The truth about impedence?????

[DanYmaL X]

I run an Acoustic B-3 Head into what I thought was a Fender 1x18 145W, 4 ohm 400 PS bass cab. Turns out as I busted the cab open yesterday, I have a Fender cab with a Sound Technologies STS618-8 which is a 400W, 8 ohm speaker.

 

What the hell is the true meaning of impedence and how the hell does it apply here? My head says 320W @ 4 ohms. I have a speaker that says 400W @ 8 ohms.

I have noticed that my 'Clip' light comes on alot, in certain situations, such as when playing a guitar through the amp, especially with the bridge pickup cranked. Or when I 'put the hammer down' on my bass...

I just recently read an article (don't remember where) about clipping and how the impedence / wattage of an amp / cab setup need to match (this is what prompted me to open my cab.) If I remeber correctly, the article seemed to indicate that a speaker's RMS rating should be lower than the amp's to avoid clipping? Or did I read it all backwards? Is the reason my setup clips so much due to the 400 watt speaker?

And if my head says 320W @ 4 ohms, what how do I figure out what I'm really getting with an 8 ohm 400W speaker?

Any help here would be appreciated... I am/was planning to add a 2x10 cab to the setup, but now I need to figure out how to match the cabs for the best results... or if I need to get new cabs altogether.

Geez, this amp thing is COMPLICATED!

 

DX

[The Fonz]

there's a viagra joke in here somewhere...

 

at any rate, i could never figure that crap out. my clip light is ALWAYS on. and i'm not al that loud. never really sounds like it's clipping anyway. if you think of a soundwave, it's nice and pretty and parabolic and round and stuff. a good smooth wave equals nice clean tone. however clipping is what you get when you're asking part of your rig to do something it can't handle and the wave gets all square on ya. the tops and bottoms look like they've been chopped off, hence "clipping." the sound you get is overdriven and muddy. blech. but when my clip light comes on i don't normally sound like that. hmmm, is it a compression thing?

[DanYmaL X]

Originally posted by Bastid E:

there's a viagra joke in here somewhere...

 

at any rate, i could never figure that crap out. my clip light is ALWAYS on. and i'm not al that loud. never really sounds like it's clipping anyway. if you think of a soundwave, it's nice and pretty and parabolic and round and stuff. a good smooth wave equals nice clean tone. however clipping is what you get when you're asking part of your rig to do something it can't handle and the wave gets all square on ya. the tops and bottoms look like they've been chopped off, hence "clipping." the sound you get is overdriven and muddy. blech. but when my clip light comes on i don't normally sound like that. hmmm, is it a compression thing?

My sound doesn't seem to be affected by the clipping either, but then, I bought this setup as a unit so I don't know if the sound would be even better with different(more appropriate?) speakers, or what... I don't usually play that loud either, and when I do I usually run direct out to a PA anyways, and use the amp more as a monitor.

DX

[jpmiii]

It gets complicated but to try and keep it simple.

 

If your amp is rated for 200 watts at 8 ohms then it is capable of putting out 400 watts at 4 ohms (roughly).

 

Two speakers wired in parallel both rated at say 200 watts at 8 ohms will become a 4 ohm load at 400 watts. So your amp has to be able to handle that load or it will clip.

 

As a rule of thumb, you always want your amp to be able to put out more power then the speaker is capable of handling or you run the problem of clipping the amp. This is not good because when an amp clips it puts out something close to a square wave which is like hitting your speaker with a hammer.

 

Never run anything that will equal a 2 ohm load because very few amps are capable of handling that unless you know for a fact that it is rated for that operation.

[DanYmaL X]

So my amp, which is rated 320W at 4 ohms is really underpowered for the 400W 8 ohm speaker... hence the clipping.

Solution: get a new amp head... Oh, yeah! I can't wait to tell my wife... oh, um, well, never mind.

Apparently I need to go with a couple of 100W cabs, or something along those lines...

Is there somewhere I can look up the formulas used to calculate this stuff?

DX

[Scoot]

Originally posted by JPMIII:

. . . As a rule of thumb, you always want your amp to be able to put out more power then the speaker is capable of handling or you run the problem of clipping the amp. . .

I'm not a gearhead (obviously), but is this correct? Or is it just worded incorrectly? Wouldn't this damage the speakers?

[Soundcrafter]

Coming from a sound guy (who's learning bass now), that is correct. You always want some more power in your amp than your speaker can take, to protect from clipping.

[Scoot]

And danymal_x, think of "ohm's" as "resistance" because that's what it is. Each cabinet has an "ohm rating", which signifies how much electrical resistance it gives to the amp. The higher the ohm rating, the more resistance it gives.

 

Amps on the other hand, CAN have several power ratings. One example is my Ampeg B5R:

 

200W @ 8 ohms

350W @ 4 ohms

500W @ 2 ohms

 

As you see, the less the ohm rating (resistance from cabinet), the more power (Watts) is allowed to flow through the speakers. My cabinet is an 8 ohm, rated at 600W maximum; so I'm only pushing 200W through my cabinet (although the cabinet can handle 400W more.)

 

And yes, there is a formula to calculate ohms for multiple cabinets, as you'll see, the more cabs, the less resistance (and thus, more power which equals more volume!)

 

1/(1/speakerA)+(1/speakerB)=total ohm rating

 

If you're using two 8-ohm cabinets, your formula becomes:

 

1/(1/8)+(1/8)=4 ohms

 

If you're using one 8-ohm and one 4-ohm cabinet, it is:

 

1/(1/8)+(1/4)=2.67 ohms

 

Hope this makes sense, and I hope it's correct! This is how I've taught myself anyways!

[_Sweet Willie_]

Originally posted by danymal_x:

Is there somewhere I can look up the formulas used to calculate this stuff?

DX

Try a search on this board for "impedance." The topic has been discussed quite a bit.

 

Generally speaking, an amp will put double the wattage into 1/2 the impedance. This is not a hard and fast rule, however, and you should always look up the specs for a particular amp. (For example, the Hartke 2000 head puts 200W into 4 ohms, but 120W (not 100W!) into 8 ohms.)

 

In most cases, if you run 2 cabs off of a single amp (excluding for the moment stereo amps), you will be running them in parallel. Two 8-ohm cabs run parallel give a 4-ohm load. Two 4-ohm cabs run parallel give a 2-ohm load. An 8-ohm cab and a 4-ohm cab give a 2.67-ohm load! I've read the formula on this board a hundred times, but now it escapes me. I'm off to look it up for you...

 

Peace.

[_Sweet Willie_]

Oooops, while I took my time writing my last post, Scootdog put up the relevant info with some good examples. I'll go back to my cave now!

 

Peace.

[_Sweet Willie_]

Originally posted by Soundcrafter:

Coming from a sound guy (who's learning bass now), that is correct. You always want some more power in your amp than your speaker can take, to protect from clipping.

Yes.

 

Go to a store and try to use heads that are identical (or very close to it) except for their power rating (e.g., the Mesa Boogie M-Pulse 360 and M-Pulse 600 or the Hartke 2000 and Hartke 3500, etc.) thru the same cab. As you drive the cab louder, you'll actually hear the difference the added headroom makes, even if you're not running either amp at full power. (At least that's the experience I had when I A/B'd both the M-Pulse 360 and the M-Pulse 600.)

 

Of course, you also want to use your ears to make sure you don't crank your amp so much that you blow your speaker from too much power! There's certainly an "overkill" factor (i.e., careful with that 1000W power amp on a speaker rated at only 50W!).

[Scoot]

Originally posted by Soundcrafter:

Coming from a sound guy (who's learning bass now), that is correct. You always want some more power in your amp than your speaker can take, to protect from clipping.

But you wouldn't want some monster amp that can push 800W at 8 ohms driving an 8 ohm/600W cabinet, would you?? I'm only pushing 200W thru my 8-ohm/600W cab, and when it clips, I turn the gain down. I've always thought of clipping as a notice that you're pushing as much power as possible, not that you're damaging anything.

[fantasticsound]

Scootdog, JPMIII did, indeed, word that phrase as he meant it.

 

You'd think that overpowering the speaker would cause damage, and it can. The usual speaker powerhandling rating is an RMS average. Cheap speakers will be tested with a sine wave. If peak wattage is tested, it will be done by quickly overloading the speaker. Peak handling is usually about twice the RMS average in this instance.

 

A good speaker (Such as JBL, EV, etc.) will be tested with wideband noise over a period of 8 hours or more. As you might guess, this is a far more rigorous test, which yields a lower power rating than would result with the cheap speaker test. It takes into account heat dissapation and vibration. Peak handling is about 4 times the RMS average with this test.

 

So back to JPMIII's point. If you take a wave, overload the amp, it creates clipping distortion. Again, the result of this is a square wave. VERY bad for a speaker.

 

On the other hand, if you push the RMS power rating of a speaker with a clean signal, even a cheap speaker should handle almost twice as much power as the rating for short bursts. The signal is far less rigorous than a square wave. So over powering with a clean signal will do no damage while underpowering with a clipped wave can blow the speaker.

 

Some corrections.

 

originally quoted by JPMIII:

If your amp is rated for 200 watts at 8 ohms then it is capable of putting out 400 watts at 4 ohms (roughly).

Not exactly. More like 1/3 more power at 4 ohms than 8 ohms from a given amplifier. Not a good idea to guess high. As you said, you'll cause damage by underpowering. Some amps won't handle a lower resistance. You end up burning up the power supply as it attempts to keep up. I like to compare this to running downhill. At some point, the downgrade that assists you in running fast with less energy becomes steep to the point your legs can't move fast enough to stay under you. Crash.. burn... Ouch!

 

If you want comparisons, check a couple of amplifer manufacturer websites for specs on their products.

 

And yes, I know if you look at some cheap Yamaha amps (particularly their PA heads and powered mixers) you can find specs that are 1:2 for 8 ohms: 4 ohms.

 

Originally posted by Danymal:

So my amp, which is rated 320W at 4 ohms is really underpowered for the 400W 8 ohm speaker... hence the clipping.

Yes... kind of. You need to find out the rating of the cabinet, not the speaker. It's possible Fender has used electronics inside to alter the input impedance of the cabinet. Most probably you're correct, and someone replaced the speaker with a more powerful one. (Is this a used cabinet?)

 

In this case, you should make sure there is no distortion produced by the amp when a clean signal is passed. If it is distorting, you're clipping the amp.

 

Most bass amps nowadays have a built in peak limiter to protect your speaker cabs. If yours is switchable, keep it on to protect that speaker. That explains most cases of, "my clip light is on all the time but the sound is clean." Beware, however, that you may be damaging the head as you're actually running at it's power limits most of the time.

[_Sweet Willie_]

Originally posted by Scootdog:

Originally posted by Soundcrafter:

Coming from a sound guy (who's learning bass now), that is correct. You always want some more power in your amp than your speaker can take, to protect from clipping.

But you wouldn't want some monster amp that can push 800W at 8 ohms driving an 8 ohm/600W cabinet, would you?? I'm only pushing 200W thru my 8-ohm/600W cab, and when it clips, I turn the gain down. I've always thought of clipping as a notice that you're pushing as much power as possible, not that you're damaging anything.

Yes, you would want that 800W amp. Just don't turn the volume knob up to 11. One way I rationalize it (and maybe wrongly so), is that if I were to ever want the full 600W sound from that cab, I'd want my amp to get there cleanly and not be straining at the extremes.

 

Clipping is your signal to turn it down before you damage anything. But if you continually have clipping at the power amp, you will be continually doing damage to your speakers. It also means you don't have as much juice as you need for your playing situation. If you're clipping once in a while, don't sweat it -- just turn down and keep playing. BUT, try A/B'ing similar amps with different power specs and you really will hear a sound quality difference thru the same speaker cab.

 

Rock on.

[_Sweet Willie_]

Neil (fantasticsound) also makes a great point about RMS vs. peak power ratings.

 

Basically, you're best off if you can avoid clipping.

 

Later.

[Scoot]

Cool, the line: "Peak handling is about 4 times the RMS average" cleared it up for me. Makes sense now. Thanks guys!

 

sd

[fantasticsound]

Sweet Willie hit it on the head with this comment:

 

Origially posted by Sweet Willie:

Yes, you would want that 800W amp. Just don't turn the volume knob up to 11. One way I rationalize it (and maybe wrongly so), is that if I were to ever want the full 600W sound from that cab, I'd want my amp to get there cleanly and not be straining at the extremes.

Remember, Scootdog, the 4x spec is only if they do the rigorous test. Figure 2x for cheap speakers. If the speaker or amp company doesn't list how they arrived at their spec, it's probably the cheap test. Makes the speaker power rating look more impressive.

[DanYmaL X]

Originally posted by fantasticsound:

...

Originally posted by Danymal:

So my amp, which is rated 320W at 4 ohms is really underpowered for the 400W 8 ohm speaker... hence the clipping.

Yes... kind of. You need to find out the rating of the cabinet, not the speaker. It's possible Fender has used electronics inside to alter the input impedance of the cabinet. Most probably you're correct, and someone replaced the speaker with a more powerful one. (Is this a used cabinet?)

 

In this case, you should make sure there is no distortion produced by the amp when a clean signal is passed. If it is distorting, you're clipping the amp.

 

Most bass amps nowadays have a built in peak limiter to protect your speaker cabs. If yours is switchable, keep it on to protect that speaker. That explains most cases of, "my clip light is on all the time but the sound is clean." Beware, however, that you may be damaging the head as you're actually running at it's power limits most of the time.

It is a used cab, and it does not look like there is or was any additional electronics... the Fender specs were 145W @ 4 ohms, from what I've been able to determine... the Sound Technologies speaker is the same model that they use in one of their PA subwoofers (thats where I found the specs for it, anyways.)

My amp does have a limiter in the form of a push/pull pot, but I haven't really been able to notice any difference when I've experimented with it.

I'm pretty confident I get the basic idea of the whole impedence thing, so here's another question.

On the back of my Amp along with all of the pre-amp and direct outs, there are two 1/4" outputs, labelled 320W 4 Ohms Min. Does this mean, each output is 320W, or combined? Is it stereo?

Forgive my seeming ignorance on this but this is the first real full featured amp I've owned. Most of my past experience has been with combos along the lines of:

1 or 2 inputs, 1 Speaker out, Headphones out,(maybe)FX loop in/out etc...

I have been unable to find ANY technical data on this amp whatsoever. I bought the set used a while back (almost a year now) when I decided to get back into the music scene after a looong stretch of missing it. (10 years, I think...)

I was fortunate enough to have made a good friend who's band needed a bass player, and I decided, what the hell, why not? So here I am, and I'm ready to build up my rig.

I really appreciate all of the input from everyone!

 

DX

[Scoot]

Originally posted by danymal_x:

. . . On the back of my Amp along with all of the pre-amp and direct outs, there are two 1/4" outputs, labelled 320W 4 Ohms Min. Does this mean, each output is 320W, or combined? Is it stereo? . . .

More than likely, it means either output is 320W by itself. And no, it's probably not stereo. The reason that there is 2 is so you can hook 2 cabinets up in parallel (see that formula I posted above for calculating ohm levels while running multiple cabs in parallel.)

 

The "320W 4 Ohms Min" means that you'll push 320W @ 4ohms, and you need a minimum of 4ohms resistance. Any amount of resistance less than 4ohms can give you an expensive smoke machine.

 

Once again (disclaimer): I think this is what it means, as you see earlier in the thread I am not the final word on gear.

[fantasticsound]

Originally posted by danymal_x:

...there are two 1/4" outputs, labelled 320W 4 Ohms Min. Does this mean, each output is 320W, or combined? Is it stereo? DX

Unless it clearly states these are two separate channels, it is combined output. The jacks should be wired in parallel. Therefore, plugging in two 8 ohm speakers would be fine. Plugging in one 4 ohm speaker would be fine. Plugging in two 4 ohm speakers is a definite no-no!

[Scoot]

Okay, from now on, nobody listen to me as far as gear is concerned. But at least I have learned a lot from this thread.

 

At least I got that formula right . . . didn't I???

[DanYmaL X]

If nothin else, Scoot, ya did open up the floor for the experts! Thanks for the help bro.

 

DX

[Scoot]

Gee, I feel better now. I'm now "Scootdog, the Forum Idiot"

 

[DanYmaL X]

If it's any consolation, Scootdog, I thought the same things... 'specially about the amp not being more powerful than your speakers... thats what started this whole thread, when I read that article that said the speakers should be lower wattage than the amp can put out... thats what happens when you go reading... gets ya in trouble every time!

 

DX

[fantasticsound]

I'd hardly call you an idiot for stating what you've learned and asking questions, as you did above.

[GeorgeR]

I don't think anyone has mentioned this yet, but impedance matching shouldn't be taken TOO seriously in a number of instances.....

 

What? HORRORS???

 

Why speak of such heresy?

 

1) Speaker impedance ratings are based on a nominal value.

 

If you've ever seen a plot of speaker impedance versus frequency, you'll observe that a typical 8 ohm 12 inch speaker may demonstrate an impedance as high as 60 ohms or higher at certain frequencies, and as low as the DC resistance of the coil at other frequencies. Speakers are highly reactive beasts, hence their rating as an impedance than a resistance.

 

2) Decibel-wise, hooking your amp up for 200 watts at 8 Ohms, or 400 watts at 4 Ohms, you actually won't hear much of a difference. You might think you do, but use a C-weighted SPL meter to measure it, and you'll be surprised.

 

Why?

 

The mathematics behind this are logarithms. The human ear isn't linear, but logarithmic.

 

With a mere doubling of power input (notice that my stating "doubling" means the power level increase is RELATIVE to a previous and lower level, not to some distinct or absolute level), the sound level increases by only 3 dB, a barely noticeable difference to the human ear.

 

The more advanced folks here know that for a sound source to sound twice as loud, an actual ten-fold increase of input power to the speaker is required (10 dB increase = twice as loud).

 

So, a popular myth among youngsters I know: A 100 watt bass amp is twice as loud, compared to a 50 watt bass amp?

 

NOPE. The difference is a mere 3 db, all other things (such as speaker efficiency) being equal and unchanged.

 

Now jumping from 50 watts to maybe 300 watts will start to have some effect. At least the sound pressure from the same speaker system will increase by about 7 3/4 dB as compared to the original 50 watt amp driving the same speaker system.

 

More importantly (ask any audiophile) is the speaker efficiency, since they're terribly inefficient buggers to begin with.

 

So........don't sweat the impedance matching and power levels TOO much, ok? At a certain point (3 dB??) there's nothing gained by the extra cost and effort!

 

But don't risk loading your amp below the manufacturer's impedance rating either, or depending on the quality of the amp, you might risk blowing the output transistors.

 

BTW, a bit of engineering stuff here.....tube amps don't really get bothered by too low a load impedance. Heck, I've witnessed and know of many a tube amp, surviving running full out with a dead short on the output lines.

 

What ticks tube amps off is running without any load at all, or run uncompensated with a capacitive load (like an electrostatic speaker) on a typical voltage based negative feedback system.

 

Capacitive loads are stable to use when amps run on negative feedback based on current feedback, not voltage feedback.

 

(Solid state amps using voltage feedback don't like capacitive loads either.)

[fantasticsound]

Good stuff, George.

[Adrian Smith]

Just when I thought I had the whole impedence thing down... sheesh.

 

Well I have a question now. 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.

 

Awesome thread, by the way.

[DanYmaL X]

Based on what everyone who has contributed to this thread has indicated... I'd say get a bigger amp! (Who wants smaller speakers?)

I know there's alot of technical this-and-that to consider, and there are factors to consider in each situation... but based on the input from this thread, the solution to my particular problem is going to be:

A) Find a 200W+/- to put with the amp I currently use, and stick it in my garage as a practice amp... (big-ass practice amp, but hey...)

B) Pick up another head @ 800W(+) and a 2x10 cab @ 200W to add to my 400W cab.

There, problem solved. Now I don't need the 100W Combo amp I was going to buy, and I can still get my 2x10 Cab, and I get to get a new amp head- not going to buy used this time-I don't like not having the factory information to fall back on. If I'd had that I would never have had this problem in the first place. But then this cool thread would have never started...(until someone else asked the same question.)

Thanks again to everyone who contributed and helped out.

DX

[alexclaber]

Originally posted by Adrian Smith:

[QB]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.

QB]

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. But watts are very cheap nowadays, you might as well go for a power amp that puts out 1000W bridged into 4 ohms allied to a separate preamp.

 

For the $750 that an SWR350X costs new you could get a Mackie M1400i or similar power amp and a second-hand SWR Grand Prix which would give you far better tone than the SWR350X and much more volume. Of course the downside is the extra weight and size, but if you carting a 4x10" anyway that shouldn't bother you.

 

Alex