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- Q&A with Robert Keeley
Howdy, Pedal Power fans! We’re glad
to be back at Musicians Hotline after a
couple of months away. During that
time we had record-breaking sales, and
recently Jacob and Heather Adams
blessed us with an addition to the
Keeley Electronics family. So, welcome,
young Mr. Samuel Quincy Adams, and
thanks to our readers and customers
for all the support.
A common usage for the FET (a type
of transistor) is as an analog switch. An
ideal analog switch should behave like
a mechanical one. First, it should be
bidirectional. Secondly, it should act as
an open circuit in the “Off” position,
while in the “On” position it should
pass a signal functioning linearly without
exhibiting any attenuation.
Turning to the stock SD-1 circuitry, FET
switching is used for the purpose of
going from the bypass mode to the
effect mode. Essentially, two FETs control
this behavior.With any electronic
part there will always be some concern
about the relationship between theoretical
hypothesis and real-world application.
This is evident in the SD-1. A common
problem with circuitry (including that
found in the SD-1) is that with the use of
FETs the bleed-through of signal can corrupt
an otherwise clean signal. The internal
nuisances of the FETs prevent the
bypass mode from operating in the ideal
manner. In the case of the SD-1, this
reduces almost any distorted signal from
the clean path in the “Off” position.
The modification developed by Jacob
Adams and Robert Keeley incorporates
the use of an additional
FET at the beginning
of the signal path in order
to help eliminate the problem of signal
bleed-through. It does so by opening
the circuit before the signal passes
through the op amp. In the case of the
stock SD-1, however, the switching
FETs appear in the signal chain after
the op amp. Essentially, the signal--
even in bypass mode--will be amplified
before it gets to the FETs. This type of
circuitry lends itself to bleed-through
problems.
The Keeley Electronics modification to
the SD-1 has been through much
experimentation with respect to the
placement of the additional FET. After
much discussion and testing it has
been determined that the best results
are achieved using a design in which
the FET is placed immediately after
the input section of the circuit and
before any circuitry intended to amplify
or distort the signal.
This modification is really an inexpensive
one, but the change it makes is
essential (especially for anyone looking
for that Zakk Wylde sound using
humbucker pickups). The best part is,
it’s easy for even the novice do-ityourselfer.
Refer to Figure 1 for a simple
drawing of the JFET and diode
modification. DIY’s will find that the
parts for this modification can be
found at any local Radio Shack store or
other electronic parts retailer. The parts
needed are one Radio Shack J201 nchannel
JFET for audio/RF switching
applications and one 1N4148 or 1N918
diode (but you can use whichever unit
they have in stock).
Note: The 1N4148 diode is under the
heat-shrink assembly in Figure 2.
Interestingly, after some experimentation
it was discovered that the internal
resistance of the JFET could be
reduced. For example, we found that
substituting the 1N4148 with a germanium
diode lowered the “on resistance”
of the JFET by approximately
25%. However, since it was such a
small series resistance no drop in
sound quality was detected.
This article and modification created
by: Robert Keeley, Jacob Adams, Dan
Hast and Marcus Purcell.
Keeley Electronics
866-846-7539
Email: fx@rkfx.com
Web Site: www.rkfx.com
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