Easy Math Free Impedance (Ohms)

[Craig Vecchione]

For whatever it's worth, if you have no load and no input on a tube amp, no damage will be done. Having no load and a given amount of input will lead to the transformer heating, possibly to the point of insulation damage and eventual failure. But this is something that you quite literally have to intentionally do with a bass amp, by playing into it with no speakers. It's not a case of "ZAP" instantaneously. It was a lot more common to have a transformer failure with an amp used for something like a stereo, where you could more easily have it on and driven with no load, and not be aware of this.

[Nate42]

Part 5: Cabs and their power ratings

 

As mentioned in the newly edited Part 1, cabs will be rated for the amount of power they can handle. The basic rule here is pretty simple: if you don't want to blow you speakers, don't give them more power than they can handle for an extended period. This doesn't mean you need to match your amps power with the cabs rating exactly though. A cab that can handle more power than you've got is never a problem, and having one that can handle a little less than what your amp puts out is no big deal either.

 

In fact, with a solid state amp, I actually contend that with an ideal setup your amp would actually have more power than your speakers can handle. This is because you presumably want to be able to get the most out of your speakers, and also speakers usually sound their best at somewhere in the vicinity of their rated power. As you play, there will be occasional peaks in your signal that are higher than your usual operating levels. These peaks won't last long enough to harm your speakers, but your amp needs to have enough power to be able to reproduce them properly. This concept is often referred to as 'head room', basically you want to give your amp a little breathing room between what it can do and where you typically run it. This of course requires you be careful with the volume knob. You can run 100W speakers with a 1000W amp all day with no difficulty, but if you crank the volume all the way up, don't be surprised when the speakers go pop.

 

Please understand I'm not saying you HAVE to have more power than you speakers can handle. You won't hurt anything by having cabs with a very high power rating.

 

With a tube amp, you typically want the cab to be able to handle more power than the amp is rated for. This is because people like to overdrive their tube amps (that's pretty much the point) and an overdriven amp by definition puts out more power than its clean rating. Also, you don't really need to concern yourself with headroom so much with tube amps, because tube amps clip in a pleasant manner, unlike solid state amps which sound like {censored} when pushed beyond their limits.

 

A note on running more than one cab: If you have more than one cab, they don't each have to be able to handle your amps full power, they split the available power. If they are of equal impedance, they split it equally. So a 500W amp with two 8 ohm cabs gives each cab 250W. If the cabs are different impedance its a little more complicated (the lower impedance cab gets more power) but still no one cab has to handle the full power of the amp.

 

Regardless of what kind of amp you have and your power ratings, always follow the golden rule: If your speakers are distorting, turn your {censored} down.

[isaac42]

Originally posted by bikehorn output transformers are the beefiest of the three(usually there are three transformers in a tube amp) transformers in a tube amp. they generate plenty of power and when there's nowhere for it to go, it heats up the transformer quite a bit. eventually, which is not THAT far off, this leads to failure of the transformer. if it's not too badly damaged, it can be rewound. if the core is shot too, the whole transformer is a write-off. Output transformers are in general an expensive part to replace. a replacement for an SVT can be $300 or more.

First, none of my tube amps has three transformers. Next, infinite resistance is not possible. The primary windings have a DC resistance, and eddy currents in the transformer will prevent the primary seeing an infinite reflectdd impedance. The power suply cannot magically develop a voltage higher than the supply is designed for. The Power transformer produces a specific output based on the input and the turns ratio. That is rectified and filtered by the power supply capacitors. Volage cannot, will not exceed this maximum supply voltage.

[Mr. Pants]

Originally posted by Nate42 Part 5: Cabs and their power ratings

 

I thought part 5 was going to be Stereo Power Amps :confused:

 

-b

[Nate42]

It was. Now there's gonna be a part 6.

[the_big_geez]

 

Originally posted by isaac42 First, none of my tube amps has three transformers. ...

 

 

 

You obviously don't have enough amps then! I have had several with 3 Xfmrs, but the 3rd one is either dedicated to filament power or used as a choke.

 

 

Originally posted by isaac42 ... Next, infinite resistance is not possible. The primary windings have a DC resistance, and eddy currents in the transformer will prevent the primary seeing an infinite reflectdd impedance. The power suply cannot magically develop a voltage higher than the supply is designed for. The Power transformer produces a specific output based on the input and the turns ratio. That is rectified and filtered by the power supply capacitors. Volage cannot, will not exceed this maximum supply voltage.

 

 

I'm with you 100% on this.

 

But it is worth pointing out that input and output ratings are relative to one-another, meaning that a transformer rated at 350v output at 110v input will produce a higher output at an input voltage greater than 110v, and vice-versa.

 

I only bring that up as a point of clarification for others.

 

 

 

 

[bassmantele]

 

Originally posted by isaac42 The power suply cannot magically develop a voltage higher than the supply is designed for. The Power transformer produces a specific output based on the input and the turns ratio. That is rectified and filtered by the power supply capacitors. Volage cannot, will not exceed this maximum supply voltage.

 

 

 

 

From alt.guitar.amps:

 

 

 

A day or two ago I read on this NG that one reason OT's

in combos last longer than they do in discrete heads is

that the latter are more prone to being run with no

speakers and thus damaged.

 

I don't quite get this. If you disconnect the speaker

from the secondary then the primary's impedance will become

really high and very little current will flow in the output

transformer. I can imagine that the output tubes might

conceivably oscillate under no load, or that the voltage

might go so high that the transformer might arc over and

thus be hurt, but those both seem a little farfetched to

explain lots of blown parts.

 

I can understand how a shorted output could lead to

runaway primary current, but not an open output ..

 

Would anyone more knowledgeable than I care to comment?

 

 

 

 

Response:

 

 

With the OT open circuit, it acts like a flyback transformer...

the collapsing magnetic field causes very high voltages to form,

which will eat up tubes, tube sockets, and the output transformer.

The OT is the heart and soul of these amps, and a really good one

costs $100-$200. Expensive enough.

 

 

 

 

 

 

From R.G. Keen's Tube Amp FAQ:

 

 

Q: I've seen circuits that use reverse biased diodes connected from ground to the plates of output tubes as "transient spike preventers". How does this work?

 

A:The 1N4007's serve mainly as an amulets against the voltage gods in this case. An inductive flyback pulse will go to literally ANY voltage until it finds a discharge path. Ideally, transients that would cause very high positive voltages on one push-pull plate would cause high negative voltages on the other plate, and the diodes on the negative going plate would clamp the voltages on the positive going plate through the output transformer. This does indeed happen for small, extremely-tightly coupled transformers. However, any leakage inductance between the two primaries prevents the tight coupling that would let the negative going diodes protect, and worse yet, it's the leakage inductances that cause the spikes on transients anyways.

 

What really happens is that the first few flyback pulses that occur will break over the 1N4007's rather than than arcing the plates on the positive side, so there really is some protection, it's not just where it looks like it is. If you're lucky, the 1N4007's break over before the transformer insulation punches through, and all is well until the 1N4007's go leaky or short. Probably better than nothing, but not a whole lot of additional protection, either. Heck, amulets are not harmful, I guess.

[the_big_geez]

Bassmantele: Good stuff, but you targeted the wrong user. Isaac was referring to POWER Xfmrs, not OUTPUT Xfmrs.

 

 

As for what was said in your post --and this thread as a whole-- It's really simple and math free: The owner's manual tells you not to do it, so don't.

 

Your car's owner's manual tells you not to run your engine without oil, so (I hope) you don't. Every now and then one must take solace in the fact that the people who built something actually know something about the subject, and trust in their advice and better judgement.

 

Common sense must also prevail. While I've never seen an owner's manual specifically state that one should not drop their product from rooftops, how many warranty claims for such an event do you think would be neccessary before the manufacturer was compelled to include this in the product literature?

 

Going back to the auto manual as an example, just read one and you'll see the point. It will be filled with banal factoids that SHOULD be common knowledge by now ("Push the button in to turn device on. Pull it out to turn off. ... Turn your headlights to low beam when traffic approaches.", etc.

 

Be thankful that the community of musicians is generally less litiginous than the car-buying public, else your manual would eventually tell you: "Never plug this device into any AC power supply and it will last you a lifetime."

[Mr. Pants]

 

Originally posted by the_big_geez Bassmantele: Good stuff, but you targeted the wrong user. Isaac was referring to POWER Xfmrs, not OUTPUT Xfmrs. As for what was said in your post --and this thread as a whole-- It's really simple and math free: The owner's manual tells you not to do it, so don't. Your car's owner's manual tells you not to run your engine without oil, so (I hope) you don't. Every now and then one must take solace in the fact that the people who built something actually know something about the subject, and trust in their advice and better judgement. Common sense must also prevail. While I've never seen an owner's manual specifically state that one should not drop their product from rooftops, how many warranty claims for such an event do you think would be neccessary before the manufacturer was compelled to include this in the product literature? Going back to the auto manual as an example, just read one and you'll see the point. It will be filled with banal factoids that SHOULD be common knowledge by now ("Push the button in to turn device on. Pull it out to turn off. ... Turn your headlights to low beam when traffic approaches.", etc. Be thankful that the community of musicians is generally less litiginous than the car-buying public, else your manual would eventually tell you: "Never plug this device into any AC power supply and it will last you a lifetime."

 

 

Or at the very least: DO NOT PUT HEAD INSIDE CABINET WHILE AMP IS ON 10

 

-b

[bassmantele]

 

Originally posted by the_big_geez Bassmantele: Good stuff, but you targeted the wrong user. Isaac was referring to POWER Xfmrs, not OUTPUT Xfmrs.

 

 

 

As I understood, he was referring back to the "no load" references in earlier posts, including mine. The flyback spike produced in no-load conditions far exceeds the voltage produced by the power transformer, so I was trying to make that point. If I misunderstood Isaac then please excuse me.

[MackemGeezer]

I struggle with amplification issues and am extremely grateful for this thread!!

 

Nate42 for president.

[mrcrow]

Originally posted by MackemGeezer I struggle with amplification issues and am extremely grateful for this thread!! Nate42 for president.

i struggle with my shoelaces..

 

 

God bless Nate42....have a magnificent christmas:cool:

[isaac42]

Originally posted by bassmantele As I understood, he was referring back to the "no load" references in earlier posts, including mine. The flyback spike produced in no-load conditions far exceeds the voltage produced by the power transformer, so I was trying to make that point. If I misunderstood Isaac then please excuse me.

You're right, and the explanations given do, indeed, refer to flyback, or inductive kick, which I identified as the only possible mechanism I could think of. I still see no reason for this to occur unless the speaker is disconnected while under load, but I don't claim to know everything.

[Fran da Man]

Easy?????

[Nate42]

Originally posted by Fran da Man Easy?????

 

Well, if you ignore the tube debate and stick to my multipart essay, its as easy as I know how to make it. A little wordy maybe but the concepts aren't hard, and no math required. I've also tried to structure it in such a way that you can skip information that doesn't apply to you. You're welcome to try to do better.

[Nate42]

bump

[Nate42]

Part 6: Stereo Power Amps

 

Usually considered part of a PA setup, stereo power amps can also be used in a bass rig. There are also some bass amps with a stereo power section (sometimes called biampable) that have many of the same features as a stereo power amp.

 

So why would you want a stereo power amp? Mostly because the watts per dollar ratio is nice and high, and because you have a great deal of flexability in how you can hook one up. Of course as we're so fond of saying, you'll need a preamp with that.

 

The basic rules are the same as with any other solid state amp, except that a stereo power amp is essentially two completely seperate amps that just happen to be in the same box. So if you hook one cab up to one channel, and another cab to the other channel, they aren't in parallel, they aren't even hooked up to the same amp, so you don't have to worry about how their impedances combine. Of course you can still put more than one cab on a channel. Since most modern amps can handle as low as 2 ohms on each channel (be sure to check your manual) you could have a group of cabs that combine to 2 ohms (say four 8 ohm cabs) on one channel, and another group of cabs that combine to 2 ohms on the other channel. Thats way more cabs than you'd ever need, but its nice to know you have that kind of flexability.

 

Here are some of the ways you can use a stereo power amp

 

dual mono: Not to be confused with bridged mono, dual mono simply means the same signal is sent to both channels. They still act as two seperate amps, they just have the same input. This is a pretty common usage for bass players. Since each channel has its own volume control, you can change your sound somewhat by controlling how much power each cab gets. This is very useful if you have different types of cabs, like a 4x10 on one channel and a 1x15 on the other.

 

biamp: a biamped rig uses a device called a crossover to split your signal into two (or more) frequency bands. Then each group of frequencies is sent to its own amplifier channel and cab. For example, you could send lows to a subwoofer, and mids and highs to a fullrange cab. This way your speakers don't have to work as hard since they only have to deal with the range of frequencies they handle best. Again, each channel has its own volume, so you can alter your tone by adjusting the volume of each channel.

 

stereo: this is stereo in the same sense as your home stereo system, where one channel is used for a speaker (or group of speakers) on the left, and the other channel for the right. You use a stereo effects processor or pedal (often chorus for bassists) to split into the left and right signals. Most bassists don't bother with stereo since the lowest frequencies aren't very directional anyway, but still you can do it if you want, and it can make for some cool effects.

 

bridged mono: To get the absolute most power out of a stereo amp, you run it in bridged mono mode. This makes the two channels work together as a single amplifier, and essentially doubles your output power. However it also doubles your minimum impedance rating, so you have to be careful. Here's an example of how you can get into trouble: suppose you have a stereo amp that can handle 2 ohms on each channel. You're running in dual mono with a 4 ohm cab on each channel. No problem, since you're running each channel at 4 ohms and its rated for 2, the amp isn't even breaking a sweat. But if you decide to run in bridged mode, the amp can now only handle 4 ohms, and the two cabinets now have to be in parallel, for a combined impedance of 2 ohms. Your amp isn't going to like that, and will (if you're lucky) overheat and shut down.

 

A note on power and impedance ratings: Like any other solid state amp, stereo power amps put out more power into lower impedances. However, as mentioned, they have the added complication of the minimum impedance being different for bridged mode and stereo mode. As an example, here's the specs of a Mackie 1400i

 

stereo:

300W per channel at 8 ohms

500W per channel at 4 ohms

700W per channel at 2 ohms

minimum impedance 2 ohms

 

bridged mono:

1000W at 8 ohms

1400W at 4 ohms

minimum impedance 4 ohms.

 

Of course you should consult your manual for the specific ratings of your amp, but this should give you an idea of the sort of thing to expect.

[RSBro]

Great stuff.

 

 

Only on the HCBF did I ever "figure out" all that ohmage crap. Now I try to explain it to my guitarist...

 

Riiiiiiiiiiiiiight.

[Nate42]

bump. I'm pretty much done at this point. I'll probably still do some kind of summary/conclusion, and I may edit a couple things here and there, but this is pretty much it I'm thinkin. Anybody feel like I left out something important?

[lug]

 

Originally posted by Nate42 Anybody feel like I left out something important?

 

 

You're gonna need a preamp with that.

[Jazz Ad]

Just let me know when you think it's ready.

[Nate42]

Originally posted by lug You're gonna need a preamp with that.

 

See part 6, paragraph 2.

[lug]

Originally posted by Nate42 See part 6, paragraph 2.

 

Drats! Caught not doing my homework again!

[Nate42]

Conclusion

 

Well, this about wraps it up. Turned out a little longer than I'd hoped, it takes awhile to explain all the common questions you see with this stuff. But despite its length I hope I've stuck with my goal of keeping it simple, the concepts aren't hard, just stick with it and you'll get it. In summary:

 

Don't run your solid state amp at a lower impedance than it is rated for.

 

If you connect two or more speakers to one amp (or one channel of a stereo amp) they are in parallel.

 

Match your tube amp exactly to the cab impedance it is expecting.

 

If your speakers are distorting, turn down. 'Underpowering' is a myth, and turning down is NEVER a bad thing.

 

Your impedance rating doubles if you run a stereo amp in bridged mono, so be careful.

[Nate42]

Appendix

 

Here's some extra stuff that, while not essential, is probably worth mentioning.

 

What was that formula for impedances in parallel? 1/Ztotal = 1/Z1 + 1/Z2 + ... , this can be extended for as many impedances as you want. Example 1/8 + 1/4 = .375, so Ztotal = 1/.375 = approx. 2.67. So 8 in parallel with 4 is 2.67.

 

 

What about wiring in series? If you know a thing or two about electronics, you know there are two basic wiring schemes, series and parallel. When impedances are in series they simply add, 4ohms + 4ohms = 8 for example. However, you don't really need to worry about this unless you are making your own cab. Individual speakers within a cab might be in series, but we're normally just concerned with the total impedance of the cab, and multiple cabs on the same amp are always in parallel. You could concievably make a special cable to connect two cabs to the same amp in series, but this is rarely done and not something novices should concern themselves with.

 

I've got a multimeter, why doesn't my cab show the right ohms? Simple multimeters measure DC resistance, which is only part of the story when you're talking about impedance. It is completely normal for a cabinet to have a resistance measurement less than its total rated impedance. So if you measure your 4 ohm cab at 3 ohms, or your 8 ohm cab at 6, or whatever, no need to be concerned. Making an accurate impedance measurement requires more equipment and know how than the average joe can be expected to have, that's why the manufacturer makes those measurements for us.