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Boss BF-2 mods

11/26/2003 9:50 PM
Matt King
Boss BF-2 mods
I was wondering about the possibility of extending the width of the sweep on the BF-2. I'm a novice, so I could be way off on my ideas. I thought that there must be some way to set a new min/max LFO swing, but I really don't know if that is the right tree to bark up. Another thought was to just swap out the 50k lin. depth pot with something of a larger value.  
What other mods are out there for this flanger? I feel that there's a great deal of potential waiting to be coaxed out of this thing.  
I'll sit back and let you Mod God's take over,  
11/27/2003 2:58 PM
Alvaro Salinas

the BF-2 have internal trimpots to calibrate the sound. Download the Schematic and play with them..  
One sure it's a BIAS trimpot for the Delay IC, Some other could be DELAY TIME (Sweep Range) and Regeneration (feedback). maybe a mix trimpot could be found inside too.  
I have a vintage Dlx Electric Mistress and have almost the same internal controls.
11/27/2003 3:24 PM
Mark Hammer

You CAN extend the width of the sweep on a MN3007/3207. The problem is that you have to go beyond use of an MN3101/3102 clock driver. The BBD itself has a high input capacitance on the clock input chips. Think of that as being like a lowpass filter. If the clock pulses come infrequently enough (i.e., low clock rate, longer delay), the BBD sees them crisply enough. Once the clock rate goes above a certain range, though (100khz or so, which is what limits the BF-2 to abou 1msec delay at the shortest), the clock signal tends to get smushed/smudged by this input capacitance, and those nice crisp rising edges that allow the internal switching in the BBD to work doesn't work so well any more.  
The solution is to buffer the clock lines in between the MN3101/3102 and the BBD. Take a peek at the various Craig Anderton flangers at my site ( ...scroll down a few pages) and you'll see examples of this in action. I have reliable reports of this permitting clock bandwidth driving an MN3007 to be pushed up to 1Mhz and beyond (which will get you ridiculously short delay times approaching through zero flanging). The trouble is that it involves pretty much a totally new design, rather than swapping a part here and there and stuffing it back in the BF-2 chassis.  
So what CAN you do that still permits you to stuff everything back in and have it work normally?  
1) C7, found way over in the corner of the board, limits low end and DC in the recirculation path. If you replace it with something 1/6 the value or smaller (so let's start with 4700-6800pf), this will chop out the lows in the recirculation path which will remove much of the tankiness when the regen is cranked and allow you to tolerate higher degrees of regeneration. Th smart way to do this is to stick the original and a second cap in series and use a SPST toggle to shunt the added cap to restore the BF-2 to original specs. If you stick a .01uf cap in series with the .047, you get a series capacitance of around 8200pf. Shunt the .01 and it goes back to .047.  
2) Most modulation boxes will produce a variation of vibrato if you cancel the straight signal. In the BF-2, this would mean lifting one end of R28, which carries straight signal to the mixing stage to be combined with the delayed signal. R28 (47k) can be found just to the left of IC1, when looking at the component side of the board with pins 1&8 pointing up. Wire this up to a SPST toggle and you can switch between vibrato and flange modes. A word of warning, though. The BF-2, like many such units, adopts a "bypass" function that simply cancels the delay signal and nothing else. When the straight signal is left untampered with, "bypassing" the effect leaves you the straight signal. Of course, if you *lift* the straight signal to get vibrato, the moment you "bypass" the effect with its own switching system, what you'll hear is....nothing.  
3) A dry/wet balance is a joy to have. Sadly, most modulation effects lack one. There are many ways to achieve it, but the simplest and least problematic is just to adjust the wet level via the input resistance to the mixing stage. In the BF-2, the delay level at the mixer stage is determined by R29 (27k). Make it smaller in value and the balance shifts in favour of wet. Make it bigger, and the balance shifts in favour of dry. Maximum notch depth and least subtlety requires a 50/50 balance so that's where most companies set it to. If you replace it with a 22k resistor in series with a 100k (linear or log) pot wired up as a variable resistor, you'll be able to get sounds ranging from a little more wet than dry (not quite vibrato, but more in that direction, and also letting you absolutely NAIL a perfect 50/50 balance when using 5% tolerance resistors in the rest of the pedal), to a flanging sound that can be made subtle in the background, regardless of where the regen is cranked to.  
4) Most modulation effects will produce something reminiscent of ring modulation when the LFO rate goes from the sub-audio range (<20hz) into the audio range). Unfortunately, in this instance, it involves modding two caps rather than one. The standard 2 op-amp LFO has a non-polarized cap in the feedback loop of one of the stages. The value of that cap depends on other aspects of the LFO design. Unfortunately, in this case, the cap value is around 16uf or so, which would be a huge stinking cap if it was a single one. To mimic this, BOSS uses a pair of 33uf electrolytic caps (C27/C28, way over to the left of the TL022), back to back to mimic this. If that cap pair were replaced with a smaller value, say around 5uf or less, you would be able to produce ring mod-type sounds when you turn the rate up (and that will *really* be where you appreciate mod 3). Producing a 5uf NP cap involves using a pair of 10uf electrolytic caps in the same manner. Note that this shifts the LFO *range*, which means that some of the same sweep rates obtainable in the one range will be obtainable in the other. Use of the 10uf+10uf pair will lose you the ultra-slow sweeps, and use of the 33+33 pair will lose you the ring-mod tones. Both will get you the in-between quasi-Leslie sweeps, though.  
6) Finally, replace C30 (just besdie the MN3102) with another value and you change the delay range. For reasons noted earlier, you can't make it sweep much shorter in delay than it currently does, but you CAN move it into chorus territory and beyond for thicker sorts of swirly sounds. A good place to start is to simply tack on another 47pf cap in parallel with the existing one, and see what that gets you. Note that most BBD-based pedals tune their anti-clock-noise filters to allow maximum bandwidth given the anticipated clock range. Extending the delay time with this mod may result in the clock frequency brought down low enough so that these filters do a poorer job of keeping clock "whine" out of the picture. Your ears will tell you. Also note that there is a very normal tradeoff between sweep width and sweep range, such that shifting the range of delay it sweeps over in a longer direction will require you to turn down the sweep width for a palatable sound.  
That ought to keep you busy for a while.  
If you need the schematic (although it IS available on-line in many places), drop me a note off-line.
12/1/2003 5:42 AM
Matt King

My cup runneth over.  
You're right, these mods will keep me very busy. I just bought a Foxrox Pandora TZF and am awaiting it's delivery. But, I think I'll hang on to my BF-2 and try out these mods. God help me, I like the little purple guy.  
12/1/2003 7:45 PM
Mark Hammer

As the imminent owner of a TZF you have evoked great envy from me. I thought Dave included too little envelope control in it for my tastes, but it's not like you run out of tricks for the thing to do that quickly.  
Do hang onto the BF-2. I am certain there will be tricks you will need both the TZF and BF-2 to do....and a lot of vitamins. Besides, although many of the same mods can be done with the TZF, you HAVE the schematic for the BF-2 and you have a warranty from FoxRox to be concerned with, so the BF-2 can serve as the drafting board.
12/2/2003 4:26 AM
Matt King

Yeah, I'm not sure when I'll be brave enough to mess with the TZF due to the mammoth price. I won the bid for this on Ebay for $350.00--this included the expression pedal. It's only two months old (supposedly), but I don't know what kind of warranty there will be on it. I'm curious as hell to what it looks like inside, but I better keep my hands out of there until its used and enjoyed some first. I don't think I could get rid of the BF-2; however, it was the first pedal I bought. My only real pet peave is the tin can sound that can overwhelm the tone if you don't set the controls right. I'm a flange junkie. I also built John Hollis' Ultra Flanger. It distorted some at first, but I got rid of that. Now it just ticks. I like it though. It has a unique sound--plus I built it--I guess that's the appeal there.  
I wonder if adjusting the TZF's envelope control is as simple as swapping out some resistor values--much like that on Craig Anderton's envelope controller's sensitivity.
12/2/2003 6:00 PM
Mark Hammer

If the tin can part of the BF-2 annoys you, that's all the more reason to shape the bandwidth of the delay and recirculated signal. Seriously, replace the low-end limiting cap with a smaller value and that should improve matters. The tin can tone (what I would call "box-ey") comes from emphasis of resonant tones related to the delay time. Trim the low end back in the recirculation loop and you can/should get something less tinny. Try swapping C13 from a .033uf to a .01uf cap to trim just a bit of bass off the overall delay signal, and drop C7 from .047 down to .01 to tame the higher regeneration settings.  
My guess is that the TZF's envelope control may well be every bit as simple as the half-wave rectifier found in many autowah's, hence as easy to modify. Dave's true goal was to make a through-zero device, and the envelope control seems to be simply a bell or whistle added on to increase control features and make it a product that more easily differentiated from the pack. That is not to diss it for one moment. Rather, as a secondary feature I think you can expect that the design will be appropriately simple.
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