How to determine speaker impedance?

January 10, 2009 11:01 AM

Vintage GUITAR AMP/SPEAKER Impedance - A friend has a (1) Wards Airline GVC-9035 combo head w/speaker (Jensen) and a (2) Mid-70's Custom 100 speaker cabinet w/2 speakers (Jensen), but he hasn't been able to find out what the impedance of the speakers are & needs help .....

Here's the deal: He want's to be able to determine the impedance (4-8-16 ohm) of the Wards cabinet, in order to match it to an amp's output impedance, but the Jensen speakers' aren't individually labeled so he can't calculate it by determining if they're wired in series or parallel. Simply putting a volt-ohmeter across the leads won't cut it, right? He's wondering if anyone knows what the output impedance of a Custom 100 head is, presuming that the Custom 100 speaker cabinets would have matched impedance.

The Ward's combo head/speaker cabinet he wants to install some snappy transformer upgrade to (and can apparently order it pre-set or can adjust for 4-8-16 ohm), but has a similar problem in that the speaker (which appears may have been replaced) has no markings to indicate what it is. Online search for schematics yields some that show the amp circuit and output impedance, but not for his model.

Any suggestions? Thanks!

posted by Pressed Rat to technology (8 comments total)

Measure the DC resistance with a multimeter (across the speaker terminals with nothing else attached). Multiply the value by 1.3. Then pick the nearest of those impedance values (4, 8 or 16). Should be close enough.
posted by le morte de bea arthur at 11:08 AM on January 10 [1 favorite]

Just disconnect the speaker, and put your multimeter across the two terminals. Some speakers will have slide-off terminal connectors, otherwise disconnect it. When you reconnect, it's good form to solder and use shrink-wrap insulation.

Most likely you will get 6 ohms, as many speakers are set between 4 and 8 so they can be used at both impedances.
posted by StickyCarpet at 11:10 AM on January 10

le morte de bea arthur, why multiply by 1.3? Maybe that explains why I usually read 6 ohms.
posted by StickyCarpet at 11:12 AM on January 10

Actually, the 1st sentence in the 2nd paragraph should say Custom 100 cabinet, instead of Wards, not that that makes any difference.....my screw up.
posted by Pressed Rat at 11:16 AM on January 10

StickyCarpet, AC impedance is not the same as DC resistance, although for speakers the two are usually pretty much proportional. I don't have a calculation for you unfortunately; I just found it in one of my old notebooks as a 'rule of thumb'.
posted by le morte de bea arthur at 11:21 AM on January 10

It might say on the back of the speaker itself, possibly on a sticker affixed to the magnet.
posted by rhizome at 12:14 PM on January 10

This guy seems to agree with le morte de bea arthur's figure of 1.3. This factor comes from the skin effect, for the curious. It's relatively easy to calculate this ratio from first principles in a theoretical situation (i.e., textbook problem), but much less practical to do it in most real-life situations, which is why such rules-of-thumb are so handy.

Note that the concept of an "AC resistance" is itself just an approximation, as it ignores things like the inductance of the coil (which means the impedance is a complex number) and the dependence of inductance on a number of things, including frequency. That doesn't make it any less a useful concept, though.
posted by musicinmybrain at 4:20 PM on January 10

nthing the 1.3 factor.

old tube amps are pretty rugged. don't play it without a speaker, it'll blow the output tranny, but it's pretty safe to plug and play.

most amps have an "Ext. Speaker" jack. Plugging an additional speaker drops the impedence (sq rt 2 or some such. been a long time), sometimes drastically, and it's pretty much factored into the circuit.

check it, use it, enjoy it.
posted by KenManiac at 5:17 PM on January 10