Tractrix midrange horn (Pt II) – drive units

In my Tractrix Midrange Horn, the loudspeaker drive units in use for the last few years are the SEAS FA22RCZ.

This driver is inexpensive but materials & build quality are excellent, as you would expect from SEAS.

I have added a tweeter, a Fostex T90A.  This tweeter is sensitive (106dB/W/m).  A Fostex T925A (more sensitive) would possibly be a better match – but much more costly.

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The Mounting Base-plate for attaching the Fibreglass Mid Horn…..

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Fibreglass Mid Horn ‘clamp’ fixing method…..

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The Tractrix Mid Horn with Tweeter…..

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(Arguably, the tweeter should be located at the centre of the horn – however this high placement has a pleasing effect on soundstage height and scale.)

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Crossover:

The chosen Crossover is a first-order ‘Solen Split’ type:

Xover

With Solen Split, the selected crossover point is at -6dB (not -3dB) and with the tweeter connected in reverse phase, the response at crossover point is almost flat, with only a slight -1.2dB dip. [Ref. Douglas Self: The Design of Active Crossovers]

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(Typical Solen Split example only)

The 0.3mH coil is air-core type, using 1.6mm diameter coated-copper winding wire.

For the 1.5uf capacitor, I decided to go for a costly Fostex CS type ‘non-inductive copper/tin foil, PPTA film’.  The ‘outer-foil’ was identified (using an oscilloscope, see Jimmy’s Junkyard) & orientated to the tweeter connection.  (Try both ways and make your own choice, the difference is significant.)

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(A black line has been marked to label Outer Foil end)

Experiments:

I tried ‘damping’ the ‘whizzer’ cone of the SEAS FA22RCZ, using foam plastic pieces (12mm approx thickness) – this is the Marc Wauters ’98 cent whizzer mod’ which he used on Lowther drivers.

98cent-whizzer-trick

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I found that the sound was very punchy and ‘taut’, but with less bandwidth (less LF and HF extension), less ‘fullness’ and ‘resonance’.  I did not prefer this – however, you may wish to try it yourself.

Addendum (10 Mar 2016):

Prior to ordering the Fostex CS capacitor,  I used 3 capacitors paralleled –  Russian K40Y paper in oil 1.0uF,  Jensen aluminium/paper 0.33uF, Angela/Jensen copper foil/paper in oil 0.25uF = total 1.6uF (as measured).  Outer Foil lead of each capacitor was identified using an oscilloscope & marked.

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