Audio Limiter Compressor

The Super Pro III Stereo Audio Limiter Compressor 

#Matched Left & Right Channels

#Optimized sidechain/gain control–low distortion

#Super Dynamic Stereo Expand circuitry

#Selectable Release Rates

#Selectable Pre-emphasis 50/75/0 uS

#15KHz Low Pass Filters

#Full LED limiting indication


This is the High Performance NRG Pro 3 series Stereo Limiter Compressor kit which is the result of much research into analogue automatic gain control design. Without a doubt, this is one of the finest Limiter Compressor units available to the constructor. With much improved circuitry, this limiter delivers a very powerful smooth sound, with none of the ‘breathing’ and roughness of some designs. The left and right channels use perfectly matched gain control devices to ensure accurate channel balance and gain tracking. And we have made sure that those ‘high energy’ high frequency sounds don’t have that ‘squashed’ and overlimited characteristic, prevalent in many designs!

Application: FM Broadasting…This Limiter Compressor will prevent overmodulation, distortion and splatter, whilst at the same time delivering a clean and loud powerful modulated signal. This unit is all you need to provide full control of the audio signal prior to transmission. Although this product has been specifically designed for use with FM transmitters, this limiter compressor is very useful for for recording purposes where it will remove the burden of having to constantly readjust the recording levels.

New Features: A brand new feature is the inclusion of Dynamic Stereo Expander circuitry. With this unique circuit activated, your stereo signal will have far more depth and you will enjoy the wide surround sound that can be achieved. Breathtaking is the only way to describe this wonderful feature. Because the expander is under full limiter control, it is dynamic and overmodulation cannot occur. Of course, the stereo expander can be switched out if the user chooses. Another new and important facility is the choice of two gain release rate settings. The fast setting will give the aggressive broadcaster the rapid response needed from a limiter, fast recovery after loud DJ transients etc. The slower setting allows the very best quality to be obtained from music, with slightly less compression than the fast mode setting. On either setting, the output quality is superb due to the much improved gain control circuitry and accurate selection of component values.Yet Another new feature is the inclusion of Switchable Pre-emphasis which means the unit is suitable for use anywhere in the world, without having to change components! The choice is 50us for UK and Europe, 75uS for USA and Canada, or no pre-emphasis at all.

Essential Features: Dual 15 Khz Low Pass Filters come as standard with this product. These are absolutely essential to prevent very high frequency inaudible sound signals going into the transmission system. They ensure that your transmitted Stereo Signal is free of unwanted ‘aliasing’ products, which can be troublesome when using CD players. These brick wall filters allow you to transmit the full audio frequency range and are vital for excellent sound quality FM Transmission. Sidechain distortion cancel circuitry ensures that the very lowest distortion figures are achieved. The gain control devices are matched j-fet devices and the bias conditions have been optimized to produce the outstanding sound quality which makes the new NRG limiter sound so crisp. And, as of November 2002, super high quality zero leakage Tantalum Capacitors are fitted on the release time circuitry, giving an even smoother controlled sound output. As with all our kits, this product incorporates Reverse Polarity protection, which prevents catastrophic semiconductor damage if the unit is connected incorrectly to the power supply. The superb audio performance is complemented visually by sidechain driven LED Limiting Indicators which provide instant confirmation of limiting/compressing action.

Easy Construction: This superb product comes complete with a top quality fibreglass PCB and all the components to enable you to construct a Stereo Limiter you will be proud of! Comprehensive instructions are provided with schematics and pictures to assist the constructor. The unit is also available assembled and tested, if you are in a hurry!

Specifications for the NRG PRO III Stereo Limiter Compressor

Type: Backward feed multi-time constant single band matched stereo

Limiting Threshold: Limiting commences at 300mV rms input (-7dB)

Output level after limiting: 750mV rms (into 10K) from 300mV thru 20V input

Attack rate: less than 1mS

Release rate: Selectable, 0.5 sec or 3 sec, both multiple time constant

Sidechain Breakthrough into audio chain: Negligible

Harmonic Distortion: Less than 0.5% at full limiting (slow release setting at 1KHz)

Frequency response: 30Hz to 15KHz within -+ 1dB (pre-emphasis accuracy included)

Signal to Noise ratio: FET input Op-Amp = >70dB s/n after de-emphasis

Pre-Emphasis: User selectable for 50uS, 75uS or none

15KHz Low Pass Filter: Toko multi-pole ‘brickwall’ type

Stereo crosstalk: Better than -60dB (stereo expand muted)

Stereo Expand: Cross linked L – R & R – L dynamic matrix system

RFI suppression: All inputs, All outputs and DC power input

LED limiting indicators: 3 per channel plus 1 power ‘on’ led

Connectors: Standard RCA/Phono input and output

Power requirement: 12 or 13.8V dc regulated at constant 250mA

PCB size: 165mm x 140mm