Development of the Capitol CM5511 grew from Ian Sefchick’s interest in classic valve dynamics processors, particularly the Universal Audio 175, Fairchild 670 and Gates Sta-Level. Rather than reproduce one of these designs, Sefchick developed a new compressor intended specifically for the requirements of mastering. His concept combined a valve front end with a solid-state back end, with component selection and construction intended to provide repeatable operation across Capitol’s mastering facilities.
The final design was completed in 2011. Its CM5511 designation combines two dates significant to the project: 1955, the year Capitol Records moved into the Capitol Tower, and 2011, the year in which the compressor design was completed.
At the heart of the gain-reduction section are vintage Mullard 6BC8 valves. These are combined with John Hardy 990C discrete operational amplifiers and Cinemag input and output transformers. The resulting design is therefore not an all-valve signal path, but a hybrid circuit combining valve gain reduction with discrete solid-state amplification and transformer-coupled inputs and outputs.
Because the CM5511 was conceived as a mastering processor, repeatability was an important part of its design. Its main parameters use precision-matched stepped controls rather than continuously variable potentiometers, allowing settings to be accurately recalled. Each channel provides stepped Input, Threshold, Attack, Release and Output controls.
The two channels can be operated independently or linked for stereo processing. On the original hardware, linking the channels makes the right channel the master control channel. The hardware also incorporates switchable sidechain filtering, allowing low-frequency information to have less influence on the compressor detector without removing that low-frequency content from the programme signal.
The compressor uses a fixed compression ratio rather than providing a front-panel ratio control. Significantly, the four CM5511 units are not all completely identical. One of the units was subsequently modified, including a change in compression ratio from 4:1 to 3:1, a different sidechain filter frequency and larger output attenuation steps. This distinction is important because later software modelling of the CM5511 incorporates characteristics from more than one hardware unit and adds controls that do not exist on the original machines.
A parallel high-ratio limiter is also incorporated into the hardware circuit. It operates at a fixed threshold on high-level signals to help prevent excessive clipping in the output amplifier and has no dedicated user controls.
Metering is provided by two large custom analogue VU meters, complemented by controls for selecting the required metering function. The front panel also provides separate fine-gain adjustment and channel-in controls. The electronics are housed in a substantial 3U rackmount chassis with engraved front-panel overlays, vintage Bakelite control knobs and custom meters.
Once completed, four CM5511 units were constructed for Capitol Studios and installed for use by its mastering engineers. The compressor subsequently became part of Capitol’s mastering chain across a wide range of projects, including work associated with the Capitol, Motown, Geffen, Blue Note and Def Jam catalogues. Its use was not confined entirely to mastering, with the units also finding applications in recording and mixing within Capitol.
The extreme rarity of the CM5511 is a direct consequence of its purpose. It was never developed as a production product for the professional audio market. The four examples were built specifically for Capitol Studios, making the original hardware effectively inaccessible outside the facility.
The Capitol CM5511 later became the basis of Universal Audio’s Capitol Mastering Compressor software emulation. That plug-in should not be regarded as an exact control-for-control representation of the physical unit. Universal Audio studied two of the four CM5511 compressors and modelled differences between them, while also adding Ratio, Mix, Headroom, Saturator, Mono Fold, Mid/Side operation and additional digital metering facilities. Those additions belong to the software product and are not features of the original CM5511 hardware.
Features
- Custom two-channel mastering compressor designed specifically for Capitol Studios
- Designed by Capitol mastering engineer and technician Ian Sefchick
- Only four hardware units manufactured
- Valve gain-reduction stage using vintage Mullard 6BC8 valves
- John Hardy 990C discrete operational amplifiers
- Cinemag input and output transformers
- Precision-matched stepped controls for accurate mastering recall
- Independent Input, Threshold, Attack, Release and Output controls for each channel
- Independent dual-channel or linked stereo operation
- Switchable sidechain filtering
- Parallel fixed-threshold, high-ratio limiting circuit
- Separate fine-gain adjustment
- Large custom analogue VU meters
- Vintage Bakelite control knobs
- Heavy-duty 3U rackmount construction with engraved front-panel overlays
- Different circuit configurations exist among the four original units