DCR Measurement

[Gerry E.]

Hi:

I had a seller of an 8 ohm speaker measure the DCR and he sent back a photo (see attachment).  I'm not familiar with that meter but it looks like he has it set on the 20K range.  So what's the DCR (.024, .24, 2.4 or 24 ohms)?  Also, whatever the measurement is, would that reading scare you off?  I don't think it matters but the speaker is a tweeter.  Thanks!

Gerry

[Paul Birkeland]

That's 24 Ohms.

I wouldn't be very excited about an 8 Ohm tweeter showing 24 Ohms of DCR.

[Gerry E.]

That's 24 Ohms.

I wouldn't be very excited about an 8 Ohm tweeter showing 24 Ohms of DCR.

Thank you PB!  Regardless of where the decimal point should be, a "24" anything would not be a good sign a good sign for an 8 ohm tweeter.  Interestingly, the other tweeter measured 22 ohms.  I wonder if these measurements indicate that the tweeter diaphragm is in a fragile state and/or on its last legs?  In case your curious, it's a pair of vintage ALTEC 3000B tweeters, known for their fragility!       

Gerry

[Paul Joppa]

I have heard that they are fragile because they are actually microphone diaphragms.

I suppose it's possible that someone found a 22-ohm microphone diaphragm that would fit ...

[Gerry E.]

The plot thickens a bit.  The seller pointed me to a listing for a pair of 3000Bs on Oak Tree Vintage.  They were sold a long time ago but the notes indicate DCR measurements of 19.2 and 19.9 ohms (see attached photo). 

This is just one other example but that makes four of these tweeters with DCR measurements greater than 19.  Again, this makes me wonder if that's what happens when these tweeters age and indicate that they might be going bad.  Of course the seller interpreted this to indicate that his tweeters measure "normal".

Gerry         

[Doc B.]

We can only guess what is going on here. A small increase like that could just be increased resistance of the solder terminations. But a lot of people don't understand how to get good contact on the test point with a test probe to eliminate added resistance at that interface. And the seller is not telling you what his meter reads with the probes touched together, so that that baseline resistance value can be subtracted from the 24 ohm reading to give you a (theoretically, at least) more accurate number.

Obviously it's easier to determine whether the measurement is acceptable if you have two drivers to compare, or if you can measure the driver with your own, known good meter. If you have two tweeters reading high but around the same value you probably have a better chance of them being good than if one reads a lot higher than the other.

So what is the answer? Buy it only if given the option to return it if it doesn't work right.

[Paul Joppa]

Here is a useful thread on another forum:

http://audiokarma.org/forums/index.php?threads/altec-3000g-tweeter-wiring-question.735818/

Note particularly the 3-page article on the second page of the forum, where an original spec of 30 ohms is mentioned (page 3). Also it seems that the post-WWII versions were mylar, not the original 0.0005" aluminum diaphragms, and (separately in the thread) the many measured at closer to 11 ohms. I don't see an explicit claim that the mylar ones were the lower DCR, but at least it's clear that the DCR varied - there were many versions!

[Gerry E.]

Note particularly the 3-page article on the second page of the forum, where an original spec of 30 ohms is mentioned (page 3).

Thank you PJ!  I have been familiar with the various 3000 tweeter models for 15 years and this is the first time I heard about a 30 ohm version.

Gerry   

[Gerry E.]

OK, so I found and this time acquired, another pair of ALTEC 3000 tweeters.  They are the later "H" version of the tweeter with DCR measurements of 9.3 and 9.9 ohms.  I also acquired the matching N3000-F crossovers.  However, I would prefer not to use the crossovers for several reasons including the possibility that they may be out of spec.  Date codes indicate both tweeters and crossovers were manufactured in 1969 which still makes them almost 50(!) years old.

My dilemma now is what capacitor value do I chose to high-pass the tweeters?  This version of the tweeter is "rated" at 8 ohms but the DCR measurements may indicate otherwise.  In my google searches for information, I came across a post from a recognized ALTEC expert.  He wrote that while they are true 8 ohm tweeters they typically measure higher DCR readings.  If I understand it correctly, the impedance that matters is the one at the crossover frequency, less so the actual DCR measurement.

So, what tweeter impedance do I use for the crossover calculator to get the cap value?  8 ohms, 16 ohms or compromise (split the difference) and use 12 ohms?  Since I have found ALTEC 3000 tweeters output to be a bit weak, as compared to my Jensen RP302s, I need to balance getting maximum output from the tweeter vs. making sure they are well protected because they are so fragile (and no longer repairable). I found a post from Joe Roberts and he recommended a 1.5 or 2.0uf cap.  Since 1.5 is safer, I could start with that (or even lower) and go from there.  Thanks.

Gerry                 

[Doc B.]

Why don't you research how to make your own impedance measurement and use an actual measured value? That's going to be the most accurate method.

[Paul Joppa]

I'll add a small technical note here. If you are using just a series capacitor, the impedance is relevant more than two octaves below the crossover frequency.

Two octaves above 3kHz is 12kHz, which is what you get with 1.5uF into an 8 ohm resistance. This is widely said to be a good way to add some sparkle to a wide-range main driver.