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UA Capitol Chambers, 175B & 176

Plug-ins For UAD2 System By Paul White
Published October 2019

Universal Audio continue to tempt UAD2 owners with officially licensed recreations of classic equipment — and rooms!

Every couple of months brings a software update for owners of Universal Audio interfaces and co-processing boxes, introducing yet more tasty new plug-ins! The latest updates to reach SOS Towers are v9.9, which includes the Capitol Chambers reverb and an update to Softube's Tube-Tech CL-1B compressor, and v9.10, which introduces emulations of UA's 175B and 176 valve compressors.

Chamber Music

Capitol Chambers is an emulation of the physical echo chambers built below the Capitol Records building in Los Angeles. These chambers have been used to add reverb to a wide range of performers from Frank Sinatra to Muse, Ray Charles to Beck, and whereas many physical echo chambers simply sound roomy, the ones at Capitol have a magical quality that just seems to improve anything you use them on. They may just be underground rooms equipped with mics and speakers but each was carefully designed, apparently with some input from Les Paul, to offer a musical sounding ambience/reverb.

UA Capitol Chambers, 175B & 176 plug-ins.UA's engineers employed the same dynamic modelling techniques they used when creating the Ocean Way studio emulation, so unlike IR-based snapshots, the user can adjust the microphone distance to achieve different results. The plug-in is licensed and endorsed by Capitol Studios, whose resident engineer Steve Genewick worked with UA on the mic/speaker setups.

Capitol Chambers is an end-to-end model that takes in the room, the speakers, the microphones and the electronic components in the signal chain — which includes Capitol's custom preamps. Of the eight chambers below the Capitol building, the four most popular (2, 4, 6 and 7) are recreated here, along with the exact same mic and speaker setups used in those rooms. The plug-in presets include historic speaker/mic/room configurations plus some new setups, again in collaboration with Steve Genewick. Numerous top-tier engineers and producers, including Al Schmitt, Frank Filipetti and Darrell Thorp, have also contributed presets.

Despite the complexity of what goes on behind the scenes, the GUI is very straightforward, with only a handful of controls. The four amber buttons are used to select which of the four chambers is active, and a graphic representation of the setup appears below. The speaker positions are fixed but the mic distance can be varied either by dragging or by using the horizontal fader. There's also a choice of four microphone types, activated by clicking the icon of the mic you wish to use.

In the lower half of the screen are controls that are largely self-explanatory: Pre-Delay up to a quarter of a second, Decay time, three-band EQ plus a variable (0 to 750 Hz) low-cut Filter, a wet/dry Mix control and a stereo Width adjustment. A Wet Solo button lets you audition the reverb only without having to adjust the Mix control.

High Density

In very general terms, the four chambers get warmer and darker as you move from the left selector button to right — though if the rightmost two spaces feel a bit too warm, you can always use the low-cut EQ to level things out. I found that the first two chambers sounded particularly good on acoustic guitar in situations where you want to retain a sense of detail without the reverb becoming too obvious. There's just a nice sense of the instrument sitting in a real space, and unlike algorithmic reverbs where 20-percent wet can be more than enough, you can afford to dial in a lot more with these chambers without washing out the dry sound. Even with the reverb time up full the result still feels natural. The warmer two spaces are wonderfully smooth and are particularly flattering when used on vocals, but all are very versatile. Having four microphone options also lets you fine-tune the character of the reverb. For those interested in such things, the small-diaphragm mics are Altec 21D and Shure SM80 models, while the fullest and warmest sound comes courtesy of the large-diaphragm Sony C37A capacitor mic and the RCA44 ribbon.

Where these chambers differ from other kinds of artificial reverb is that the reverb tails are very dense with no prominent early reflections, and as the rooms are not huge, the reverb density builds up very quickly. By contrast, plates also have a fast build-up of density and no hard early reflections, but there's always some noticeable metallic coloration due to the physical properties of the metal plate. Rooms also introduce coloration depending on their material, shape and dimensions, but the design of these particular rooms produces a smooth reverb where any remaining coloration sounds very natural.

This is one of those plug-ins that the more you use it, the more you appreciate its potential to enrich your mix without making its presence too obvious, and it's already become my try-it-first plug-in for adding reverb to acoustic instruments. Clearly it isn't the tool to use if you want 20-second long ambient washes, but if you need to sit your dry tracks in a real space or add a moderate-length, natural-sounding reverb or a shorter room ambience, I really can't fault Capitol Chambers.

Where these chambers differ from other kinds of artificial reverb is that the reverb tails are very dense with no prominent early reflections, and as the rooms are not huge, the reverb density builds up very quickly.

Limiting Factors

Dating back to the early 1960s, Universal Audio's 175B and 176 Tube Limiters can lay claim to being the first purpose-built studio compressor/limiters, predating the better-known 1176LN. Built using valves and substantial audio transformers, it comes as little surprise that both units add a certain warmth to the sound, but they also have enough flexibility to offer both very snappy or slower attach times. Both units have been used on many classic records, from the Rolling Stones and Led Zeppelin to Aretha Franklin and Miles Davis, and LA's Sunset Sound Studios still have a rack of the originals.

UA 175B plug-in.

Though closely related and both offering independent attack and release settings, the two units have different feature sets. Most notably, the 175B has a fixed 12:1 ratio, while the 176 has four switchable ratios of 2:1, 4:1, 8:1 and 12:1. Designed by MT 'Bill' Putnam Senior, the circuit uses a feedback compression topography resulting in more gentle compression at lower input levels graduating to more assertive limiting at higher levels, making this a very versatile design. It is also possible to turn off the gain reduction by setting the Attack knob to its Off position in order to use these limiters purely for their analogue signal path character.

For the plug-in versions, UA modelled the best-sounding original examples they could find, and have added modern features that make them more practical in a DAW environment, specifically Sidechain Link, Dry/Wet Mix parallel processing, and user-customisable Headroom. As usual there's a wide range of artist presets to get you started.

Beyond The Originals

The operational paradigm is familiar from the 1176: the Input knob determines the amount of drive into the compressor and the Output knob is used to compensate for any level changes. Attack serves its usual function in determining how quickly the compressor reacts once the input signal has exceeded the internal threshold, while Release sets the time it takes for the gain to return to its initial level once the input has fallen back below the threshold. The VU meter can be switched to display input, output or gain-reduction values.

The control layouts for the 175B and 176 are identical, the main difference being the function of the leftmost rotary switch, which acts as a 10dB boost for the 175B and as a four-way ratio switch for the 176B. This means that the 175B, with its fixed ratio, is best suited for when hard limiting is needed whereas the 176 can be used as either a compressor or limiter depending on how hard you push it, using the large Input control knob to set the amount of compression. As with any compressor, setting a very fast attack and release time can alter or distort the actual audio waveshape, but some engineers find this gritty distortion useful on certain sound sources. The original was a mono unit, and the added Sidechain Link switch gives the choice of linking left/right gain reduction or allowing independent L/R channel gain reduction when used on a stereo track or bus.

UA 176 plug-in.

Another non-original feature, the HR Headroom control, adjusts the internal operating reference level to 4, 8, 12, 16, 20, 24 or 28 dB — the default value is 16dB. Another virtual trim pot labelled BAL (Balance) replaces the independent plate and cathode bias current balance trims found on the original hardware. Its default position is the optimal setting, but changing it can add a thump to the attack of a signal, which can occasionally be useful. UA recommend that this control not be automated as 'DC settling artifacts' can occur. On the original hardware, the Input and Output knobs always step in 2dB increments, with separate Input and Output Vernier knobs allowing ±2dB of fine adjustment. The plug-in uses a revised approach where continuous gain knobs can be switched to operate in steps if you prefer.

Attack works conventionally (fastest is fully clockwise) over a range of around 100 microseconds to 1 millisecond. That fast attack is impressive for such an early design, and makes the 175B a very effective limiter. The newly added Mix control below it balances the dry and compressed sounds for setting up parallel compression effects. Actual attack time varies with the selected ratio on the 176, with the higher ratios having the faster attack times. Release also works conventionally, again with clockwise being fastest.

Smooth Operators

Like so much early equipment, these two units have real character that can be employed in a very musical way, and either plug-in can be used for adding attitude to rock drums. However the 176 also works very well on bass, guitars and vocals, adding a smooth (or less smooth if you push it hard) assertiveness to the sound. Dial back the compression and the 176 can be more restrained, too, adding a useful but not overdone sense of warmth. Bypass the gain reduction and the coloration of the signal path is quite subtle unless you push it into deliberate distortion, but then that's as it should be: those early audio pioneers invariably did their best to get the best technical performance out of valves and transformers, and distortion was never designed in by intent.

Once again it seems as though some of those early designs really did have some kind of magic about them, and UA appear to have captured the essence of these two particular units in great detail. Do you really need another compressor? As usual, you can have two weeks to try any of the new plug-ins added to a UAD update, so use them and judge for yourself!


  • Both Capitol Chambers and the two valve limiters are excellent emulations of the originals.
  • Capitol Chambers offers an organic-sounding alternative to conventional reverb.
  • All the new plug-ins are very easy to use.


  • None, as long as you can still find room in your plug-in folder for new reverbs and compressors!


Once again UA have come up with superb emulations of vintage kit that still has a lot to offer in the modern studio.


Capitol Chambers £260; 175B & 176 Collection £229. Prices include VAT.

Capitol Chambers $349; 175B & 176 Collection $299.