18" Coaxial Drivers

[darkmatter]

A rare beast such as the BTH K-10A 18" can be had at a price but are rare.

Are there any modern 18" alternatives available?

[ScottL]

No high performance ones that I can recall, although I speak under correction. Big coax drivers do tend to have a few issues, especially as size increases. As driver dimensions increase so the XO frequency needs to drop. Since there are very few 18in units that can even get near 1KHz with any grace at all, the tweeter needs a shed-load of power-handling capacity to cross low enough to avoid major breakup modes from the main cone, and to avoid obvious changes in dispersion / polar response. That usually means 4th order slopes (minimum), so the cost quickly spirals if quality is the goal. Like most other things, there's a point of diminishing returns. 15in - 16in coax drivers are on the edge; larger tends to be a step too far, except under 'cost no object' conditions.

[darkmatter]

No high performance ones that I can recall, although I speak under correction. Big coax drivers do tend to have a few issues, especially as size increases. As driver dimensions increase so the XO frequency needs to drop. Since there are very few 18in units that can even get near 1KHz with any grace at all, the tweeter needs a shed-load of power-handling capacity to cross low enough to avoid major breakup modes from the main cone, and to avoid obvious changes in dispersion / polar response. That usually means 4th order slopes (minimum), so the cost quickly spirals if quality is the goal. Like most other things, there's a point of diminishing returns. 15in - 16in coax drivers are on the edge; larger tends to be a step too far, except under 'cost no object' conditions.

Agreed, from research I have carried out, it appears that the BTH K-10A and its progenitor made by Western Electric, were the only 18 inch coaxes made which had excellent performance.

Of the available coaxial units 15" seems like the largest that could offer potential HiFi performance

in particular two models from Great Plains Audio their model 604-8H-III (http://www.greatplainsaudio.com/two_way.html) and the 604E series 2 look ideal but alas not available in the UK.

This unit seems possible and is readily available in the UK.

The Beyma 15XA38ND (http://www.beyma.com/products/coaxials/115XA38ND8)

It offers a good response and set of parameters.

However one comment you made about 18" Bass units really interests me, and again reseached it appears to be the only one 18" unit I can find has an extended midrange response, which is this model from

Precision Devices the PDN.18BR40 (http://www.precision-devices.com/Product-Details/PDN18BR40)

I wonder how high this unit could be crossed over?

DM :)