Amp/Speaker/Ohmage question

[hamerhead]

Say you have a speaker cabinet of unknown ohmage, and you have an amp head with a 4/8/16 ohm selector switch.

Which setting is safest? And why?

[cynic]

The setting that's closest to the meter reading when measuring the resistance of the speakers/cab?

"The impedance (4 OHM, 8 OHM and 16 OHM) is the resistance the amplifier (power amp) has to overcome to produce sound energy. Amps are designed to run "efficiently" at one impedance or another. Obviously, if an amp provides the same voltage, no matter what, a larger impedance MAY mean lower drive current to the speakers. As speakers are primarily current driven devices, an 8 OHM amp driving 16 OHMS MAY be "softer" but ONLY due to the fact that the current flowing in the speaker/amp circuit is halved. 
Conversely, an 8 OHM amp driving a 4 OHM load MAY be louder but that's due to the fact that the amp/speaker circuit has twice the current (the ability to do real work) flowing. This impedance mis-match MAY destroy your power amp. MORE CURRENT FLOWING = MORE POWER REQUIRED. LOWER IMPEDANCE = GREATER POWER REQUIREMENTS. Amps spec'd at 8 ohms, running 4 OHM loads have to dissipate more power in the amp... if it's not made to do that it can destroy the amp."

[seeker]

Note what cynic means by "closest".   If you measure the resistance of a 16 "ohm" cab, it will be actually be a bit less; 14.7 or so. 

(The rest is reactance, but it's all rolled into impedence, blah, blah, blah.)    

[hamerhead]

Say you have no idea what the ohmage is, no meter to check it with, the label has fallen off the speaker, and the cabinet is hermetically sealed with no visible fasteners to allow it to come apart in any way.

Then which setting is safest?

 

ETA: by safest, I mean least likely to cause damage to the amp.

[bubs_42]

12 hours ago, hamerhead said:

Say you have a speaker cabinet of unknown ohmage, and you have an amp head with a 4/8/16 ohm selector switch.

Which setting is safest? And why?

4ohms, you want the ohms on the amp to be lower than the ohms of the speaker. OR you lazy (*^(& you can just use a meter on the speaker and just know what it is. Lower Ohms on a speaker than the amp causing the amp to push harder than it needs to, kind of like putting your finger over a water tap. You create to much resistance and poof there goes your transformer. 

[crunchee]

1 hour ago, hamerhead said:

Say you have no idea what the ohmage is, no meter to check it with, the label has fallen off the speaker, and the cabinet is hermetically sealed with no visible fasteners to allow it to come apart in any way.

Then which setting is safest?

 

ETA: by safest, I mean least likely to cause damage to the amp.

How would you know if the speaker is actually hooked up to the jack, if you can't get at it?  Check first with a meter to make sure there is a load of some type on the jack, then test it with something cheap and solid state...it might not even have to be an amp, maybe a radio will do.  Don't use anything expensive and/or with tubes to plug it into, if you don't have to, at least at first.  Any good shop that works with speakers, amps, and/or sound systems should have the equipment to be able to check and test this speaker.

[Jeff R]

A budget multimeter is a metric shitload cheaper and easier to source than finding a good amp tech and getting a shat amp serviced.

The present version of the cheap meter I use weekly for over 20 years now (purchased in the 1990s at Radio Shack) can be currently found for $7 at Home Depot.

[hamerhead]

Say I were on a desert island in the middle of nowhere with a speaker cabinet and no access to anything but an amp head with a ohm selector switch.

[Thundersteel]

As Bubs said, use the 4-ohm position. That way, if the cabinet is 4 ohms, then you're all set. If the cab is 8 or 16 ohms, then the amp won't be overloaded.

[Steve Haynie]

If I had $7 on desert island I would use it to buy bottled water instead of an ohm meter. 

[cynic]

4 hours ago, hamerhead said:

Say I were on a desert island in the middle of nowhere with a speaker cabinet and no access to anything but an amp head with a ohm selector switch.

16ohm setting...you're going to need a fire.

[bubs_42]

Look guys, he is A. Just that stupid or B. Not willing to except that i'm right. I'm torn on which one it really is, but i'm leaning toward the latter. I'm feeling nice today.  

[hamerhead]

There. I 'liked' your answer. Happy?

It was a legitimate question. I thought a lower ohmage speaker would cause a higher ohmage amp to run hot.

[django49]

Get a meter. Resistance is futile! Ohm my Gawd!

[tomteriffic]

Yep, spend the seven bucks on a goo-roo.  Make sure to hire a full-bore 16 OM goo-roo and not one of those chintzy 4 OM ones you find on eBay.

 

[Dave Scepter]

https://m.harborfreight.com/7-function-digital-multimeter-63604.html?utm_referrer=direct%2Fnot provided

[hamerhead]

I have a decent meter but don't always have it in my pocket. Occasionally an available cabinet isn't marked and taking it apart isn't an option (like in the middle of a gig). Knowing the best setting to use in that situation is helpful.

Thank you all for being so helpful.

 

Except Shawn.

[bubs_42]

29 minutes ago, hamerhead said:

I have a decent meter but don't always have it in my pocket. Occasionally an available cabinet isn't marked and taking it apart isn't an option (like in the middle of a gig). Knowing the best setting to use in that situation is helpful.

Thank you all for being so helpful.

 

Except Shawn.

Fine, you can forget about all that Go Fund Me Money for a meter. 

[hamerhead]

Really? A Go Fund Me? How nice! I take back all those aweful things I was saying about you.

Now cough up the cash.

[django49]

1 hour ago, hamerhead said:

Really? A Go Fund Me? How nice! I take back all those aweful things I was saying about you.

Now cough up the cash.

I am a skeptic. Typos tend to occur with Swawn. Are you sure that was not a Go Fu** Me fund?

[tomteriffic]

With a meter in your pocket, you just need to plug a short speaker cable into the cabinet and take a resistance reading.  It'll be less than the actual speaker impedance but as noted before, it'll be close (somewhat lower) to a nominal impedance.  The series resistance added by the cable will be negligable.