Here the first picture is our first victim, Seas L18RNX/P before we start to misuse and modify it beyond repair. In the second picture we have got rid of the phase plug. This is easily done by turning it until it gets loose. Easiest way to do that is by first drilling a hole into the side and use the drill to turn. Third picture is just a close shot of the pole piece / voice coil former after removing the phase plug. There is some residual glue that have to be removed before gluing or otherwise attaching the inner capacitor surface (aluminium cylinder) to the top of the pole piece.
Here is the outer cylinder of the measurement capacitor made of 0.1mm 2-sided glass fibre "board". Close shot shows the strip that is used to get through the voice coil former without short-circuit. Remember, there is voltage in the kV-range, so care should be taken that the board and other parts can withstand it. There is still additional work to be done with scissors...
First we have to drill some holes to the voice coil former to get the outer surface connections to the "outer world". The strip in the glass fibre board will be drawn through this hole. In the other side there is similar hole to make connection to the other side of the board (earth). Voice coil former itseld should also be earthed to get lower noise. The pictures above are actually from other prototype (Seas L17REX/P), but driver is very similar to L18RNX/P.
There are some pictures of the measurement system without gluing them in place. First picture is a closeup view of the inner cylinder made of aluminium, second is the outer cylinder, third shows how they will approximately be installed (just thrown in at that stage).
Here are the leadout wires (of the measurement capacitor's outer cylinder and shield). Click on the pictures for high resolution versions. The green stuff on the cone and spider is just spray-on PCB-coating I used for the board, where these leadout wires are connected (sorry, no picture of that). This PCB only contains simple 1st order RC-filter (10 MΩ, 100 nF)to filter the high voltage connected to the measurement capacitor. Connections look quite terrible in these shots, but they work (at least for a while).
Here is the final assembly (well not exactly, you still need dust cap...). Outer cylinder is glued to the voice coil former and inner one is sandwitched between two pieces of PCB (pertinax, as it is easier to machine). Top PCB layer is connected to earth through the grey wire. Sandwitched inner cylinder is connected to operational amplifier circuit that gives you voltage proportional to the velocity of the cone. Ventilation is done through the hole in the pole piece and inner cylinder.
Above is one alternative way of constuction. There outer cylinder is also made of of aluminium sheet. Inner cylinder is attached to the driver's pole piece with a bolt that goes through the whole magnet assembly. This is a bit dangerous construction since there is no shielding around the high voltage cylinder. The white thing at the outer cylinder-voice coil former-junction is nomex-insulating paper. Also problem with this construction is the lack of ventilation hole in the pole piece.
All pictures are links to high resolution versions.
This is the speaker as described in the PDF document
Description of prototypes
with SEAS L17REX/P driver, 4th order linkwitz riley
crossover and transconductance amplifier for the
bass-midrange driver. Black coaxial cable with BNC-connector
comes from the measurement capacitor.
Two PCB's are:
Magnet system. The aluminium cylinder on top of the pole piece is original one that will be replaced with similar "sandwitched" one that is presented in SEAS L18RNX-prototype.
Voice coil former before attachment of the outer cylinder of the measurement capacitor.
Voice coil former with outer cylinder of the measurement capacitor attached. This version is made from aluminium sheet.
Some pictures of the "complete" system and electronics. Second picture is early version of the electronics, third is more "integrated" version of the same.
You may read more about the prototypes and measurements from Description of prototypes