Speaker Cab Impedance Question

[HJW]

I just bought a multimeter to check the impedance of a guitar cab. 2x12's.

Checking it via a cable plugged into the cab, it shows 12.9, so I assume it's a 16 ohm cab. I then pulled one of the speakers and tested the terminals and it read 6.9. So, 8 ohms.

But the speaker is wired to the other one. So does it matter that it is wired to the other speaker ? Or should I disconnect the other speaker and check it again ?

[Robert M. Martinelli]

Hi,
Using a multimeter will allow you to check a speaker's coil DC resistance, not its impedance. Impedance is the result of a complex function which involves both capacitive and inductive reactance, so it's also frequency-dependent.

In a speaker coil, capacitive reactance is practically non-existent, but inductive reactance is significant.

That's the reason why, measuring a coil's DC resistance, you will never get the measurement to match the nominal impedance, you normally get a somewhat lower reading.

OTOH measuring a speaker's coil DC resistance is a good and fast way to state its condition.

As to your question, the two speakers are connected in series, so you can measure each speaker's coil DC resistance without opening the series connection, because, while measuring one, the other speaker does not "appear" at the meter's probes.

Things get different with speakers connected in parallel, in that case you have to open at least one of the connections to get a correct measurement from one of the speakers.

Hope this helps

Best regards

Bob

[HJW]

Hi,
Using a multimeter will allow you to check a speaker's coil DC resistance, not its impedance. Impedance is the result of a complex function which involves both capacitive and inductive reactance, so it's also frequency-dependent.

In a speaker coil, capacitive reactance is practically non-existent, but inductive reactance is significant.

That's the reason why, measuring a coil's DC resistance, you will never get the measurement to match the nominal impedance, you normally get a somewhat lower reading.

OTOH measuring a speaker's coil DC resistance is a good and fast way to state its condition.

As to your question, the two speakers are connected in series, so you can measure each speaker's coil DC resistance without opening the series connection, because, while measuring one, the other speaker does not "appear" at the meter's probes.

Things get different with speakers connected in parallel, in that case you have to open at least one of the connections to get a correct measurement from one of the speakers.

Hope this helps

Best regards

Bob

Ok, thanks Bob. I want a 4 ohm cab, so I'll wire them parallel. I suppose I can do this by simply switching the wires on either speaker ?

[Robert M. Martinelli]

Hi,
be sure to connect the paralleled speakers "in phase", otherwise the cab's response at the lower frequencies will suffer.

HTH

Best regards

Bob

[HJW]

Hi,
be sure to connect the paralleled speakers "in phase", otherwise the cab's response at the lower frequencies will suffer.

HTH

Best regards

Bob

Ok, got it. A friend is on the way over w/ a soldering iron.

Appreciate you taking time to reply Bob.

Hugh

[Enzo]

Two speakers in series require three pieces of wire. One from the amp output hot to the + on speaker A, one from the amplifier ground to the - on speaker B, and a third wire from the - of A to the + of B. probably what you already have.

Wiring two speakers in parallel requires four pieces of wire. The two from the amp go to speaker A + and - terminals, then the two extra pieces go from speaker A to speaker B. + to + and - to -.

[HJW]

Two speakers in series require three pieces of wire. One from the amp output hot to the + on speaker A, one from the amplifier ground to the - on speaker B, and a third wire from the - of A to the + of B. probably what you already have.

Wiring two speakers in parallel requires four pieces of wire. The two from the amp go to speaker A + and - terminals, then the two extra pieces go from speaker A to speaker B. + to + and - to -.

Ok, thanks. I didn't see your post until I rewired the cab, but that's exactly how I did it.

[HJW]

One last question.

I don't want to use the internal speaker. Here is the speaker out on the power amp chassis. It's a phono jack. (click on pic to zoom)



And here is the internal speaker with the blue wire going to the extension speaker jack.....







So, to run a 4 ohm cabinet using the extension jack, can I simply disconnect the wire from the negative terminal of the internal speaker ?

[Enzo]

Internal speaker? This is not a 2x12 cab?


I see tubes. DO NOT operate a tube amp with no speaker connected.

Is this internal speaker 4 ohms? is the amp set up for a 4 ohm load? If you want to use an external cab in place of the internal speaker of a combo, just connect the cab in place of the internal speaker. If there is an extension speaker jack, it has to be parallel with the internal speaker wiring.

Yes, pulling one wire off the speaker will disable it.

[HJW]

Internal speaker? This is not a 2x12 cab?


I see tubes. DO NOT operate a tube amp with no speaker connected.

Is this internal speaker 4 ohms? is the amp set up for a 4 ohm load? If you want to use an external cab in place of the internal speaker of a combo, just connect the cab in place of the internal speaker. If there is an extension speaker jack, it has to be parallel with the internal speaker wiring.

Yes, pulling one wire off the speaker will disable it.

It's an old Sano combo. It used to have 2 8" speakers in it as well, but they are gone. The internal 15" is 8 ohm, and the tech that last looked at it said a 4 ohm load is fine with this amp.





I just want to run the extension cab by itself until I locate replacements for the 8's. I'm just making sure that disconnecting the negative lead to the internal and running the extension cab won't hurt the amp.

[HJW]

Here is a later model.

http://www.gearslutz.com/board/attac...2&d=1244828901

Not sure if you can make out how it's wired.


mine still has the remnants of the 8's wiring



if the 8's were 8 ohm wired in series/parallel, then the original internals configuration may have been 8 ohm and an 8 ohm extension cab was recommended ?

[HJW]

I've got this adapter that will allow me to totally disconnect the internal speaker, but unless I secure the speaker cable to the amp's cab with a bracket there will be a lot of stress on the phono jack.

[HJW]

Internal speaker? This is not a 2x12 cab?


I see tubes. DO NOT operate a tube amp with no speaker connected.

Sorry, I kind of changed course mid-thread.

But, that is my concern, I know not to run it without a load. I just don't know enough about amps to determine if that negative lead coming from the speaker out on the power amp chassis needs to remain hooked to the speaker the positive lead is attached to.

Let me ask it this way. Is the negative lead coming from the speaker output simply a ground that can be cut, and the extension speaker ground takes it's place ? I assume if it does, then the internal speaker is out of the circuit and simply a terminal at that point ?

I think you have already answered my question, yes it does, but I just want to make sure you understand what I was asking. I know I was vague and jumped to a different topic. My apologies for not being clearer.

Hugh

[Steve Conner]

Let me ask it this way. Is the negative lead coming from the speaker output simply a ground that can be cut, and the extension speaker ground takes it's place ?

Since there's only one wire going to the speaker's negative terminal, I think this is a fairly safe bet. Edit: Suggest you try the amp with the extension speaker, and make sure sound comes from both the internal speaker and the extension, before you disconnect the internal one.

[HJW]

Since there's only one wire going to the speaker's negative terminal, I think this is a fairly safe bet.

Ok, thanks Steve. I know the extension jack works, so I guess I can go ahead and cut the negative lead and power it up.