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.