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Multi-stage Compression

Tips & Techniques By Matt Houghton
Published October 2022

Multi-stage CompressionPhoto: Universal Audio

When and why might you want to reach for more than one compressor to process the same signal?

Every engineer needs to know how to use compressors to manipulate an audio signal’s dynamics. Often, you’ll be able to access the control you want with a single compressor, and you should certainly start by figuring out what you can achieve that way (check out Sam Inglis’ SOS November 2019 article if you need advice on how to operate a compressor. As you clock up more mixing miles, though, you’ll increasingly encounter situations where no one setting seems to work for the whole duration of the song. It’s often possible to solve such problems by using two compressors, and there are also some interesting mix strategies which employ multiple compressors. I’ll explore a number of such techniques in this article.

First, it’s worth pointing out that many of us already use multi‑compressor setups without giving it a second thought. If you use mix‑bus and subgroup compression (eg. on the drum bus), then plenty of your sources will flow through at least three compressors: one for the source, one at the group bus, and one at the stereo bus. Some of us use compression while recording too, and other processors might compress the signal as part of their sound; amps and cabinets, tape saturation and distortion effects all do that. And lots of us use a compressor and a limiter on the stereo bus or as part of a DIY mastering chain: a compressor to ‘glue’ the mix together, and perhaps give it a little more ‘punch’ or ‘solidity’, and a limiter (which is simply a high‑ratio compressor) to rein in peaks, preventing ‘digital overs’ and perhaps buy headroom to allow the signal level to be raised.

One To Another

With all that in mind, let’s start with another compressor‑plus‑limiter technique. A time‑honoured tactic is to use a UREI 1176 FET compressor and an Teletronix LA‑2A tube optical compressor in series on a vocal. These analogue compressors have different characteristics. The 1176 can be set to act very fast, its transformers add some character, and it’s pretty versatile too, with a choice of ratios and adjustable attack and release times. The LA‑2A can sound lovely on vocals: smooth and ‘natural’, with a slight thickening effect from the transformers and a pleasant, subtle euphony from the tube amplification. But it’s far less controllable than the 1176; you can switch the ratio between Limit and Compress, but most of its behaviour is automatic, varying with the complexity and level of the signal. So, while an LA‑2A in compression mode can sound beautiful on a vocal, it’s not uncommon that, having set it up to sound its best for most of the track, you later notice points at which a higher signal level triggers too much gain reduction and makes the release a little too sluggish.

A classic vocal combo: the slower‑acting LA‑2A optical compressor can often perform its desired role more easily when preceded by a faster compressor such as the 1176.A classic vocal combo: the slower‑acting LA‑2A optical compressor can often perform its desired role more easily when preceded by a faster compressor such as the 1176.

In this classic vocal compression chain, you’d place an 1176 before the LA‑2A, with the 1176 set with a fairly fast action and high‑ish ratio (it could be anywhere from 8:1 to the all‑buttons‑in mode, depending on the style) to bring those problematic peaks down by, say, 3‑4 dB. This allows the LA‑2A to perform more consistently. By the way, there’s no reason you can’t use this chain on other sources, or use these compressors the opposite way around (the LA‑2A set to Limit and the 1176 providing the less aggressive compression). You could use different compressors entirely if you wished — various successful producers have suggested combining the 1176 or another fast compressor (ie. FET, VCA or digital) with a Fairchild 670, RCA BA6A or UA 175 (all variable‑mu types and slow in comparison with FET and VCA ones) in similar fashion. But a helpful guideline is to make sure the first compressor is the faster‑acting one, and that it has a higher threshold than the second.

This isn’t the only reason you might chain two compressors in series, of course. By using two (or even three!) compressors with different attack and release times, you can often target the gain reduction of each processor at specific qualities in a sound. For example, you might have one focusing on the note onsets of an acoustic guitar, and another with a slower attack that aims to control the ‘bloom’ in a longer note when a string is picked that bit too hard. That said, to my mind, multiband compression or better still dynamic EQ (itself technically a form of compressor!) is a better tool for that sort of job.

Sidechain Tactics

If you want to make the main compressor’s job easier but don’t want to process the sound as in the 1176/LA‑2A serial compression approach, an alternative option is to compress the sidechain control signal.If you want to make the main compressor’s job easier but don’t want to process the sound as in the 1176/LA‑2A serial compression approach, an alternative option is to compress the sidechain control signal.

Though the 1176 and LA‑2A combo can work well, the first compressor does have an impact on the sound. If you’d like to prevent the LA‑2A getting too overenthusiastic with the gain reduction without changing the louder parts, you can set it up to respond to an external sidechain signal, and insert your faster compressor on this sidechain signal. Most DAW software makes this easy to set up: just create a mult (duplicate track) of the part you’re processing, route the mult to the vocal compressor’s sidechain input (being sure not to also route it to the stereo mix bus!), and then place your faster compressor on this mult. This will stop the main vocal compressor from overreacting to the louder signals, but those signals themselves will pass through unscathed.

While we’re on the subject of sidechain processing, a far more common tactic is to EQ the sidechain...

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