Slim speakers
100W Hifi speakers with transmission line acoustics.


I built these speakers to go with my "40 Watts Velvet sound" Valve Amplifier.
With a 1/4 lambda frequency of 35 Hz (How low can you go?) the transmission line results
in a very warm characteristic sound greatly matching the "Valve Sound"


The speakers are from the high-end series of Tonsil (, good drivers built with conventional technique.
If these are not available you can find a replacement which fits the specification below:

speakers.pdf ( 66kB )

Case construction

The case is made of 18mm MDF or multiplex; all panels are glued tightly, except for the top panel
which is fixed with 6 screws. In this way the filter section (placed close to the speakers) remains accessible.

All panels must fit tightly; any gap will result in audible noise because
of the high air pressure in the transmission line.
The gap between the slanted panel in the middle and the top panel can be filled with silicone.

Tip: Before screwing first drill a narrow hole to prevent the panel to crack.

The slanted panel must be covered with thick foam at the speaker side to muffle lateral resonances.
Apart from that you can add some wadding behind the speakers.

Construction drawing:

Filter construction

The passive filter is second order (12dB/oct) and has a cross-over frequency of about 2.5 kHz.
A pcb is not necessary, the filter components can be "hard wired", all parts can be secured in the case with silicone.

Make sure that all parts are secured firmly, loose parts may cause audible noise!

Filter schematic:

All capacitors are 100V MKT type; In order to reach the large values (10uF, 3u3) many
smaller capacitors are placed in parallel. (I used 470nF & 330nF capacitors)

The 390uH coil is an air coil; the 2.2mH coil in the low-pass part has a ferrite core.

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