DIY Audio 3: Sound Skulptor CP4500
Updated: Jul 2, 2020
Looking at this compressor immediately should give away what unit it was designed from. The layout of the panel, the size of the unit, the shape of VU meter, all points at one stereo compressor; the SSL 4000 bus compressor. Ever since the SSL 4000 board was released, engineers have been using this VCA compressor to "Glue" together the music running through the board. It became so sought after that SSL eventually decided to release a standalone unit. The schematics for the original compressor can be found online in many different places, often times the schematics are used to diagnose problems with equipment so, many individuals have released their take on this vintage piece of gear. I won't go into the differences between this clone and others because you can find that info online elsewhere. I will instead get into the review.
MAKING THE KIT: First off, Sound Skulptor makes very nice kits for DIY enthusiasts. Their silkscreen doesn't include values for components so you will need to look at a components list to find where everything goes. But the PCBs are very nicely made, and I found the layout easy to work with. The components come separated by types and there is a guide made that you can follow to make the product. Since there are two channels you fill two of the boards with almost the identical layout, I fitted diodes, resistors and capacitors on the two main boards at the same time and then followed the instructions for the rest of the build. The final steps of the build is to adjust potentiometers to make sure the VU meter, threshold and output level are correct. Two things for this. One, a standard 500 series chassis isn't great to work in while turning potentiometer screws. Lots of people sell extenders for 500 series, or test jigs. Use these. you will have an easier time with calibration. Second thing is that I found my VU meter (a cheap multimeter that is getting old) didn't have a high enough resolution to do it accurately. You can adjust the steps to go into an interface and monitor the return signal through pro tools (this is how the OLA5 works from DIYre) or get a better VU meter, which I did.
THE GLUE: To test this out I ran a couple different samples through it, my voice, a mixed song and a stereo guitar. What I heard was that sweet VCA distortion through all of these samples, sometimes for better, sometimes worse. Now for those who are wondering if this unit just sounds like fuzz box full of distortion, no, it is clean. But if you listen closely you can hear this "sound" which is pulled over each of the sources, and this is that "glue" sound that tons of hits were mixed through. I tested the sound vs the Waves plugin version and found the sound was very similar, the CP4500 has a little more midrange than the plugin does (though that could also come from the converters that I ran it through). I found the side chain filters to be the biggest game changer though. Particularly with bass heavy music the high pass filter is a god-send. The High frequency lift I didn't find as useful, but I bet it would be nice for cymbals.
FINAL VERDICT: This is a very capable box, looks great, sounds great, does exactly as it should to the mix. The build wasn't difficult, but if it is the first compressor you have done it might be confusing during the calibration stage. I think I would have preferred that the meter didn't go all the way -20 since I usually use it between 1-7 dB of compression. But with the addition of the side-chain filters this thing rocks. I would love to pit it against some other clones and maybe against a real one just to see how it sounds.