Mod 3: Tweak Feedback in Your SD-1 or TS9
Tools and Parts for This Mod
• .1 μF, .22 μF, and .47 μF film capacitors
• 1k Ω 1/4-watt resistor
• 10k Ω 1/4-watt resistor
You can adjust the tonality of
an SD-1 or a TS9 in many ways
simply by using different resistor-and-capacitor combinations
for the components in the large
oval in Fig. 4 and Fig. 5.
Fig. 4: SD-1 Gain Stage (left). You can achieve myriad tones with a Boss SD-1 by varying the values of the resistor and capacitor shown inside the large oval. Fig. 5: TS9 Gain Stage (right). Altering the values of the resistor and capacitor shown here inside the large oval can yield a wide variety of tones with a Tube Screamer.
This component combination
(aka the feedback to ground, or
4.5V in this case) helps set the
gain, as well as what frequencies
get amplified and clipped by
the op-amp (the triangle thingy
in the schematic). A stock TS9
is set to clip around 720 Hz.
Lowering the value of the resistor
will provide more gain, but it
will also change what frequency
is getting clipped. If you don’t
want to change the pedal’s tone,
you have to change the capacitor
value with the resistor value. You
can also squeeze some bass out of
the pedal by adjusting the value
of the capacitor in this combo.
Table 1 shows some values that
I suggest you try. If you want to
play around with the values and
frequencies a bit more, I suggest
visiting muzique.com/schem/filter.
htm. This website has a great
frequency calculator for resistor/
capacitor pairs.
Note: The TS9 and SD-1 are very
similar in this part of the schematic,
so all of the same mods apply. Just
be careful with the SD-1: If you
increase the gain too much without
adding the proper circuitry, the distorted
signal will start to bleed into
the bypassed signal. If you run into
this problem, you can find mods to
rectify the situation online.
Before we jump into the actual
mod, let’s look at Figures 4 and 5
again. See the lone circled resistor
in each schematic (R5 in the
SD-1 circuit, and R7 in the TS9
diagram)? This resistor sets the
minimum gain when the drive
knob is turned all the way down.
I suggest changing it to a 10k Ω
in both pedals—it’ll enable them
to clean up a lot better.
Okay, let’s replace the SD-1’s
R5 resistor, the TS9’s R7 resistor,
and the C3 capacitor and
R6 resistor in both the Boss and
Ibanez pedals.
Photo 11 (left): Replacing your Super Overdrive’s R5 resistor with a 10k Ω part will enable you to clean up the signal more. Also, swapping the C3 and R6 components with different values will vary the available gain and which frequencies get amplified and clipped by the op-amp. Photo 12 (right): Swap your Tube Screamer’s R7 resistor with a 10k Ω resistor for a more pristine minimum-gain setting. You can also vary the C3 and R6 values for different levels of drive, as well as to change frequencies the op-amp clips and amplifies.
1. Locate the minimum-gain
resistor in your SD-1 (R5
in Photo 11) or TS9 (R7 in
Photo 12), desolder it, and solder
in a 10k Ω replacement.
2. Then test your pedal.
3. Locate C3 and R6 on your
SD-1 or TS9, desolder them, and
replace them with different values
based on the chart above or perhaps
a recipe you come up with
using the widget at muzique.com. Note: If you’re modding your
SD-1, don’t be afraid to remove the
gunk that’s globbed all over C3.