Boss
SG-1, the “mystique” of the vintage pedal…
SG-1 pedals are expensive because
they are rare, not because they are “vintage-and-awesome”. They are rare because nobody bought them way
back when they were made because they were… “different”.
So what does that mean?
Technically, the SG-1 looks
like this:
The heart of the swell effect
is the FET.
Imagine things start out with
no signal coming into the pedal, and everything sitting at “zero”. In this state the FET is turned fully on
(zero volts on the FET gate turns it on) and the input signal is shorted to ground. No sound will come out of the pedal.
If a note is
plucked on the guitar, the following things happen:
1) The input buffer passes signal, but the signal can’t pass to the output
buffer because the FET is shorted to ground.
2) The amplifier and rectifier take the input signal and turn it into a DC
voltage that it proportional to the strength of the input signal. The attack control circuit then applies a
“ramp” voltage to the FET which slowly turns the FET off. As the FET turns off, it allows more and more
signal to pass to the output buffer and we have a swell. Technically, the attack control contains an
RC filter and the attack knob adjusts the “R” part of the filter – which
changes the time-constant.
3) The signal decays and eventually the voltage coming out of the rectifier
falls to zero turning the FET fully on again which mutes the sound and the
system is ready for the next swell.
The sensitivity knob is used
to control the gain of the amplifier.
Modern guitar pickups have high output and they drive the gating
amplifier properly with very little gain needed – usually setting the
sensitivity control to around 9 o’clock is about right. Increasing the
sensitivity will eventually saturate the gating amplifier and the swell effect
will be lost.
There are quite a few
implications from this design.
Only
one of the knobs changes the sound of the pedal. Only the “attack” knob is designed to change the sound of the pedal. The
“sensitivity” knob is used to match the pedal to the output level of the
instrument connected – it is essentially a set-once-then-forget
kind of control, it is not a control that you would use dynamically to change
the character of the sound.
The
attack control is somewhat limited. The attack only goes out to about 300ms which
gives a nice bit of a swell, but not as much as you might expect (300ms is
about the time it takes to say the word "and" if you say it
quickly). Also, it is hard for the ear
to tell the difference between 300ms (knob fully clockwise) and 150ms (knob at
Held
notes decay “strangely”. If you hold a note, the decay of the note gets “lumpy” when
it starts to tail off. This is because the amplifier/rectifier pair does not
produce a smooth output at low drive levels.
The
tracking might not be as good as you expect. Longer attack times will
require slower picking, possibly with gaps between the notes. When using longer
attack times, careful adjustment of the sensitivity will be required to get the
best results. Strong, even picking will be required.
The
pedal is slightly noisy – there is a hint of fizz or “hash” that can be heard
along with the decaying note. In the Boss design, the amplifier is driven “to the rails” and every
time output voltage of the opamp is hits a rail it puts a tiny noise into the
power supply. Add up all these tiny bits
of noise and you get just that hint of “fizz” in the audio in this pedal. This is completely normal and unavoidable
with the Boss design.