ES-600 Biradial with RCF ND-850 1.4 Compression Driver

In this blog post I will be looking at the RCF ND-850 1.4 compression driver mounted to the ES-600 Biradial Horn.

RCF ND-850 mounted to the ES-600 Biradial (original prototype)


ES-600 Biradial (original prototype)



RCF ND-850 1.4 Specifications




  • 3" Diaphragm
  • 1.4" Throat 
  • 500HZ - 20kHz 


Below is the raw frequency response of the on-axis at 1 meter mic distance.  


Raw on-axis Frequency Response for RCF ND-850 on ES-600 Biradial 

Below is the % Distortion Sweep showing D2, D3, and D4.  As you can see distortion is at 0.10% across it's bandwidth.




% Distortion for RCF ND-850 + ES-600 Biradial 

Burst Decay 

The burst decay shows very clean output especially near cutoff.  However there is breakup which occurs starting at 10kHz.

Burst Decay for RCF ND-850 + ES-600 Biradial 

Off-Axis Coloured Polar Map

Off-Axis Coloured Polar Map for RCF ND-850 + ES-600 Biradial 

Subjective listening Impressions 

Because of the breakup occurring at 10kHz this driver exhibited some harshness in the upper treble.  However when used as a midrange only driver in a 3-way setup the driver really comes into it's own.  The midrange is palpably realistic and has genuine audiophile sound quality.  There is plenty of lower midrange authority.   The ES-600 Biradial horn exhibits very well behave off-axis even up to 16kHz which makes this horn suitable for a 2-way type system.  However because of the breakup which starts at 10kHz this driver in stock form is really only a dedicated midrange driver.

Potential for modifications 

This driver is a candidate for modification like I've done with other compression drivers.  I decided to install a small amount of 30ppi open cell foam into the rear chamber so that it gently presses on the center of the diaphragm.  


30ppi open cell foam placed under rear cover

RCF ND-850 with rear cover removed exposing 3" Titanium diaphragm 



Below shows the resulting measurement comparison with and without the foam.  





RCF ND-850 without rear chamber foam 

RCF ND-850 with foam in rear chamber 

As you can see below there is no difference in the measured frequency response.



Frequency response overlay with (green) and without (red) the foam 


Since my microphone only extends up to 20kHz I was not able to get a distortion sweep comparison for the frequency range of interest (10kHz-20kHz).





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