Thick tape saturation. Punchy compression. Warmth for your whole mix.
Slam drums into thick saturation, warm a vocal, and glue your whole mix together. The Empirical Labs FATSO Tape Simulator puts tape, compression, and high-frequency smoothing under one set of knobs. From subtle color to total destruction, you’re mixing with the definitive FATSO, developed with its designer, Dave Derr.
- Get the same tape and saturation processing heard on recordings by AC/DC, Pearl Jam, Soundgarden, and others
- Access iconic tones with FATSO Jr or get deeper compression control with the FATSO Sr
- Glue your buses and mixes together with Buss, G.P., Tracking, and Spank settings
- Slam drums, make bass pop on small speakers, or warm up a vocal instantly
- Beyond-hardware features include sidechain filtering, variable transformer saturation, and deeper compression parameters
FATSO Jr
In this article
FATSO Functional Overview
FATSO combines saturation, distortion, warmth, and transformer emulation with compression to provide vintage processing reminiscent of the tape sound.
Four processing types
The FATSO was designed to integrate frequencies in a musical manner and provide foolproof vintage-sounding compression. Generally, it is difficult to make the unit sound unnatural due to its vintage topology. FATSO provides four types of processing: Saturation and Distortion, Warmth, Tranny, and Compression.
Saturation and distortion processor
This is a distortion generator associated with the Input knobs. Anytime you pass a signal through the FATSO, it passes through the saturation and distortion processor, except in bypass. This processing is useful to softly but instantly clip peaks and transients, allowing a higher average level. Aggressive distortion can also be achieved through the same controls.
FATSO uses the principles of triode distortion in tube circuits, which produce second- and third-order harmonics in varying ratios. Analog tape also saturates in this manner. Third-order harmonics are induced in the FATSO by increasing the level through two discrete distortion circuits. Second-order harmonics are also added, especially while compressing in the FATSO.
The FATSO's input clipping gives the same result. These lower-order harmonics form "the octave" and "the octave and a fifth" to the fundamental musical tones, providing "musical" distortion. Harmonics above the second and third get increasingly harsh and unmusical, and therefore should be lower in amplitude (<-60 dB) to prevent dissonant overtones. The second-order harmonic is considered to be the warmest and most "consonant" harmonic distortion.
Warmth processor for softer high frequencies
This circuit simulates the softening of the high frequencies that occurs with analog tape. As Warmth is increased, overly bright signals and transients are attenuated. The time constants are very nearly instant, so the high frequencies return very quickly after a loud burst.
The Warmth circuit is by far the most complex part of the FATSO, implementing a unique high-frequency (HF) gain control circuit or HF limiter. It is very unobtrusive in operation – it gets in and out of the way very quickly. The desired result is akin to the HF saturation that analog tape exhibits when the HF amplitude interacts with the tape recorder bias to produce "self-erasure" of certain frequencies. The nature of the filter allows the corner frequency to move as attenuation occurs.
There is only one control for Warmth, but there are other ways to control the overall action of this circuit. If you are using the compressor, adjust it first, because the compressor affects the operation of the Warmth control. There is heavy interaction between the compressor and Warmth settings. The best way to think of the Warmth control is as a compressor affecting the high frequencies, with a setting of 7 having the lowest threshold and the most Warmth, and responding most quickly and often to high-frequency content.
Tranny processor – transformer and tape head emulation
The Transformer ("Tranny") circuit emulates the non-linear transfer characteristics of vintage audio transformers and analog tape by introducing low-frequency harmonic distortion, peak saturation, and phase shifts. Designed to process signals with fundamental frequencies below 150 Hz, the circuit generates higher-order harmonics to enhance perceived low-frequency energy without increasing peak levels.
This harmonic synthesis leverages psychoacoustic principles to improve low-end audibility on band-limited playback systems, allowing deep fundamentals, such as the 41 Hz low E on a bass guitar, to translate on smaller speakers incapable of reproducing true sub-bass frequencies.
The hardware implementation utilizes an internally buffered, switchable topology engineered to replicate the saturation profiles of large-core vintage output transformers. By soft-clipping transient peaks while increasing RMS energy, the processing lowers the signal's crest factor and increases perceived overall loudness. Audibly, this produces subtle midrange presence and improved low-frequency articulation, expanding perceived bass response while maintaining control over overall system headroom.
Compression processor
These are not your typical automatic leveling devices. Empirical Labs compression is smooth and sweet, but punchy and in-your-face.
There are essentially four discrete compressors in the FATSO: Buss, General Purpose (G.P.), Tracking, and Spank. Switching modes simultaneously sets the compressor threshold, ratio, attack, and release. This control is designed to provide an easy-to-set, yet versatile group of compression curves. The release curve of all types is logarithmic, meaning it lets off quickly at first and then slows. This release curve is a big part of the FATSO's compressor sound.
Note: Threshold, attack, and release values can be modified in the FATSO Sr.
Buss
Buss mode (green LED lit) is a very gentle 2:1 ratio with slow attack, fast release, and a very soft knee. One to four dB of gain reduction is typical for this compressor type. Five or more dB of Buss compression is hitting it hard!
G.P.
General Purpose mode (yellow LED lit) is a medium-attack, slow-release type that sounds pretty invisible while able to maintain a consistent RMS level. The slow release will not pull things into your face unnaturally.
Tracking
Tracking mode (green and yellow LED lit) is an 1176-type compressor that is great for tracking instruments and vocals during the recording process or during mixdown.
Spank
Spank mode (red LED lit) is an aggressive limiter-type compressor, specifically designed to emulate the nice squeeze of console talkback compressors from the 70s and 80s, but with added fidelity. Note that Spank's aggressive nature will tend to dominate when combined with any of the other modes.
FATSO Controls
General notes about FATSO controls are below, followed by a detailed description of each channel-specific control, the global controls, and the FATSO Sr controls.
Mono / stereo operation
The FATSO is a two-channel device capable of running in stereo or dual-mono modes. Controls for both channels can be linked for ease of stereo operation when both channels require the same values (see Link Controls), or unlinked when dual-mono operation is desired.
Each of the channel functions has its own separate group of controls (one set each for Channels 1 and 2). Since the controls for each of the two channels are identical, they are detailed only once.
Note: When the FATSO is used in a mono-in/mono-out configuration, the Channel 2 controls have no effect and the LINK parameters are disabled.
Pushbuttons
All FATSO pushbuttons are momentary. The value of the parameter increments by one step each time the button is clicked (holding the button down does not continue to increment the value). The value cycles to the beginning when the end of the range is reached. Clicking on the control LED indicators has no effect, with the exception of the LINK parameters.
Tip: Shift+click any pushbutton to decrement its value by one.
Channel Controls
Note: The channel controls are identical for Channels 1 and 2.
FATSO input controls
The Input knob defines the signal level going into the plug-in. Higher levels result in a more saturated signal. Levels above 0 VU provide dramatically higher distortion characteristics, especially when clipped (as indicated by the Pinned LED).
When the compressor is active, higher input values also result in more compression, as indicated by the gain reduction meters.
Note: This control has no effect when Bypass is active.
THD indicators
The Total Harmonic Distortion (THD) LEDs provide some reference operating levels. The yellow "0 VU" LED light indicates around 1% THD, and the red "Pinned" LED indicates 5% THD or more. These LEDs are an excellent guide to where the sound is in the "Grunge Department." You will find that the harmonic distortion is generally more obvious on overall mixes and complex programs. On individual instruments, sometimes 10% distortion sounds "fat" and "analog" and isn't heard as distortion at all.
Compressor mode
The COMP button defines which compressor mode is active. See Compression processor for a description of the modes.
Spank mode can be combined with any of the other three modes for a total of seven available compressor modes.
Note: Generally speaking, the Input and compressor mode controls should be set before the other FATSO processor settings, because of the high degree of interaction between the compressor and the other processors.
Mode LEDs
The three Mode LEDs indicate the active mode. Refer to the list below for each specific value. The compressor is inactive when all Mode LEDs are off.
- All Unlit: Compressor inactive
- Green: Buss
- Yellow: General Purpose (G.P.)
- Green + Yellow: Tracking (most versatile ratio)
- Red: Spank
- Red + Green: Spank + Buss
- Red + Yellow: Spank + General Purpose
- Red + Green + Yellow: Spank + Tracking
GR meter
The Gain Reduction Meter displays the amount of gain reduction occurring within the FATSO compressor, expressed as negative dB values.
Note: At extreme settings, the GR Meter may indicate gain reduction is occurring even when the compressor is disabled. This behavior is identical to the hardware unit.
Warmth
This button defines the Warmth amount. Warmth simulates the softening of the high frequencies that occurs with analog tape saturation (see Warmth processor for a detailed description). Higher values increase the Warmth, as indicated by the Warmth Meter.
Values of 1–7 are available. The current value is indicated by the arc of Warmth LEDs. Warmth is off when all LEDs are unlit.
Warmth meter
The Warmth Meter is a very accurate display of the amount of high-frequency attenuation, as defined by the Warmth button. The meter shows the amount of HF gain reduction occurring at 20 kHz.
Note: At extreme settings, the Warmth Meter may indicate activity even when Warmth is disabled. This behavior is identical to the hardware unit.
Bypass/Tranny
This button is a multifunction control that cycles through Tranny on/off and Bypass modes. The currently active mode is indicated by the adjacent LEDs (LEDs are unlit when Tranny is off but processor is enabled).
Tranny (green LED)
The Tranny processor is active in this mode. The Tranny circuit adds frequency "rounding," low-order clipping, intermodulation distortion, and transient clipping. On FATSO Sr, the Tranny amount can be set with the Tranny Level control.
Note: Disabling Tranny reduces processor usage. In the UAD DSP plug-in, UAD DSP usage is reduced when DSP LoadLock is disabled. If DSP LoadLock is enabled (the default setting), disabling Tranny will not reduce DSP usage.
Tranny Off (red and green LEDs off)
In this mode, the Tranny processor is inactive but the other processors are active. This mode requires less processor usage or UAD DSP usage than when Tranny is active.
Bypass (red LED)
All FATSO controls and processing for the channel are inactive in this mode.
Note: Processor usage or UAD DSP usage is not reduced in Bypass mode. If you want to reduce UAD DSP usage when bypassing both channels of the FATSO, use the Power switch instead.
Output
The Output knob controls the signal level that is output from the plug-in.
Note: This control has no effect when Bypass is active.
Global Controls
The global controls are not channel-specific; they apply to both channels.
Link Compress
You can link the gain-reduction processor sidechains for Channels 1 and 2 with the Link Compress function.
To activate Link Compress, click the LINK COMPRESS text or LED on Channel 1, on the left. The feature is active when the LED is illuminated.
In typical use on stereo signals, Link Compress should be active so the stereo imaging is maintained. If the compressor is inactive (Compressor mode), or when FATSO is used in a mono-in/mono-out configuration, this control has no effect.
Important: Unlike the other controls for Channels 1 and 2, which are identical on Channel 1 and Channel 2, the Link COMPRESS function is on Channel 1 only (not to be confused with Link CONTROLS, which is on Channel 2 only).
Link Controls
The parameter controls for Channels 1 and 2 can be linked using the Link Controls function.
To activate Link Controls, click on the LINK CONTROLS text or LED on Channel 2. The feature is active when the LED is illuminated.
Note: Although the Channel 1 and 2 Warmth and Tranny controls are linked when the Link Controls feature is active, the actual Warmth and Tranny processors are not stereo linked. This behavior is identical to the original hardware.
Controls linked
The Link Controls feature is provided for stereo operation when both channels require the same values. When enabled, the right channel controls "snap" to match the left channel control values, and modifying any channel control causes its stereo counterpart control to move to the same position (Channel 1 and 2 controls are ganged together in this mode).
Important: Any unique Channel 2 parameter values are removed when the Link Controls feature is enabled.
Controls unlinked
Unlink the controls when dual-mono operation is desired. Channel 1 and 2 controls are completely independent in this mode, and automation data is written and read by each channel separately. The Link Controls feature is disabled when the FATSO is used in a mono-in/mono-out configuration.
Important: Unlike the other controls for Channels 1 and 2, which are identical on the Channel 1 and Channel 2 of the interface, the Link CONTROLS function is on Channel 2 only (not to be confused with Link COMPRESS on Channel 1).
Power
The Power toggle switch determines whether the plug-in is active. It is useful for comparing the processed settings to the original signal. When Power is in the Off (down) position, plug-in processing is disabled, processor and UAD DSP usage is reduced, and all LEDs are unlit.
Note: UAD DSP usage is reduced only when DSP LoadLock is disabled. If DSP LoadLock is enabled (the default setting), disabling Power will not reduce DSP usage.
Click the lower portion of the switch to disable the plug-in; click the upper portion to activate (or click+drag up/down on the switch).
FATSO Sr Controls
FATSO Sr adds additional controls for the compressor and Tranny.
Note: Both the FATSO Sr and FATSO Jr plug-ins use the same amount of processor or UAD DSP usage. The additional controls in the FATSO Sr do not use any extra processing power or UAD DSP.
Threshold Level
This knob enables manual threshold control of the FATSO compressor. Higher values lower the threshold, and therefore increase the amount of compression. A value of 5 is the unity setting.
The Input control also affects the compression threshold. Generally speaking, set the amount of desired signal saturation with Input first, then adjust Threshold as desired.
Filter (HP SIDE FILT)
Filter regulates the cutoff frequency of the filter on the compressor's control signal sidechain. When active, frequencies below the filter value are not passed to the sidechain. Values of 60 Hz, 120 Hz, 240 Hz, 480 Hz, and Off are available. The filter slope is 6 dB per octave. When the compressor is disabled, Filter has no effect and its LED turns off. When the compressor is enabled, Filter returns to its original value.
Tip: Removing low-frequency content from the sidechain can reduce excessive gain reduction and/or "pumping" on bass-heavy audio signals without reducing bass content of the audio signal itself.
Note: The Filter parameter affects the control signal (sidechain) of the compressor only. It does not filter the audio signal.
Attack
Attack sets the amount of time that must elapse once the input signal reaches the threshold level before compression is applied. The faster the attack, the more rapidly compression is applied to signals above the threshold.
The available attack time values are 0.9 ms, 10 ms, 30 ms, 60 ms, and Default (unlit). The unlit behavior depends on whether or not the compressor is active. These behaviors are described below.
Note: Attack values are approximations. Actual attack and release times may vary depending on the compressor mode selected.
Attack LEDs unlit – compressor active
When the compressor is enabled and all Attack LEDs are unlit, the attack characteristic of the active compressor mode in FATSO Jr is used. This "default" FATSO Jr behavior can then be manually overridden with the Attack button. However, when Spank mode alone is active, Attack cannot be modified. When Spank mode is combined with another compressor mode, Attack can be changed, but the results are typically very subtle.
Tip: After experimenting with other time constants, you can return to the default attack setting of the FATSO Jr if desired by cycling the attack control until NO LEDs are lit.
Attack LEDs unlit – compressor inactive
When the compressor is disabled and all Attack LEDs are unlit, the button is disabled.
Note: This control has no effect when the compressor is inactive, or when it is in "pure" Spank mode (see Compressor mode).
Release
Release sets the amount of time it takes for compression to cease once the input signal drops below the threshold level. Slower release times can smooth the transition that occurs when the signal dips below the threshold, which is especially useful for material with frequent peaks. However, if you set too long a Release time, compression for sections of audio with loud signals may extend to lengthy sections of audio with lower signals.
The available release time values are 0.05 s, 0.1 s, 0.2 s, 0.5 s, and Default (unlit). The unlit behavior depends on whether or not the compressor is active. These behaviors are described below.
Note: Release values are approximations. Actual attack and release times may vary depending on the compressor mode selected.
Release LEDs unlit – compressor active
When the compressor is enabled and all Release LEDs are unlit, the release characteristic of the active compressor mode in FATSO Jr is used. This "default" FATSO Jr behavior can then be manually overridden with the Release button. However, when "pure" Spank mode is active, Release cannot be modified. When Spank mode is combined with another compressor mode, Release can be changed, but the results are typically very subtle.
Tip: After experimenting with other time constants, you can return to the default release setting of the FATSO Jr by cycling the release control until NO LEDs are lit.
Release LEDs unlit – compressor inactive
When the compressor is disabled and all Release LEDs are unlit, the button is disabled.
Note: This control has no effect when the compressor is inactive, or when it is in "pure" Spank mode (see Compressor mode).
Tranny Level
This control determines the amount of Tranny processing (see Tranny processor for a detailed description). Higher values make the Tranny effect more prominent. Increasing the Tranny Level also increases the signal THD (see THD indicators), and the sensitivity of the Warmth processor. A value of 5 is the unity setting.
Note: This control has no effect when the Tranny processor is inactive.
LF Sat LED
The LF Sat (Low Frequency Saturation) LED indicates the amount of LF saturation in the Tranny processor. Higher Tranny Level values increase the LF saturation.
FATSO Jr Presets
When loading presets created on the FATSO Jr into the FATSO Sr, the parameters that are unique to FATSO Sr are set to their default control values. The default values of the unique FATSO Sr parameters are: Threshold/Tranny knobs at 5, and Filter/Attack/Release buttons off.
Empirical Labs EL7 FATSO Jr hardware
All visual and aural references to the FATSO and all use of EMPIRICAL LABS's trademarks are being made with written permission from EMPIRICAL LABS, INC. Special thanks to Dave Derr for assistance with this project.
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