info on stevens compression drivers

[enochRoot]

ok...i know they aren't lansing stock, but you guys seem to be the most knowledgeable in this arena. does anyone have any info at all on the earlier stevens tru-sonic compression drivers, and multicell horns? i am really interested most in sensitivity. any listening impressions would be great too.

[Steve Schell]

Bob Stephens is a towering figure in the history of this type of equipment. He was part of the team that built the Shearer Horn System at MGM Studios in the mid 1930s. Specifically he designed and supervised construction of the multicellular horns. He left MGM in 1938 to found Stephens Tru-sonic.

His multicellular horns were always top quality tar filled units. He may actually be the father of the tar filled horn (due to his MGM work), but I haven't been able to verify this. His company manufactured a large variety of horns with various cell configurations and cutoff frequencies all through the 1940s, 1950s and into the 1960s.

His compression drivers are considered to be very good, roughly the equivalent in performance to Altec and JBL units or very nearly so. The earlier ones were field coil. They all used a spun aluminum diaphragm with integral half roll compliance, methods pioneered by Jim Lansing for the Shearer project. Stephens' phasing plugs are the drilled hole or "pepper pot" style. Hal Cox knew Bob Stephens years ago, and told me that Stephens once told him: "The holes in the phasing plug should really be tapered, but I drill them straight. People seem to like the drivers so I am going to continue to make them that way."

The main problem these days is that no replacement diaphragms are available for the compression drivers. If you have drivers in good condition, they should continue to perform well with attention to proper crossover slopes (at least 12dB/oct.) and cutoff frequency.

[enochRoot]

great...thanks steve! about the sensitivity...are they more on the "professional" level (like jbl, altec, etc) in the 110db region? or more like a "home" level (ie: 100db)? funny about the phasing holes too. knowing a better way to do it, but doing it the easy way as the public doesn't notice the difference. hehehe.

[Steve Schell]

Can't say for sure since I've never measured any, but I would expect the sensitivity of these drivers to be very high. They were definitely high grade professional equipment, which in the good old days was also pressed into service for high quality consumer products.

[enochRoot]

great...thanks! that was my *assumption*, but we all know what happens when one assumes ;-)

[Earl K]

Hi

Here's a real nice series of photos showing a complete disassembly of the Stephens TruSonic model 815HS driver.

:)

[Earl K]

And a few more ;
:p

[Earl K]

- Here are a few pics of a different ( earlier ? ) compression driver / with a small multicell attached.

:)

[Zilch]

Well, many thanks to whoever took those beautiful pics.... :thmbsup:

[enochRoot]

yeah...those are great!

[pelly3s]

i wish someone made some aftermarket parts, i have a mint frame for a coax sitting in my shop and nothing to do with the damn thing

[Steve Schell]

I smile when I see those spun aluminum Stephens diaphragms. They look remarkably similar to the early Lansing Mfg. Co. diaphragms that Jim Lansing reportedly spun with his finger. They are obviously a legacy of Bob Stephens' involvement with Jim Lansing in the Shearer Horn project.

It really is not too difficult to make diaphragms this way. I made some for a ring radiator tweeter project a while back. The duralumin sheet is clamped around the perimeter, over a tool machined to the shape of the desired diaphragm. Spinning in a lathe, an implement is applied to the aluminum on the outer diameter and gradually brought inward to the center. A beautiful diaphragm results, conforming exactly to the tool. A tiny hole needs to be present in either the aluminum or the tool, to let the air out.

Altec Lansing shifted to the hydraulic forming method in 1944 when they incorporated a tangential compliance in the diaphragms for the 288 and 802. These could not be spun. Jim Lansing developed the manufacturing process for them; he was still with Altec at the time. I have a few 288 diaphragm pressings, taken from Jim Lansing's workshop, which were given to me by the Lansing family. They are not quite perfect, displaying a bit of radial puckering around the outside of the dome. Jim obviously perfected the process shortly after, as the early production 288 diaphragms are flawless.

The half roll compliance of the Stephens and Lansing diaphragms, and also the JBL 375/2440, has a single broad low Q resonance which extends the performance of the driver a bit on top. This behavior sounds smooth and inoffensive to the ear. More complex compliances such as tangential and diamond pattern tend to have a greater number of breakup modes which extend the driver response but tend to create metallic sounding colorations. Long live the half roll compliance!

[Alex Lancaster]

:) If I remember correctly, the Hydroforming was done with high pressure Nitrogen; I´ve often wondered if Nitrogen migrated thru the thin Aluminum, and caused the age-embrittlement process We all dread, Aluminum Nitrate is hard and brittle; Ideas?.

[Earl K]

I´ve often wondered if Nitrogen migrated thru the thin Aluminum, and caused the age-embrittlement process We all dread, Aluminum Nitrate is hard and brittle; Ideas?.

Well Professor : Sounds like a good theory . :)

I would think that sometime between the start of WW11 and the beginning of the "Space-Race" , all metallurgy problems/effects like this would have been addressed . OTOH, nobody really cares about us audio idiots and "our appetites for metal" :p