Monacor SP200X + Fountek NeoCD3
Copyright © Troels Gravesen

A quick and easy 2-way with 90 dB/2.8V sensitivity.

The Monacor SP200X is a cost-effective fullrange driver, built on wellknown premises and also sharing some of the bad qualities like a thin dust cap over an un-vented and un-damped magnet polepiece. The small whizzer cone really doesn't do much as can be seen from the frequency response profiles below, and it's better cut off.
The response in the 2-10 kHz region is - as often seen - 5 dB above average level and it doesn't sound good at all - at least on-axis. The sensitivity is around 90 dB/2.8 V and the voice coil resistance is high, making it an ideal partner for valve amps. Listening to the SP200X - as is - is special and a lot of people swear to this approach, having a driver without any crossover components at all. To me it's a narrow, beaming midrange supplemented by poor treble dispersion and I don't like it. It's possible to add a series notch filter to smooth the response in the treble region and 0.82 mH\\2.5 uF\\10 ohms (all in parallel) make a nice and smooth response from 1-10 kHz.

However, I wanted to hear what the Fountek NeoCD3 ribbon tweeter would do to the overall performance of this driver and it doesn't eliminate this special "fullrange" sound, having a large cone doing most of it - up to 2.5 kHz - but it certainly improves the treble considerably. The filter is 4th order and if you think 4th order kills transient response, think twice. The SP200X is not as "fast" as the 9710, but it comes close. I won't elaborate much on what I mean by fast, but it's mostly linked to what happens in the midrange. When a drummer hits the snare drum hard, you will recognise the difference between the 9710 and the SP200X. The 9710 is phenomenal in this respect, but the SP200X may have an overall more neutral sound. Right now I dismantling a 9710 driver to see how it is made and this will be reported soon. This was a driver ahead of its time.

A classic fullrange driver made from long fibres pulp and with a coated fabric surround.
The whizzer cone is removed here and a phase plug (from a SEAS driver) inserted.
Some of the very thick rubber holding the surround was cut off. Not particularly nice looking.

The unmodified driver.

The Cabinet

The cabinet is the same as used for the Philips 9710 driver:
37 litres and a vent tuning of 45 Hz. Internal depth is 280 mm.

Use bracing to increase rigidity, but don't overdamp the cabinet.

I've had a suggestion on a Voigt horn for the SP200X from Rodolphe in France and he mails this link (page 6):
http://www.cinetson.org/phpBB2/viewtopic.php?t=2778&highlight=sp200x

Some day I may try this option - long time since I did a quarterwave...

The Crossover

Crossover and LspCAD prediction of frequency response.

Measurements

And here's real life performance. Very close to LspCAD prediction.
Red = SPL, Blue = minimum phase.

Reasonably smooth roll-off characteristics and a nice phase tracking.
Point of crossover is at 2400 Hz.

Impedance and phase of individual drivers. The ribbon with 100 uF in front.

FR of modified (red) and un-modified driver (blue).

Horizontal dispersion at 0 (red),10 (blue),20 green),30 (yellow) and 40 (purple) deg.

Vertical dispersion from 1 metre distance at a level between B and T:
Red = on-axis, blue = +10 cm, green = +20 cm, yellow = -10 cm, purple = -20 cm.
This is much better than anticipated and these two graphs suggest an overall
even power response, which was also expected from the 4th order filter.
But the SP200X is doing much better than expected.

Step response. Note the smooth decay of the second peak.
The SP200X really performs well.

Impedance of system. If you add an LCR circuit to the crossover, you get
a very valve friendly performance with a basic impedance of 7 ohms. A rare sight.
The LCR is: 10R + 18.2 uF + 0.82 mH (all in series across input terminals)