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Impedance etc

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Fantastic thread, thanks for all the useful posts. I have a q that seems to be coming from the "other end" - i.e. about using a head with a LOWER impedance than the cab. Bear with me while I paint the picture. I have a Tecamp Puma 1000 and Barefaced Big Baby inbound from Legion on here. This will be my main rig. I've also just acquired an RH450 which I was going to use with my Epifani 8 ohm 500w 2 x 10 for smaller gigs and as a back up. Then I thought about replacing it with a cab which coud be used on its own with the RH450 or together with the BB, possibly a BF Midget. However the BF cabs are 8 ohm. The RH450 is rated 4 ohm and only has one output. I guess I'm going to have to have dedicated cabs for each head, as it's not good to mix 8 ohm and 4 ohm cabs together (with the Tecamp), and if I use an 8 ohm cab (BF or Epifani) with the 4 ohm rated RH I will only be getting half the power out of the head? Advice/opinions/suggestions welcome! And BTW I haven't won the lottery, am selling stuff to fund this! :)

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I see bass direct offering deals one of which is....... A GB shuttle 9.2 + a 4X10 (600w 8ohm) + a 2X10 (300w 8ohm). now...... If you rig all this up your cabs are down to 4 ohms & the amp will be capable of pushing 900w. Assuming that in reality the master on the amp won't be past 12 o'clock is this still a mismatch or is it workable?
Cheers, John.

Edited by pockethammer

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This rig isn't mismatched.

If the volume controls are linear and 12 o'clock really is half power then each cab is seeing just over 200 watts, and my guess is that in reality you will be using less than that. In this rig more speakers equals more volume rather than more watts from the amp.

My Aguilar TH500 is supposed to have the same power module as the GB 9.2 and I'm loud enough through a good 212 with the master at about 10 o'clock.

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Depends on the definition of 'mismatched'.

When both cabs are connected to the amp it will capable of outputting 900W at full volume. Because each cab has the same impedance that power will be split evenly between them, so 450W to each cab. the 600W cab will obviously be able to cope but the 300W cab may not. In that sense, I'd say it was fair to say it's a mismatched rig.

I entirely agree that [u]in practice[/u] the amp will probably not be run at full volume so it won't actually matter, but that's a different issue.

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[quote name='flyfisher' timestamp='1388945674' post='2327953']
....the amp will probably not be run at full volume so it won't actually matter, but that's a different issue....
[/quote]

Pockethammer asked if this rig was workable, and it is, so it's [i]the[/i] relevent issue.

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Ahem, he actually asked "[i]is this still a mismatch or is it workable[/i]?"

Clearly [u]it is[/u] a mismatch and as such one of the cabs [u]could [/u]be damaged by high volumes.

The rig is only 'workable' providing the volume is kept within the limits of the smaller cab, which will have to be determined by guesswork and assumptions, as you've already pointed out.

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Not really.

Putting cabs together in modular rigs has an advantage in that the cabs can be used separately on smaller gigs or combined for the larger gigs.

Also you'll get more volume and better tone not from running an amp flat out but by using more speakers.

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[quote name='pockethammer' timestamp='1389024772' post='2328943']
Cheers guys,
I did think that the 300w cab would be in danger, just wanted it confirmed. Seems stange that they would bundle all that together.
[/quote]

to be fair tho would each cab be getting 450w, so id pit the 210 on top but im pretty sure that it would be fine, id imagine if you used it on its own with the head it be over loaded aswell

andy

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Nothing wrong with modular rigs (I've got two :D ), but it's preferable if you don't have to worry about power handling limitations whatever the combination.

But as long as you are aware of the limitations and never forget or never trust anyone else to use it then you'll probably be fine.

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[quote name='0175westwood29' timestamp='1389031373' post='2329068']
to be fair tho would each cab be getting 450w, so id pit the 210 on top but im pretty sure that it would be fine, id imagine if you used it on its own with the head it be over loaded aswell

andy
[/quote]

If the amp is rated at 900W into 4 ohms then a good rule of thumb is that it'll deliver 2/3rds into 8 ohms - so about 600W in this case.

Thus, the 600W cab on its own will be fine at any volume but the 300W cab would be at risk on its own.

Again, probably all OK if used carefully, but the risk is there.

It's a bit like the difference between having a red line on a car rev-counter that you have to be careful not to exceed or having a rev-limiter that will make it impossible to do any damage.

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[quote name='chris_b' timestamp='1389030304' post='2329051']
Not really.

Putting cabs together in modular rigs has an advantage in that the cabs can be used separately on smaller gigs or combined for the larger gigs.

Also you'll get more volume and better tone not from running an amp flat out but by using more speakers.
[/quote]

It was my thinking to have a rig for all occasions, I'll be the sole user & I'll keep a keen eye (& ear) on the levels. I've been using a 350w combo up to now (Ampeg ba500) but it's limited on the larger stage & is getting on in years now, time for a change.

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[quote name='pockethammer' timestamp='1388937880' post='2327820']
I see bass direct offering deals one of which is....... A GB shuttle 9.2 + a 4X10 (600w 8ohm) + a 2X10 (300w 8ohm). now...... If you rig all this up your cabs are down to 4 ohms & the amp will be capable of pushing 900w. Assuming that in reality the master on the amp won't be past 12 o'clock is this still a mismatch or is it workable?
Cheers, John.
[/quote]It's a mismatch. To be matched the 2x10 should have twice the impedance of the 4x10, so each driver receives the same power. That means a 4 ohm 4x10, and an amp capable of handling a 2.7 ohm load. It may be workable, but it's not ideal. The position of the master, BTW, doesn't indicated how much power the amp is producing.

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[quote name='flyfisher' timestamp='1389032014' post='2329076']
If the amp is rated at 900W into 4 ohms then a good rule of thumb is that it'll deliver 2/3rds into 8 ohms - so about 600W in this case.

Thus, the 600W cab on its own will be fine at any volume but the 300W cab would be at risk on its own.

Again, probably all OK if used carefully, but the risk is there.

It's a bit like the difference between having a red line on a car rev-counter that you have to be careful not to exceed or having a rev-limiter that will make it impossible to do any damage.
[/quote]

i know that but anyone shout know not to drive the crap out of a small cab anyway, perfectly fine to use it with both cabs ill probs end up if i can get on in time the 210 obc cab

[quote name='Bill Fitzmaurice' timestamp='1389043000' post='2329302']
It's a mismatch. To be matched the 2x10 should have twice the impedance of the 4x10, so each driver receives the same power. That means a 4 ohm 4x10, and an amp capable of handling a 2.7 ohm load. It may be workable, but it's not ideal. The position of the master, BTW, doesn't indicated how much power the amp is producing.
[/quote]

limiting yourself even more then tho id say they have a gd small/ medium/large gig rig there very gd for the money, and also if we all used cabs that could take our heads power i would never get to use my terror.

andy

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I've used this simple rule for years:
1) The amp output in ohms should equal the specified ohm rating of the loudspeaker.
2) If your ears are bleeding, it's all probably got a little excessive.
3) My amp goes up to 11, so it's louder than yours....right?

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Hi guys, bit of advice needed...

I'm wanting to get another wizzy 10" to pair with my existing one and an MB200. As they are 4ohms can I get a cable or something so the amp would see the two cabs in series and therefore an 8 ohm load?

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You could try a PM to ...

[url="http://basschat.co.uk/user/12-obbm/"]Obbm ...[/url]

... or ...

[url="http://basschat.co.uk/user/7835-kiogon/"]KiOgon ...[/url]

... who may be willing to oblige..?

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[quote name='CamdenRob' timestamp='1416909312' post='2614826']
Hi guys, bit of advice needed...

I'm wanting to get another wizzy 10" to pair with my existing one and an MB200. As they are 4ohms can I get a cable or something so the amp would see the two cabs in series and therefore an 8 ohm load?
[/quote]

Assuming that it's all Speakon I have such a cable that I made use with my long departed Schroeder cabs.

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[quote name='Dad3353' timestamp='1416917903' post='2614955']
You could try a PM to ...

[url="http://basschat.co.uk/user/12-obbm/"]Obbm ...[/url]

... or ...

[url="http://basschat.co.uk/user/7835-kiogon/"]KiOgon ...[/url]

... who may be willing to oblige..?
[/quote]

Good shout Dad... and here's the man himself;

[quote name='obbm' timestamp='1416919360' post='2614974']
Assuming that it's all Speakon I have such a cable that I made use with my long departed Schroeder cabs.
[/quote]

Ah it's good to know it can be done. When I get hold of a second wizzy 10" I'll drop you a pm if you could make one up for me?

All my other speaker and instrument cables are yours anyway so i might as well continue the tradition...

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[size=4][sharedmedia=core:attachments:167485][/size]

[size=4]Keep it in the family, eh..? :) [/size]

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[quote name='spectoremg' timestamp='1427068347' post='2725518']
Apologies for duplicating any previous questions but:
4ohm cab + 4ohm cab driven by a single 4ohm amp.
How do I do it?
[/quote]

With difficulty. :)

Two 4Ω speakers sat on the back of an amp will present a 2Ω load - not good.

The only way of doing it will be to have an adaptor lead made up that'll put the two 4Ω cabinets in series.
That'll give you an 8Ω load (if it's a valve amp then you might need to set it to 8Ω, if it's a transistor amp then it doesn't matter).

The downside is that you're going to get far less volume as the amp is expecting to run into a 4Ω load and you'll probably not gain very much as a consequence.

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[quote name='spectoremg' timestamp='1427068347' post='2725518']
Apologies for duplicating any previous questions but:
4ohm cab + 4ohm cab driven by a single 4ohm amp.
How do I do it?
[/quote]

You can connect the cabs in series (using a special lead or junction box) which will give you an 8-ohm load. Although will get somewhat less power from the amp, in practice it may well sound louder than running just one 4-ohm cab on its own.

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