Interesting Micro-Flanger mod and a resulting question

[Mark Hammer]

I recently bought one of these at a local music store "garage sale".  It was kind of grungy-looking - familiar for those who have ever had to deal with the decomposed foam pad in older MXR pedals - and was missing a bottom plate.  But hey, it has an SAD512D and he only wanted $15.  Nice sounding flanger.  I don't know what the more recent reissues were using for BBDs, but here is a video of a vintage one, crumbling foam and all: 

I cleaned it up nice and thought I'd attempt a mod.  The stock unit, just like the plastic box flanger I have from the Commande series (also using an SAD512D) has a Rate and a Regen control.  Nice sounds but, much like the MXR Phase 90, opts for a single fixed sweep width, rather than providing a control.

The schematic shows that the LFO is essentially identical to that in the Phase 90.  When I've built Phase 90 units for myself, following tips that others here have provided, I have generally installed a 2 or 3-position switch to select between values of what is shown as R41 on the schematic.  On the Phase 90, increasing the value of that same resistor reduces the sweep width.  It doesn't change the start or low point of the sweep, but lowers the high point of the sweep, in a manner that is better for faster gurgly sweeps.  I thought that would be a nice addition for mine.  I replaced the 1M resistor with 2M7.  Imagine my surprise when I found it did nothing for the width of the sweep but changed the delay range instead.  I gather the clock is somehow current-controlled.

I settled on a 3-way switch, replacing the 1M R41 unit with 3M0, and using the switch to patch either a 1M5 or 1M0 resistor in parallel.  That gets me R41 values of 3M0, 1M0, and 750k.  That translates into a "thicker", more chorus-like delay range, stock, and a "drier" sounding shorter delay range.  The latter is particular attractive, and the former removes the need for a chorus.


But here's my question: why does that same resistance alter the sweep width on the Phase 90, but not in the Micro-Flanger?

[armdnrdy]

Possibly because R41 is connected to a converter Q4 & Q5 which is connected to a "clock."

In the phase 90, that resistor is limiting current to the JFET gates.

I think to add a Depth mod to the Micro Flanger, you would connect a pot to ground to attenuate the LFO signal.

Most flangers...good ones at least, have the depth pot "mixing" between the clock and Manual circuitry.

Try tacking a pot (variable resistor) to ground at the LFO side of R41.

[armdnrdy]

You can see how a depth control is implemented here:

http://www.diystompboxes.com/smfforum/index.php?topic=65872.0

[Scruffie]

armdnrdy is on the mark, looking at the MXR-117 schematic might help understand it as they're fairly similar in that area of the design.

For the record the new MXR micro flanger uses a 'v3204' so a custom 512 stage BBD, presumably made by coolaudio from the v prefix (I asked Steve at smallbear to ask coolaudio about getting some but I don't think anything ever came of it).

[Fender3D]

R30 is a nice depth-contol-wannabe (R41 @ wiper) 

You won't enjoy its whole range though... maybe 2 or 3 resistors and a switch...

while R39 is a better Delay control.

If you're planning to mod depth, you might as well reduce a bit C15

[Mark Hammer]

The Range switch I installed works well, even it doesn't do what I thought.  On reflection, I was kind of thinking that the voltage divider created by R30/R41 might be useful, which corroborates your suggestion for dickering with R30. 

I'd preobably need to replace the pots with smaller ones, and use smaller knobs' to leave any room for one or more additional toggles.  Those big pots and big knobs don't leave a lot of room for accessible mods.

Incidentally, I don't know about the more recent reissues, but the chassis on this one is NOT cast aluminum.  1) It's no Tonecore pedal but dang it's heavy for a 1590B.  2) My stepping bit had to fight its way through that sucker; it fought back.