Modifying Audax TW034XO-P47N 34 mm dome
Copyright 2010 Troels Gravesen

The aim of this study is to modify the Audax TW034XO-P47N tweeter, making it suitable for points of crossover in the 1-2 kHz range. Using the TW034 with the usual point of crossover in the 2-3 kHz range does not to my experience justify the trouble of modification.

Pay notice to the possible need for liniarising impedance when taking point of crossover this low.
For the modified dome here 1.2 mH + 26.7 uF (22+4.7) + 8R2 will make a flat impedance.
Whether this is needed or not will depend on the actual point of crossover and not least the slope of the high-pass filter.
Using TW034 from 1.5 kHz with an e.g. first order filter may soon fry your voice coil, so be warned!

Audax TW034XO-P47N comes with an solid center pole piece. A dome of foamed rubber is glued to the pole piece providing the necessary damping of the cavity behind the dome.
Below the voice coil gap is a fairly large un-damped cavity with only little (negative) impact on performance.

My initial thought was that overall performance could be improved by venting the pole piece and adding a damped rear chamber as is common practice. This turned out not to be the case. Venting the center pole piece is essential, but that's about it. No extra chamber was needed although everything wasn't perfect yet.
Drilling 4 x 4 mm holes to the cavity behind the voice coil proved beneficial and in particular when this cavity was coupled to a small highly damped chamber.

 



Left: TW034 dome. Right: The damping material on pole piece feels like foamed rubber.


The finished tweeters. Some black paint might do well...Next pair will be nicer.

Modification


12 mm hole is drilled in center pole piece and chamfered 45 deg to a depth of 3 mm.
4 x 4 mm holes are made at back plate at a diameter of 38 mm.

- next pair -


Magnetic gap is covered with alu tape to reduce amount of debris getting stuck here. Completely avoiding this is not possible.
This alu tape didn't perform well. It took an hour/each to clean the magnet system.
Next time I'll seal the magnet gap with a ring of Superfix and wait for the next day. Superfix can always be removed again.
Preventing iron chips from drilling the rear holes to ventilate magnet gap is not possible, so some work can be expected here.


I usually start out with a 4 mm drill and proceed with 6, 7, 8, 9, 10, 11 and finally 12 mm. Last thing is chamfering the hole as seen to the right.
Allow the magnet system to cool in between. If iron parts get too hot, it may loosen the glue holding it all together.


Cleaning the magnet gap is a drag! To the right the cleaned magnet system.


Left: Rear view with the 4 x 4 mm holes to the voice coil gap. Make a circle of 38 mm for guide.
Right: A slightly chamfered 3 mm felt ring is placed on center pole piece and MDM3 is used to dampen center pole piece hole.


Routing for the rear panel.


Left: Felt rings in place. Right: The finished magnet system. Seal routing center hole with a drop of glue.

Performance


Left: TW034 as-is (red) vs. modded magnet system. Right: Performance of two modded tweeters.


Left: CSD (40 dB scaling) of un-modded tweeter. Right: CSD (40 dB scaling) of modded tweeter.


Left: Impedance of tweeter as-is (yellow) and two modified domes (red and blue).
Note no bump at 2.2 kHz and Fs has been lowered from 900 Hz to 700 Hz.


Left: TW034-modded step response. Right: Same ETC plot. Overall a well-damped system.