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.


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


Fibreglass Mid Horn ‘clamp’ fixing method…..


The Tractrix Mid Horn with Tweeter…..


(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.)



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


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]


(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.)



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


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.



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.


 Update 2018: Fostex T925A tweeters (replacing Fostex T90A)


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