Tube/Valve EQ
PULTEC EQP- 1A Passive Valve EQ
TL AUDIO EQ1 Dual Valve EQ
Thermionic Culture The Swift 2 Channel Valve EQ
Valve or Tube EQ's are pieces of hardware analogue outboard, that use vacuum tubes to amplify the analogue audio signal. The EQ circuit is Passive - this means it works by attenuating (reducing the volume) the audio signal. While this preserves audio quality, the undesirable side effect is a reduction in volume, and so the vacuum tubes are used to amplify the signal. Vacuum tubes are desirable for processing analogue signals as they naturally add second order harmonic distortion. This means it adds a second note an octave above the original, which sounds harmonically pleasing.
Valve EQ's are often used to add 'warmth' and 'colour' to the signal, enhancing the musical aspects of the signal and having a thickening effect. This works well on low frequency material such as kick drums, bass lines, and other drum elements. It will add some warmth and thickness to any signal passed through the unit however.
The Pultec EQP-1A was first developed in the 1950's, and is still widely copied in modern EQ outboard units today, which tells you a lot about its desirable and versatile characteristics.
Classic Console EQ
Although this is derived from EQ modules found on high end mixing desks, these are now widely available as outboard channel strips, and in the 500 series module format. These are solid state devices, and are often desirable due to the 'musical nature' of the chosen frequency points. Before the creation of 500 series modules people would often buy up old classic mixers, and re-house the various desk modules for use as single channel outboard processors. This was possible due to the modular nature of high end mixing desks.
The 1073’s three-band EQ has a fixed 12kHz high frequency shelf band along with switchable low and mid range bands with cut and boost controls. Below those is a passive third-order (18 dB/octave) high pass filter.
The API 550b is another EQ derived from a classic console. Its four EQ bands are overlapped significantly to aid in dual roles as problem solver and sweetening device with each band offering seven switchable filter frequencies that span four-to-five octaves. These frequencies, purposely selected to be musical rather than numeric, were selected by an experienced "who's who" list of the industry's most proficient engineers. API also pioneered the 500 series 'Lunchbox' format, a much more affordable entry point in to high end channel processing.
The SSL E-Series is based on the classic E-Series mixer from the late 70's/early 80's. Again this now comes in a more usable format, coming in both 500 series format, and SSL's own 'lunchbox' style format, the X Series.
The Trident 80b follows the same idea, transferring the EQ modules from the classic Trident A-Range mixer in to both channel strip and 500 series format.
These modules have stood the test of time due to the high quality manufacturing processes, and the time spent developing each module to work in the most musical way possible. The fact that they are still widely produced today using modern manufacturing processes but keeping true to the original design demonstrates the versatility and usability of these classic pieces of outboard processing.
Digital EQ
With the advent of DSP - Digital Signal Processing - EQ was able to move away from the analogue domain and in to the digital world. This came with many advantages and disadvantages. The most obvious benefit with digital EQ was the ability to recall settings, something that isn't possible with analogue EQ's. the use of upsampling and very high bit rates means that digital EQ can preserve signal quality, and so there should be no obvious quality change when using digital signal processing. Internally within a DAW processing usually makes use of 32 or 64 bit floating point calculations, giving incredibly high internal resolution, and upsampling means they are often working at much higher sample rates than the DAW setting, all of which ensures that no errors or audio artefacts are introduced.
Unlike analogue, digital EQ is completely transparent, and should not colour the sound in any way. This makes them incredibly precise when compared to the natural anomalies found with analog circuitry such as harmonic distortion.
Every DAW comes bundled with its own plugin EQ, and third party plugins that often use higher quality algorithms are widely available.
Outboard digital EQ was only limited by the quality of processors available at the time, and these outboard units have been superseded by much more powerful computer based systems, making digital outboard much less common.
Due to the rapid increase in computing power (see Moores Law) EQ plugins now provide much higher quality signal processing than was previously available. Bundled EQ plugins now match the quality of previously expensive third party plugins.
This vast increase in computing power has also led to the rise of analogue modelling - the ability to recreate separate components of analog circuitry in the digital domain - and this technology allows people to create digital models of classic analogue outboard. As well as standard plugins, equipment such as hosted UAD plugins provide incredibly accurate models of analogue equipment, by offloading the processing power needed from the computer to a separate DSP card contained in third party hardware.
While internally digital signal processing is very high quality, the biggest limitation, or area for concern, lies in the initial conversion quality from the analog to the digital domain. In order to achieve high quality results, studios invest in expensive, super high quality interfaces, providing the very best analogue to digital conversion, allowing them to take advantage of a range of both analogue and digital equipment.
Linear Phase EQ
Standard EQ plugins are sometimes described as 'Minimum Phase' EQ's, as they can cause phase issues with material passed through them. The end result of this is often inaudible, but can sometimes manifest as phasing at certain points in the frequency spectrum. This can result in a 'smearing' of the sound - a lack of clarity introduced by the processing. To combat this, linear phase EQ introduces a delay, or time shift, to prevent any phasing caused by the EQ process.
While this preserves audio quality, the very nature of the process will cause latency - a time delay to the processed signal - moving the audio signal out of time with other material. To achieve Linear Phase, the whole audio signal is delayed, compared to a standard EQ where the delay will only occur at the processed points of the frequency spectrum. To achieve this requires a large amount of processing power, which is one of the drawbacks of using linear phase EQ. Additional processing power is needed to try and dal with the latency issues caused as a side effect of the process.
Linear phase EQ can either come in a plugin format, but is also available as very high end outboard equipment, such as the Weiss EQ1-LP Linear Phase Mastering EQ.
Match EQ
Match EQ allows you to analyse the content of a sound source to create a template of the frequency spectrum, and then generate an EQ curve that will make another sound source match these frequency characteristics.
In other words, I could analyse a commercial mix in a similar style and genre to my own material, and create an EQ curve that would match the characteristics of my mix to the source material.
In theory this sounds like a very simple way to make everything sound great, but it isn't perfect, and so cannot be used as a magic wand, but it does give you a great insight in to the balance and frequency spread of your material, to give you an indication as to where your own mix may be lacking.
It can also be used on individual elements, to make one kick drum match the frequency profile of another for example.
Graphic EQ