Altec small format diaphragms: Aluminum vs. Symbiotik



I've been playing around with Altec drivers for almost 20 years and have never questioned the dictum of using only aluminum diaphragms for high-fidelity applications until now.



Altec Aluminum Diaphragms


Altec 20275
Aluminum voice coil
DCR = 9 ohms

The 16 ohm Altec 20275 is the original diaphragm fitted to my cherished green hammertone 802D. This is the Altec diaphragm I am most familiar with and to my ears its tonal balance favors the midrange more than the later 34852, which is known for better high frequency extension.

I speculate that late 40s to 50s wrinkle charcoal gray/brown and late 50s to 60s hammertone green 802s, 804As and 806As command premium prices IF they are fitted with original and intact 20275s, which are now unobtainium. By early to late 70s, black 802s and 806s were fitted with either the 16 ohm 34852 or the 8 ohm 34647.

Altec 34852
Copper voice coil
DCR = 12.5 ohms

My Altec 605B Duplex should have been originally equipped with a 20275 but by the time I acquired it, this 34852 was already fitted. Maybe that's why I still got it cheap ;)

As previously noted in my 605B experiments, I hear a significant sonic difference between a 34852 and a 20275, so much so that I cannot recommend their use as a stereo pair. Jeff Markwart has more 20275 vs. 34852 info if you scroll down to the bottom of this page

GPA 34852
Copper voice coil
DCR = 11.5 ohms

AFAIK, Great Plains Audio is the only source for Altec diaphragms made to original Altec specifications. Even if they have a website, they prefer to do business the old fashioned way. I always call Bill Hanuschak to place an order and never had problems.

I haven't had a chance to compare this GPA 34852 to the Altec 34852 because the 605B Duplex was left behind in the attic;( But there's no doubt that this GPA is the real thing - very dynamic, with clean and clear harmonic overtones up the wazoo!

Altec Clone


Chinese Aluminum Diaphragm Clone
Copper voice coil
DCR = 11.9 ohms

For kicks, I made an offer to one of the Chinese vendors on eBay and got this pair of 16 ohm diaphragms for about $24/pair, shipped. These clones will tide you over in a pinch. However, even a casual comparison to an Altec original or GPA diaphragm revealed smeared harmonic overtones and compressed dynamics.

Spot the fake



Left picture (clockwise from left) - older version Altec diaphragms used screw terminals on a raised platform, later Altecs and GPAs have tabs, Chinese clones do not have a raised platform for the screw terminals. 

Right picture - The raised platform in the Altec uses tapped machine screws whereas in the Chinese copy the screws are self-tapping.


Altec Symbiotik


I recently acquired a couple of unloved 70s era 802 and 808 drivers, some fitted with Symbiotiks. 

Two abused Altec diaphragms:
Symbiotik on the left and aluminum on the right

The Symbiotik was Altec's initial response to the 60s demand for a higher power handling diaphragm. As pictured above, due to the rigidity of the aluminum suspension, it can shatter when overdriven. I can only surmise that the Symbiotik  diaphragm, which uses a polyimide/mylar type suspension, evolved from a research and development project at LTV by John Hilliard and Jonas Renkus.


Early + Later Symbiotik diaphragms

In early Symbiotiks, the abrupt 90 degree angle voice coil termination (left) also caused failures in the field due to metal fatigue. I repaired this early Symbiotik by carefully flowing solder between the broken ends with a low temperature soldering iron. This issue was rectified in later production Symbiotiks (right). 

Uncovered: Altec > Emilar


Top = early Emilar with brown mylar suspension
Bottom left = later Emilar with gray mylar suspension
Bottom right = Altec Symbiotik

I have written favorably about Emilar and Renkus-Heinz compression drivers, and noted that their extreme top end was a bit reticent compared to Altec drivers with aluminum diaphragms. Let's see how the earlier Symbiotik technology stacks up.

Top = later Altec Symbiotik with tabs and
improved termination
DCR = 6.7 ohms
Bottom = early Altec Symbiotik with
screw terminals
DCR = 7.5 ohms

In retrospect, Altec aficionados may have been hasty and/or unkind in their initial reception of the Symbiotik. Even if the Symbiotik rolls off earlier than its aluminum brethren, it still reproduces proper harmonic overtones. It sounds very sweet, airy and open, with none of the smearing and compression I hear from the Asian clones. A Symbiotik equipped 802/808 sounds more organic than its descendants from Emilar and Renkus-Heinz. To my ears, the Symbiotic diaphragm has a niche in Hi-Fi and may well be the antidote to those who find the aluminum diaphragm a bit aggressive. 

Unfortunately, there are no modern replacements for Symbiotik diaphragms. You'll have to find original units that did not suffer catastrophic meltdowns. Although it's just a hunch, the Radian replacement diaphragm for Altecs should have Symbiotik DNA because of the Emilar/Plus One connection.

Octave RTA by Onyx


Since affordable audio measurement apps have become available, I got this $5 app for my iPhone as suggested by my buddy J-Rob.

Test set-up

The Altec 414A housed in a 3.5 cu. ft. repurposed Madrid (861) bass-reflex cabinet is driven full-range, J-Rob style, while the Altec 32A metal horn + driver combo is protected by the crossover + EQ below.

Crossover + EQ

I was quite surprised to see pretty decent RTA results since I've been building/tweaking crossovers for over 20 years without a computer simulation program or an RTA.

Altec 802-8D + GPA 34852

Altec 808-8A + Chinese 16 ohm diaphragm

Altec 808-8A + Symbiotic

Emilar EA175-16 (alnico)

Emilar EC175-8 (ferrite)

These graphs pretty much represent how these driver/diaphragm combinations perform except the Chinese clone which looked good but sounded rolled off, compressed and smeared as mentioned above. Unlike human ears, an RTA will measure sound waves but will not discern unmusical harmonic overtones. Even my wife thought the Chinese diaphragm sounded muffled. Although she argues that if I can afford only a $12 diaphragm, I better be happy with it ;(

It goes to show that it is much easier to satisfy audio measuring equipment than human ears. And I'm lucky to be blessed with a tolerant wife ;)


Comments

  1. I tried a pair of the Radian diaphragms on some 15" Urei co-axials which had JBL HF drivers, they were a big improvement over the stock titanium JBL diaphragms. Probably worth the price of admission just to compare them to the rest of what's available in the current market...

    Roscoe

    ReplyDelete
    Replies
    1. That's good to hear Roscoe! Ti and Be diaphragms are controversial to those weaned on classic Altec/JBL/WE designs ;)

      My only problem with the Radian 1228-8/16 (Altec replacement) is, they cost the same as GPA aluminum diaphragms ;(

      JE

      Delete

Post a Comment