Goodmans Axion 150 mkII, 16 ohms
Copyright 2007 Troels Gravesen
       

In search for suitable drivers for an open baffle 3-way, these Goodmans came up and I took my chances and bought them. Buying vintage drivers on eBay is gambling and in particular when we're talking drivers some 40 years old; drivers having paper surrounds and paper voice coil formers. Paper may over time take up moisture and we run the risk of rubbing voice coils and suspensions having lost compliance. TS-data may be way off what they were back then. The drivers reported here were so and so, but suitable for what I had in mind.

The cones were in good shape and not too much voice coil rubbing. Magnets were in perfect shape (alnico may weaken over time). This could be concluded from TS data measurements. These 12" drivers from Goodmans were meant as full-range units and came with an odd trumpet whizzer cone (seen below) and I immediately removed these. The driver basket can be removed from the magnet structure simply by removing four screws and this helps a lot in restoration as the voice coil now can be inspected from all sides. Assembling the speaker again is tricky business as the magnet gap is very narrow, 1 millimeter. Normally we can add thin plastic sheets between the voice coil and center polepiece, but not good enough here. The tolerances are so narrow that I had to run a sine wave (40 Hz) through the units while tightening the screws again. Still some voice coil rubbing and I sanded the inside of the voice coil while running the 40 Hz sine wave. This way small paper surface irregularities can be removed until it runs smoothly. Take care when doing so as the voice coil former is very thin and removing paper will weaken the cylinder. More details on this can be found in the JBL L26 file, the JBL bass driver here having the same problem.
Both the spiders and corrugated paper surrounds were soft and had lost elasticity, thus not returning the cone to proper rest position. The spiders were given a DAMAR treatment and the paper surrounds a thin layer of coating material. Initial Fs was around 25 Hz and after treatment the Fs was up to ~38 Hz, well in accordance with Goodmans' data as seen below.
The reason for insisting on 16 ohms drivers for the open baffle project is that if we want to stay close to a system impedance of 8 ohms, we simply need drivers with an Re of min. 10 ohms.


Close to what the Axiom 150 mkII would look like new.


The whizzer cones were removed from the drivers.
The whizzer cones are made from some heavily impregnated rigid paper
and a fabric dust cap was glued to the throat to protect the voice coil.

Measurements

TS-data after restoration of surrounds and recentering of voice coils. Now, a BL factor of 20 is not an everyday sight and these drivers are extremely sensitive. Calculated 97 dB/1W/1m. And measurements confirm this.


Original Goodmans data.

Two things from the above LspCAD modelling: The Speaker Unit Editor is a great feature allowing you to check your measurements by cross calculation. As can be seen there's a good correlation between data. My Tannoy test cabs will serve as initial cabs for these drivers and a decent alignment can be a achieved giving a good response down to 50 Hz. 150 liter would take the response down to 40 Hz, but these drivers will be tried in an open baffle construction. Preliminary "test" can be seen below.

Left: The impedance curves tell a lot about driver performance. As can seen, the Fs of the restored drivers are fairly good, but compliance of suspensions are not exactly the same, one driver having a more narrow impedance peak compared to the other. All the ripples on the graph reveal suspensions not working properly, but it's been seen worse, much worse.
Right: Impedance of driver in my 95 liter test cabs. Vent tuning is 45 Hz = two holes in front panel of 75 mm diameter.


Frequency response of two drivers in test cabs, quite nice.
Reading normalised for 2.8V at 1 meter distance.
Sensitivity = 96-97 dB/2.8 V.

The following three graphs were made in LspCAD just to get an idea of how easy - or difficult - the Axioms would be in more conventional applications. Above modelling for a potential two-way system with a point of crossover around 2.5 kHz. I've tried setting up a two-way system with the Axiom bass and Wharfedale Super 3 tweeter with a point of crossover around 5-6 kHz and got a reasonably flat response. The sound wasn't bad at all. Having some 550 cm^2 cone area for midrange is special and the transient capability of these drivers is phenomenal.


Above modelling a point of crossover around 800 Hz. A fairly smooth decline of the Axiom response is seen.
The minor notch at 2 kHz can easily be handled by by-passing L1011 with an RC circuit.


Simulation of a 3-way system with the Fostex FE126E for mid and
Wharfedale Super 3 for treble. Doesn't look bad at all.


Here's my open baffle test made from whatever I had at hand in my workshop.
Taking the Axioms down to floor level and getting decent room-gain is special.
The bass may not go deep, but you have the feeling of extension all the way down.
Running the Axioms full-range and playing Nils Lofgren/Acoustic Live is chocking.
You get a feeling of having never heard the dynamics of the recording before this.


So much on the Axiom 150 mkII drivers.

Go to: OB7, Axiom 150 mkII in open baffle experiment.