A few easy Boss SD-1 Super Overdrive Mods

The Boss SD-1 is a very good sounding overdrive, economically priced and is one of the most famous tubescreamer type circuits available. Many boutique pedal builders have used this type of circuitry to 'design' their own pedals complete with new fancy paint job and the assumption that their version is better. While it does perform exceptionally well on it's own, we can do different changes to make it better, even making it into something that would rival some of the best boutique overdrive pedals available.

First, let's talk about what makes the circuit 'tick', and what the signal is doing stage by stage.

The signal comes in and passes through R1 and C1 (input cap) and passes a resistor to 4.5v (R2) which is the bias for the transistor (Q5) being used for an input buffer.

After the input buffer, the signal travels through C2 and past R4 which keeps the opamp biased by supplying voltage from 4.5v.

This first gain stage has a 1meg pot for gain with 3 diodes in assymetrical orientation. R5 which is connected to the gain pot sets the minimum gain and C3 and R6 together set gain as well as dictate the frequencies that are boosted and clipped, in this case 723hz and above. D4, D5, and D6 are diodes which clip the signal in an assymetrical fashion.

The signal leaves this opamp and goes through R7 and passes a cap (C4) to ground which is a low pass filter reducing highs at about 720hz and above. Another low pass filter is formed from the tone control which connects at this same spot and goes through C5 and R8 to ground. This low pass filter is variable depending on where the tone knob is set. This is also the same type of tone control the Ibanez Tubescreamer uses though the values are different.

The signal goes into the next opamp stage where it meets the tone control which actually boosts the highs as well as rolls off the highs (as mentioned in the last paragraph). R9 here helps to set the gain along with the tone control itself while C6 rolls off even more highs.

The signal leaves through C7 and R10 and into a 10k pot acting as the volume pot. The signal leaves the volume pot, goes through R14, C8 and then into the output buffer. After this buffer it goes through R16 and C10 then to the output jack.

“Boutique” Overdrive mod:
C1: 1UF
C2: .055UF
R6: 1K
C3: .22UF
R5: 10K
GAIN POT: 500K AUDIO TAPER
R7: 1K
C4: .15UF
C5: .15UF
TONE POT: 5K LINEAR
R9: 1K
C6: REMOVE TOTALLY (DON'T REPLACE WITH ANYTHING)
C7: 22UF (POSITIVE SIDE GOES TOWARD OPAMP)
R10: JUMPER
LEVEL POT: 100K AUDIO TAPER
R14: 1K
C8: 22UF (POSITIVE SIDE GOES TOWARD TRANSISTOR)
C10: 22UF (POSITIVE SIDE GOES TOWARD TRANSISTOR)

Bass Guitar mod:

C1: 10uf (POSITIVE TOWARD TRANSISTOR)
C2: 10uf (POSITIVE TOWARD OPAMP)
C7: 10uf (POSITIVE TOWARD OPAMP)
C8: 10uf (POSITIVE TOWARD TRANSISTOR)
C10: 10uf (POSITIVE TOWARD TRANSISTOR)
R6: 1K
C3: 2.2uf
C4: .0068uf
C6: REMOVE TOTALLY (DON'T REPLACE WITH ANYTHING)
LEVEL POT: CHANGE TO 500K AUDIO TAPER
GAIN POT: CHANGE TO 500K AUDIO TAPER

“More bass, less mids” mod:

C2: .1uf
C3: .15uf, for more bass go to .22uf or even .33uf
R6: 2.2k
C4: .0068uf or for more clarity go to .0047uf
C6: REMOVE TOTALLY (DON'T REPLACE WITH ANYTHING)
LEVEL POT: CHANGE TO 500K AUDIO TAPER

Other mods that can be done alone or in combination:
“Volume booster” mod:

C2: .1uf
C3: .68uf
R6: 2.2k
D5: Connect two 1n4148 diodes together in series.
D6: Connect two 1n4148 diodes together in series.
D4: Connect four 1n4148 diodes together in series.
*Alternatively, remove D4, D5, and D6 altogether and do not replace with anything.
C4: .0068uf or for more clarity go to .0047uf
LEVEL POT: CHANGE TO 500K AUDIO TAPER
*For use, turn “level” control all the way up, use the “gain” control as the volume

“Better low gain” mod:
R5: 4.7k

Diode Changes:
D4, D5, and D6 can be switched out with all different types of diodes and combinations of them. For example, many folks like this change:
Example 1:
D4: 1n4148 and a 1n34a germanium diode connected in series
D5: 1n4148
D6: 1n4148 and a 1n34a germanium diode connected in series

Example 2:
D4: 1n4148 and a LED connected in series
D5: 1n4148
D6: 1n4148 and a LED connected in series