Robin-II
HiVi
M5a with Denovo Mini Elliptical Tweeter
13 Liters Bass Reflex Satellite
by
Michael Chua
This design is free for DIY. Not for Commercial
Use.
This article may not be published in part or full without
the express permission of AmpsLab.
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Denovo Mini Elliptical Tweeter
When I first came across this tweeter, I was quite skeptical.
An Elliptical
Horn loaded tweeter with a Neo Magnet for a paltry sum
of $15. I've always been disappointed with sub $20 tweeters
but this particular tweeter originates from the maker of
the famed SEOS Waveguides, DIY
Sound Group. It's inconceivable that they would jeopardize
their reputation for a budget tweeter.
When
I took delivery of this tweeter, I was immediately struck
by the high build quality. The plastic horn was well moulded,
no burr at the edges to file off. The compression tweeter
looks correctly aligned at the throat too.
An
Impedance Sweep of the Mini Elliptical revealed what I was
up against. As expected, the main resonant peak centers
at about 2.2kHz. Since there's not much damping, it looks
like a LCR network is mandatory. |
Fig
1 - Impedance Sweep of Mini Elliptical Tweeter
Mini Elliptical Frequency Response
The next step is to see the response of the Mini tweeter.
The Black trace in Fig 2 is the Raw response without any
protection capacitor. Sweep is with Bass removed.
The
Red trace is with a 12dB filter with EQ and LCR. Crossover
frequency is at 3.5kHz. The Black trace is normalized
to the Red trace for easy comparison.
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Fig
2 - Frequency Response of Mini Elliptical Tweeter
(mic 1 meter Tweeter Axis | 5ms Impulse Window | 1/12 oct smoothing)
Mini Elliptical Distortion
With the above 12dB network installed, the Mini showed
remarkably low distortion. So much so that I dispensed
with testing it at 18dB.
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Fig
3 - Mini Elliptical Tweeter Distortion
3.5kHz 12dB/oct passive XO with EQ and LCR
(mic 20 ins On Axis with tweeter | 5ms Impulse Window | 1/12 oct
smoothing)
Robin-II
Frequency Response
Fig 4 is the Frequency Response of the Robin-II. Due to
room limitations, measurements below 500Hz should be disregarded.
The
M5a network is a conventional 12dB filter (fc=3.5kHz)
with a zobel. As is visible in the violet trace, the M5a
exhibits two peaks, one at about 4.3kHz and the cone breakup
peak at 9.0kHz. The upper frequency peak is not a problem
but the 4.3kHz peak is near the intended crossover frequency
of 3.5kHz. This presents a serious issue. If I were to
adjust the tweeter level to cross at 3.5kHz acoustically,
it will cause a higher peak at 4.3kHz. To compound matters,
the SPL from 2kHz to 5kHz will be raised too. This must
be avoided at all cost.
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Fig
4 - Robin-II Frequency Response
(mic 1 meter Tweeter Axis | 5ms Impulse Window | 1/12 oct smoothing)
Robin-II
Distortion
Fig 5 is the Distortion plot of the Robin-II. The distortion
peaks at 1.5kHz and 3.0kHz are from the HiVi M5a midwoofer.
For these distortions to vanished, the M5a needs to be crossed
at about 700Hz, which makes it quite absurd. Fortunately,
I am unable to pick up these distortions with music playback.
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Fig 5 - Robin-II Distortion
(mic 20 ins On Axis with Tweeter | 5ms Impulse Window | 1/12 oct
smoothing)
Robin-II
Dispersion
The Robin-II Horizontal Polar response (Fig 6) is outstanding.
Frequencies from 4kHz upwards are well controlled. Music
is virtually even within a +/- 15° window. |
Fig
6 - Robin-II Horizontal Polar Response
Robin-II
Impedance & Phase
Fig 7 is the Impedance and Phase plots of Robin-II. The
valley between the two peaks is at 50Hz. This is the box
tuning frequency. Minimum impedance is 8 ohms at 300Hz.
Where the M5a crosses over to the Mini tweeter at 4kHz,
Phase is +30°. Impedance at this point is at 18 ohms.
At the lower end, the phase angle of -60° at 90Hz,
impedance is at 36 ohms. At 70Hz, the phase angle is +45°
at 20 ohms.
Though
the Phase angle is not as linear as I would like it to
be, the Robin-II does not present a difficult load for
modern day power amplifiers.
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Fig
7 - Robin-II Impedance & Phase Response
Robin-II
Crossover Network
Classical
filter theory dictates that when the Low and High Pass
networks are both 2nd Order, the tweeter's polarity must
be inverted for correct summing. However, this only holds
true if the driver is a pointsource.
The Robin-II is the ideal candidate for 2nd Order filters.
Due to the depth of the Mini tweeter, the voice coils
of both the tweeter and the midwoofer are almost in alignment
in the vertical plane. This was proven true during the
SPL sweep. The tweeter must be connected in reversed phase
for proper summing.
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Fig
8 - Crossover Network of Robin-II
the sound of Robin-II
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The
Robin-II is exceptional in price and performance. There's
absolutely no grain or brightness on playback. Midrange
stands out clearly in the mix. Thanks to the Denovo Mini
Elliptical tweeter, the highs are smooth with no hint of
harshness.
The
Robin-II will be right at home as nearfield monitors for
vocal recording. For music playback, the M5a is rather bass
shy. Supplement the Robin-II with a subwoofer crossed at
about 90Hz (24dB/oct) and the system will rock.
next
> Robin | Robin-II | Robin-IIa
Oct
1st, 2013
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