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CS-1aThe Boss CS-1 is a optical compressor made in the late 70’s and early 80’s by Roland. Optical refers to the gain control element used in the circuit to create the compression effect.  Optical compressors  have a particular sound that is dependant on the properties of the device used. The devices from different manufacturers have different attack and decay characteristics, and there are variations even with the same type device.

There were at least two different circuit variations used in the CS-1 and several service notes that included “factory mods”. Factory mods are circuit changes that the engineer publishes in the service notes so Boss dealers could perform the mods on customer pedals to fix an issue or change the performance.  Typically these “mods” are included in production pedals at some point. One of these engineering changes on the CS-1 was to change a resistor that reduced the output of the pedal. cs1compmod2 The service notes indicate a date of December 1978 for this change. It also specifies that the factory applied changes to serial no. 7700 and higher. Many players find the CS-1 is not capable of unity gain or any boost capability due to this reduced output. The simple fix is to undo the resistor change in any CS-1. I took it a little further and increased the output level by lowering the resistor below the original value. The resistor is shown at the right. It is normally a 150k (brown-green-yellow) that connects to pad 16 and the .033 green cap on the right. Replace the 150k resistor with a 47k resistor. For more output use a 10k resistor.



  1. hello, I did this mod to my CS-1 and it worked perfectly! thank you. this is one of my favorite pedals for bass, but I havent used it in years due to the volume drop.
    thanks again!

  2. Funny to find this one here.
    Had a CS-1 in my youth, my father who was a electronics engineer told me after a short look at the pedal to change that resistor, had the bug ever since back then….

  3. This mod definitely works… I tried the 47k first and it got unity gain or just above. Might be enough for most folks. I ended up with the 10k mod and there is loads of headroom. This pedal is far from transparent, but that’s why I love it. Now I can bump the front of an amp as well as compress. Great info and thanks much…

  4. Hi! thanks a lot for this. Do you have any advice to reduice the noise… ?

    • There are not a lot of options to reduce the noise in the CS-1. Keep the sustain control as low as possible and don’t use the treble mode. Also be sure to use a high quality power supply. I run mine at 12VDC to help increase the headroom and lower the noise floor. Your best path to lowest noise is to use a modern design compressor like the Diamond or Empress. The CS-1 is older technology that just can’t perform like newer designs.

        • guitar511
        • Posted January 7, 2015 at 9:44 pm
        • Permalink

        This mod (at 10k resistance) worked perfectly for me too. What a difference! I heard there was a mod for this unit that would turn the momentary LED into an ‘on’ LED when circuit is on. Any suggestions and thanks for the improvements so far.

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