Skip to main content

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 ;)


  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...


    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 ;(



Post a Comment

Popular posts from this blog

Altec 2-way horn system redux

Truth be told, I was never fully satisfied with the performance of the Altec 2-way which is why I never uploaded the old article in the Arkiv.
Let's recap how the 2-way horn system evolved...

The project was conceived in mid 1998 because I needed a monitor system that was more efficient than the OB/755 so I could listen to SE amps with 2 watts output and below.
I commissioned my cabinet maker neighbor to build 614 bass reflex cabinets and loaded them with Altec 419A 12" drivers + 811 horns (early version/heavy sand cast) and 804A compression drivers gathered from hamfests and radio shows. Crossover was a textbook 1st order/6dB/octave hinged at 900 hz and the horn padded down to match the sensitivity of the low frequency driver.

Eventually I acquired proper 12" woofers - Altec 414As, which lack the upper midrange nasties of the 419As. The 414As have great midrange quality even if they lack the bass end of their larger 416/515 brethren.

802D compression drivers replaced t…

2017 Update: Hi-Fi Room

One year progress report

Garrard 301 + Ortofon RMG 309/SPU GME or SME 3012 + Denon DL103 Altec 4722 MC step-up
Digital line level sources processed  through UTC A20 transformers

JEL Stereo Preamp DX
JEL Stereo SE2A3 DX amp with Tango NY15S OPTs
Altec 2-way Altec 414As in 3.5 cf bass reflex enclosures + Altec 32A horns with 802D compression drivers
Mono Neat P58H 4 speed idler turntable + Velvet Touch viscous damped tonearm with Denon DL102 or GE RPX cartridges

je2a3 mono integrated amp

Stereo > Mono Line Level Mixer

Mixing line level stereophonic signals into full dimensional monophonic ;)
Here's a DIY project for mono aficionados that's so basic, I should have uploaded it many years ago. But other things got in the way.

Just like an MC step-up, these are entirely passive devices. 

The simplest way to convert stereo to mono is to connect left and right channels via a Y connector. But there's a more elegant way than just shorting two channels.

Resistors can blend two signals into one better by...

...using Allen-Bradley carbon composition for warmer sound ;)
We can end the mixing quest here but...

...the most elegant way of mixing two channels as was done in the studios by audio pros of yesteryears was through transformers.

This single UTC A-20 transformer was in a box lot of tubes and parts I picked up at a radio show. It's a high quality transformer designed for mic, mixer or line matching applications. Instead of trying to find a mate for MC step-up duty, I repurposed it for a g…