The Audax AP170Z0 Aerogel mid/bass driver.

The Audax Aerogel drivers are excellent mid bass woofers.  However they have a big peak first break up mode, at 3.8 kHz of 9db in magnitude.  This is associated with a rise in response at 1kHz.  This very large peak requires notching out.  However with careful crossover design, excellent performance is obtainable from this driver.  At first sight this driver does not look promising, but with notch filter and tweaking the Qs of the filters superior performance is possible.  A speaker with good bass performance and excellent smooth detailed mid range is posible, in a very cost effective speaker.

Last fall while in England, my father asked me to put together speakers for their sun room.  There are always a collection of drivers around my parent’s house.  I used the Audax AP170Z0 and the Scanspeak D2905/95 tweeters.  I used these because they were handy.  A superior speaker resulted from this design.

Here is the woofer alignment, for an optimal 1 Cu. ft. box.

Here is the alignment for a smaller 0.43 cu.ft. box

This is the Crossover Circuit.

I have modified the circuit for the Vifa D27TG 05/06.  This is available from parts Express.

The driver has an F3 of  46Hz in a 1 cu.ft box.  This is good performance, although the driver is displacement limited from 50 to 110 Hz. In this range spl is limited to 100db.  above that spl. of 107db is possible.

There are two impedance humps at 30 and 75 Hz, with the minimal impedance at F3, so tuning is optimal.

A vent flared at both ends, 3″ in diameter, six inches long, slows air velocity to 18 m/sec.

The crossover is electrical second order, however it combines with the driver slopes to make a composite fourth order filter.  Both tweeters are down at least 24 db at resonance (Fs).

The woofer impedance is equalized by Req and Ce and the break up mode peak dealt with by Rm, Lm and Cm.

16 SWG air core inductors  should be used for L1 and L2.  A good quality iron cored inductor should be used for Lm.  Good quality polypropylene caps should be used, such as Solen caps.  Resistors should be wire wound 15 watt.

The phase response of the crossover is unusually good.  The mid band response is excellent, including across the crossover region.  Some degree of diffraction compensation has been achieved, about 3db.  This is usually what is achievable without adding a second driver.  Room gain should in any event do the rest, unless the speaker is a very long way away from room boundaries.

The impedance is 5.5 ohm though most of the range with a peak to 24 ohms in  the crossover range.  These speakers should be regarded as 6 ohm speakers.

Allowing for component insertion loss, notching the woofer peak, and providing for some diffraction compensation, the sensitivity of the speakers is 86db 2.83 volts at one meter.  This is very typical for a two way using drivers of this size.

This makes a very nice speaker.

I have also worked out a crossover that has minimal changes for the SEAS 27TDC.  This tweeter is about $10 more, but well worth it.

If you want to do a two and a half way tower, then here is the optimal box alignment with an F3 of 46 Hz.

I have designed a crossover using the Scanspeak tweeter.  This should be a very good speaker and well worth the cost of the Scanspeak tweeter.  The diffraction compensation starts at the optimal point of 600 Hz, and provides +6db of compensation for the diffraction loss by 175 Hz.  This speaker should have good robust tenor register.

Here is the 2.5 way crossover using the SEAS 27 TDC tweeter.  The phase response is excellent.

I’m adding and MTM design for this driver with a choice of two tweeters.  This requires a third order crossover at 1.8kHz.

Here is the box alignment.  Allowing for the volume of driver crossover and braces,the final volume Vt should be around 1.3 cu.ft.  The port should be 4″diameter X 10″long flared at both ends.  You can also use the two cu. ft. box from the two and a half way tower, above, for extended bass.

The crossover circuits for the Vifa D27TG 05/06  and the SEAS D27TDC are the same.   The SEAS D27TDC is excellent value for money, and in my view worth the slight extra cost.

This is the response with the Vifa D27TG 05/06

This is the response for the SEAS D27TDC

Any of these designs should make an excellent set of speakers.  The crossovers for the MTM set up should have optimal lobing for this configuration.  Vent velocity of these MTMs is an excellent 18m/sec.

Allowing for insertion loss, and notching the woofer peak, overall sensitivity of these MTM speakers will be 90 db 2.83 volts 1 meter and 87 db 1 watt 1 meter.

The impedance is very uniform at 4 to ohms throughout, except for a slight peak t0 15 ohms at crossover.  This design is impedance compensated and should be a pretty easy load for any amp or receiver able to drive a four ohm load.

50% of the internal surfaces of these enclosures should be covered with a Rockwool like product or Black Hole.  The internal surface of the back of the enclosures should be fully covered.

Any of these speakers if properly constructed should give results equivalent to speakers in the 1 to 2 K range per pair.

Here is a parts list and sources, for the two and a half way with the Scanspeak D29.  The component values that change when using the SEAS 27 TDC tweeter are in the brackets, and those alternate values that change when using that tweeter can be found by clicking within the brackets.  Note the specified values and source values in some cases differ slightly.  However there is no significant change in performance.

C1   Two required

C2   Omitted

C3   Two required.  (version with SEAS 27 TDC tweeter)

C4   Two required

C5   Two required

Ce   Four required

Cm  Four required

L1   Two required

L2   Omitted

L3   Two required (version with SEAS 27TDC tweeter)

L4   Two required

L5   Two required

Lm  Four required

Rp1 Two required (version with SEAS 27 TDC tweeter)

Rp2 Two required (version with SEAS 27TDC tweeter)

Req Four required

Rm  Four required

Circuit Board  Two required

Terminal cups  Two required

Ports 4″ flared  Two required

The total cost of the above list is $323.94

Crossover parts for the MTM.  Note that the parts for the Vifa and SEAS tweeters are same.












Circuit Board

Terminal Cup


The cost of this MTM crossover and other parts is $130, so for a pair, $260, for mains and center $390.

This is the parts list for the crossover for the 1 cu. ft and 0.43 cu.ft book shelves.  This is for the Vifa and SEAS tweeters.  The different values for the Seas tweeter are in brackets.








Rp1 (SEAS 27 TDC)




Circuit Board

Terminal Cup

Port 0.43 cu. ft.

Port 1 cu. ft.

The cost is around $100 each.  So two mains or surrounds $200.  Two surrounds and rear backs $400.

So the total parts cost for a system using 2.5 way mains, MTM center and four bookshelves is $1228 using the Vifa tweeter and $1298 using the SEAS.

Using MTM mains and centers the cost is $1164 using the Vifa tweeter and $1234 using the SEAS tweeter.

So the most expensive system averages $185 per speaker for all parts, and the cheapest $166 per speaker.

So building with MDF, you should be able to build two towers, a center and four surrounds for around the $1500 price point, which is outstanding value.

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