API 2500 Bus Compressor

The API 2500 models a two-channel compressor's discrete signal path, including transformers and API op amps. Its usefulness goes beyond reducing peaks on a mix: knee, detector voicing, and stereo-link behavior determine how the compression moves with the arrangement.

NEW operation derives detection from the uncompressed signal, while OLD uses feedback from the compressed path. In OLD mode, the gain stage also sits inside that control loop, so changing makeup gain can change reduction. THRUST shapes the detector's frequency response rather than equalizing the audible signal, reducing the tendency for bass to dominate compression.

Start with a fully linked stereo pair and restrained threshold and ratio settings. Choose the timing around the groove, then explore THRUST and knee before changing the link percentage. Partial linking can add channel independence but may change the image. The IN switch separates compression from circuit coloration; BYP bypasses the hardware model, and Power disables processing. Plug-in-only Mix and Headroom extend the workflow without requiring extra routing.

Manual — sections: API 2500 Controls; Tone Section; Link Section; Output Section.

• Threshold: +10 to −20 dB, decreasing clockwise for more compression. Its LED brightens with reduction. Actual parameter values can differ from silkscreen positions; consult the host parameter display for precise values.
• Attack: 30, 100, or 300 microseconds; 1, 3, 10, or 30 ms. Shorter times catch initial peaks; longer times retain more impact.
• Ratio: 1.5:1, 2:1, 3:1, 4:1, 6:1, 10:1, or infinity:1. Choose the amount of compression above threshold before fine-tuning the threshold itself.
• Release: The fixed selector offers 50, 100, 200, or 500 ms, 1 or 2 seconds, plus Variable. Only Variable enables the second knob's continuous 50 ms–3 second range.
• Knee: SOFT, MEDIUM, or HARD sets how gradually compression begins around threshold.
• Thrust: NORMAL leaves detection unfiltered. MEDIUM reduces low-frequency and emphasizes high-frequency detector content while retaining a flat middle region. LOUD applies the full tilt, approximately −15 dB at 20 Hz and +15 dB at 20 kHz. These are sidechain changes, not output EQ.
• Type: NEW is feed-forward; OLD is feedback. Recheck reduction after makeup-gain adjustments in OLD because gain participates in its control loop.
• L/R Link: IND gives independent channels; 50, 60, 70, 80, 90, and 100% progressively link them. Use 100% to minimize level-driven image shifts. Mono operation locks to IND.
• Link Shape: HP, LP, or both filters determine which frequencies contribute to inter-channel linking. This is distinct from THRUST and has no effect in mono.
• IN / BYP / Power: IN disengages only compression, retaining circuit tone. BYP bypasses the modeled hardware path. Power disables the processor and can release DSP when LoadLock permits.
• Make-up Gain: Automatic mode compensates as compression changes. Engage manual mode to use the 0 to +24 dB gain knob; the knob is inactive in Auto.
• Mix: 0–100% dry/wet, default 100%; use reduced wet contribution for parallel bus compression.
• VU Meter Source: IN, OUT, or GR selects the two meters' readings. These indicate averages, not peaks; 0 VU in level modes represents +4 dBu.
• Headroom: 4–28 dB in 4 dB steps, default 16. Lower numbers increase headroom; higher numbers encourage compression and color. Avoid automation because of possible temporary gain jumps.

Manual — sections: Compressor Section; Tone Section; Link Section; Output Section; Other Controls.