The Robin-X is about HiVi RT1C planar tweeter. I’ve always wanted to try it with HiVi M5a. What kept me back was the cone breakup of this mid-woofer. I wasn’t sure whether the crossover can suppress the breakup enough when I cross it at 4kHz. Why 4kHz? The RT1C demands it. Having worked with it before, I found the RT1C distorts below 4kHz.
Fig 1 – HiVi M5a RAW Response. No smoothing applied. Measurements below 500Hz are in Nearfield.
The M5a seems to breakup at about 8kHz in Fig 1. There’s a burst in energy at 9kHz followed by a steep roll-off. I’m quite sure that will not present any problems once I roll-off the M5a at 4kHz. What is more worrisome is the peak at 4.5kHz. That is very close to the crossover.
Fig 2 – M5a with Low Pass filter
In Fig 2, the Blue plot is the M5a with a BSC and a Low Pass filter. The 9kHz peak is down -15dB which is not too bad. Discounting this peak, the M5a roll-off is about 20dB/oct from 5kHz to 10kHz.
Fig 3 – Response of M5a with RT1C
The Red plot is the RT1C with a 4kHz crossover. The two drivers appear to intersect acoustically at 4kHz.
Fig 4 – Summed Response of the M5a and the RT1C
The Black plot is the summed response of the M5a and the RT1C. The +3dB gain at the crossover region looks like a textbook 2nd order Butterworth network.
Fig 5 – Robin-X Frequency Response
For clarity, I omitted the M5a Low Pass and RT1C High Pass responses. The Black plot is the Robin-X frequency response. Note that this is without any smoothing applied. Moreover, no flush mounting for both the M5a and RT1C.
Fig 6 – Robin-X response with the RT1C wired in revered phase
This is where it got difficult. It took me a while to get the null looking like this. This is a good indication that the M5a and the RT1C are crossing properly.
Fig 7 – Robin-X Impedance
Power amplifiers will love the Robin-X. The minimum impedance is 7.5Ω at 250Hz. In the power hungry bass region, it is only 10Ω at 50Hz. Electrical phase is within a +/- 60° window from 10Hz~100Hz.
Robin-X in play
I tested the Robin-X with a variety of music, from female vocals to strictly instruments. In the end, I couldn’t find any fault. There was a decent amount of bass coming out of the M5a. That alone is sure to impress visitors. I did not hear any veil over the mid-range. Voices came out clear and well separated from the mix. And that RT1C planar tweeter. What more can I say. It sounds absolutely wonderful.
If you are planning to build your first 2-way using a 5″ mid-woofer, I strongly recommend this Robin-X. She won’t disappoint. The RT1C cost a bit more but take my word, it’s worth it.
November 21, 2017Projects