Any electronics types out there?... impedance question...

[boogiemon]

i just had a chat w/customer service at MB, & was told that running my amp w/a 8ohm speaker in the 8ohm jack & another 8ohm speaker plugged into one of the 4ohm jacks will cause the amp to run too hot.

Why is this so when the manual states that using a single 8ohm speaker in either the 8 or 4 ohm jacks is ok?

i know that running a pair of 8's each plugged into the 4ohm jacks is the "correct" config, i'm just wondering why the 8 & 4 ohm jacks can't be used at the same time? For that matter, what about running 3 speakers at once, all 8 ohms, one in the 8 ohm jack & the other two in the 4 ohm jacks?

thnx.

 

[chennie]

I don't have proof for I don"t have the schematic at hand. but assuming that the customer service is right. (and I firmly believe they are right) the following if one uses backward analysis:

The 2x4 ohm output are wired internally parallel, while the 1x8ohm output is independent wired from the 4 ohm outputs.

This gives the following consequences: One can jack up a 4 ohm cab to the 4 ohm output, two 8 ohm jackets to the 2x4ohm outputs or a 8 ohm jacket in the 8 ohm output without having mismatches.

Now for your question: If the mentioned internal wiring is correct that means that you have a mismatch when you hook up the 4 and 8 ohm outputs with both an 8 ohm jacket which is not safe. I also believe that the connecting all the three outputs will cause problems in this case. But I am not sure, because the mismatch rules kinda differences for amps.

 

[oyster]

WTF??? :wink:

 

[boogiemon]

Thanks for trying, but this doesn't really help.

What is it exactly that makes a 8s <-> 8j & 8s <-> 4j config unsafe?

(s=speaker, j=jack)

Now for your question: If the mentioned internal wiring is correct that means that you have a mismatch when you hook up the 4 and 8 ohm outputs with both an 8 ohm jacket which is not safe. I also believe that the connecting all the three outputs will cause problems in this case. But I am not sure, because the mismatch rules kinda differences for amps.

 

[domct203]

Don't think of it a matching the jack to the speaker. The speaker jacks are TOTAL load (impedence). If you have one 8 Ohm cab then the load is 8 Ohms, use the 8 Ohm jack. If you have two 8 Ohm cabs then (in parallel) the load is 4 Ohms, use the 4 Ohm jacks.

It's OK to use a speaker that is a higher impedence ( ohms ) but never lower ( 8 ohm load (speaker) in the 4 ohm jack is OK, 4 ohm load in the 8 ohm jack BAD). Don't let the amp run at less than 4 Ohms. The lower the load on the OT, more current will try to flow through the amp, possibly causing some damage before the fuse blows.

Dom

 

[Russ]

If you are running 2 separate 8ohm speakers use the two 4 ohm jacks. That will properly load your amp. If you run one 8 on 8 and one 8 on 4 it is an improper match and your amp will run hot. Your amp should be 4 ohm stable but you are not putting the expected load on the amp running 8 and 4. You can run a 16 into 8 and an 8 into 4 and you should be fine but not 8 into 8 and 8 into 4. Also do not try to run your amp with all of your jacks loaded. This will certainly not be good. If you want an explanation in numbers search Ohm's law and apply it. Mesa amps are not 2 ohm stable, this is why you cannot run two 4 ohm cabinets. The load you present with 8 into 8 and 8 into 4 doesn't drop below 4 ohms but it is not how it is expected as wired when you present it to the amp.

 

[boogiemon]

Russ, thanks. you seem to be knowlegable in this area. i know the amp should not be pushing too few ohms, but what do you mean by: "The load you present with 8 into 8 and 8 into 4 doesn't drop below 4 ohms but it is not how it is expected as wired when you present it to the amp"?


thnx.

If you are running 2 separate 8ohm speakers use the two 4 ohm jacks. That will properly load your amp. If you run one 8 on 8 and one 8 on 4 it is an improper match and your amp will run hot. Your amp should be 4 ohm stable but you are not putting the expected load on the amp running 8 and 4. You can run a 16 into 8 and an 8 into 4 and you should be fine but not 8 into 8 and 8 into 4. Also do not try to run your amp with all of your jacks loaded. This will certainly not be good. If you want an explanation in numbers search Ohm's law and apply it. Mesa amps are not 2 ohm stable, this is why you cannot run two 4 ohm cabinets. The load you present with 8 into 8 and 8 into 4 doesn't drop below 4 ohms but it is not how it is expected as wired when you present it to the amp.

 

[Russ]

The way the jacks are wired the amp expects a certain ohm load to be presented to it depending upon what jacks are in use. If you do not use the correct ohm load you can cause damage to your amp. A mismatch in an upward direction gets you somewhat darker tone. A mismatch in a downward direction gets you a repair ticket and bill. The easiest way to maintain the proper ohm load is to only use 2 speakers/enclosures at once. Maintain 8 ohms on each of your two speakers/enclosures and use the two 4 ohm jacks. If you use a 16 ohm speaker/enclosure put it into the 8 ohm jack and run an 8 ohm speaker enclosure into 4 ohms. This would be the safe way to use the 8 ohm jack and a 4 ohm jack. If you run an 8 ohm speaker into the 8 ohm jack with another 8 ohm speaker in the 4 ohm jack you are creating a scenario that the amp is not wired for. From using this jack configuration you are trying to tell the amp that the speaker in the 8 ohm jack is in fact 16 ohms. When it begins powering that speaker it will not be able to tell that it is in fact an 8 ohm speaker. That is how it was explained to me by Mesa.