monob

MONO OB

As suggested by Brian Clark monaural playback is best experienced from a less directional speaker. Since I do not have the woodworking skills to build a Lowther TP-1 clone or budget for the driver while doing spring cleaning in the basement I found three 14" x 72" dismantled 3/4" plywood shelving units destined for the dumpster. I cut them in half giving me six 14" x 36" panels enough material for a pair of 42"W x 36"H open baffles with folding wings that can stand on its own and fold-a-way for easy storage. In practice an OB acts in the room like pseudo dipoles therefore less directional than the Altec 2-way.

The Siemens Bayern is my inspiration for this folding open baffle. I found it at a Japanese website years ago but do not have the dimensions.

I was going to use piano style hinges but the longest I found at Lowe's was a 30" flimsy looking one selling for $8 each so I opted for cheaper but heavier duty generic cabinet hinges and used packing tape to fill the gap. The center point of the speaker is 21" from the floor. My initial listening was done without the 7" x 13" top plate and the sound was congested in the midrange and lacking in definition in the bass.

Folding the wings further back decreases bass response which reinforces my earlier finding that a tall and narrow baffle looks good but does not deliver the goods. Baffle width is crucial for decent bass response. In the early days Altec engineers advocated adding wings to the 825 VOT cabinet and come to think of when I owned Magnepan SMGas in the 80s the manual also suggested its use to extend the bass frequencies.

Altec 605B Duplex

Front baffle donated by a vintage EV cabinet with added "wings".

The Altec 604/ 605 series has significantly gone up in price but there are still sleepers out there. I'll start with the following vintage 12" co-axes worthy of consideration for not much cash outlay just to encourage people to be adventurous.

Jensen DL220 - late model ferrite magnet equipped Jensen with a variant of the RP302 horn.

A bit hot in the top end for my taste even with judicious adjustment of the L-pad. Excellent bass extension the equal of the Altec 414 in 614 cabinets. But there is a narrow "suck out" in the upper midrange which leads me to believe that the woofer to tweeter integration can be improved. I have not found a way to access the crossover maybe it requires carefully peeling away the metal decal at the back of the frame. Sounds best driven by a classic topology amp like the 6C6-SE2A3. Those who are used to modern speakers will probably like the tonal balance and presentation of this driver over all the others in this survey.

University 6201 - phenolic diaphragm horn tweeter with alnico magnets.

I played around with crossover point ran the woofer full range but realized stock form is best! Those guys at University knew how to maximize the potential of the drivers in this unit. This is a sleeper with a "sweet" phenolic horn tweeter (4401) extended enough in the top end to make late 40s-50s Duke, Basie, Goodman and young Sinatra recordings come alive. There's a hint of vintage "phenolic" coloration when driven by the 6C6-SE2A3 amp which keeps it from the Altec class ameliorated by using a wider bandwidth amp like my SE2A3dx. Bass is slightly less extended than the DL220 but has good definition.

Lafayette SK58

I modified the crossover slightly to get rid of some upper midrange harshness caused by the lower limit of the cone tweeter by adding another 4uf cap in series with the stock 4uf paper in oil can type. This shifted the crossover an octave higher. The presentation is similar to the DL220 without the hot top end and less "snap" due to the cone tweeter.

Unbranded - alnico magnet with cone tweeter, maybe a Quam or Utah OEM?

Very good balance tonally with an almost seamless transition between woofer and cone tweeter but good only before ~ 10 khz and the response drops drastically. Really fine midrange, bass is tight and fast but has the least extension in this syrvey. At some point I might try using a phenolic cone horn tweeter like Jensen RP103 to add some air or maybe replace the tweeter with a Jensen P35. However for listening exclusively to 78 rpm this is perfect!

After several years of listening to various vintage drivers I can safely say that if coherence is a priority go for a full range driver or a 2-way co-ax with a cone tweeter for obvious reasons - much easier to mate two cones. A horn loaded tweeter gives a better sense of dynamic contrasts and the edge in transient response but be prepared to work on interfacing with the direct radiator woofer. It is always a compromise.......

Associated equipment used: Rek O Kut B12H, viscous damped tonearm, GE RPX and VRII cartridges and JEL mono preamp with variable EQ.

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