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Oh, That’s My Buss Compressor – Sidechain Filtering

In case anyone has been wondering what that the hell my header graphic is, it’s taken from my SB4000’s circuit board. This versatile stereo compressor is a do-it-yourself project modeled after (and enhanced from) Solid State Logic’s G Series center compressor, who’s presence on countless hits has rendered it legendary. Yes, sure, one could just buy this for a measly $3,699, but I decided to build my own – with additional features – for about 3 grand less. It’s currently strapped across the main buss, and can be heard on virtually every mix on this site. Here’s a slideshow of the build process.


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The SB4000 includes a peak-reading gain reduction meter (yes, the analog meter shows peak reduction within the limitations of its own ballistics). Controls provides are as follows:

  • Ratio: 2:1, 4:1, and 10:1
  • Attack: 100µs, 300µs, 1ms, 3ms, 10ms, 30ms
  • Release: 100ms, 300ms, 600ms, 900ms, 1.2s and Auto
  • HPF: Off, 60Hz, 90Hz, 130Hz, T1 and T2.

So what’s the difference between the XLogic and an SB4000? Most noticeably, there’s no auto-fade. I’m not crying over it, because though you sacrifice auto-fade, you receive internal sidechain tilt functionality. To understand sidechain tilt, we need to address sidechain filtering.

Traditional high-end compressors often provide a switchable hi-pass filter, allowing you to help prevent low frequency energy from triggering gain reduction. This is useful, for example, if your mix has a raging kick drum and you don’t want your whole mix being compressed when it strikes. You can high pass up to 100Hz or so and remove the kick’s fundamental frequency (presumably where it presents the most energy) from the compressor’s sidechain, allowing the compressor to respond to material much less than it otherwise would. The picture to the left is a snapshot; notice the strong presence of low frequency material. Now, look at the image below. It represents a hi-pass filter, at 100Hz 3dB down point, at a slope of roughly 12dB per octave. Enabling it would reduce the low frequency content your compressor “sees.”

While the SB4000 provides that functionality, it provides two “Tilt” settings, which look more like this:

If you’re not a logarithmic person, just imagine that graph as a diagonal line going up from left to right. You’ll notice that while this curve does begin to reduce frequencies below 5kHz, it also makes the sidechain more responsive to frequencies above 5kHz – things like harmonics, cymbals, sibilance, shakers, and lots of other things that begin with an S. The result is a highly transparent option to increase loudness, bring out detail, and shape the dynamics of your mix.

Expect some more posts with video demonstrating my SB4000 in action. In the meantime, visit the Prodigy Pro forums for info on building your own. If you are proficient with a soldering iron, pay attention to detail, and can stumble through a schematic when necessary, I highly recommend this build. Be warned – it’s not for the feint of heart. But if successful, you’ll have created with your own hands a very highly respected piece of gear that could be one more piece to the “big sound” puzzle.

And yes, it didn’t take me long to swap out those ugly black knobs I first bought for shiny fluted ones.


5 thoughts on “Oh, That’s My Buss Compressor – Sidechain Filtering

  1. Great post and explanation of the sidechain tilt. I’m also building an SB4000 and documenting the process on my blog. Glad to see your unit is working functionally!

    Posted by joestulpin | November 29, 2011, 19:26
  2. Where did you order the case from? And did you punch your own holes or get it pre drilled? I just ordered parts and the pcbs. Super excited!

    Posted by charlievelarecording | May 6, 2012, 16:37
    • Hey! Its been a while, but from what I recall Ruckus had machined a few face plates with pre-drilled holes for the potentiometer posts and meters. You still needed to carefully drill 4 holes for the screws which attach the face plate to the case. On the chassis itself you’ll need to drill holes to secure the PCB and toroidal transformer, and as my case had “lips” on the front, I needed to use a dremmel to file down concave indents for the power switch and pots.

      Posted by dan | May 21, 2012, 14:00


  1. Pingback: Oh, That’s My Buss Compressor – Video Demo « Audio Engineers are Dorks - October 13, 2011

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