Altec 421A impedance question

[spkrman57]

I know that JBL produced many drivers that were actually 10 ohm in spite of being labled 8 ohm or 16 ohm.

I have not run into that situation with Altec products until now. I bought (4) Altec 421A's and measured the DCR of all of them and they were all really close to 4 ohm.

I then ran a impdedance curve (WT2) and discovered they were roughly 5 ohms and rose slowly as frequency increased. They were only 7.5 ohms at 1khz.

Is this something anyone else has run into before?

I'm also making a crossover for a pair of 421A's in Altec 815A dual 15" horns with Altec 288's on 203B multi-cell horns. So I'm thinking of running the 421A's in series for 10 - 12 ohms.

This is for a good friend of mine.

Thanks in advance for any and all comments/info on this subject!

Also, this is my 1st post on the forum.

Regards, Ron

PS: I know some of you know me from other forums.

[spkrman57]

I measured one of the drivers in case anyone is interested:


Re - 4 ohms
Fs - 38 hz
Qms - 4
Qts -.3
Qes - .33
Zmax - 52 ohms
Le - .49 mH
Vas - 8.5 cu ft
Eff - 3.86 %
Sens - 97.8 db/watt/1 m

I ran impedance sweeps on the rest only.

Thanks, Ron

[bowtie427ss]

Just my .02, and i'm no expert. But, with a passive network my main concern with impedance would be what it measures at my target XO freq.

Remember, impedance ratings are almost universally an average of the driver's curve within it's intended BW. An 8 ohm rated speaker will likely have just 2 points(frequencies) where it's impedance is 8 ohms.

IMO, series connecting the woofers is not the best recipe for fidelity. However, the intended amplification may rule out parallel making series a necessity.

[GM]

I then ran a impdedance curve (WT2) and discovered they were roughly 5 ohms and rose slowly as frequency increased. They were only 7.5 ohms at 1khz.

So I'm thinking of running the 421A's in series for 10 - 12 ohms.

I've various 421 measurements from way back along with some more recent ones done by others and they all have somewhat lower Fs, Qts and close enough Vas, Le with ~5.3-5.5 ohms Re, so wondering if yours are either aftermarket re-cones or have some later Altec (GPA?) generic kits.

Le is low enough that running them in series should be fine if matched pairs measure very close in efficiency, otherwise at high power, one will dominate, turning the other into an expensive fuse.

GM

[spkrman57]

to GM and Bowtie427SS.

I bought these from a reputable source and they test close to each other T/S parameter wise.

I'm going to use these in series and use 16 ohms as a starting point. They will be installed in Altec 815A horns and powered by a 45 SET amp (1.8 wpc), so power issues will not be a concern.

The impedance at 500hz (crossover frequency) is approx 6 ohm, so will be approx 12 - 14 ohm in series.

This will be not be home hifi nor PA application so I'll be able to customize a crossover that should work well.

Current configuration is using (4) JBL D130's in the horns and 10 ohms each and in series using a Altec N500D crossover designed for 16 ohms.

Thanks to both of you for your input. You both should know me from Ak and LH forums!

Regards, Ron

[bfish]

Hi Ron,

All the 421s I've checked were in the 6.5 to 7 ohm range. Altec did use some 4 ohm kits in the 422 and 423 which wouldn't be unthinkable to find in some 421s. Cone # 21466.

[spkrman57]

Thanks bfish for chiming in here!

The following codes are what I have:

Cone: 2153 3
Frame 391147
"AL/94" on frame edge also.

I have never seen the T/S parameters for the Altec 421A that made sense vs the ones I took with these drivers.

This will be a interesting invenstigation as to what is actually going on here. The only thing I could come up with is someone reconed these with 4 ohm voice coils and everything else is stock.

Only question is: what voice coil was used????

I still plan on installing these in my friends system and see if they do what we want them to!

Regards, Ron

Hi Ron,

All the 421s I've checked were in the 6.5 to 7 ohm range. Altec did use some 4 ohm kits in the 422 and 423 which wouldn't be unthinkable to find in some 421s. Cone # 21466.

[bfish]

You're welcome Ron.

21533 is certainly the correct number format for a mid-late 70s Altec kit, the '75 reference has numbers all around it. Unfortunately, the next available reference is many years later, after they changed the numbering format.

Would be interesting to know if they had the earlier aluminum coil forms, or the Kapton forms of the 8HII.

[spkrman57]

You're welcome Ron.

21533 is certainly the correct number format for a mid-late 70s Altec kit, the '75 reference has numbers all around it. Unfortunately, the next available reference is many years later, after they changed the numbering format.

Would be interesting to know if they had the earlier aluminum coil forms, or the Kapton forms of the 8HII.

I don't know anyway of finding out what the coil forms are made of without tearing them apart which won't happen unless one of them has a failure of some sort.

I'll see what the previous owner knows about the past history of these units if possible.

Regards, Ron

[GM]

Frame 391147

Hmm, late 1971 vintage and being a MI driver a later re-cone with a generic 'one size fits all' 421 kit seems a distinct possibility, so hopefully the previous owner can shed more light on these.

GM