This mod is not really meant for maxing out at either 0 or 1M - there is simply added tonal options if you bypass a bit less or a bit more with the cap. The added range of the 1M resistor makes this mod a flexible as possible (0 or 1M are the settings I liked the least...) - try 600-800M settings and 300-500M settings, those I liked the best, depending on the guitar I was using. This mod is not really about preserving a Marshall-ish tone, although you get it with the 500M setting, but rather getting a bit more flexibility from the circuit - irrespective of the tone Ed was shooting for with the original design.

Mod #4 is adding gain and some highs and mids to the last gain stage. This is mostly noticeable when the gain pot is turned less than half (as noted above)  I like the lower gain settings with the original schenatic so I didn't keep this mod in. I didn't notice much other than the gain increase, if you turn up the gain pot you get pretty much the same thing. So unless you used the mod on a footswitch as a gain boost,  for the lower settings of the drive control there isn't much point.

For me, this mod seems to work best with the other mods in place, and with the right pickups. With PAF-like pickups there is enough range in the original signal to benefit from some added upper mids/gain.

[John Lyons]

Nice! The "new AMZ presence control" linked above looks like the one. Only a couple value changes and adding a "Body" pot.
I'll have to give that a test drive.
Thanks for reminding me off that one Steben.

John

[alfafalfa]

Hi Mojotron,

I tried your mods in January and have liked them ever since. Of my three BSIAB's this is the one I like best.
Thank you for those  great improvements !

Alf

[mojotron]

Hi Mojotron,

I tried your mods in January and have liked them ever since. Of my three BSIAB's this is the one I like best.
Thank you for those  great improvements !

Alf

Thanks alfafalfa! 

I'm not sure if I improved the BSIABII - I think the stock BSIABII is an awesome pedal - but these mods do add some flexibility for dealing with a variety of guitars/amps and speakers situations.

[Steben]

Nice! The "new AMZ presence control" linked above looks like the one. Only a couple value changes and adding a "Body" pot.
I'll have to give that a test drive.
Thanks for reminding me off that one Steben.

John

No prob.

I'ld like to use this thread again to express my admiration for the efforts all made that contributed to the design of the BSIAB.
I guess it's the closest mankind has ever contributed in solid state galaxy to emulate an amp tone. Especially the way it emalutes it from clean to distorted, a feature no other circuit has ever done in such a way.

It should be the standard preamp in all SS amps...

[Xavier]

I'm rediscovering my BSIAB these days. Now I have different amps, 2 Randall RG200G3 combos, which are a hell lot bassier than my previous Marshalls, and now the BSIAB sounds correct............I have the BSIAB for 80's metal and the Boogey for modern metal.

I have built up a new one and I will use some of Mojo's mods, I'll try to post samples.

Funnily, I have just found out when opening my previous one that I used for 2 years, that I used a 15nF cap in C9  , so probably it's no wonder why i found it dark and thin sounding ................it was my first build, now I'm really excited again to see how it sounds.................

[mojotron]

Great - it will be interesting to see what your impressions are.

[Xavier]

After 3 days of debugging I consider myself defeated ........ Changed all JFETs and still it doesn't work........I have given my crunch box clone to the other guitar player in my band in return, and I'm not touching it...........my BSIAB V1.0 with the wrong cap still sounds good, nevertheless I'm afraid of opening it up  . One day I'll try again.again..........

[tackleberry]

I finished a BSIABII just a few days ago. Had the same non working problem. It took a magnifying glass to find a bad solder joint. Just a thought. I used the tone stack calculator to substitute some some values in the tone controls to loose some of the mid scoop. The original tone cap values made any thing less than 5 on the tone pot drop the volume to next to nothing really fast.