API 500 - essence

True Class A Optical Compressor with differential drive sidechain

For those unfamiliar with the 500 Series, the system is based on an API* designed modular format whereby different modules can be assembled into a frame or rack like the API* Lunchbox or 500VPR units as well as The Revolver 2 Slot Frame. The user can assemble preamplifiers, equalizers and compressors of their choice to cover a wide range of applications.


The essence is a True Class A Optical Compressor designed to fit the API* 500 VPR Series rack frames offering a great feature set in a compact package. The essence will find application with those seeking a musically transperant compressor for all tracking and mixing tasks, and it sounds particulary outstanding when processing voice and bass tracks. The essence - the Heart 'n' Soul of your next recording!


Buzz Audio currently produces four products that are compatible with the API* 500 Series module frames. These products feature a completely new design whereby our high speed True Class A discrete amplifiers are combined with very high specification Lundahl** audio transformers. The resulting sound is something we believe to be new, unique and a true advancement in the art of analogue audio electronics. All these modules will add intensity and vigor to your recordings!

The essence control circuit design is based on the Buzz Audio SOC-1.1 Stereo Optical Compressor but the main audio path utilises our BE40 and BE50 True Class A amplifiers along with high spec Lundahl* input and output transformers. The input transformer is directly coupled to the Buzz proprietary Opto gain reduction element (completely passive input), amplified and then balanced via the output transformer - a very simple but gutsy signal path!

The essence control circuitry utilizes the Differential Drive Side Chain (DDSC) topology that was developed for the SOC-1.1. This system, which processes positive and negative audio waveform in seperate paths, provides the incredibly musical characteristic you will hear with this unit. The DDSC design also provides for a much faster attack time compared to most other LDR based optical compressors.
Simple side chain equalizers have been provided to allow the user to adjust the sensitivity of the compressor at high and low frequencies. In addition, a side chain insert point is provided for more complex EQ and compressor "keying".

The essence consumes 2 slots within a 500 Series rack. The input/output (I/O) connectors associated with the first slot carry the main audio path. The I/O connectors associated with the second slot are used for a side chain insert point (send and return). The insert can be switched in/out of circuit and monitored via front panel switches.

Stereo Matching
We go to a lot of trouble to match up the LDR's used in our optical compressors but due to the inherent variances in these devices we now offer the essence in stereo matched pairs. Ask your dealer for more detail. For more information on linking two essence units, please read this.

Rack Compatibility

The essence power consumption of 120mA is higher than other 500 series modules, but because this demand is spread over two slots (60mA each) it should work fine in all available racks.


>   API* 500VPR Series rack module format - plug in and play!

>   Advanced analogue technology - not a clone or copy of products from the past.

>   Powerful fully discrete hi-bias, wide bandwidth, differential Class A amplifiers.

>   Passive transformer coupled input, transformer coupled output

>   Side chain insert point

>   Side chain monitor selector

>   On board side chain EQ

>   Unique Differential Drive Side Chain design

>   May be linked to multiple essence units within one rack, more info

>   Switchable 10 segment VU meter


Frequency Response with no gain reduction = 14Hz to 107kHz, +/- 1.5dB
Maximum Input Level = +25dBu
Maximum Output Level = +27dBu
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
Sidechain Max Input/Output Levels = +22dBu
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

* API is a registered Trademark of Automated Processes Inc.