In this blog post we look at the RCF ND850 2.0 compression driver. This driver features a 3.00" diaphragm and a copper shorting ring for low distortion.
The copper shorting ring shown below which reduces inductive rise and stabilizes the magnetic field in the gap against voice coil current induced flux modulation.
The phase plug is sintered together from pressure cast metal. It appears high quality and even has enlarged openings for the two central rings compared to the outer opening.
The titanium diaphragm uses a vented and damped surround. As you can see the surround overlaps the diaphragm by a significant amount.
With the driver mounted to the ES290 Biradial I conducted an impedance sweep measurement which is shown below. The driver's mechanical resonance is centered at 500Hz and shows a flat impedance above that. We see breakup starting at around 8kHz.
Below is the raw frequency response for the ND850 2.0. The response is very linear across it's bandwidth with very little irregularities until we get to the breakup at 8kHz. Sensitivity is high at 110dB.
Burst decay is also very clean up to 8kHz.
Harmonic distortion at 85dB is -65dB for H2 and -82dB for H3 if looking at the 1kHz region. I would like to see H2 a little lower here.
Increasing to 95dB we see H2 rise to -55dB which is not what I like to see.
Conducting IMD at 85dB reveals -78dB IMD in the 500Hz region rising to -68dB above 1kHz. 5kHz shows -65dB performance.
Raising the test signal to 95dB we see IMD at -65dB for 500Hz and -60dB for the 5kHz region.
Removing the Rear Cover
I removed the rear cover and secured the diaphragm using thick rubber washers as shown below.
I then conducted an impedance sweep. We see that the FS drops significantly from 500Hz to 300Hz!
Conducting a frequency response test we see the response improve even more. This is a very linear response!
Looking at harmonic we don't see much change. H2 is still extremely low at 0.056%.
Raising the test signal to 95dB SPL results in H2 at 0.15%.
Re-testing IMD we see -78dB across the spectrum until breakup at 8kHz. This is a new record for low distortion.
Even at 95dB SPL we still see IMD below -65dB. This exceeds my performance target for sound quality by a wide margin.
Below is the schematic for crossover No.1981 which I plan on integrating with bass cabinet 1798 (not shown). The goal here is to integrate CT440 super tweeter at 8kHz and create the required target response in the midrange for system integration.
The RCF ND850 2.0 achieves a nice target curve that is completely flat and devoid of any issue. Integration with the Ciare CT440 tweeter was quite easy to achieve.
The step response below shows good alignment between the ND850 2.0 and the CT440. Coherence is excellent.
The RCF ND850 2.0 exceeds my target for sound quality by a wide margin. Subjectively the driver sounds excellent.