resistive divider to get a split supply, I
stepped up to the plate and used a TI
rail splitter part (part #TLE2426). This
IC works exactly as advertised and is
only the size of a TO-92 transistor.
The advantage of adding this to
your BOM over using a resistive
divider is twofold: (1) very low power
dissipation, which is the major flaw of
resistors as a divider; and ( 2) it
maintains an even division of the rail,
regardless of load current for each
polarity. The latter is the real issue
with onboard electronics — especially
as batteries get weak.
However, even if you still wanted
to go with two batteries for two
polarities, then I would put them
outside the guitar and have a jack
mounted through the rear cover plate
to connect the power to the PCB.
There are some neat four-cell AA
battery boxes that are about the
width of a wide leather guitar strap.
Just re-wire the box as two sets of
two batteries and you have a ±3V
power supply.
(FYI, contrary to your assertion,
there is no headroom issues with a
±3V supply, considering the low
output level of the pickups).
I have one such battery box that
also has a belt clip — if you want to
wear it instead of having it on the
strap. Not sure where I bought mine,
but there’s a very nice one with a
power jack, switch, and aluminum
belt clip at www.batteryspace.com/
BatteryHolder10xAA5x2With526AW
GWireLeads-RoHSCompliant-2.aspx.
If you went with this box, I'd
simplify things and go with a single
polarity battery box (which preserves
the nice power jack on this unit), and
use the TI rail splitter IC on the PCB.
The beauty of the battery box
scheme is that you can use a small
diameter jacketed cable from the
battery box to the back of the guitar,
and it will not be intrusive.
Servicing the batteries during a
gig becomes a non-issue. (Heck, you
can even have a spare battery box in
the wings, if needed.)
The only negative issue with this
scheme is that you have to use a
right angle style jack on the guitar's
back plate or your body rubbing
against the guitar body will eventually
cause a cable failure.
Something like a stereo 3. 5 mm
right angle plug and cable can be
stolen from a $3 pair of headphones
for the task, which is nice since the
plug is molded to the cable for strain
relief. This is still a weakness in the
overall power supply cabling method,
however.
So, my best solution was to lose
the local (to the guitar) power supply
altogether. What I did instead was to
change the guitar's 1/4" jack from a
mono to a stereo version. Then, the
cable from my guitar to my pedal
board needs to become a stereo
cable, as well.
However, this is not a big
sacrifice and it’s easy to keep a few
spare cables in your gig bag in case
one fails.
The purpose of the added
conductor is to use the new ring
terminal on the three-cond plug as
the positive supply voltage to the
guitar's internal PCB. The tip stays as
the signal terminal.
With the power supply now off-loaded from the instrument, I never
have a battery issue and I can get a
well filtered and well regulated
constant voltage to the PCB (since I
have a very stiff home-brewed 9V
power supply built into my pedal
board).
This last scheme is a big
architectural improvement, as it
makes the roll-out of my guitar
electronics agnostic to any guitar I
own since the space issues of how to
fit batteries into the instrument and
keep them stably located are now
gone.
Plus, with a line-powered DC
source of supply, I will always have
full voltage available to the pre-amp,
making low rail dropout concerns a
non-issue. (BTW, I keep a gel cell on
trickle charge in parallel to the line
powered 9V DC power supply, just in
case the venue's power quality is
poor.)
Another advantage: I also never
have to open the instrument's control
cavity again, which preserves the
integrity of the wood body by
not exercising the cavity cover's
READER FEEDBACK Continued from page 7
72 November 2014
