The Roland SH-101 while an awesome sounding synthesizer, unfortunately suffers from digital noise on its output. The main source of this noise is the current flowing through its DAC combined with bad grounding. The Tubbutec SH-1oh1 mod reduces the noise floor below that of the original (amongst a lot of other features), but noise is sometimes still an issue especially with bass sounds.
This is the instruction of an easy to perform – and reversible – modification that reduces the noise substantially, by about 10dB.
Here is a short audio demo of SH-101 noise reduction mod. I turn the mod on and off to show the difference between the noise levels. Note: This is incredibly amplified, half of the noise probably comes from the recording equipment. In the quiet parts (noise mod on) one can actually hear the VCA bleeding a little bit (the hum in the low end is the VCO). This is impossible to hear normally which shows how well the noise is reduced.
The modification consists of soldering a single wire to achieve a better ground connection across the circuitry. The following pictures show how to solder the wire. I used two wires in parallel to get a better low impedance connection, but you can also use one thicker wire. One side goes to the leg of resistor R63 , the other side is soldered to the ground connection on the jack board. (Ignore the two short wires here, these are a fix of a broken connection ) Doing it this way there is not even the need to dissemble the SH-101, only removal of the back metal cover is necessary to access these points.
The bad grounding in the SH-101 comes from the fact, that they chose to use a single layer board in order to save costs. Ground current from the digital part, and most prominently from the DAC is flowing all the way across the board, introducing noise into the VCO, the VCA and the output amplifier.
If you would like to improve your SH-101 even further, check out our SH-1oh1 mod.