Speaker OHM question

[Guest HRB853370]

Ok, I know that an OHM is a unit of resistance. When connecting an extension speaker cabinet to an amplifier, does the load resistance increase, therefore do I need to go out of the amplifier jack that says 16 OHM, rather than 8 OHM? And does it matter if the amplifier I am going out of has two speakers, versus one?

[kbp810]

The resistance of the cabinet depends on the ohm rating and wiring style of the speakers - for example, two 8 ohm speakers wired in series will give you a 16ohm load (e.g. you would want to use the 16 ohm output from the amp); while the same two 8 ohm wires in parallel would give you a 4ohm load (e.g. you would want to use the 4 ohm output from the amp).

 

As far as resistance is concerned, the amount of speakers connected will only be impacted by the same factors in the above example; however, the power of the amp will be divided across the speakers. So essentially there will be less power per speaker, but the increase in the amount of air being pushed creates an increase in perceived volume; which is why generally speaking 2x12 will sound louder than a 1x12 under a similar power load. (thats just a generalization though, there are other factors that can come into play)

[kbp810]

While on the subject, I figured I might as well add a few quick tidbits as to why it matters:

 

The output transformer in an amp has a primary and secondary that are intended to be (at least somewhat) balanced. I say "as least somewhat", because the rating of a speaker is rarely it's true value, for example a speaker with a DCIR of 5.9 ohms would still be labled as and considered to be an 8ohm speaker.

 

As a generalized example, let's consider an amp with two 6V6's and a single 8ohm output going to an 8ohm speaker. The pair of 6v6's in push/pull will want an 8k primary load... so our output transformer would have an 8k primary and an 8ohm secondary.

 

If we disconnect the 8ohm speaker and put in a 4ohm speaker instead... our reflected primary load is now also halved. So we have a 4k primary and 4ohm secondary... the 4k load on the 6V6's is more than they like so we are now pushing them harder than they are intended to be pushed. The tubes will likely fail soon (though the amp might very well sound louder and awesome up until they do!)... the Output transformer will also likely run hot and may potentially fail as well.

 

If you disconnected the 8 ohm speaker and put in a 16 ohm speaker, we have the opposite effect... the primary would now be 16k. This would run the tubes cold, meaning less volume and likely wouldn't sound very good... while the tubes are generally not going to be harmed in this scenario, this can still be hard on the OT. (though generally speaking in the above two scenarios it's safer to have used the 16 ohm instead of the 4 ohm load)

 

An output transformer with multiple secondary outputs (4, 8, or 16 ohm outputs), has multiple windings... so if we use the same pair of 6v6 scenario above this means that the OT will still reflect an 8k load so long as you have a 4ohm on the 4ohm tap, an 8ohm on the 8ohm tap, or a 16ohm on the 16ohm tap. If you connected a 16 ohm on the 4 ohm tap you would have the same sort of problem as before, where as the primary reflected load would be altered, and a 16ohm on a 4ohm tap would be very bad! If you used more than one output at a time, again you would run into the same problem of changing the primary.

[Steiner]

Additionally, every speaker output from an amp (that I've encountered) is wired in parallel. So, if you're adding an extension cab to a combo amp, the impedance changes. So if you connect a 16ohm extension cab to a 16ohm combo (with speakers from both plugged in), the impedance drops to 8ohms.

 

I've read that generally playing 1 tick away (e.g., an 8 ohm speaker at 4 or 16 ohm setting) is ok and I've gotten away with it in the past. The louder and longer you play a mis-matched amp the hotter your tranny gets. Sooner or later you're bound to "let the smoke out."

[GuitArtMan]

Ok, I know that an OHM is a unit of resistance. When connecting an extension speaker cabinet to an amplifier, does the load resistance increase, therefore do I need to go out of the amplifier jack that says 16 OHM, rather than 8 OHM? And does it matter if the amplifier I am going out of has two speakers, versus one?

If you are connecting an external speaker to a combo with an internal speaker, the outputs are wired in parallel so the load actually decreases. For example, 16 ohm internal speaker, 16 ohm external speaker, 8 ohms total so set you amp for 8 ohms. If your amp as an 8 ohm internal speaker and the extension cabinet is 8ohms, that's 4 ohms total so set your amp for 4 ohms.

[Guest HRB853370]

If you are connecting an external speaker to a combo with an internal speaker, the outputs are wired in parallel so the load actually decreases. For example, 16 ohm internal speaker, 16 ohm external speaker, 8 ohms total so set you amp for 8 ohms. If your amp as an 8 ohm internal speaker and the extension cabinet is 8ohms, that's 4 ohms total so set your amp for 4 ohms.

 

ok, this is all helpful. So lets test myself! If I have a 30 watt amps, one with single 12 inch at 8 ohms, if I go into a single 12" extension cabinet, the ohmage will actually be 4 ohms?

 

And if my combo amp has 2x12 instead of 1x12 (which makes it a 16 ohm output), if I go into a single 12" ext cab, the ohmage will be 8 ohms?

 

Did I pass?

[mars_hall]

Ok, I know that an OHM is a unit of resistance. When connecting an extension speaker cabinet to an amplifier, does the load resistance increase, therefore do I need to go out of the amplifier jack that says 16 OHM, rather than 8 OHM? And does it matter if the amplifier I am going out of has two speakers, versus one?

 

Question 1) Tell us the amp model to get an exact answer since output labeling varies.

 

On typical amplifier speaker outputs, adding an extra extension speaker will actually decrease (not increase) the total resistance seen by the amplifier, since it puts the sets of speakers in parallel. This combination impedance is what is typically seen on most amplifier output labels. Therefore you will want to put the ext cabinet in the 8 ohm output. This assumes that the internal speaker impedance matches the external and both internal and external are 16 ohms.

 

Question 2) Depends on what "matter" means. If the amp is a Marshall and has two speakers, the internal speaker impedance seen by the amp is 8 ohms since both 16 ohm speakers are typically wired in parallel. Look at the internal speaker lugs. On each speaker there will be a painted dot or + near the speaker lugs.

 

If the wire from the dot/+ from one of the speakers is connected directly to the dot/+ of the other speaker, then they are in parallel and the impedance seen by the amp is 1/2 that seen on a single speaker's label.

 

If the dot/+ lug of one speaker does not directly connect to the dot/+ lug of the second speaker then the speakers are probably in series and the impedance seen by the amplifier is the sum of the impedance seen on the speaker labels.

 

Hope this is clear enough but you will have to focus on what exactly has been said.

[kbp810]

ok, this is all helpful. So lets test myself! If I have a 30 watt amps, one with single 12 inch at 8 ohms, if I go into a single 12" extension cabinet, the ohmage will actually be 4 ohms?

 

And if my combo amp has 2x12 instead of 1x12 (which makes it a 16 ohm output), if I go into a single 12" ext cab, the ohmage will be 8 ohms?

 

Did I pass?

For #1 above - if the internal speaker is 8 ohms, and the external speaker is also 8ohms, and then considering they will be in parralel, yes you would have a 4ohm load.

 

For #2 above - would need to know the ohm rating of each speaker, and then the manner in which they are connected to know for sure. If your 2x12 had two 8ohm speakers wired in series they would be 16 ohms, and then add an external speaker cabinet that is also a 16ohm load; and again considering they will be in parralel, then you would indeed have an 8ohm load.

[byrdland]

ok, this is all helpful. So lets test myself! If I have a 30 watt amps, one with single 12 inch at 8 ohms, if I go into a single 12" extension cabinet, the ohmage will actually be 4 ohms?

 

And if my combo amp has 2x12 instead of 1x12 (which makes it a 16 ohm output), if I go into a single 12" ext cab, the ohmage will be 8 ohms?

 

Did I pass?

The combo amp (with two 12's would be 16 ohms if the two speakers are each 8 ohms and wired in series. If they are each 8 ohms and wired parallel, the load on the amp is already at 4 ohms. I have a Fender Super Twin with two 12 inch speakers at 8 ohms each wired parallel. That is a 4 ohm load the amp sees. I do not hook up any other speakers to the amp. (Don't really need to anyway). 4 ohms is low enough.

[kbp810]

This site might help explain it better - they have visual diagrams and calculators

 

Wiring Calculator

[Guest HRB853370]

The combo amp (with two 12's would be 16 ohms if the two speakers are each 8 ohms and wired in series. If they are each 8 ohms and wired parallel, the load on the amp is already at 4 ohms. I have a Fender Super Twin with two 12 inch speakers at 8 ohms each wired parallel. That is a 4 ohm load the amp sees. I do not hook up any other speakers to the amp. (Don't really need to anyway). 4 ohms is low enough.

 

I will refer to that wiring diagram, however let me simplify my question with an example: I have a Peavey C 30 with 1x12 rated at 8 ohms. On the back, next to the extension jack, it says 16 ohm min load. My extension cabinet has a 1x12 8 ohm speaker in it. If I hook up this extension cabinet, which will be in parallel, the resultant resistance will be 4 ohms, right? Is this safe for the amp?

[mars_hall]

From what I read in the Peavey manual for the C30, the external speaker must be 16 ohms

[lilkuku]

[byrdland]

I will refer to that wiring diagram, however let me simplify my question with an example: I have a Peavey C 30 with 1x12 rated at 8 ohms. On the back, next to the extension jack, it says 16 ohm min load. My extension cabinet has a 1x12 8 ohm speaker in it. If I hook up this extension cabinet, which will be in parallel, the resultant resistance will be 4 ohms, right? Is this safe for the amp?

 

It sounds like Peavey doesn't want you to take the total load on that particular amp lower than 8 ohms. If the original speaker was 16 ohms, and the "minimum load" on the extension speaker is 16 ohms, that would be an 8 ohm load on the amp. If the speaker in the amp is 8 ohms, you already have an 8 ohm load on the amp. I know people that run Classic 30's into two twelve cabinets at 4 ohms. It works, and I guess they may hear a little more gain. i believe an amp tech might tell you that running an amp in ways it wasn't built for may reduce the life of your output transformer. I've been told by an excellent amp builder that doing things like pulling a couple of tubes out of a high power amp with 4 output tubes (to run it at lower output) will also reduce the life of the OT.