Cab Impedance

[SPHDS]

As the title says, this is a (probably very stupid) question about bass cabs and their impedance.

I know you shouldn't go below the minimal impedance of an amp (i.e.if the amp is rated for 4 ohm minimum, don't run it with an 8 and a 4 ohm cab in combination, as this will make a 2.67 ohms set up...unless you want fireworks)

My question is, is it possible/simple to make a 4 ohm cab an 8 ohm cab?

Cheers, and sorry for any stupidity.......!

Sam

[barkin]

What drivers are in the cab? 4x16 Ohm, 4x4 Ohm, 2x8 Ohm...?

[Mr. Foxen]

Doesn't matter what drivers are in the cab, you can't change from 4 ohm to 8ohm without replacing them.

[geoham]

[quote name='Mr. Foxen' timestamp='1361148901' post='1982004']
Doesn't matter what drivers are in the cab, you can't change from 4 ohm to 8ohm without replacing them.
[/quote]

This is mostly true. The only exception I can think of is disconnecting some of the drivers. For example, say you have 4 x 4 ohms wired 2 series / 2 parallel - , this would give you 4 ohms in total. Disconnecting one of the parallel circuits, leaving you with just two in series would give you 8 ohms. You could make this switchable if you wanted.

One application where I suppose this might be attractive, would be if you had an 8 ohm cab and a 4/8 ohm switchable cab.
Take out the switchable cab set to 4 ohms for small gigs, and both cabs for bigger gigs. Assuming both cabs were 4 x 10, have the disconnected speakers towards the bottom, and the remaining 6 speakers are raised off the floor.

To address your question - it might be possible if you are willing to sacrifice half of your drivers. Should be simple enough after you've figured out the circuit - just snip a couple of wires! Or a lot more complex to make switchable.

You'd be easier just buying a new cab - why can't money just grow on trees?

[barkin]

Duh! Don't post when I should be in bed...!

[flyfisher]

I'm guessing the question is because Sam wants to add a second 8ohm cab and still have a 4 ohm total load.

A simple way to achieve this would be to add a high-power 4 ohm resistor in series with the 4ohm cab, though of course this won't generate any additional sound energy and will just be a waste of power. But it would work electrically and the amp will be OK.

But would there be any point? Presumably the additional cab is desired to move more air, but would this make much difference is a significant portion of the amp's output power is just being used to heat up a big resistor? I doubt it.

Edited February 18, 2013 by flyfisher

[SPHDS]

Cheers boys,

The idea is, as FlyFisher deduced, to add another cab to my existing rig (an Ashdown Combo), which is running at 8 Ohms with the internal 2x10s, I have seen a few cabs for sale (1x15/2x8, 1x15/2x10 combinations...you get the idea), but they all seem rated at 4ohms. So if I rigged a 4ohm resistor is series would it really sap loads of power, seeing as it is already running at 8 ohms on it's own? Or would it just be worth me just finding an 8ohm cab (I really was looking for a 15" to compliment the 10's, but these combined cabs are pretty cheap and was wondering if it was worth the bodge.....!)

I know I could just disconnect the combo's speakers and run through the other cab,but almost seems a waste of a good set of speakers into the combo.....

And finally.....sorry, I'll shut up in a minute......I know there is a lot of debate on this and it will probably run and run.....but would it be better to pair the existing 2x10s with a 15", or would this muddy the sound, and should go for more 10s, or would a 15 add a better bottom end.....!

Cheers

Sam

[Urban Bassman]

If you're planning to run a single 15" as an extension cab then I'm afraid there's not much you can do to change the impendance. If you like the cab then you're best bet is to change the speaker for an 8 Ohm, I guess one benifit of this is that when you come to resell you can offer either. But I'm sure that there must be an 15", 8 Ohm Ashdown cab out there in Basschat-land for sale.

To answer your other question I used to run 2x10 and 15" cabs in a Peavy Rig and more recently a Trace Elliot 2x10 combo and 15" Ext Cab and had no issues with muddy sounds. Much would depend on the sound you're trying to achieve. It's a fairly common set up.

[barkin]

[quote name='Urban Bassman' timestamp='1361189721' post='1982375']. But I'm sure that there must be an 15", 8 Ohm Ashdown cab out there in Basschat-land
[/quote]

There's one in the For Sale section as we speak

[Bill Fitzmaurice]

[quote name='SPHDS' timestamp='1361188722' post='1982355']
would it be better to pair the existing 2x10s with a 15", or would this muddy the sound, and should go for more 10s, or would a 15 add a better bottom end.....!
[/quote]Mixing drivers is like a box of chocolates, you never know what you're going to get. Trying before buying is the only way to go.

[brensabre79]

I would go for a matching 2x10 Ashdown cab, and by matching I mean the same drivers inside etc. i.e. an extension cab designed to go with your combo. Anything else, even if you get the right impedance, is like a game of roulette to be honest.

[flyfisher]

I only suggested the 4-ohm resistor thing for completeness. It would work, and wouldn't damage the amp, but I wouldn't try to pretend it's a great idea.

With that extra bit of information from Sam, I'd say the best answer, by far, is to get an 8ohm cab. Ashdown 210s and 115s come up pretty regularly on here and they are all 8 ohms and will perfectly complement Sam's Ashdown combo.

As for which would be best, I'd agree with Bill that the options really have to be tried because the results will be subjective.

FWIW, I have an Ashdown Mag300 head and have two Mag210 cabs and one 115 cab. I've tried them in various combinations but cannot honestly say I've noticed a big difference between them, certainly nothing that the eq knobs couldn't sort out. I've gigged a single 210, a 210 + 115 and both 210s and they've all done the job.

Maybe I have cloth ears? Or perhaps it's because I don't use gold-plated mains plugs and unobtainium speaker cables?

[Thatguy59]

I've used an ashdown 210 combo plus a 115 ashdown extension once or twice and it is a pretty good setup, once was at a gig with provided backline and the other was a rehearsal studio which I used a few times, I'd say that getting the 115 is the best all round option from what I've heard, with the 210 close behind, however if you ever sold the combo for a seperate head I'd rather have a 115 cab than a 210.

[Mr. Foxen]

[quote name='geoham' timestamp='1361175244' post='1982100']
This is mostly true. The only exception I can think of is disconnecting some of the drivers. For example, say you have 4 x 4 ohms wired 2 series / 2 parallel - , this would give you 4 ohms in total. Disconnecting one of the parallel circuits, leaving you with just two in series would give you 8 ohms. You could make this switchable if you wanted.
[/quote]

Running some drivers and not others that share a box is a pretty bad idea, the unpowered drivers work as passive radiators and mess up the box tuning, meaning the remaining ones can fart out and die really easily.

[geoham]

[quote name='Mr. Foxen' timestamp='1361200672' post='1982619']
Running some drivers and not others that share a box is a pretty bad idea, the unpowered drivers work as passive radiators and mess up the box tuning, meaning the remaining ones can fart out and die really easily.
[/quote]
I won't disagree with you on that one. Mainly my pedantic side coming out thinking purely about the maths side of things rather than practicality.
It's not impossible to convert, but there are easier and much better sounding solutions out there.

In any case, now that we know exactly what the scenario is - I'd recommend just adding an Ashdown Mag 210 or 410. These are 8ohm cabs. You can get 4 ohm versions, but they have a different name, so you should be able to spot them easily enough (214 and 414).

Of course, you'll see plenty of people mixing 15s and 10s. It will give you a different sound - whether it compliments or ruins your sound is subjective! Sticking with the matched cabs as recommended by brensabre79 should just give you more of the same sound, along with the benefits of having speakers at a higher level - definitely the best bet if you like what you have already.

[icastle]

[quote name='flyfisher' timestamp='1361180296' post='1982161']
A simple way to achieve this would be to add a high-power 4 ohm resistor in series with the 4ohm cab, though of course this won't generate any additional sound energy and will just be a waste of power. But it would work electrically and the amp will be OK.
[/quote]

'High power' being the operative words here.

The resistor needs to have a power rating equivalent to the rating of the speaker you are replacing it with.
There's no point in stick a little 0.25W resistor from Maplins in there, it'll burn out before you get to the chorus.

[flyfisher]

Indeed, and such resistor would probably need to be mounted on a heating as well.

Edited February 19, 2013 by flyfisher