CP4500 Stereo Bus compressor for 500 series-DIY Analog Audio
  • CP4500 Stereo Bus compressor for 500 series-DIY Analog Audio
  • CP4500 Stereo Bus compressor for 500 series-DIY Analog Audio

CP4500 Stereo Bus compressor for 500 series

$481.62 VAT incl.
$481.62 VAT excl.

The CP4500 is a Stereo bus compressor based on the design that was originally used in a legendary British console, adapted to the 500 series format of API*. It is famous for adding punch and glue to the mixes.

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The CP4500 is a Stereo bus compressor based on the design that was originally used in a legendary British console, adapted to the 500 series format of API*. It is famous for adding punch and glue to the mixes.

The CP4500 adds a few features to the original such as an equalizer in the side chain with three high-pass frequencies and a high boost section which enhances the spectral balance of the output. It also adds a Mix knob that allows mixing blending dry and wet signal for parallel compression.


    • Very faithful to the original design,
    • Four paralleled THAT VCA's (plus one in the side chain) per channel,
    • True stereo side chain,
    • Stepped Ratio, Attack and Release,
    • Side chain EQ with 3 high-pass frequencies and one high boost filter,
    • Wet/Dry mix control for parallel compression,
    • Ultra-low distortion and noise audio op-amps,
    • Analog gain reduction meter,
    • High grade electronic components throughout,
    • High grade double sided PCB with plated holes,
    • High precision aluminum knobs,
    • Available assembled or as a full kit (everything included).


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1: RATIO Rotary switch that sets the compression ratio to 2, 4 or 10.

2: ATTACK Rotary switch that sets the attack time from 0.1 to 30 milliseconds.

3: HP Inserts a 50, 100 or 200 Hz high-pass filter in the side-chain. Produces a Fat sound and eliminates bass driven pumping.

4: RELEASE Rotary switch that sets the release time from 0.05 to 1.2 second. The AUTO position is a program dependent automatic release time.

5: Meter Analog meter displays the dB gain reduction.

6: IN Compressor bypass (hardware by relay).

7: THRESHOLD Potentiometer that sets the input level at which compression begins.

8: HF LIFT Inserts a high boost filter in the side chain. Combined with the High-Pass it evens the audio energy in low and high bands and gives a very natural sounding to the compressed output.

9: MAKEUP Potentiometer that restores the signal level after being compressed (make up gain).

10: MIX Potentiometer that mixes the direct unprocessed signal with the compresses signal (0 to 100%) for parallel compression.

Frequency response of the side chain filters

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What's in the kit?

Absolutely everything!

  • Passive components (resistors, capacitors, inductors),
  • Active components (diodes, transistors, LED's, IC's),
  • Front panel elements (switches, potentiometers, analog meter, knobs),
  • Connectors, relays,
  • PCB's,
  • Front panel and side panel,
  • All hardware.

You bring the solder (use good quality solder, 1mm diameter max) and the Lunchbox.

Difficulty level

The CP4500 kit is a medium difficulty kit. Nothing difficult but a rather large number of components.

The setup is simple but requires using a good quality AC voltmeter.


Specific References

Technical specifications

Measure Conditions Value
Noise input shunted with a 50Ω resistor
Low pass @30kHz
<-93 dBu
Maximum output level before clip f=1kHz +27.5 dBu
Dynamic range >120 dB
Frequency response Deviation=+/-0.2 dB 10Hz - 100kHz
Idle supply current No input signal V+ : +130 mA
V- : -125 mA
Input impedance f=1kHz 24 kΩ
Maximum input level +27.5 dBu
Output impedance 50Ω
THD + Noise f=1kHz without gain reduction < 0.005%

Demo of the Soundskulptor CP4500 stereo bus compressor

Another demo of the Soundskulptor CP4500



Great kit to built for a classic compression

It's quite impressive to read schematics, to put together all the elements of a genius' piece of audio hardware, and it's quite rewarding to listen to it in action!

I really enjoyed to follow the guide and assemble this CP4500 kit: everything is so carefully prepared by SoundSkulptor, the documents and the components. The calibration procedure is quite straight forward.

I already have an ssl bus comp clone, and ...plugins. This compressor is renown for it's punch and glue, kinda transparent compression, within small amounts of compression. But with right gain structure and heavy compression, harmonic distorsion occurs .The 4 options to taylor the sidechain signal are very efficient to skulpt the compressed sound to taste: i love the CP4500.

Thanks you SoundSkulptor

Martin W

Sounds amazing!

Fun to build and sound just amazing. Use it a lot on the master bus. Even when I'm just listening to ready made music through my monitors I've noticed I often prefer to send it through the CP4500 (with basically no compression engaged, but still let the audio pass through) just because I perceive it sounds better then. Might be ghosts in my head, or the circuits give the audio some mojo. Great kit!

Paul Tietze


This is an extremely versatile and excellent sounding compressor. I bought this unit already assembled and could not be happier. It does the SSL Glue thing and much, much more. I like it even more than the original, More options and possibilities. Highly recommended!

Tommy Hedin

Love at first sight

I love everything about Soundskulptor.

The documentation i excellent, the kits are very well designed in order to be easy to build. And the products bring excellent value for your money.

The CP4500 is no exception. Every knob change the sound in a noticable way, and the result is almost always musical. This SSL-style buscompressor beats every plugin available (my opinion), and I use it on every song as a glue compressor in mastering.

I just wished I have ordered this product years ago.


Great Kit

This kit is a 5 out of 5 in every category. Quality parts, great design, comprehensive instructions, all at an attractive price point. Instant mix bus glue. The unit sounds great and I couldn’t be happier. Highly recommended.

Lee Campen

Incredible value and quality throughout

- Very high quality parts and components
- Extremely well packed and organized
- Excellent, detailed instructions
- Thoughtfully laid out and designed
- Fun to build if you have basic soldering skills
- Best price for this circuit
- Looks gorgeous!

As my recording and mixing skills have grown, I have become increasingly interested in outboard gear. I have a few pieces of 19" rack gear but I have wanted to get into 500 series for the space savings. The issue for me has been the astronomical cost for quality gear.

I have a good deal of experience building guitar effects pedals and amps so I was confident that I would do well building kits to fill my lunch box. After extensive searching I found Sound Skulptor and I was truly surprised to find this level of quality at a price that is realistic for me. The CP4500 really stood out to me as an almost too-good-to-be-true kit, so I took the plunge and bought it.

Shipping to the US was surprisingly fast and when I opened the box I was immediately impressed with the packaging. Everything was expertly sorted and perfectly fitted into the custom cut foam internal packaging. Small parts and components came sorted into zip lock baggies and larger pieces were well protected. There was not a single scratch on the faceplate, knobs or meter, and after laying out every part, there was absolutely nothing missing.

The PCBs are thick and of excellent quality. They are double sided and through-hole plated, and there were no discernible defects anywhere. The layout and space makes it very easy to solder. I never felt like I was at risk of a solder bridge or that the components were so close that I might melt the top of a cap etc. All of the IC sockets are the high quality mill-max style, which I greatly prefer. Resistors are 1% tolerance and they have nice thick leads. All box caps are WIMAs and all ceramic caps are MLCCs. There are NO poor quality passives in this kit. Everything is top notch.

I highly recommend following the advice in the instructions to lay out all of the resistors, measure one from each set and mark the ammo reel tape. Then lay them out in order of resistance so you are less likely to accidentally misplace one. I did all resistors for all three boards before moving on to the taller components. Populating the resistors was by far the most tedious part of the build, and loading all of the tiny MLCC caps isn't much fun either, but the build really clicks along once you get through that phase.

Setup and calibration was very straightforward and easy to do. All you need is some way to generate a 1kHz sine wave and a DMM. I have a function generator but you could use a synth from your DAW or look for the link in the instructions for an online resource.

At the end of the process I ended up with a perfectly working, quality piece of gear that I am proud to have built and that I expect to use on my mix bus for everything! Once I have the money saved up I plan to build a pair of the EQP501s to go with it. I can't wait!

So, is there ANYTHING not to like about this kit? I hate to admit that there is ONE sticking point. In my opinion, the control pots really let this kit down. Others have noted that they feel a bit wobbly, being that they are plastic shaft and the shafts are pretty long. I agree with that criticism. But not only do they wobble, they just don't have the silky smooth feel that I'm used to. Am I afraid that they might break. No. Do they perform their function? Absolutely. They just don't have the feel that conveys quality. If I were building this to sell to someone else I would consider drilling out the holes in the faceplate and panel mounting better feeling pots. Time will tell if this bothers me enough to change them. Honestly, I'll probably just keep them as they are.

When you consider the fact that this unit is about 1/5 the price of the "real deal" and that, apart from the minor issue with the control pots, this unit screams quality, I can't recommend this enough. If you have successfully built at least one guitar pedal or similar project, you can do it. If you have never soldered before or if you aren't good at paying attention to detail you may want to pass or just buy one pre-built. It's still a phenomenal deal.

Great job Sound Skulptor!


transparent, punchy, flexible!

It was my first Sound Skulptor build and it was great fun - perfect documentation, I have to applaud JPK for his efforts! When I messed up a connector, JPK was of great help providing links for re-ordering part. He answered so quickly. I use the CP4500 on drum buss mostly, not on the mixbus. Compared it against UAD SSL4000 G Bus Compressor and it sounds a bit lighter, not that "gluey", but very punchy and more open. I was surprised how it worked as a buss compressor on backing vocals. Very transparent! Great unit.

Small Steps Mastering

Excellent guide, great compressor

Had a blast putting this one together. All the documentation was super helpful, especially the printed resistor colour code guide. Ran into some trouble upon testing, and with prompt responses from Jean-Pierre I was able to find an unsoldered SMD pin. Calibration was easy, and it sounds great. Love how this sounds on master bus duties, and the mix knob is truly the magic ingredient for this compressor.


It's fun to build this awesome glue compressor

For some time I had been tempted to start using 500 gear and also to start building equipment. Having built this compressor and enjoying its performance for the first time has been the perfect start for me. I am completely hooked now - because:

This compressor can glue perfectly. from electronic music to pop music - from master sum to drum bus - everything I throw at it gets glued without destroying dynamics. If I had to decide again, I would always choose this stereo compressor for the characteristics and the features: side chain, highpass, wet/dry mix control - this compressor is just awesome in versatility and in getting results quickly.

Now for building it - if you are patient, willing to work on it for a few hours/day for less than a week, and if you enjoy building stuff, you might be able to pull it off even as a beginner. Binge watch tutorials about soldering, learn using a voltage meter, buy good quality solder, temperature regulated iron, a pcb holding tool, a soldering mat, a good pair of cutters and tweezers, a desoldering pump and off you go!

JeanPierre was super fast and helpful when I had a question and the kit was spot on. superior documentation, super thought out packaging, premium components and last but not least - a great design of the boards and the front.

I highly recommend building and using this compressor.


This thing is fantastic

I have both a GSSL4000 with the DBX VCA modules as well as a Neve 33609, both are fantastic bus compressors, but I find that the Sound Skulptor 4500 compressor is my favorite. Neither he Neve or the GSSL have the high pass filtering or the capability of parallel compression. The high pass filter alone is worth the price of admission. This thing plays with the big boys and kicks sand in their face! And for the price, I am tempted to get a second for my Aux bus.



First of all, as a kit it is fantastic! The kit itself is a good quality kit, well designed and easy to build. Just follow the instructions (great instructions) and you will be good.
The sound and functions are as good as you can get, you get the SSL sound and the features.
Good work with this one!

Andi Jones

Creative Interpretation of the SSL4k

I Had only built a DI box and two preamps before getting this kit, so I was a bit nervous about critically ruining the project.
No surprise: it did not work when I first tried to calibrate it. However, Jean-Pierre was so helpful in patient with helping me troubleshoot, and within a week it was working perfectly! Thank you so much J.P.!
Now about the gear itself:
I've never had the chance to work with a hardware SSL before, but I have used a number of good hardware compressors before, and many software emulations of an SSL, and this is one of the best compressors I've ever used. The short attack times are remarkably versatile (though an additional 60-100ms option would make the mastering engineer in me very happy). The release curve is super smooth in general, especially for a VCA. And the sidechain filters can turn this little box into a timbral balancing tool, as well. I have a feeling this will be on almost every project I come across for the foreseeable future.

Udo Terlisten

A No-Brainer

Very easy to build,easy use plus some nice extras (HF) compared to the originals.
Sounds exactly the way I wanted it to hear.
Have built two of them meanwhile.Looking at the kit price this is a no-brainer.
Highly recommended!

Daniel McM

Easy build and great useability

A bit scared off by the component count and being the first time soldering on a PCB, it took me surprisingly only 8hrs to finish this build, although I highly recommend a good iron and following the instructions closely.
The calibration worked great too.
The sonic quality is very good to my ears and flexible for all different kinds of soundsources.
It does subtle works for most of its uses, but this unit is also not meant to radically change your track! Just giving it the final polishes and nice overall glue.


Cant believe I got it right the fist time!

No rework. No headaches. No freakouts. Well detailed build guide and a great sounding unit. While I've never used the OG SSL this bad boy definitely wins over the UAD emulations for me!

Write your review

CP4500 Stereo Bus compressor for 500 series

The CP4500 is a Stereo bus compressor based on the design that was originally used in a legendary British console, adapted to the 500 series format of API*. It is famous for adding punch and glue to the mixes.

Write your review