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Point 75i
Copyright 2005 Troels Gravesen

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SEAS L22RN4X-P + W15CY001 + Expolinear NDRL81


Acapella SE and Point75i aside.

During the last 1 year I've kept contact with Martin who bought my Point75A speakers and from time to time we have been discussing a possible upgrade in order to get better bass performance and a little more warmth and weight in lower midrange. Analysing the response from the W15 and 18W/8535 drivers in LspCAD made it clear that some improvement might be possible. Fitting a larger bass driver into the smaller Point75 cabinet (compared to the Acapella LWJ) would take a special driver if the low-end response shouldn't be sacrificed. An 8" Vifa polypropylene driver was tested, but due to low membrane weight and a huge magnet the F3 would rise some 6-8 Hz and this made a noticeable reduction in bass performance. The SEAS 8" alu drivers had been on the shelf since the Acapella LWJ upgrade and these drivers have a 43 grams cone weight, a reasonable Qt of 0.32 and would fit nicely into the approx. 28 litres volume giving a F3 of 36 Hz. Quite impressive. These L22NR4X/P drivers can be used in relatively small cabinets giving an impressive low-end response. The trade-off is efficiency, but the 18W/8535 never was an efficient driver and some improvement in efficiency might be possible after all. So, it was decided to replace the Scan-Speak drivers with the SEAS alu driver and as can be seen from the picture below, the driver holes needed some routing to get the new drivers in place.
Initially I thought that I only needed to make a new bass low-pass section for the SEAS (how naive!), but things developed and it appeared that an overall modified crossover would make it possible to increase performance significantly - and numerous hours went into modelling and testing a whole range of options. Crossover version 19 was where it finally stopped and I'm pleased to say that I like these speakers so much that I'm considering building a pair of Point75Ai for myself. I've always been fascinated by the challenge of making small speakers sounding big, and this is one of the kind.

Should anyone consider the upgrade, most of the old crossover components can be reused by unwinding the coils to new values, except for a new 1.0 mH coil needed in the mid-section. The overall phase-tracking between drivers has been improved and the NDRL upper response has been flattened a bit to give an overall smoother treble. Attenuation of the mid and tweeter has been reduced to give a rise in efficiency of 1-1 dB. The vent and port-tuning was left as is, but in a possible new incarnation I would lower the bottom panel like the Acapella SE to gain another ~2 litre volume and I would use a larger vent diameter - 60 mm ID - and a length of 190 mm get a vent tuning of approx. 33 Hz.
I've really enjoyed having the Point75As back and I couldn't resist trying the JP3 and G2Si ribbons also, and it appeared that both the Fountek JP3 and the Aurum Cantus G2Si can readily substitute the NDRL/ATD ribbon. These drivers have similar frequency response and apparently also the same transformer unit "driving" the ribbons (impedance). No basic changes to the crossover, only attenuation had to be adjusted. The JP3 and G2Si require a 4R7-5R6 input series resistor to match the W15 mid-driver. Simple as that. Please don't ask which sounds the best. The NDRL has a wider horizontal distribution if this may be an issue. But I would use the Fountek JP3 - half price compared to the NDRL, at least here in Denmark.


Enlarging driver hole to fit with L22RN4X/P.

The Crossover


Modified crossover for Point75i.
Red = real resistors, "empty" resitors = internal resistance of coils.
Note: inverted polarity of tweeter.

Components for a pair of crossovers:

2 x 0.18 mH, 0.9 mm wire, 0.23 ohm, tweeter, I-0606
6 x 0.15 mH, 0.9 mm wire, 0.21 ohm, tweeter, I-0604
4 x 1.0 mH, 1.4 mm wire, 0.28 ohm, mid, I-0060
2 x 1.5 mH, 0.9 mm wire, 0.76 ohm, mid, I-0628 (this coil has intentionally high resistance)
2 x 2.7 mH, trafo, 1.2 mm wire, 0.12 ohm, bass, mo-125420

6 x 4.7 uF, pp, tweeter and mid, mcap-00470
4 x 6.8 uF, pp, tweeter, mcap-00680
4 x 10 uF, pp, mid, mcap-001000
2 x 33 uF, polyester, bass, I-1451
2 x 22 uF, pp, mcap-002200
2 x 100 uF, electrolytic, bass, I-1085

2 x 10 ohm, 5 W MOX, tweeter, I-1944
2 x 3.3* ohm, 5 W MOX, tweeter, I-1932
2 x 4.7 ohm, 5 W MOX, tweeter, I-1936
2 x 1 ohm, 5 W MOX, tweeter, I-1920
2 x 4.7 ohm, 10 W MOX, mid, I-1982 (must be 10W series resistor for mid)

* 2 x 4.7-5.6 ohm for Fountek JP3 or Aurum Cantus G2Si (only input series resistor (R-1021)

Getting coils with the proper internal resistance is always a problem and I receive many mails asking questions about this.
The inductance (mH) tolerance is usually very low and I have bought coils from many sources and never found them to vary more than +/- 1-2%. Very good.
The resistance of the coils can from my experience vary +/- 10-20% without causing serious trouble, i.e.:
The 0.18 and 0.15 mH coils may vary from 0.1-0.3 ohm internal resistance without affecting performance.
The 1.0 mH coils may vary from 0.2 to 0.4 ohm.
The 1.5 mH coil may vary from 0.4 to 1.0.
The 2.7 mH coil may vary from 0.1 to 0.3 ohm. Don't go higher than 0.3 ohm or you will loose efficiency and bass performance.
Obviously the capacitors and resistors should be exactly as specified, but as they are all standard values this shouldn't cause trouble - although some strange capacitors values are seen in the US and a few other places.

Measurements:


Left: Nearfield bass spliced with m midrange+tweeter. Point of crossover between bass and
midrange is around 400-450 Hz.
Right: SPL response from midrange and ribbon, here from 800-22000 Hz. Blue = minimum phase.


Left: Impedance of Point75i, here both speakers. Right: Point75i step response with marked peaks from tweeter, midrange and bass.


Red = NDRL tweeter with 3R3 series resistor. Blue = JP3 ribbon with 5R6 series resistor.
As usual the tweeter level should be fine-tuned upon final set-up, so better have extra resistors ready, e.g. 3R9/4R7/5R6/6R8.