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API 2500 Stereo Bus Compressor

Stereo bus compressor


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SKU: API2500 Categories: , Brand:


The API 2500 Stereo Bus Compressor features a patented Thrust control for extra punch, variable stereo link, selectable knee and detector operation

One of the most well-known and widely used bus compressors, the API 2500 is most at home across the 2-bus of rock mixes where chest-thumping sonics is the order of the day. Some bus compressors are known for adding a smooth glue, and others for filling out the midrange, but punch is domain of the API 2500. It accomplishes this feat with API’s patented Thrust feature, which balances musical energy across the frequency spectrum so that low-frequency energy won’t cause the compressor to suck the life out of your mix. The API 2500 also has a variable stereo link control that switches from dual-mono operation to 100% stereo linking in six stage. The advantage of this control is that one channel receiving more musical energy won’t cause the other channel to slave to it and needlessly compress.

API 2500 stereo bus compressor unique features: 

  • Tone section lets you dial in compression characteristics
    • 3-position selectable knee
    • Patented Thrust control balances frequency range and adds punch to a mix
    • Selectable feedback and feed-forward detector
  • Variable L/R link control adjusts percentage of stereo linking per channel
  • Shape control prevents low frequencies from linking left channel to right and vice versa
  • Tilt adjustment equalizes uneven compression that may offset stereo compression

API 2500 stereo bus compressor in one take:

  • All-discrete, fully balanced design ensures high fidelity
  • API 2520 Op Amp
  • Variable threshold between -20dBu and +10dBu
  • Selectable ratio, attack and release times
  • Soft, medium or hard knee for easy to sharp compression
  • Auto or manual gain make up
  • 1/4″ balanced side chain inputs
  • Output clips at +28dBu

INSIDE THE BOX—API 2500 Stereo Bus Compressor

 Tone section with Knee, Thrust, and Type controls

 Knee—This control sets the point at which compressor begins to reduce the gain applied to the input signal. In the Hard position, gain reduction begins at the set ratio point and is an abrupt, or hard transition. This setting is good for percussion. The Medium setting has a slight fade-in to the set ratio. The Soft setting has a gradual fade-in to the set ratio, and is good for program material.

 Thrust control—The patented Thrust circuit is only found on API compressors. It inserts a high-pass filter in the input of the RMS detector circuit, which limits its response to low frequencies. In Normal mode, no filter is applied, so that the API 2500 responds like a traditional compressor. When Medium is selected, slight attenuation is applied to the low frequencies, a little boost is added to the high frequencies, while the mids remain flat before going into the RMS detector. The result is taming of pumping from lows and peaks from the highs. When Loud is selected, a gradual linear filter is applied that’s down 15dB at 20Hz and up 15dB at 20kHz, applying equal energy to the RMS detector. Loud reduces pumping form low-end compression, increases high-frequency compression, resulting in an increase in low-frequency punch and uniform overall compression. When you want chest-thumping punch, Loud is the mode to select.

Type—This control sets the point where the signal for the RMS detector comes from. In New mode, the compressor operates in the same fashion as most modern VCA-based compressors, in which the sidechain is fed directly from the input. This is called feed-forward compression. When the Old position is selected, the RMS detector gets the signal from the output of the VCA, and then feeds the VCA a signal based on a set ratio of that signal. This type of compression is called feedback compression, and is how the older API 525, UA 1176, and Fairchild 660 compressors worked. The New mode is harder, while the Old mode is very smooth. When Soft knee, Loud Thrust, and Old is selected, can barely compression in action.

Additionally, the output stage has an auto gain-makeup button that lets you vary the threshold or ratio while automatically maintaining a constant output level.

Link and output section

Variable L/R link control: The 2500 is a dual-channel design that can be configured for stereo compression or for use as two separate units via a single compression setting. However, it doesn’t end there. Most compressors only allow full stereo linking, but the 2500 allows for linking from 0 to 100% in six steps. Each channel’s detector still works independently so that heavier compression on one channel doesn’t result in unwanted compression in the other.

Shape: This control works in tandem with the link control, adjusting the shape of the voltage mixing between the two channels. There are two filters; a high-pass filter eliminating the lows, and a low-pass filter eliminating the highs. A combination of both filters creates a band-pass filter. The value of this circuit allows the Link control voltage not to include certain frequencies, for example, preventing the low frequencies from linking from the left side to the right side, etc. When the different combinations of the filters are used, sounds like sharp percussive instruments on one channel will not couple to the other channel and cause it to compress. Think of a drum kit, with hi-hat on the left and floor tom on the right. If the right channel triggered compression when the tom was hit, in traditional compressors, the hi-hat would be pulled down as well, even though it doesn’t present anywhere near the energy of the floor tom.

In and Bypass: The In switch is a soft In/Out button that defeats the compression action silently, but allows the signal to pass through the 2500. The Bypass switch is a hard-wire relay that routes the signal directly from the input XLR to the output XLR, without going through any electronics. If the power fails to the 2500, the Bypass automatically engages, preventing any loss of signal during the power fault.

Gain: When the Gain switch is in, the gain pot controls the amount of make-up gain to compensate for the compressor’s gain reduction. In the out position, the output make-up gain is automatically maintained regardless of the position of the threshold or ratio controls. If the input signal is below +4dBu, rotating the threshold control will increase the output level until the output level is around +4dBu. As the signal gets compressed, it will maintain an even output level, allowing threshold and ratio adjustments without having to re-adjust make-up gain. This is very useful when an adjustment needs to be made without disturbing the output level to tape or air (the API 2500 compressor was used in the famous ATI Paragon and Paragon II broadcast consoles). This function is similar to the API 525 ceiling control

OUTSIDE THE BOX—API 2500 Stereo Bus Compressor

Starting from the left side of the 2500, standard compressor controls include threshold, attack, ratio, release, and variable release. The Tone section has three selector buttons and indicators for threshold knee, detector Thrust, and tone type. The Link section has a 7-position control ((0, 50, 60, 70, 80, 90, 100) and three-way filter shape button. The Output section has buttons for In, Bypass, and makeup gain, and a stepped manual gain pot from 0dB to 24dB. Finally, two front panel VU meters with meter source button, show input, output and gain reduction level, next to which is the tilt adjustment.

Rear-panel I/O includes balanced XLR In/Out and 1/4″ insert jack for channels 1 & 2.

Put a 2500 on your 2-bus and get the punchy mixes that only API can deliver. For more information, call or chat online with your Westlake Pro Sales consultant today.


API 2500 Stereo Bus Compressor specifications:

  • Power Consumption, Quiescent: 22.8 watts
  • Dimensions (W x H x D): 19″ x 1-3/4″ x 10-1/2″
  • Size (Boxed for Shipping): 23-1/4″ x 6-1/2″ x 16″
  • Actual Weight: 9.48 lb.
  • Shipping Weight: 13.38 lb.

Additional information

Weight 14 lbs
Dimensions 24 × 7 × 16 in

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