impedance change

[Mr Bassman Sax]

Right guys I have my 4 ohm x-treme head, an 8 ohm cab and a 4 ohm cab. The 8 ohm is a 411 nd and the 4 ohm is a 2x15. I'm sure you can see the impedance problem here. To do this would fry the head. How can I change the impedance to something safe? I don't want to swap the wiring on the 15's to a 16 ohm as the output will be too low.

Any suggestions?


If I can pull this off it will be a six and a half foot stack!!!!!!!

 

[BulletProof]

4 ohm resistor with high heat dissipation in series with the 4 ohm cab making 8 ohms.. 8) I remember very old cabs actually using bulbs to accomplish this. The easy fix would be to add a pot/switch in the loop (rated for the max current it will see - around 10A in this case), which will allow you to switch between 8 and 4 ohms as necessary.

 

[Eberbachl]

4 ohm resistor with high heat dissipation in series with the 4 ohm cab making 8 ohms.. 8) I remember very old cabs actually using bulbs to accomplish this. The easy fix would be to add a pot/switch in the loop (rated for the max current it will see - around 10A in this case), which will allow you to switch between 8 and 4 ohms as necessary.

Then you're just dissipating a portion of your amp's output as heat without any volume.

What's the point?

The only real way is to re-wire the 2 x 15 cab for 16 ohms.

 

[BulletProof]

Then you're just dissipating a portion of your amp's output as heat without any volume.

What's the point?

The only real way is to re-wire the 2 x 15 cab for 16 ohms.

Agreed, it's just dissipating heat, but more overall power will get to the speakers. <----- Point 8)

Scenario A:
411 nd @ 8 ohms/15x2 @ 4 ohms ~ 2.7 ohms = short

Scenario B:
411 nd @ 8 ohms/15x2 @ 16 ohms ~ 5.3 ohms, each cab gets a lot less that 500w, and the 15x2s get half of what the 411nd gets (much less than 333W).

Scenario C:
411 nd @ 8 ohms/15x2 @ 8 ohms (resistor) = 4 ohms, each cab gets 500W rms, resistor dissipates 1/3, but the 15x2 gets 2/3 of the power that the 411nd gets (more than 333 W).

Peace.

 

[Eberbachl]

Scenario B:
411 nd @ 8 ohms/15x2 @ 16 ohms ~ 5.3 ohms, each cab gets a lot less that 500w, and the 15x2s get half of what the 411nd gets (much less than 333W).

At 5.3 ohms, the head will produce well in excess of 400w.

Scenario C:
411 nd @ 8 ohms/15x2 @ 8 ohms (resistor) = 4 ohms, each cab gets 500W rms, resistor dissipates 1/3, but the 15x2 gets 2/3 of the power that the 411nd gets (more than 333 W).

Peace.

Each cab does not get 500w.

The 411nd gets 250w, and the 2x15 cab gets 250 (half each).

Further, inside the 2x15 cab, the resistor gets 125w, and the speakers only get 125w.

...so - the 411nd gets 250, and the 2x15 really only gets 125w, for a total of 375w.



The important thing is not what's going to each cabinet so much as the actual wattage sent to real speakers instead of resistors.

With the 8 + 16 ohm cab, the head will produce more than 400w total (at 5.3ohms), but with the resistor idea, although the head will produce 500w, only 375w of that will be seen by speakers, and the remaining 125w dissipated as heat. A dead loss.

 

[BulletProof]

At 5.3 ohms, the head will produce well in excess of 400w.

I think it's more like 800W, since it is a 10.1 we're talking about, which means the power, split 2:1 for the 411nd:2x15, the 411s will see 533.3W and the 15s will see 266.7W (133.3 each)

Each cab does not get 500w.

I may be a n00b, but from what I understand, the X-treme 10.1 gives 1000w rms @ 4 ohms, meaning if you have 2 8-ohm cabs, they get 500w each.

The 411nd gets 250w, and the 2x15 cab gets 250 (half each).

Further, inside the 2x15 cab, the resistor gets 125w, and the speakers only get 125w.

...so - the 411nd gets 250, and the 2x15 really only gets 125w, for a total of 375w.

Following from that, that would mean the 411nd would get 500W and the 2 15s get 333.3 (166.7 each)... which is more than the 16-ohm scenario, with the resistor dissipating another 166.7 W.

The important thing is not what's going to each cabinet so much as the actual wattage sent to real speakers instead of resistors.

With the 8 + 16 ohm cab, the head will produce more than 400w, but with the resistor idea, although the head will produce 500w, only 375w of that will be seen by speakers, and the remaining 125w dissipated as heat. A dead loss.

Again from what I understand, the X-treme 10.1 produces 1000w and not 500 as in your statement.

To summarize:
8/16, est 800W rms output, 411nd gets 533.3W, 15s get 266.7W
8/8, 1000w rms output, 411nd gets 500W, 15s get 333.3W, 166.7W dissipated.

So the real question is does he want to sacrifice some power to the 411nd in order to get it back on each of the 15s!!!


:lol:

 

[Eberbachl]

Is it an X-Treme 10.1 or a 5.1?

The 5.1 makes 500w, the 10.1 makes 1000w.

It doesn't matter at all - the maths is still the same.

Either way, it's more efficient to use a 8+16 ohm cab than the resistor idea.

Check it out:

X-Treme 5.1 (500w):

With an 8 ohm cab and 16 ohm cab @ 5.33 ohms, the head makes more than 400w as seen by the speakers.

With the 8 ohm cab plus the phony 8 ohm (4ohm+4ohm dummy load) cab the speakers only see 375w whilst the rest is wasted as heat.

X-Treme 10.1 (1000w):

With an 8 ohm cab and 16 ohm cab @ 5.33 ohms, the head makes more than 800w.

With the 8 ohm cab plus the phony 8 ohm (4ohm+4ohm dummy load) cab the speakers only see 750w whilst the rest is wasted as heat.




Not only is it better to use the 8+16 idea from the above perspective, also consider that the amp isn't working as hard to produce that sound (because it's working at 5.33 ohms instead of 4 ohms), and will therefore run cooler - lessening the strain on internal components.



EDIT: I just checked the OPs sig - it's a 10.1 head. Still...it doesn't make a difference.

 

[BulletProof]

Mathematically, it does make a difference. The real test is to try it out in a real-life scenario. Worst case is that the resistor blows and cuts the circuit to the 2x15, with no harm to the amp.

 

[Eberbachl]

Mathematically, it does make a difference.

....no - it doesn't make a difference. It still stands either way that more power will be seen by the speakers with either a 500w head or a 1000w head if the 8+16ohm cab is used than the resistor idea.

Following from that, that would mean the 411nd would get 500W and the 2 15s get 333.3 (166.7 each)... which is more than the 16-ohm scenario, with the resistor dissipating another 166.7 W.

The 2x15s make 4 ohms, and the resistor is four ohms, meaning that the resistor will consume the same power as both 15" drivers, not just one of them.

More power is consumed by the resistor than you think.

To summarize:
8/16, est 800W rms output, 411nd gets 533.3W, 15s get 266.7W
8/8, 1000w rms output, 411nd gets 500W, 15s get 333.3W, 166.7W dissipated.

correction: 8/8, 1000w rms output, 411nd gets 500W, 15s get250w, 250w dissipated as heat - not 166.7.



Power seen by speakers in 8/8(with dummy load) configuration: 750w

Power seen by speakers in 8/16 configuration = +800w, with the benefit of head running cooler at 5.33 ohms, and power not being dissipated as heat (a bad thing).

 

[anarchyok]

Interesting discussion guys.

Eberbachls right on this one - i give my maths and science teacher tick to his workings. He's on it.

Trust me guys, if there is one person on this forum who knows his amps and impedances etc, its Eberbachl!!

Now luke, about that corvette you're giving me

 

[Eberbachl]

Now luke, about that corvette you're giving me

:lol:

Yeah Jono - I might get you a Corvette Keyring if you're lucky.

 

[Mr Rabble]

When do you need to use both cabs together? :shock:

Anyway, if you don't go too far with the master volume, it's ok to use your head at 2,67 ohm.

 

[Eberbachl]

I understand what you're saying about not going too far on the master dial Mr Rabble - but I don't think it's good advice.

I say stick to the impedance limit described by your amp manufacturer.

 

[Mr Bassman Sax]

Both cabs would be used together at a gig to make quite an imposing stack but I've backed off the idea as it's just too much work to fit the whole rig in a small French car. It does fit amazingly but takes twice as long to load just for the sake of one more cab. So I think I'll save up for another 411ND at some point.

But thanks for the advice guys. I'll see how much I remember for if it comes in use later on. Did hurt my head to begin with. My job doesn't require much thinking so when I do need to think it's harder than it used to be. Not good.

 

[CaryTheLabelGuy]

Eberbachl,

I was wondering, how would you rewire a 2x15 cabinet with 2, 4 ohm speakers currently wired in parallel to make 16ohms? I would be interested in trying this, as i'm looking to take a 4ohm 2x15 cabinet and make it 8ohm or even 16ohm.

Thanks man!

 

[Eberbachl]

Eberbachl,

I was wondering, how would you rewire a 2x15 cabinet with 2, 4 ohm speakers currently wired in parallel to make 16ohms? I would be interested in trying this, as i'm looking to take a 4ohm 2x15 cabinet and make it 8ohm or even 16ohm.

Thanks man!

Hi Carey,

You can't do that.

A 2x15 cabinet with a total impedance of 4ohms has two 8 ohm 15" speakers wired in parallel. If they were two 4 ohm 15" speakers wired in parallel, it would be a 2 ohm cabinet

If you wired those speakers in series, the impedance becomes additive, to give you a total load of 16 ohms.



You can't make two 4 ohm speakers into 16 ohms I'm afraid. If you had four you could, but not with two.



You have a 4 ohm 2x15, so you can rewire it in series to give you a 16 ohm cabinet.

 

[CaryTheLabelGuy]

Eberbachl,

I was wondering, how would you rewire a 2x15 cabinet with 2, 4 ohm speakers currently wired in parallel to make 16ohms? I would be interested in trying this, as i'm looking to take a 4ohm 2x15 cabinet and make it 8ohm or even 16ohm.

Thanks man!

Hi Carey,

You can't do that.

A 2x15 cabinet with a total impedance of 4ohms has two 8 ohm 15" speakers wired in parallel. If they were two 4 ohm 15" speakers wired in parallel, it would be a 2 ohm cabinet

If you wired those speakers in series, the impedance becomes additive, to give you a total load of 16 ohms.



You can't make two 4 ohm speakers into 16 ohms I'm afraid. If you had four you could, but not with two.

I'm sorry....i said that wrong. I have a 2x15 cabinet with two 8 ohm speakers, wired in parallel, for a total impedance of 4 ohms....

So, basically, i would need to re-wire the speakers in series..(i.e. from the jack, to the first speaker, then run the wires to the second speaker from the first). This would give me a total of 16ohms.

This seems easy enough!

Thanks man!

 

[Eberbachl]

Yeah - you can totally rewire it in series to give you a 16 ohm cab.



Of course you'll be using less wattage from your head as a result - remember to bear that in mind.

 

[CaryTheLabelGuy]

Yeah - you can totally rewire it in series to give you a 16 ohm cab.



Of course you'll be using less wattage from your head as a result - remember to bear that in mind.

Of course, i'm more than willing to take a hit on my wattage to be able to run this cab @ 16ohms.