Ohm mismatch?

[SteveD]

The first part of this question should really have been pitched to Santiago in the JVM410 forum but I'm sure he won't mind if I answer this.

Here is the pertinant extract from the JVM manual; -

15. SPEAKER Outputs

There are 5 speaker outputs available on the
rear panel. They are labelled according to the
intended impedances:
16 ohms: connect any 16 ohms guitar cabinet to this
jack.
8 ohms: connect a single 8 ohms guitar cabinet or
two 16 ohms guitar cabinets.
4 ohms: connect a single 4 ohms guitar cabinet or
two 8 ohms guitar cabinets.

WARNING, although the JVM amplifier has 5
speaker outputs, never attempt to connect more
speakers than rated. The safe combinations are
1x16 Ohm, 1x8 Ohm, 1x4 Ohm, 2x16 Ohm or
2x8 Ohm. Any other speaker configuration may
stress the power amplifier section and in
extreme cases may lead to valve and/or output
transformer failure.

I suppose it would be more specific to say you can use the 16 ohm socket OR the 8 ohm sockets OR the 4 ohm sockets; NEVER use all of them simultaneously!!

And if you want a much simplifed version to the second half of the question; -

The relationship between amplifier and load for solid state and valve is as follows.

A 100W valve amp will put out 100W into 16, 8 or4 ohm loads as long as the impedance is matched. This is because it is coupled through an output transformer with 16, 8 and 4 ohm tappings.

A solid state power amp is connected directly to the load (usually 8 or 4 ohms) into which it will drive an optimum current to give it's nominal power output. You can see from this that if you go too far - eg. a 1 ohm load, then the large resulting current (because there's less resistance) will cause the output transistors to burn out unless there is a protection circuit implemented.

[mcholley]

Steve...

I've said it before, and I'll keep saying it - We're Glad You Are Here!!!!

Mark

[slowpokerhino]

This is a quote I found on the web, and we all know much that is worth. :? Just wondering if this is true.

"Just so you know, you can run an 8 ohm load from your amp into a 16 ohm speaker with no problems. I think it actually sounds better running a load like that."

So, if this IS safe to do, I could theoretically run my head @ 8 ohms into the mono 16 ohm input of my cab?

[SteveD]

This is a complicated subject and can warrant a book in its own right. Speakers are an active load and their impedance is heavily frequency dependant. They have inductance, capacitance and resistance which looks like a nightmare to an amplifier output. So, depending on the frequency, the dynamic impedance can be primarily capacitive with a resistive component or primarily resistive with an inductive component etc. You get the idea!! Add to this the phase alteration that the different components bring to the equation and it gets very interesting indeed.

The impedance of an active load is a nominal figure generally quoted for a standard freqency, usually 400Hz and the impedance is usually at its lowest at this frequency and primarily resistive. From this you can see that it isn't critical to 'match' the amplifier to the load (as it will only occasionally 'line up') but it is advisable to set the figures the same for optimal performance.

The worst thing you can do to a valve amp is drive it hard with no load attached.

[domis]

I run a 16ohms cab into my VM the the ohm switch at 8.
It sound good and it plays little lower than the 16ohm setting, sometimes i switch from 8ohm to
16ohm on the fly to see what the best result depending on the size of the stage.

So, from what i've read its not a harm to the amp to play with 8ohm on a 16ohm cab, right?
and is it ok to switch from 8ohm to 16ohm without turning on the standby or even turning off the amp?

I play with the master at 3 o'clock at all times.

[SteveD]

It wouldn't be advisable to change the selector switch while there is sound coming through the amplifier but ok if you're not playing at the time.

[FJA Mods]

I run a 16ohms cab into my VM the the ohm switch at 8.
It sound good and it plays little lower than the 16ohm setting, sometimes i switch from 8ohm to
16ohm on the fly to see what the best result depending on the size of the stage.

So, from what i've read its not a harm to the amp to play with 8ohm on a 16ohm cab, right?
and is it ok to switch from 8ohm to 16ohm without turning on the standby or even turning off the amp?

I play with the master at 3 o'clock at all times.

You should run your amp with the correct impedance match.
Jerry

[SteveD]

I run a 16ohms cab into my VM the the ohm switch at 8.
It sound good and it plays little lower than the 16ohm setting, sometimes i switch from 8ohm to
16ohm on the fly to see what the best result depending on the size of the stage.

So, from what i've read its not a harm to the amp to play with 8ohm on a 16ohm cab, right?
and is it ok to switch from 8ohm to 16ohm without turning on the standby or even turning off the amp?

I play with the master at 3 o'clock at all times.

You should run your amp with the correct impedance match.
Jerry

This is always the best advice but for some reason there are certain people who will ignore it. :

[domis]

well, i would follow the advice if i knew it would hurt my amp :P

[nielsdaugaard]

Hi steve i accedently came to run my vm head at 16ohm with 2 16ohm cabs, would that hurt the amp? :shock:

[SteveD]

Hi steve i accedently came to run my vm head at 16ohm with 2 16ohm cabs, would that hurt the amp? :shock:

Answered your PM.

[steep]

Hi.
Just to be sure.
When running two 8ohms speakers i parallel (4ohms), the VM 2466 should be set at 8ohms or 16ohms???
Or should these speaker be in series to make them 16ohms in total??
And when running two 16ohms speakers in parallel (8ohms) the VM 2466 should be set at 8ohms or 16ohms???


Just can't seem to get that! :oops:

[SteveD]

The impedance switch on the amp should be set to match the total impedance of the load.

There is actually an unused 4 ohm tap on the 2466's output transformer which can be used at the expense of one of the other settings, however, wiring two 8 ohm speakers in series to give 16 ohms (which in turn matches one of the impedance settings already on the switch) would be less intrusive to the amp's electrical configuration.

Running two 16 ohm speakers in parallel to give 8 ohms means setting the 2466 to match it i.e. 8 ohms as well.

There's lots of stuff on the net about series and parallel resistance calculations which may clarify it for you. I believe I have previously posted a link in another thread in this very section to a suitable educational site, here is the thread, see my post for the link; -

http://vintagemodern.informe.com/viewtopic.php?t=104

[steep]

HI SteveD.
So if I was thinking of using the spare 4 ohm tap on the transformer, is it just to move the jumper from 16 ohm to 4 ohm??
I guess that would be the green wire on the 16 ohm tap.
Is there a tonal difference in series the 8 ohms to get 16 ohms, compared to parallell the two and run in 4 ohms???

And the plot thickens.....

[SteveD]

Yes, you could just move the 16 ohm (or the 8 ohm for that matter) connector on to the 4 ohm connector and that would be it with maybe a label on the rear panel impedance switch reflecting the change. Bear in mind though that modifying the amp from stock is not 'officially' recommended and ultimately it is your choice if you want to risk jeopardising the warranty.

The only way to be sure which cinfiguration suits you is by trying it all ways but bear in mind you won't be able to A/B the results (unless you have two cabs).

Personally I would recommend the 16 ohm configuration.