So I’ve been wanting to understand and make “my own” compressor for a long time. Turns out this time has finally come.

After having researched the various ways of implementing compression, I decided I wanted to try a design based on a VCA chip. Looking at well known compression pedals of the past I was unable to find one that used this type of chip (I may not have researched this extensively), so I instead went to the makers of a chip I have used before, THAT, and started looking at the data sheets and design documents. 

A friend made me aware of a number of studio compressors using that same chip, so I ended up looking at the SSL 4000 stereo mixbus compressor. While this thing has a bit more to it, I wanted to cut it down to the essential compression circuitry, and see how that would work out as a guitar/bass compressor.

The SSL circuit does indeed look quite similar to the THAT data sheets and designs, so I cross referenced both, and came up with what you see here. After much testing and tweaking, while I will probably never be entirely happy with anything I make, I think it’s quite cool now.

Tried on both guitar and bass, and according to my friend who is a much better player than me, sounds especially killer on bass.

Project PDF with trace cut diagram etc.: VCA compressor 1.0

15 thoughts on “Kompressor

  1. Hello! I was wondering if you could tell what exactly the THAT IC should be? The schematic says 2180, but the build notes say 2181. Furthermore, there are variations on the THAT 2180/2181: the A, B, and C variant. Namely they vary in relation to equivalent input bias current and output offset voltage, according to the datasheet. Would appreciate anyone weighing in on this 🙂

  2. I tried to redesign it to work off of guitar signals. It sounds like you want something like the original circuit.

  3. Excellent!
    Thank you for this design.

    – Is it designed for guitar, or can it be used as a regular buss compressor, or on drums?

    – Is there any way to add a potentiometer or a switch to lower the bass in the detection circuit (so it would compress less quickly when playing bass heavy material)?

  4. Thank you Harald.
    Yes,There is loud noise at frequencies above 20khz.
    I’m going to try a lot of possibilities.

  5. Hi Keigo,
    Yes, my build is apparently fine, though I haven’t measured it’s output for inaudible frequencies. This might be none from the 7660S chip, which is known for producing high frequency switching noise.

  6. Hi Harald,

    I just finished mine, and even managed to make it fit in a smaller 1590B enclosure. There is not anymore room for a battery, but there has been room for the giant 250v 4.7uF film capacitor i ordered by mistake. I wanted to replace the 4.7uF electrolytic ouput cap with a small film one, but I did not check the size of the one I ordered.
    Anyway it sounds very good, and the range of settings is huge.
    After the great OC-2 clone I cherish, thanks again for another great fun project.

  7. I have two questions:

    A) What does the trimmer do?

    B) How would one connect a gain reduction meter to it?

  8. I’m so happy to see some action in this blog again. Thanks for sharing that piece of art with us. This will definitely get my project list. Which is basically full of your projects by the way ?

  9. You are THE man! I recently heard about the SSL compressor, so this is a real treat! Thanks for sharing the project and welcome back! 😛

Leave a Reply

Your email address will not be published. Required fields are marked *

Time limit is exhausted. Please reload CAPTCHA.