Master your understanding of compression:
COMPRESSION DEFINITION || OPTICAL || FET || VARI-MU (Tube) || VCA || DIGITAL || DIODE-BRIDGE || MASTER BUS COMPRESSION
While compression can simply be understood as the process of evening out the loudest and softest parts of an audio signal, different types of compressors work differently. Everyday plugin emulations that we use in our DAWs are primarily modeled on the five main types of audio compressor circuitries. One of the most widely used types is VCA compressors. VCA, which stands for voltage control amplifier, uses a control voltage to modulate the amount of gain applied by the compressor. In this VCA compressor article, we are going to dive deep into the inner workings and applications of VCA compressors by using sound examples created with some of the most popular VCA compressor plugins in the market.
Contents
What Is A VCA Compressor? (TL;DR)
A VCA is an analogue compressor that uses control voltage(CV) to modulate the volume of an input signal. By using an IC chip as a semiconductor, the incoming signal is then routed separately to the output path and a detector path, where the VCA compression occurs. Housed in an IC, the VCA is highly fine-tuneable and repellent to unwanted distortion.
You can read more here, if you’re interested in the application of VCAs in synthesis.
The voltage-controlled amplifier or VCA deals with two input signals and an output signal.
- Carrier Signal: The first type of input signal, which is known as the carrier signal, accepts and allows bipolar signals, which are comprised of negative and positive amplitudes to pass through the input. however, this carrier signal is dependent on the positive values of the control voltage, or second input signal.
- Control Voltage input: The CV input, which is also termed the Modulator signal, is the second type of input signal, which carries a uni-polar positive signal in a VCA. The CV, which carries a positive amplitude, alters the amplitude of the carrier signal in proportion to its positive values.Since the amplifier will only accept and read a positive signal or positive bias flowing through the CV input, the values have to be more than zero. If the incoming values are biased to zero, the modulator signal doesn’t function, and we get no signal at the output.
- Output Signal: The output signal is an instantaneous product of the carrier signal and CV. So the output signal’s values are calculated by multiplying the amplitudes of the two signals at each instant in time.
Is the VCA an Amplifier or an Attenuator?
Since the ratio of the variable gain never exceeds 1, the VCA circuit works more like an attenuator than an amplifier.
We’ve loaded up a block diagram of the famous API 2500 VCA compressor, so you can get a visual idea of connections inside a VCA compressor.
Basically, higher the levels of modulator signal or CV that passes through the VCA compressor, the more the carrier signal that flows to the output. We can understand this better by laying out the three possible scenarios that occur inside a VCA compressor:
- Negative CV: When the CV values touch zero or go below zero, the output receives no carrier signals.
- Maximum Value: Within every VCA, there will be a specific positive value, when the entire carrier signal makes its way to the output.
- CV Attenuation: Negative values don’t send signals and at a positive maximum value, everything gets through. So what about all the values in between?These are the values of the carrier signal that get attenuated by the CV.
To understand the nature of gain reduction inside a VCA, we’re going to dive deep into the functioning of the circuitry and signal flow inside a standard VCA compressor.
How does the VCA circuitry work?
In most standard VCA Compressor/Limiters, we have a series VCA and an RMS level detector, along with some OP-Amps, that form the heart of the design. They work well in tandem as a controller and detector respectively. The RMS detector gives a logarithmic DC output, while the VCA receives commands for operating gain control in an exponential format.
The VCA Compressor Design. The great thing about the VCA compressor design is that the compression gain, ratio, and threshold, can be all controlled independently. The RMS Level Detector manages the time constants automatically.
The Signal Chain. The signal first flows from the VCA compressor and the first OP-Amp (OP1). The VCA generally generates an output signal which is multipliable in value (Decibels) to the input signal. The OP Amp-1 transforms this signal into a voltage, in accordance with its own Feedback resistor, R6.
In the case of Resistor, R1=R6, the voltage input equals voltage output, with the pin3 control port being at 0V or unity gain. The gain is inversely proportional to the Voltage fluctuation here.
A gain reduction of 1dB is always accompanied by a voltage increase of 6mV and an increase in gain by 1dB is always accompanied by a decrease in voltage by 6mV.
So in a nutshell, the output level is only modulated by the input signal and the CV at pin3.
RMS Calibration. The input of the RMS is a virtual ground and receives its signal from Resistor, R7. The circuitry is designed to have an output of 0V, when the input voltage reaches -10dBV. The RMS detector’s output voltage is directly proportional to the incoming signal.
The values are calibrated exactly opposite to the gain reduction we saw earlier. So a 1dB reduction in input causes a 6mV reduction in the output. A 1dB boost in the input causes a 1dB boost in the output too.
Threshold Control. The second OP-Amp, OA2 is connected to the RMS in a way that when the volatge at the RMS is negative, the OA2’s output is positive. But how does this relate to the voltage at the threshold?
When the voltage at RMS is more than 0, the output at the second OP-Amp turns negative. But, the voltage at the threshold constantly follows the voltage at the RMS with a -1dB gain. So the equation reads like this:
Vth = –Vrms for Vrms > 0 V.
So, in simple terms, the voltage at the threshold is always negative for the RMS when the voltage at RMS is greater than unity gain or 0V.
The basic idea behind the threshold design in the VCA is that, the voltage at the RMS should be equal to 0V at the Threshold. This means that the voltage from the second OP-Amp would only pass through based on the condition that the voltage at the input signal is higher the input level.
Since values have to cross the 0dB mark in this case, the voltage at the threshold represents the dB level of the input signal above the threshold.
VCA Compression: The voltage at the threshold is scalable by using the compression control in a VCA before it passes through the other parts of the circuitry. The compression control in a VCA helps mirror certain output values in divisible fractions.
For example, when the values are set to max, the voltage at the threshold precisely mirrors the output at the third OP-Aamp, OP3. Every setting below max would have the OP-Amp3 mirror a divisible fraction of the threshold value.
So in simple terms, at maximum compression values, we get:
Vopamp3 = Vth
So the voltage at OA3, can be viewed as the Voltage at the input Vin, above the threshold at 6mV/dB.
As we apply these equations, we find that for every 1dB of gain boost at the voltage input above the threshold, there is a 6mV boost at the OA3. This results in the overall gain reduction at the VCA by 1dB. Why does this happen?
The 1dB gain reduction occurs as the voltage at OA3 is passed through pin3 of the VCA, which navigates the gain at -6mV/dB.
So for example, if the compression is set at the halfway mark, the gain will reduce by 1dB for a 2dB increase in the input signal above the threshold. Thus, the output would increase by 1dB for every 2dB increase in the incoming signal.
Compression Ratio: Now coming to the compression ratio, we can understand it better with an equation and a definition.
Increase in the number of decibels above the threshold of the output signal in comparison to the input signal, is termed as the Compression Ratio.
Compression Ratio: ∆ Vin/∆ Vout
In the equation above, Vin and Vout denote the decibel increase in the input and output signals, above the set value of the threshold. The compression ratio is 1:1 when the compression is at its least, and goes to ∞:1, when set to max.
If you’re interested, you can find more interesting compressor diagrams here.
Gain Reduction: Since a DC offset is added to the gain control, the static gain amounts to 6mV/dB. By passing positive or negative 15 Volts of supply, the gain control cause the variation at Op-Amp3 to vary by positive or negative 123mV.
This reaction in the circuitry is instrumental in allowing the gain to fluctuate between +20dB and -20dB. This fluctuation is used by the compressor to make up for the attenuation of signal level caused during VCA compression.
Nature of VCA Compression: Basically the RMS detector in the compressor recreates a decibel representation of the input. This triggers the VCA to act immediately, based on the information about the gain sent from the RMS detector.
An operational rectifier, which converts a bi-directional alternating current(AC) into a uni-directional direct current(DC), is used in the threshold detector in the VCA compressor.
Since the rectifying nature of the component is quite sharp and steep, we generally end up with a sharp knee when the signal hits the threshold.
Trim Functions: The two trim pots used in the design play an important role in the sound. Their primary objective is to set the VCA for minimal distortion, and the RMS detector for the least amount of ripple. The first trim pot is mainly used to control the distortion and feedthrough in the VCA, and the second for balancing the symmetry of the rectifier.
How Does An VCA Compressor Sound?
Since VCA compressors act on Peak signal input, they are known for being extremely fast. But unlike the FET compressors, which are also known for their speed, VCA compressors don’t add too much colour by squashing all the transients. They in fact, smother the incoming transient peaks in a way that can make them sound more ‘smooth’ and ‘clean’.
Owing to this, VCAs are way more versatile than FET compressors for mixing applications as they can come across as being quite ‘transparent’.
However, the way you crank a FET is not the same way you treat a VCA. Since they are quite sensitive, care must be taken to not go overboard with them. FETs will add character when cranked. But VCAs might just lose the liveliness of the track when cranked beyond moderation.
By being able to react much faster than Opto and Vari-Mu compressors, VCAs are a preferred choice for drum busses, parallel compression, and master busses. Their speed makes sure that they catch all the information and their predictable nature ensures that you can use them on a group of instruments.
VCA Sound Comparison:
In the VCA vs FET vs Opto sound comparison below, you can hear how the electric guitar sounds much more open in the VCA. The FET adds attitude by crushing the sound, making the transients more tameable.
The Opto, which uses non-linear topologies has a much slower attack and release, which creates more intimacy in the midrange. But the Opto doesn’t open up the high frequencies like the VCA.
The VCA, which works specifically on the micro-dynamics, makes the guitar more breathable.
Dry Electric Guitars:
VCA: (Waves dbx160)
FET: (Waves CLA76)
OPTO: (Waves CLA2A)
We chose this guitar example as its easy to notice the release on the notes in the higher strings. The FET is too fast, while the Opto doesn’t completely open those notes up.
Scenarios Where VCA Compression Is Useful:
Since the circuit boasts of a voltage controlled amplifier, there is more control over the attack and release values of the VCA. Known for being predictable and repeatable, VCAs are great on busses. The response to compression itself is quite smooth, which makes it an ideal candidate for mastering applications too.
Dealing with transient peaks and dynamic rhythmic material requires a compressor to be heavy-handed, with extremely quick transient detection, so that nothing slips by. VCAs can do exactly that and can tame the overall dynamic range to be more manageable, while still being fairly transparent.
VCAs must be avoided if you’re dealing with an unmodulated dynamic segment in your track. Since the sensitivity to transient variation is the VCA’s USP, they’re not very useful in dynamically steady passages.
How Do VCAs Sound And React On Different Instruments?
We’ve used the dBX16o and API2500 VCA compressor plugins with the RMS detection side chain circuit applied in a ‘Feed-Forward’ style to gain control. Since it’s the same circuitry explained earlier, you can easily follow along with our sound samples.
While Dbx160 VCA hardware models moved onto soft-knee compression later, we’ve used the Waves Dbx160 emulation, which uses the original hard knee.
Drums:
Since the auto attack and release are reacting to the envelope of the input signal, we’re working with a single compression ratio knob and a threshold in a VCA. We’ve set the compression ratio to 4:1 and the threshold level to 1 Vrms(-18dBFS).
We’ve set the Output gain to around 9.5dB. While using the high pass filter on the side chain at 90 Hz is a famous drum trick, we didn’t engage it here as we wanted to preserve the rhythmically changing dynamics of the kick.
We’ve used parallel compression on the drums here. Since the Dbx plugin has a mix knob, we directly dialed around 76% into the mix. While adding ‘noise’ or hum is generally recommended for analogue characters, we found our hi-hats losing rhythmic clarity while doing so. So we’ve set the noise to 0%.
If you pay attention to the snare in the sound sample, you’ll notice how the skin of the snare seems to have been stretched out tighter. The drumsticks sound like they’re hitting the exact resonant frequency of the membrane, which means that all the even harmonic overtones are aligning.
In the sound samples, you can hear how the rhythm of the hi-hats gets more defined due to the speed of the VCA. The track is in 6/8 time signature. The hi-hat mainly accents on the 5th beat of every 2nd bar. In the dry example, you can hear how the tail sustains for longer, giving the track a looser feel.
Drums (Before):
Drums (After):
With the DBX160 VCA engaged, you can hear how the accent of the hi-hat is defined. The amplitude of the body is raised, with a 6dB gain change. But the VCA releases fast enough to get us back to the original amplitude, without completely losing the tail. You can still hear the hi-hat continue from beat 5 to 6 softly.
This preservation of micro-dynamics, which affect the vibe of a groove is what makes VCAs so sought after.
Bass:
The Bass part used here follows a melodic contour and thus we wanted to make sure the muddiness was removed. Our dry signal was occupying more space in the mix due to its lack of definition.
We wanted to change that by bringing the bass ahead in the mix and spreading itself over the piano with more musical overtones. Initially, we cranked the compression ratio to around 7 with threshold at 0.2Vrms.
This made the bassline stick out, but the body of the bass felt cut out from its release tail. The overtones were separated and each stroke hit with a thud.
This is a problem to be aware of, with VCAs, that they don’t handle distortion well after a certain point. By dialing back the compression to 4.71, and leaving the threshold at 0.3Vrms, we found the overtones aligning with the fundamental. The bassline felt musical and wide, but still transparent.
Bass (Before):
Bass (After):
We dialed in parallel compression at 50% with the mix knob. We also bought the noise up to 50% to add some hum. We increased the input to 3dB and left the output gain at 2.3 dB.
The same bassline with a 1176 FET would’ve bought in more attitude but would’ve distorted in a way that wouldn’t gel well with the soft piano in the background.
Vocals & Vocal Buss:
As you can hear in the before example that the guitar is transient- heavy, and interrupts the vocals. In this example, instead of boosting the vocals with a FET compressor, we decided to soften the track by smothering the vocals on top of the transient heavy guitar.
Notice how the VCA compressor maintains the transparency of the vocals while lending power back to the vocals. In this example, we have used VCA compressors on the vocal bus and the individual vocal track.
Vocals (Before):
Vocals (After):
We set a threshold of -8dBU and an attack of 1ms on the individual vocal track. We dialed the radio to 1.5:1 and the release to 0.5 seconds.
We have engaged the API 2500 famous thrust mode. The thrust, in the API 2500, limits the compression in lower frequencies and adds compression to the frequencies passing through the hi-pass filter.
By setting the thrust to loud, we manipulated the compression algorithm which draws a gradual line across the frequency spectrum. With 15dB gain reduction at 20 Hz, and 15dB gain increase at 20kHz, we reduced the interference of artefacts in the lower frequencies. This makes the compressor highly effective on the mid and high frequencies.
Vocal Buss (Before):
Vocal Buss (After):
We have added the API 2500 on the Vocal bus as well. The threshold is set to 4dBU, while the attack is set to 0.3 ms. The ratio is set to 3:1 and the release is set to 0.2 seconds. The thrust is again set to loud and a hard knee is chosen.
By selecting the feed-forward mode in all three examples, our sound examples follow the circuitry explained before.
Popular VCA Compressors:
Here’s is a short list of popular VCA Compressors that have become a studio staple over the years:
- API 2500+ Stereo Bus Compressor
- SSL The BUS+
- UAD dbx 160 Compressor/Limiter
1. UAD dbx 160 Compressor/Limiter:
Compatibility: macOS 10.14+ Mojave onwards. UAD, UADx, LUNA, UA Connect compatible with M1/M2 processors. Windows 11+. Windows 10 Anniversary Update necessary for Thunderbolt connections with UA devices. 64bit only.
Price: dbx 160 Compressor / Limiter: $1500 (for hardware)UAD dbx 160 Compressor/Limiter $99 (for emulation)
2. SSL The BUS+:
Compatibility:Â macOS 10.15.7+ Windows 10+. Windows 21H2. 64bit only. AAX Native, AudioSuite, VST, AU, SoundGrid.
Price:Â SSL The Bus+Â $2899Â (for hardware)Â Waves SSL G Master Buss Compressor:Â $29Â (for emulation)
3. API 2500+ Stereo Bus Compressor:
Compatibility:Â macOS 10.15.7+ Windows 10+. Windows 21H2. 64bit only. AAX Native, AudioSuite, VST, AU, SoundGrid.
Price: API 2500+ Stereo Bus Compressor $3130 (for hardware) Waves API 2500: $35 (for emulation)
FAQ
What Are The Disadvantages of VCA Compression?
Since VCAs are quite transparent, beginners feel the need to boost them in order to hear the compression. While VCAs are truly transparent at moderate levels, they tend to grab at your signal once you boost them. Sounding aggressive in chunks, they tend to upset the continuity of the contours of the amplitude of your signal.
VCAs also tend to pump in the lower frequencies, when you dial in too much compression. As we saw in our sound examples, the API 2500 has an additional thrust control to handle this low-frequency pump.
Due to this, the disadvantage is that VCAs cannot be pushed as much as FET.
However, since VCAs are extremely cheap to build and provide predictable and repeatable results, their negatives are often overlooked.
What’s The Difference Between VCA and FET Compression?
The primary difference between VCAs and FETs lie in their transistor design.The VCA’s transistors are placed in an IC and react to the input signal by modulating it with a control voltage. FET compressors work with the electric field in a way that more incoming voltage results in lowered resistance in the drain circuit, leading to gain reduction.
In the FET design, the electrical charges are added to the voltage to calculate the variation in gain. Since the FETs are working on the principle of a gate, we have the voltage pushing through the gate, which causes a reduction in the resistance.
So in an FET, the higher the voltage passing through the gate, lesser the resistance. Lesser the resistance, more the gain reduction applied.
However, VCAs and FETs are quite similar in application due to their fast response. They can both be used for fast moving transients and dynamically rich material.
But, a FET will add character to your sound and colour it by squashing your transients. A VCA, on the other hand, will make your track sound smooth, clean, and transparent by smothering your transients.
So if you’re looking to add character, grit, and colour on isolated instruments like acoustic guitars, rock vocals, or snares, its better to go for a FET compressor. But if you’re looking for a lot of control over a group of instruments in a drum bus or a master bus, with a wide range of response times, you’re better off going for a VCA.
Are Plugin Emulations of VCA Compressors Good Enough?
While we see a plug in emulations are a great fix, the low end on the plugins can sound drastically different from the hardware units. While some can be transparent and clean, other plug in emulations add a lot of unnecessary artifacts in the low end. Also due to their versatility, hardware units are preferred over plugins.
VCAs are also used as bus compressors which impact the whole track. Since they are not a one trick pony, getting a VCA hardware compressor isn’t a bad investment.
That being said, a lot of VCA plug in emulations sound excellent. While the attack and release functions might not sound the same, some plugin emulations provide excellent width and depth to the audio, which is what you should be looking for while choosing a plugin emulation.
For example, the UA dbx160 sounds smoother than the waves version. The UA dbx160 has an elaborate low end, which is very noticeable.
The waves DBX160, on the other hand, adds compression, similar to the hardware in the mid range, while adding more brilliance in the high end.
Sai is a full-time music producer located in India, and is head of Faculty at D7 Media Institute. He is the most passionate music production guru I’ve ever had the pleasure of meeting. Fantastic at sound design, mixing, and recording, Sai heads most of the review content, as well as dabbling in some mixing and mastering content too here at WCS. Give Sai any topic and he could write forever about it. He has over 10 years of experience working in the industry and has earned both Music Production and Music Composition & Piano degrees.
