A Different Kind of Jazz Box

"Jazz box" is a colloquial term for any archtop guitar.  However, if you put a few electronics in a project box, and, ta-da!  You've got yourself a good accessory capable of bringing out the best of your instrument.



I developed this project because I wanted the muted tones of a jazz guitar plus the output level of some of my other instruments.  However, as with almost every other modification I perform, I wanted everything to be reversible so that I could always switch back to the stock tone.  This accomplishes all of the above.

Suggested options
Like I said, this is a very simple design, but it is possible to obtain still more options from it will only a few additional components.
  • Varitone.  A selectable collection of capacitors that change your tone.  Depending on how it's wired, a varitone can cut or pull out different frequencies (i.e., it acts as a low pass or notch filter).  Jazz players in particular tend to want to cut the high end of their instruments, and this module can serve to do that with more specificity than conventional tone knobs.  If you haven't read about this device already, see this page.
  • Bypassable inductor.  Again, refer to the varitone page for the specifics, but the idea here is to be able to add or remove the inductor from the varitone.  This allows it to act in the aforementioned capacity (pun?) as either a low-pass or notch filter.  Having an inductor keeps some of the high end while pulling out specific frequencies.  Conversely, without an inductor, all the frequencies are dropped above a point determined by the capacitor.  An example of the latter case is the tone switch common to many Gretsch guitars which toggles between two positions commonly (and perhaps pejoratively) known as "muddy" and "muddier."  However, being able to remove the inductor is generally useful for most jazz and some blues guitarists who tend not to require a high end but are seeking a different tone for a particular piece.  (Note: Before anyone else asks, I don't know what value this one is.  It came as part of a kit I bought years ago.  There aren't any markings on it.)
  • Variable knob.  This isn't a volume knob in the traditional sense, more like a wet/dry mix for the influence of the varitone in the circuit.  Like, say, the capacitor selected pulls out just the frequencies you want, only the effect is a little too strong.  This cuts that back.  If you place a potentiometer (recommended: ~50 kOhms) between the varitone and the rest of the circuit, you will be able to adjust the influence of the varitone itself.  This won't affect your overall volume anymore than turning down your tone knob would, just how much of the signal the varitone works on.  The option can also be a good means of comparing the default (i.e., bypassed) tone to that of the varitone-influenced one.  In some cases this could be faster than flipping back through all the settings between the cap you're testing and the bypass position.
  • Pre-amp.  For all the guitars I have, I tend to run everything through the same set-up: A Digitech 2120 processor going to a Fender Ultimate Chorus.  The signal processor gives me loads of tonal options, but a basic issue that I have to adjust manually each time is the overall input level.  My "heavy metal" guitars and any active instruments have a higher output, so I tend to get more tone out of them relative to the somewhat muted response of the weaker pickups in the Gretsch 6120 or the D'Angelico.  Great tone, both, but they need a bit of a boost to push them into the range I like.  This is the EMG PA2 preamp which has a simple Off/On toggle on the face of the box, but there is also a trim pot on the board to set the level as I need it.

The schematic
This is essentially the same idea as the varitone box, only with the addition of the potentiometer to vary the influence of the varitone and the inductor bypass switch.  The preamp is not included in this schematic.  If you add one, you can follow the instructions included with it, but the concept is easy enough: input, output, and connections to the battery.  Everything else about the box is completely passive except for the preamp (obviously), so this could be a battery-free device if you desire and can live without the boost.

As usual, wire colors are arbitrary.

From left to right: 

  • positive and ground sides of the original circuit approaching the jack (you can make this an input jack if you're putting it in a box the way I did here.
  • variable knob (i.e., varitone level pot)
  • varitone (only four caps pictured; I have 11 in mine)
  • inductor bypass switch
  • inductor
  • output jack

See also...



Here's a look at the completed box.

Top row: Varitone selector, varitone knob, inductor bypass.

Bottom row: Preamp switch, true Off switch (cuts the battery).


A look inside.  There are two batteries to give the preamp more "headroom" (i.e., higher input range so it doesn't clip).





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