Compressor-types

We give you an overview over the different compressor-types.

VCA Compressor

The VCA-Compressor (VCA = Voltage-Controlled-Amplifier) is the the most common of the 4 compressor-types.

Actually, You can use The VCA compressor anywhere; as a sum compressor or in a channel strip.

In general, however, one can say that a VCA compressor is very well suited to processing transient-rich and percussive signals due to its easily adjustable timing values. For example, drum recordings and drum subgroups.

VCA-Compressors are good for:

• Bus-Compression
• Drums
• Transparency
• In some cases for Vocals

Common VCA-Compressors

• UAD dbx 160 (very personal coloring, a lot of character, considered one of the best, many use him on the DrumBuss)
• NEVE 33609
• SSL Bus Compressor
• API 2500
• UAD Precision Buss Compressor
• Distressor

Properties

• The most widely used type of audio compressor.
• Very accurate and variably adjustable compression.

Opto/Optical Compressor

An attribute that is always associated with the optical compressor:

Musicality.

This means that the compressor still works largely inaudibly even with strong compression. The Opro-Compressor is very useful on vocals or solo instruments because of it’s incredibly transparency. But you can also use it as a limiter during recording, for example to protect an analogue or digital recording medium from clipping. Here, too, the opto-compressor scores with transparency and “inaudibility”.

Common Opto-Compressors

• UAD Teletronics LA-2A

Properties of a Opto / Optical Compressor

• “inaudible”
• “musically”

Opto/Optical Compressors are good for:

• Vocals
• Solo-Instruments
• As a Limiter during Recording

How to Use a Opto / Optical Compressor

The attack times of an Optical Compressor are around 20 ms (fast, but not ultra-fast). But the release times are highly dependent on the nature of the signal and it can take several seconds for the signal to return to its original level.

Tu adjust an Optical Compressor is therefore very simple:
Turn up the peak reduction until the desired number of dB is pushed away and make up for the loss in loudness with the gain control.

Which Teletronics LA-2A do I use for what?

	LA-2A Silver	LA-2A Gray	LA-2
Year	late-’60s	mid-’60s	early-’60s
Characteristics	Generally appreciated for its faster time constant and its treatment of transients	For material requiring a medium-speed compression	The slowest response time and a distinctive “mellow” sound.
Good for	vocals, keyboards, guitars	drums, percussion, bass guitar	vocals, strings, horns

More Tips & Tricks about the LA-2A:
https://www.uaudio.com/blog/la-2a-collection-tips-tricks/

FET Compressor

FET Compressors were designed as a faster alternative to Opto & Variable Mu. A FET-Compresor emulates the behavior of a tube via a special transistor circuit.

Common FET-Compressors

• UAD 1176 Series

Properties of a FET-Compressor

• In contrast to the tubes, the FETs work much faster, so much shorter attack times can be achieved
• not very transparent
• dirty
• gives character and (if necessary) some aggressiveness

The three 1176 UAD2-Plugins

• The Rev A “Bluestripe” represents the original Putnam FET limiter design, complete with its higher distortion and unique FET gain amplifier characteristics.
• The Rev E “Blackface” covers the early ’70s Brad Plunkett “LN” (Low Noise) era, with variations including a more linear compression response, transistor gain amplification, and a change to the program dependence. Less distortion than Rev A.
• The 1176AE: Similar sound to Rev A, but new, gentler compression ratio with 2:1, 4:1, 8:1 and classic 20:1 for hard limiting. Expanded range of “multi-button” compression and classic “all-button” mode with 2:1 ratio.

FET-Compressors are good for:

• Signals with a lot of transients
• Snare
• Bass

Grit-Trick wiht the 1176

Due to the short control times, you can even distort the signal with an 1176 compressor. This is called the grit trick: If you turn the attack and release times to the shortest possible values, the signal is simultaneously compressed and begins to sound distorted. You can really bang if you combine the grit trick with the all-button mode. This works particularly well with DI tracks from electric basses, but you can also enrich the signal with a little “dirt” with vocals.

Good to know:
Attack times on the 1176: “7” is fastest attack time

Tube / Vari-Mu Compressor

The attack times achieved by a Tube-Amp are too slow to keep very fast transients under control, by the time the compressor has reacted, the signal peak has already rushed through. But what the tube compressor can do very well, however, is to smooth out the dynamic differences on a larger scale.

Vari-Mu stands for Variable Mu (MU = The amplification factor or voltage gain) is another name for a classic tube-compressor.

The archetype of the tube compressor is the Fairchild 670 (or 660 in the mono version).

Common Tube / Vari-Mu Compressors

• UAD Fairchild 660
• UAD Fairchild 670
• UAD Manley Variable Mu 

Properties of a Tube / Vari-Mu Compressor

• Because of the non-linearity, tube compressors naturally have a soft-knee circuit.
• slow attack times
• “analogue” sound

Tube / Vari-Mu Compressors are good for:

• Master Bus
• Bus
• “analogize” digital sound
• smooth out the dynamic differences on a larger scale
• UAD Fairchild 660 for Vocals, Guitars, Bass, Saxophone …
• The attack times of a Tube-Compressor are maybe too slow for drums

How to Use a Tube / Vari-Mu

Because the ratio of a signal depends on the input volume, you can’t adjust the ratio on a tube compressor. With the Input and Threshold controls you can control the amount of compression. The output control determines the output signal of the compressor.

The Fairchild parameters

With a tube compressor, the amount of compression is determined by the input and threshold controls. A tube compressor is not the fastest, with the Fairchild the shortest attack time is 200 milliseconds. The release times for the Fairchild are between 300 milliseconds and a full five seconds.
The output control determines the output signal of the compressor.

Attack- and Release-Times of a Fairchild

The control for selecting the five timing settings does not work from slow to fast, rather five different settings for attack and release times are hidden behind the switch positions.

Time Constant	Attack Time	Release Time
Position 1	200 microseconds	300 milliseconds
Position 2	200 microseconds	800 milliseconds
Position 3	400 microseconds	2 seconds
Position 4	800 microseconds	5 seconds
Position 5	200 microseconds	Program dependent: 2 seconds for transients 10 seconds for multiple peaks
Position 6	400 microseconds	Program dependent: 300 milliseconds for transients 10 seconds for multiple peaks 25 seconds for consistently high program level

Ratio of a Fairchild

The ratio of a Fairchild compressor cannot be adjusted, it depends on the input volume.
The gain reduction starts with a very low ratio of between 1:1 and 2:1 for smaller peaks and gradually increases to a ratio of up to 20:1 for loud input signals.

By rolling back the “DC-Threshold”-Control you can change default-soft-knee to a harder knee of a Fairchild.

If you want to dive in a bit deeper, we recommend you this 5-Minute UAD-Tip from UAD itself:

Diode Bridge Compressor

diode bridge circuit originated in the 1960s, but only a few manufacturers used it.

As the name says, these compressors use a diode-bridge, also known as a diode-ring as the variable gain-reducing element.

Properties of a Diode Bridge Compressor

• faster response time than a typical opto-compressor
• less distortion than a typical FET
• Fat/”Vintage” Sound

Tube / Vari-Mu Compressors are good for:

• Vocals, Drums and pretty much everything…
• To add analog sound and vibe to your mixes.

Common Tube / Vari-Mu Compressors

• Neve 2254 (Waves V-Comp is an Imitation of the 2254)
• AMS Neve 33609
• Rupert Neve Designs 5254