Hardware compressors have been a key part of a producer/engineer’s arsenal since they were first created. Today, we’re lucky enough to have countless replications of hardware compressors available in our DAW, with no limits on how many we can use at once.

But what makes these hardware compressors different? How can compression come in so many different ‘flavours’? Isn’t it just reducing the volume after all?

Well, in the analogue world there are different ways to achieve that result, and one of the keys is in the “detector circuit”, the part used to decide how and when to lower the volume of the signal.

I’m going to talk about 4 hardware compressors that I use in almost every mix, what makes them different, and what that means for me when I pick between them.

 (Sadly, my gear budget doesn’t stretch to the hardware yet, so we’re using software emulations for now!)

1.          SSL G Bus Compressor

One of the most used hardware emulations, this compressor started its life in the SSL mixing desks of old, and with the G series desk (which is hugely sought after, and used on countless hits) it quickly it became a huge deal. I don’t think there’s a person alive who hasn’t heard it!

The SSL is VCA (Voltage Controlled Amplifier) compressor with a feedback detector circuit.

Feedback Quick Explainer: There are two types of detection circuit, feed forward and feedback. The distinction here makes a huge difference to the sound of a compressor’s gain reduction. It’s pretty simple:

·      Feed Forward compressors split the incoming audio signal in two, sending it to the attenuator and the detection circuit

·      Feed Back compressors split the outgoing audio signal (after the attenuater) in two, sending it to the output and ‘backwards’ to the detection circuit

This difference has a huge effect on the how fast a compressor ‘feels’.

If you hear someone talking about “glue” compression, this will be what they are referring to. A gentle bit of gain reduction on the master bus helps to make the track feel a bit more cohesive, as if the drums are nestled in to the rest of the music, but without losing their clarity.

Adjusting the release time can add a huge amount of energy to a drum heavy track, and using longer attacks will keep your low end from squashing the rest of the track.

The bx_Townhouse plugin lives on my master bus about 50% of the time, and starts with the settings in below. This plugin models an early prototype of the SSL bus comp I believe, but there are SSL G options from Waves, UAD, and even SSL themselves. My favourite though is the bx_Townhouse plugin by Brainworx. It’s a model of the rack version of the G compressor, before the SSL racked version existed, and it does something to the low mids that I love!

These are my default settings for the Townhouse compressor - I'm always looking for between 2 and 4dBs of gain reduction

2.        API 2500 Bus Compressor

APIs consoles and processors for me have always had a punchy, crisp and clear sound. This compressor is the other 50% of my master bus time! I use this compressor when the tonal balance of the stems is already great, and I just want to add punch without messing up the clarity.

My favourite part of this compressor is the Tone section, which gives you a huge amount of control over your sound. The “THRUST” (Filter on my Lindell plugin) control is reason enough to demand you try this out!

Thrust Control Quick Explainer: From my understanding, the thrust control adds a tilt EQ to the detector circuit, which boosts highs and cuts lows around a fixed frequency. This causes the compressor to respond less to low frequencies and more to high frequencies.

If you’re working with music where sub bass is prominent, this setting is an absolute must. It will allow you to compress harder, and maintain the excitement in the top end, without the low end squashing everything

Combined with the option to choose feed-forward or feed-back compression, a set of knee controls, and even control over how similar the left and right compression is, the 2500 is surely one of the most versatile compressors about, and a must have tool in a mixer’s arsenal.

There are of course a number of 2500 plugin models online, but my favourite is the Lindell SBC from Plugin Alliance (a lot of my faves are from there!). I’ve used the Acustica Pink version too, which is incredible but very CPU intensive!

My default settings for the 2500 compressor - I'm normally looking for 3-4dB gain reduction when used on the mix bus

3.        Urei 1176

Probably one of the most talked about compressors ever, the 1176 is a FET based feed-back compressor who’s characteristics are fast and aggressive. This is the compressor you use to slam a vocal, to rough up those drum overheads, to squash that bass guitar. Its sound comes from its crazy fast attack (up to 20 microseconds!) and program dependant release (which releases transients faster than sustained sound). All in all this can make for some very fast, but very smooth, aggressive compression.

The 1176 has four selectable ratios, but engineers quickly discovered you could jam all four buttons in at once, which gave you insane compression with some great distortion. This is used all over the place now, and is one of my favourites when it comes to drum room mics, or parallel drum bus compression.

There were a number of revisions to the hardware unit, which we mostly see in the form of different face plates. We know these as Black, Blue Stripe, or Grey models. The Blue Stripe models have been made very popular by Chris Lord-Alge, who’s particular unit is modelled by Waves and loved particularly for aggressive rock vocals.

I love the way these compressors give a bite to the upper mids, while making things sound firm, in your face aggressive!

My Default 1176 - the attack is at its slowest setting, release fastest

4.      Teletronix LA2A

You can’t mention the 1176 without its partner in crime, the LA-2A. combine these two on a vocal and you’ll hear great results every time!

In stark contrast with the 1176, the LA2A uses tube amplification rather than FET transistors, and an optical detection circuit. This gives the feeling of a silky smooth, warm tone that almost softens what you put into it.

Optical Compression Quick Explainer: Simply put, the incoming audio signal powers a light, which changes with the level of the signal. A light-dependant resistor then controls the gain reduction. Easy!

This normally results in a slower attack and release time, as non-LED lights take time to warm up/cool down!

I use the UA LA2A plugin for this one. Whenever I’m looking for smooth, gentle compression that will warm up the source audio a bit, this is what I pick. It’s also one of the easiest to use, with just three controls!

UA's LA2A plugin