pro-audio products

optical compressor for 500 series

A versatile compressor for 500 series frames that takes optical compression to the next level for all forms of music production. Indulge yourself in it's smooth and creamy sound!

500 series compressor

design features

vocal compressor

control genius

The Essence attack & release settings have been carefully selected for a varied dynamic palette and the intelligent auto modes allow for really smooth "set and forget" operation. But unlike the slow LA designs, this compressor can also be fast! The soft knee opto action is perfect for smoothing the dynamics of vocals and a stereo pair will find immediate use to glue together stems, synths and the mix buss.

audio path quality

The audio input is coupled directly to our custom made LDR gain reduction element via a Lundahl LL1540 input transformer. This is followed by a discrete signal path using our class-a BE40 amplifier and BE50 high current line driver that excites the Lundahl LL1517 output transformer. All this creates a very warm low end sound along with a creamy top end finish.

creative versatility

The Essence is the only 500 series compressor (that we know of) to feature a side chain insert point so you can apply equalisation to the compression characteristic to control certain parts of the audio spectrum. The insert can also be used to "key" the compressor from another sound for all sorts of creative pumping effects. In addition, there is on board side chain eq allowing you to quickly adjust the compressor sensitivity at high and low frequencies.

For critical stereo operation we recommend you order a factory matched pair.

other 500 series modules - Elixir mic preamp | Tonic inductor equalizer

controls and functions

bass compressor

GAIN - sets the output gain from 0dB to +15dB.
DRIVE - sets the amount of compression applied to the signal, clockwise rotation means more compression.
ATTACK - 4 position switch with attack time settings of 1mS, 15mS, 70mS and our excellent Auto mode.
RELEASE - 6 position switch with recovery time settings of 100mS, 200mS, 400mS, 800mS, 1600mS and the intelligent Auto mode..
RATIO - 4 position switch sets the depth of compression from light to heavy, 2:1, 5:1, 10:1, 20:1 all with the soft knee action.
METER - can be switched to read input level, output level and gain reduction. A jumper on the PCB is used to set the peak level reference from +4dBu or +14dBu.
BYPASS - toggle switch used to hard wire bypass the essence.
S.C. MON - allows you to hear the side chain signal (the side chain eq) and the side chain insert return signal.
S.C. HI EQ - applies 8dB at 10kHz of boost or cut to the side chain signal and therefore to the compression.
S.C. LO EQ - applies 8dB at 100Hz of boost or cut to the side chain signal and therefore to the compression.
LINK - using the 500 frame link system, two Essences can be linked for stereo operation.
S.C. INS - this switch engages the side chain insert point which are the input/output connectors associated with the second slot the Essence uses.

user comments

Steeve Body

kick compressorIt is a really good comp, it definitely has a different vibe than the SOC-1.1. It reacts to the signal in the same way but is more colored tone wise, input and output transformers being the reason. More bass than the SOC also more punch in some ways, just a tad more aggressive in tone. S/C EQs and side chain input are also a good addition, really good meter as well! The SOC is more of a classic smoothing device whereas the Essence is a little more forward and snappier, on percussion elements especially. Works on everything, great for tracking bass and vox I found. Also good on drum groups, kick and snare, and for the bucks honestly you can't go wrong. Another 500 series winner!

product review

Mix magazine by Barry Rudolph

Buzz Audio's essence is a single-channel optical compressor module with many custom features usually found on larger, rack mounted compressors. One Essence module fits into API's 500 Series rack — either the newer 4- or 6-slot Lunchboxes, or the 10-slot 500VPR unit. The unit occupies two slot spaces: Slot one's XLRs are for audio I/O; slot two's XLRs provide I/Os for a sidechain insert.

Like the Buzz Audio SOC 1.1 Stereo Optical Compressor (see the review in March 2004), the optical gain-change element uses a Hewlett-Packard Quad LED light block and four specially selected, light-dependent resistor elements. The light block is driven differentially so that two LDRs control the positive audio peaks and the other two control the negative. The main audio path uses Buzz's BE40 and BE50 discrete Class-A amps and Lundahl I/O transformers.


The densely packed front panel has a large 10-segment LED VU meter, seven toggle switches and five human-sized control knobs that are larger and feel better than the typical knobs used on API modules. Essence's operational precision is evident by the complete set of clearly marked front panel control knobs, including drive (threshold), ratio, output gain, attack and release.

There are switches for hardwire bypass, stereo or surround linking, sidechain insert I/O, sidechain monitor select, high- and/or lowpass filter insert to the sidechain path, and input/output/gain-reduction source select for the level VU meter. The VU can be internally set to read either standard +4 dBu or +14 dBu, with the latter better suited for use with a DAW.

The high- and lowpass sidechain filters have a "B" (boost) position and a "C" (cut) position. These are shelving filters with 8 dB of maximum boost/cut at 100 Hz and 10 kHz, with their corner frequencies both starting at 600 Hz, which is the classic Baxandall topology used in many EQ circuits.

snare compressor A HARDWARE PLUG-IN

Essence went to work as soon as I plugged it into the studio's API rack. It went right in with no problems; you will have to secure it with screws so that the rear-edge connectors stay firmly seated. Essence pulls a total of 120 mA — 60 mA for each slot — which is well within spec for modern API racks.

My first tests were on individual drum tracks recorded flat into Pro Tools. Normally, I would not use an opto compressor on percussion because usually the attack times (if they are available) are too slow. Essence has a 1ms attack time that was actually too fast! At 1 ms with a 20:1 ratio and the drive control set halfway or higher, the snare hit's "stick" portion was completely removed. This type of special effect is usually associated with compressors that use VCAs or FETs.

The 15ms attack position was more realistic-sounding, producing a nice "pop" with each strike on the drum. Between the drive control setting and the excellent range of release times, you can dial in as much drum ring out as you want. I used the 200, 400 or 800ms positions, depending on tempo and whether the drummer played grace notes or fills between back beats.

On vocals, cleanliness is important, especially if you use a lot of compression. Essence's sound sparkles even using a 10:1 ratio and 10 dB or more of gain reduction. No compressor is transparent, but when I selected proper attack and release times, there were no noticeable bad-sounding artifacts. I used the "C" position on the sidechain's LPF to lessen the amount of compression caused by intermittent plosives and the subsonic low frequencies from my male singer.

guitar amp compressor Electric guitars sounded good when compressed by essence. I'm not generally into "auto" positions, but the auto-release time position worked fabulously on a funky rhythm guitar track. With the attack time set to 15 ms, there is very little dulling, even under about 6 dB of gain reduction. On guitars, primarily a midrange instrument, I used the "C" positions on both the low and high sidechain filters to narrow the frequency bandwidth available to essence's opto element. This setup produced a dense guitar sound without sounding overly clamped.

For bass guitars, I switched to the "B" mode in the sidechain's lowpass filter to make the unit more sensitive to low frequencies. The result was increased precision: You can fine-tune essence to exactly meet the needs of a bass guitar. Using a 2:1 ratio, 70ms attack and 800ms release, the ultraclean opto sound kept the Fender P bass sounding full and natural.


Essence packs a lot of easy-to-control power into just two API 500 slot spaces. At first, I had two immediate impressions: It is very clean-sounding and extremely fast for an opto-based compressor. I also liked the solid feel of the switches and knobs, along with the large VU meter that always indicates level and gain reduction, even in bypass. It functions cleanly for smooth and fast compression, making this $1,200 (MSRP) unit a first choice for any dynamic processing — from subtle to severe.


Frequency Response with no gain reduction; 14Hz to 107kHz, +/- 1.5dB
Maximum Input Level; +25dBu
Maximum Output Level; +27dBu
Side chain Max Input/Output Levels;+22dBu
Total Harmonic Distortion
   Measured at 0dBu input, no gain reduction, make up Gain 0dB; 100Hz = 0.1%, 1kHz = 0.015%, 10kHz = 0.002%
   Measured at +10dBu input, 20dB gain reduction, make up Gain +15dB, Attack AUTO, Release 16; 100Hz 0.1%, 1kHz = 0.015%, 10kHz = 0.005%
Residual Noise; -95dB A wtg, 150ohm source Z, make up Gain 0dB
Make up Gain Range; 0dB to +15dB
Maximum Available Gain Reduction = 25dB
S/C HI EQ Boost/Cut; 8dB @ 10kHz
S/C LO EQ Boost/Cut; 8dB @ 100Hz
Main Audio Input Impedance; 20k ohms, bal or unbal
Main Output Impedance; 100 ohms bal or unbal
Sidechain Send Output Impedance; 100 ohms unbal
Sidechain Return Input Impedance; 44k ohms bal or unbal
Size; (3"Wx5.25"H). Fits 2 spaces in API* 500VPR Series rack format
Power requirements; 120mA +/- 15-18 volts DC, as supplied by rack power supply.

Specifications are typical of a production unit and are subject to change without notice. 0dBu reference = 0.775 volts RMS

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