The truth about impedence?????

[_Sweet Willie_]

Originally posted by alexclaber:

Originally posted by Adrian Smith:

I plan on purchasing a head that puts out 350W RMS @ 4-ohms and a 4x10 cabinet that handles 700W RMS @ 4-ohms. Will this be safe, or should I get a higher output head? SWR 350x and Goliath for the curious.

It will be safe as long as 350W into that cab gives you enough volume - the goliath is a pretty efficient cab so it may be ok.

 

Alex

It is okay to have an amp rated lower than your cab, as long as that's all the power you need (like Alex said). 350W into a 4x10 (whatever its rating) is usually pretty darn loud. I've always sort of believed that 400W and a 4x10 will be plenty for almost any playing situation except for really big rooms or if the other folks in the band are both plentiful and plentifully loud.

 

It can't hurt, though, if you can afford it (Alex's proposed pre and power amp set-up sounds relatively comparable pricewise), to have extra power.

 

Peace.

[Adrian Smith]

Cool, thanks. I figure I can always add a power amp later to the 350x if I need it, but I know that 300+ through a 4x10 will be enough (thanks for reassuring me though!).

[Greg A]

Wow. A lot of info here. Nice work, all.

Here's another wrinkle: I've only noticed clip lights come on due to too much input to the amp head; i.e. plugging an active bass into the passive input. Correcting this situation eliminates the problem.

So it would seem to me that some clipping problems may have more to do with overloading the preamp than it does with power amp/cabinet issues. I thought this might be important to mention, especially for the "My clipping light is always on" crowd.

Please correct me if I am totally out to lunch or saying what you already know.

[_Sweet Willie_]

Originally posted by Gerrg:

Wow. A lot of info here. Nice work, all.

Here's another wrinkle: I've only noticed clip lights come on due to too much input to the amp head; i.e. plugging an active bass into the passive input. Correcting this situation eliminates the problem.

So it would seem to me that some clipping problems may have more to do with overloading the preamp than it does with power amp/cabinet issues. I thought this might be important to mention, especially for the "My clipping light is always on" crowd.

Please correct me if I am totally out to lunch or saying what you already know.

There is preamp clipping and power amp clipping. I think that you can get away with some preamp clipping in ways that you can't with power amp clipping. (If I've got it correct, and I'm no techno wiz with this stuff -- so feel free to correct me!) Generally I try to set my input gain at a level where the preamp clipping indicator lights up occasionally (sometimes lighting a little more frequently if I'm trying to overdrive the tube pre), but I try never to have my power amp clipping indicator light -- if it does, I pretty much immediately turn down the volume a little. Power amp clipping sends those dangerous square waves at your speakers.

 

However, others with more knowledge on the nature of preamp clipping vs. power amp clipping can explain the differences better than I!

 

Peace.

[Theodeus]

Let me see if I got this straight...

 

1 It´s a good thing to have more power in the head than in the cab.

 

2 When you add two cabs @ 8 ohm you get the owerall resistance of 4 ohm. That means it would get louder?

 

3 And I´m lost. Is there any site where you can read this straight from the basics?

[rumpelstiltskin.]

1. correct.

2. correct for two reasons. the more important is that you have two speaker cabinets creating sound. the other is that your amp will be creating more power.

3. i may have to take care of that with its own thread.

 

robb.

[bassplayer5217]

i am a little confused too (played combos whole life)

 

- wouldnt u want to have ur head provide the cab with no more than the RMS rating of the cab? shouldnt not supplying maximum power not be bad for the cab?

 

- if i put a 200 W @ 8 ohm head on a 300 W @ 8 ohm cab, that shouldnt clip or be bad based on logic, that the wave will be smaller and not square?

[_Sweet Willie_]

Originally posted by bassplayer5217:

- wouldnt u want to have ur head provide the cab with no more than the RMS rating of the cab? shouldnt not supplying maximum power not be bad for the cab?

Not supplying maximum power is great, as long as you can still generate the loudness you need for the rehearsal or gig.

 

Originally posted by bassplayer5217:

- if i put a 200 W @ 8 ohm head on a 300 W @ 8 ohm cab, that shouldnt clip or be bad based on logic, that the wave will be smaller and not square?

All is well unless you consistently need more than 200W -- not an occasional volume spike, but rather continuous playing that pushes the amp past its rating. This will clip your power amp -- "clipping" the nice curved waves so that they're flat! Everything is groovy as long as 200W is plenty to get the job done. However if you had a head doing 500W into 8-ohms, you'll get the 300W the cab can handle without straining the amp and get a little louder.

 

The earlier discussion in this thread about RMS vs. peak power ratings should help explain some of this.

 

Maybe these examples will also help, and I don't own any of the gear I'm using as examples here -- I just used the SWR website to get the data:

1. I could play an SWR 550x (550W RMS @ 4 ohms) into a 4-ohm Goliath III 4x10 (700W RMS) for club gigs. My likelihood of launching one or more of the speakers into outer space because I'm putting too much power into the cab is slim. My likelihood of clipping the power amp, creating dangerous and damaging waves for my cabinet, is also slim because I will probably always have plenty of volume for the club and never need to push my cab up to either its RMS or its peak rating.

 

2. I have a SWR Workingman's 2004 head (160W into 8 ohms) and a SWR Goliath Jr. III 2x10 cab (350W). I don't have to worry much about destroying my speakers from too much power, but if 160W into that 2x10 isn't getting me enough volume at a gig, and I push my amp really hard to try to get louder, I might clip the power amp, sending dangerous waves to my speakers. This is different than frying my speakers by overpowering them -- this has to do with distorting the signal sent to them in undesirable ways (different than, let's say, tube overdrive).

 

3. I have an SWR 750x (750W into 4 ohms) and a 4-ohm SWR Triad 1x15/1x10/horn cab (400W). I have little chance of clipping the power amp, because I have lots of power at my disposal and won't need to crank the amp's volume. I have little chance of destroying my speakers by overpowering them because I won't be cranking out 750W. I might get up to 400W RMS, which would be plenty for plenty of gigs, and I will get to those 400W without straining my amp.

 

I hope that this helps. If you listen to your rig, you can avert much danger by using your ears to detect when it sounds like it's straining.

 

As I always, I defer to more tech savvy folks to explain, clarify, or correct what I've written -- since I learned most of it from them via this forum!!!

[_Sweet Willie_]

Originally posted by robb.:

3. i may have to take care of that with its own thread.

 

robb.

Good idea -- rig tech reference thread: "tech lessons" similar to Jeremy's "theory lessons."

 

Go for it!

[japhy4529]

Originally posted by Theodeus:

Let me see if I got this straight...

 

1 It´s a good thing to have more power in the head than in the cab.

 

2 When you add two cabs @ 8 ohm you get the owerall resistance of 4 ohm. That means it would get louder?

 

3 And I´m lost. Is there any site where you can read this straight from the basics?

One general rule of thumb that I have learned, is to make sure that the total cabinet(s) Impedance meets or exceeds that of the Amplifier.

 

Now this is an easy rule to follow if you have only one cabinet. If you have an amp with a rating of 4ohms, purchase a cabinet that is 4ohms as well. However, if you purchase a second cabinet at 4ohms your total combined ohm rating will be 2ohms and you will be below the "safe" ohm rating for your amp.

 

So, when you are out shopping for Bass Cabinet(s), keep your amplifier's lowest rated impedance in mind. If your amp is rated @ 4 ohms and you plan on running two cabinets eventually, you should purchase an 8 ohm cabinet. If your amp is rated @ 2 ohms, you may purchase a 4 ohm cabinet. When you go back to pick up that second cabinet, remember KISS (keep it simple stupid) and pick up a cabinet with the same ohm rating.

 

If you are connecting two cabinets to an amp with parallel ouputs, make sure that the combined impedance does not drop below that of the amplifier. If it does, you are in trouble!

 

Here is the "real world" explanation for all of this (paraphrased from the Mackie 1400i manual).

 

If your amp is wired in "series", simply just add the loads of the speakers to find the total ohms (i.e. if you are running two 4 ohm cabinets, your load will be 8 ohms.

 

If your amp is wired parallel (like the Mackie 1400i), divide the ohm value by the number of speakers (i.e. if you are running two 4 ohm cabinets, your load will be 2 ohms). This can get tricky if you have cabinets with different ohm ratings, so I will let one of the experts chime in with the calculation for that one. It's also in the Mackie 1400i manual, which you can download from their website. It's hard to translate that one into UBB code!

 

Good luck!

 

tom

[bassplayer5217]

thanks willie, that is great!

 

one last comment: as long as ur 750 watt head isnt cranking out that full 750 into the 400 cab, it is all good, unless u put the master up high (8-9) and play hard, then u blow the cab.

 

that is correct, right?

[SplitNick]

Now I am blown out of the water...I am looking at getting the Avatar 212 which is stated to handle 900 watts (maybe inflated?)...I can not afford to replace my head yet...It only does 220 watts at 4 ohms...Now, would I be safe for a while or should I just scratch my plan and save more money?

[_Sweet Willie_]

You're okay as long as 220W will be enough power (assuming the 2x12 will be 4 ohms). Use your ears and make sure you're not clipping the power section of your amp.

[SplitNick]

See that is the thing...I know if I run the gain at full my LED's will be lighting up like fireworks, but it isn't going to be anymore than if I ran it full with a 400 watt 4 ohm cab or 900 watt 4 ohm cab, right? Basically, I just need to not expect a 900 watt sound just because that's the speaker rating...Expect a 220 watt sound coming through a different speaker.

[_Sweet Willie_]

Right. Your amp will put 220W into 4 ohms no matter how much power the cab can handle. A given cab may be more or less efficient, though, meaning that your watts may produce more or less volume. This, however, has to do with the cab's efficiency and not its power handling.

 

Just don't clip your power amp, or the power amp section of your all-inclusive amp head.

 

Peace.

[alexclaber]

Originally posted by Sweet Willie:

You're okay as long as 220W will be enough power (assuming the 2x12 will be 4 ohms). Use your ears and make sure you're not clipping the power section of your amp.

Even if you push that power amp right into clipping the cab won't be seeing more than 450W which is less than half the rated power. The only way you could blow a 900W cab with a 220W amp would be by boosting the bass below the tuning frequency thus causing speaker over-excursion.

 

However, a lot of the extra power that you get from pushing the power amp into creating a square-ish wave will be in the high end - this will blow your tweeter (or it's fuse/bulb if it's protected).

 

Alex

[Dave da Dude]

Just a note here (pun intended).

 

I believe that "Impedence" is slightly different than "Resistance", which is measured in ohms. I'm not sure of the units of "Impedence".

 

I'm not sure of the difference or the significance, although I might be able to look it up if anybody cares.

 

Dave

[rumpelstiltskin.]

well, first, the only pun would be mixing up "impotence" (im - po - tens) with "impedance" (im - pee - dens). but we're not juvenile like that, are we?

 

right. anyway, resistance and impedance are both measured in Ohms. resistance is the DC part of impedance, and reactance is the AC part. you don't hear about reactance very often, because it's just not something the average player needs to know about. but if you add up resistance and reactance, you get impedance. they're all measured in Ohms.

 

robb.

[jreed]

I'd like to continue on this one a bit because I just found out the watt rating on my cab.

I have a SVT 400T and a V-4 cab(200 watts 8omhs), I can momo bridge the head for 400 watts @8omhs, so that would be 400 watts going into a 200 watt cab, from what this thread says that is a good thing, as long has I dont turn up all the way, right?

[alexclaber]

Originally posted by jreed:

I have a SVT 400T and a V-4 cab(200 watts 8omhs), I can momo bridge the head for 400 watts @8omhs, so that would be 400 watts going into a 200 watt cab, from what this thread says that is a good thing, as long has I dont turn up all the way, right?

Right-ish. If you start hearing distortion turn down but up that point you'll be fine even if you've got the volume knobs maxed out. If you've got your amp cranked so it's just below clipping on the peaks, because bass is much more dynamic than either a simple sine wave or program music the average power going into the cab is likely to be a lot less than 400W.

 

In fact, with typical program music it'll be less than 200W and with bass playing even less than that. If you're playing slap although your amp may be reaching maximum output on the peaks (probably about 800W instantaneous power) the average power going into the cab may be as little as 50W or less.

 

Alex

[fantasticsound]

I re-read this thread for the first time in several months. Something occurred to me that might help non-technical people understand serial vs. parallel wiring and impedance.

 

Parallel wiring is a classic "Y" jack configuration. + and - from the power amp go separately to each + and - on each speaker.

 

Serial wiring is a big loop. + from the power amp goes to + on the first speaker. - on the first speaker goes to +[/] on the second speaker, and so on until the - of the last speaker in the chain connects to - on the power amp.

 

So why does parallel wiring yield less resistance than that of each component? Simple. A "Y" gives the electrical signal several "tubes" to travel down. If one "tube" has (x) resistance, than two "tubes" will give more space to flow, yielding less resistance to that flow. It helps to think of this as physical resistance in a plumbing network.

 

Series wiring, on the other hand, forces the electrons to flow through each speaker to complete the circuit. This makes resistance additive. Think of it as multiple obstacles for you to push out of your way. Add another obstacle and your ability to push is further impeded[/]

 

Another comment; Note, if any speaker in a series connection fries, it cuts off all flow to all the other speakers. In parallel wiring, each speaker is a circuit unto itself, in that it will continue to reproduce sound when another speaker on the entire circuit is compromised.

 

Does this make sense to you, or am I adding further confusion. (I know what I'm talking about... but I don't always explain myself with clarity. )

[_Sweet Willie_]

Originally posted by fantasticsound:

Another comment; Note, if any speaker in a series connection fries, it cuts off all flow to all the other speakers. In parallel wiring, each speaker is a circuit unto itself, in that it will continue to reproduce sound when another speaker on the entire circuit is compromised.

You know what really helped me understand the differences between parallel and series wiring? Electric Christmas tree lights.

 

Growing up we had sets of lights wired in series and sets wired in parallel. Now I think all my parents' sets of lights (approx. a half dozen or so) except one are parallel.

 

Why? You blow a light in the series set, and all the lights go out. Then it's a real pain in the ass to figure out which of 30+ bulbs was the culprit. With the parallel sets, if a light blew, it was the only one that went out, and easy to find and replace (and the rest of the lights kept glowing and keeping the tree lookin' pretty ).

 

Here's a practical example that demonstrates that not all 8-ohm 4x10 cabs are necessarily the same:

Let's say you want to build an 8-ohm 4x10 cab using 4 8-ohm drivers. You could take one of two paths --

(1) You can wire two pairs in series, thus having two pairs at 16 ohms each. Then wire those pairs parallel -- resulting in an 8-ohm final load.

(2) You can wire two pairs parallel, thus having two pairs at 4 ohms each. Then wire them in series -- resulting in an 8-ohm final load.

 

In scenario (1), if you blow one of your speakers, one of the pairs will stop functioning, but the other pair will keep pumping sound (albeit at 16-ohm impedance). In scenario (2), if you blow one speaker, both pairs go out since the pairs were wired together in series.

 

I think this strays from the original specific topic of this thread, but it is related to the direction Neil was headed. I don't know if my words explain this well-enough, but there was once a diagram and explanation of this in a BP mag. I'll try to figure out which issue if I have time.

 

Peace.

[fantasticsound]

Originally posted by Sweet Willie:

...You know what really helped me understand the differences between parallel and series wiring? Electric Christmas tree lights.

 

Growing up we had sets of lights wired in series and sets wired in parallel. Now I think all my parents' sets of lights (approx. a half dozen or so) except one are parallel.

 

Why? You blow a light in the series set, and all the lights go out. Then it's a real pain in the ass to figure out which of 30+ bulbs was the culprit. With the parallel sets, if a light blew, it was the only one that went out, and easy to find and replace (and the rest of the lights kept glowing and keeping the tree lookin' pretty )...

And that's why you gentiles have a built in advantage in electronics.

 

I never had a christmas tree to help explain serial vs. parallel electrical circuits!

 

Chanukiahs use candles!

 

But yes, I was made aware long ago of the use of parallel wiring for Christmas lights.

[_Sweet Willie_]

Originally posted by fantasticsound:

And that's why you gentiles have a built in advantage in electronics. ... Chanukiahs use candles!

 

But who needs electricity for lights when miraculously the oil keeps going and going and going?!

 

Although I have yet to see an oil-powered bass rig.

[J Reed]

This thread might help me, I am staring to understand this stuff.

I have two cabs one SWR with 'in' and 'out'.

the other - Ampeg v-4 ONE jack on the back.

so, would series be into the SWR, then out from the SWR into the Ampeg?

I think Parellel would be each cab with its own cable into the two jacks on the head.

[_Sweet Willie_]

Originally posted by J Reed:

This thread might help me, I am staring to understand this stuff.

I have two cabs one SWR with 'in' and 'out'.

the other - Ampeg v-4 ONE jack on the back.

so, would series be into the SWR, then out from the SWR into the Ampeg?

I think Parellel would be each cab with its own cable into the two jacks on the head.

Nope. Sorry.

 

The SWR cab's in/out jacks are most likely wired in parallel (inside the cab), meaning that running into the SWR and then to the Ampeg will still be a parallel set-up. It is rare for a rig to be set-up in series, and the number of outputs you run from the head vs. daisy-chaining cabs does not tell you whether you have a parallel or series set-up.

[J Reed]

So, the cab has to be wierd in series?

With mine then, either way I hook them up will be parallel?

[_Sweet Willie_]

Yes, the cab would need to be wired in series or you can build your own speaker cables to link the cabs in series. Basically, I don't think it's worth it.

 

The Nemesis combo amps from Eden used to be (and maybe still are) equipped with an extension jack wired in series, so that if you added an extension cab you would increase the overall impedance of your rig.

 

Most amp output jacks and cab in/out jacks are wired in parallel. My guess, without looking up the specs on your gear and not knowing what head you're using, would be that however you combine your head and the two cabs, assuming regular speaker cables and common parallel wiring scenario, you will be running in parallel. If your cabs are both 8 ohms, you will have a 4 ohm overall impedance. If they are both 4 ohms, you will have a 2 ohm overall impedance. If one if 4 ohms and the other 8 ohms, you will have a 2.67 ohm overall impedance. As a sidenote, most amps don't like loads lower than 4 ohms, but there are definitely some that can handle it.

[J Reed]

I get you now. Mine probably are parallel since they dont say otherwise. The head is a SVT 400T. so I can get 400 watts mono'd @8omhs and then just use one cab @full power, right?

[getz out]

Originally posted by Sweet Willie:

The Nemesis combo amps from Eden used to be (and maybe still are) equipped with an extension jack wired in series, so that if you added an extension cab you would increase the overall impedance of your rig.

You are correct regarding the Nemesis combos... quite ingenious, actually. Get the most of the amp as a combo. How often do you use an extension with a combo?

 

Actually, I wonder how a powered extention cabinet would sell in the Nemesis line? That would be interesting. If I didn't have my "big rig," I would probably be interested in a powered 2x10" cabinet to throw under my Nemesis 210P.