• TalkBass has been independent since 1998. Add your voice.
    Create a free account to reply to discussions, view embedded media, and browse with fewer display ads.
    Join free Log in
    Want zero display ads or expanded classifieds tools? Compare plans.

Impedance: By the book or close enough for rock and roll?

JimmyM

JimmyM

Apr 11, 2005
127,257
125,180
Plymouth NC
Disclosures
Endorsing: Yamaha, Ampeg, Line 6, EMG
#1
So we all have lots and lots of cabs and we want to mix them all together in as many different ways as we can. Trouble is, the Impedance Police come along and say you can't do it. Most solid state amps require at least a 4-ohm load. And tube amps require an exact impedance match with their input jacks. Fair enough. Nobody wants to damage their amps so we dutifully follow the rules.

Now I've never tried to run a solid state head at less than 4 ohms, but I have often used SVT's into one or two 8-ohm cabs with nary a lick of trouble. Yet I keep hearing the Impedance Police threatening to kill my amp if I do.

Now I am by no means an expert, so that's why I'm throwing this out there for wide discussion. But what I've read about impedance says it's constantly changing with the load your rig gets from the instrument, and it can vary wildly with every note, and that a speaker's rated impedance is merely a mean number. And no less of an authority than Jess Oliver, the inventor of the Ampeg B-15, recommends having the original 8-ohm speakers reconed with 4-ohm kits because...well here, let's let him tell you:

"After being in the amplifier business for many years, I found out you don't always have to follow the book rules. When it comes to speaker load on a tube amp, I've found you can load a tube amp down to saturation without damaging the amp. You just get more effective power out of the amp. Just as long as the secondary of the output transformer "sees" a path, some resistance, it doesn't matter how low that resistance is, as long as it sounds good, and four ohms is not below the saturation point."

So between the fluctuations in impedance and the extra effective wattage you get out of the amp, I'm starting to see no reason to follow the Laws of Impedance whatsoever. Yeah, OK, maybe keep it down to 2 ohms just to make sure you don't go below saturation long enough to damage the amp, but from everything I've gathered, it doesn't seem to be as crucial as it's made out to be at all. On the other hand, every single manufacturer still has impedance specs that they tell you will damage the amp if you don't follow them to the letter and nobody seems to be in any great hurry to change it.

So the question here is why are we told that you have to change output transformers in tube amps to use speakers with different impedances, and why can't you run below 4 ohms in a solid state amp? Are there real risks involved, or is it just caution to keep people from doing something ridiculous like stringing 10 speakers into one jack? At the very least, it sounds to me like you could run a 2-ohm load at all times in any amp without damaging it, which I would gladly settle for.
 
#2
Lots of solid state amps will run two ohms..

Anyway my '75 Traynor YBA-1 "wants" to see 8 ohms but I always run it with 4. Hasn't failed me yet. The Hammond transformers in it are huge and the amp is built like a tank, I'm sure it'll be fine. Close enough for rock and roll indeed. I would not load it down to 2, it wouldn't sound that great first of all, not to mention a lot harder on the tubes, etc.

I would not run it with a 16 ohm load either, I won't get into explaining why. And I wouldn't run a 4 ohm solid state head at 2, but I'm sure modern solid state heads have protection built in so it probably would just shut down if it got too hot.
 
#3
Who writes your material for you, Jimmy? (Evidently someone with too much time on their hands). I mean, 2-ohm capable heads and power amps have been around for decades. That aside, don't let a lack of general knowledge and more importantly, specific cases, stop you from advocating things that are just too likely to cause misunderstandings and problems for newbies. By all means spread the ignorance! ; }
 
#4
That was uncalled for by a longshot, dude.

Yes, I know lots of ss amps are 2-ohm capable. But lots of them aren't, or claim not to be. My Markbass head for example...the instructions very clearly read not to use anything less than a 4 ohm load. So do my SWR and Ampeg hybrid heads. So it's a legit question and your response was out of line.

And so what if I have too much time on my hands...doesn't happen much these days so better to expand my knowledge than go watch cartoons.
 
#5
Ah, gee, I really didn't mean to hurt your feelings. But it can be serious business to pass around ideas that others might take as gospel and end up then causing equipment failures or at least downtime at gigs.

As far as 2-ohm SS gear, again, tons of it has been around for decades. And conversely, some newer amplification stuff has had trouble with loads that are supposedly whatever-ohm nominal, what has been termed "difficult loads" when the blame gets passed and nobody wants to admit they engineered a little outside the nominal bounds for one reason or another (dollars, or performance shortcut).
 
#7
Thank you. That's all I wanted to know, because I plan to do some cabinet testing and light impedance mismatching once I get my SVT speakers reconed. Hey, it's all I've got to do in Florida with the bugs and the annoying tourists and the old people driving 45 on the interstate. You live in God's country with hardly any annoyances at all!

Ander, Devo James Brown says "Don't freak, be happy!"
 
#8
Jimmy, these questions are meant in all seriousness and without even a hint of attitude (it's often hard to tell on the internet):

Do you understand exactly what Jess Oliver is saying in that quote? If you do, you will understand why it is okay to run amps with output transformers below the rated impedance in certain situations.

Solid state amps are another issue entirely, where heat is the major enemy. Do you understand how running a solid state amp below its rated output generates excess heat within the amplifier and how that heat can affect the various components?

There are lots of really well written publications on these topics that are relatively easy for the novice engineer to understand. I will recommend one written by another forumite "Dave Funk's Tube Amp Workbook." This is a book that will take you from the basics into design considerations that will help you understand the answers to your questions and mine in a very straight forward way. I have lots of other books here that are much more detailed, but Dave's book will teach you enough so you can decide if it is worth reading more on the subject to feed your curiosity.
 
#9
If I understood everything I've read lately, I wouldn't be asking these questions...geez! Do you guys really think that's such a no-brainer that I shouldn't even ask about it? Me think am dum now.
 
#10
JimmyM said:
If I understood everything I've read lately, I wouldn't be asking these questions...geez! Do you guys really think that's such a no-brainer that I shouldn't even ask about it? Me think am dum now.
Click to expand...

No, it's not a no brainer, but in order to understand it you need to understand it, which requires some background information. The reason people without the background information are told to follow the "rules" is because they are rules for a reason and you are better off following them unless you understand the effects of breaking them. If you are asking for a shortcut, it is follow the rules. If you are asking for more info, there is lots out there to digest that is written a lot better than any of my posts.
 
#11
Well now that I've been sufficiently talked down to and chastised for asking a question that could have been very simply answered by saying, "Too much heat for solid state" (like I'm supposed to know that...I'm a musical genius, not a scientific genius :bassist:), let me ask another dumb question that invites even more scorn and pity:

Say I have an SVT head. Say I don't want to run the giant 4-ohm speaker cab. Say I'd rather run two 8-ohm cabs at 4ohms. Since the speaker outs are factory set to take 4-ohm cabs, does that mean that I would have to daisy chain the two cabs in order to have a 4-ohm load on the head, or would plugging each cab into the separate speaker jacks still drop me to 4 ohms?

Again, this may be grade school stuff to a lot of you, but it's not to everybody, so lighten up and try to remember you're talking to a grown man.
 
#12
Jimmy, there's no difference electrically between plugging the two cabs into two outputs on the head (they're connected in parallel) or daisy-chaining the cabs (they're still connected in parallel). The two 8 ohm cabs will result in a 4 ohm load either way.

The key thing about your original question is how you interpret the specs published by manufacturers. When they say "This SS head is okay with loads down to 4 ohms", I interpret that as them saying, "This amp has been designed in such a way that the current flowing through the output stages shouldn't be enough to cause internal damage when you connect any cab or combination of cabs that has a rated nominal impedance of 4 ohms or more - but any lower than that may mean damagingly high current could flow in some performance situations, so we don't advise risking it and it won't be our fault if you do and damage results."

That would be one mouthful to print in every set of specs! But, if you accept that that is what they're actually saying, then the best policy to me seems to be "better safe than sorry." I personally wouldn't run any head at 2 ohms unless the manufacturer explicitly stated that the amp could work with this. If you halve impedance from say 4 ohms to 2 ohms (nominally), this will mean about 41% more current flowing on average for the same power output (and that's ignoring any increase in power owing to the reduced load). And current generates heat, so that sounds pretty risky to me.
 
#13
Thanks Bill. That was a nice refreshing change. Hey, it was a tard question, but I'm a tard with one of the very first SVT's ever made, and I want to have fun with different cabs when I get it out of the shop but I don't want to blow it up, either. So realizing that I'm a tard, I asked.
 
#14
ModestCargo said:
I would not run it with a 16 ohm load either, I won't get into explaining why.
Click to expand...
Can you explain why? I'm curious...

Edit: Also, if heat is one of the main factors in running Solid States at lower than their rated, surely someone could devise an after market cooling system to cope with the heat? I mean...If you can over-volt computer CPU's and slap on a massive heatsink to suck out the excess, isn't the same possible on an amp?
 
#15
J.Stone,

All engineering reasons aside, do you really think there would be a reason to run a 1-ohm nominal load? Because you can already run dual two-ohm loads with almost any pro power amp. And a hella lot of heads.

And if they added weight bulk and cost for this cooling system (would be clumsy for that to be aftermarket too), would it pay the vendors back in the marketplace?
 
#16
When it comes to appropriate loads SS and tube act as opposites. SS amps have a minimum load, tubes a maximum, and it all has to do with current, which is the source of heat in the output stage. As you lower the impedance load on an SS amp the current draw on the output transistors goes up, and along with it heat. When you lower the impedance load on a tube amp the current draw on the output tubes does not go up, because they're isolated from the load impedance by the output transformer. Too high or low an impedance load on the output transformer can cause problems, but generally not within a 2:1 ratio of the rating, in either direction.
 
#17
J.Stone said:
Can you explain why? I'm curious...

Edit: Also, if heat is one of the main factors in running Solid States at lower than their rated, surely someone could devise an after market cooling system to cope with the heat? I mean...If you can over-volt computer CPU's and slap on a massive heatsink to suck out the excess, isn't the same possible on an amp?
Click to expand...

Yes and no. The output devices, and power transformer, have a thermal resistance. Even if the outside case of these devices is held to ambient temperature, at increasing power levels, the innards of the devices will eventually heat up beyond their limits. Then the only option is to cool the heat sink below ambient temperature.

Going to higher power requires decreasing that thermal resistance, which means bigger power transformers, more output devices, or output devices in different packaging.

There is also an issue of the internal resistance of the amplifier circuit. As the load impedance gets smaller, the amplifier's impedance becomes a larger fraction of the total, and the efficiency of the amp decreases. Lowering that internal impedance also requires... bigger power transformers or more output devices.
 
#18
Hey Jimmy.... I'll give you this comparison.

I wouldn't trust a Crate bass amp to run at 2 ohms. But I'd trust an Ampeg SVT3 to run at 2 ohms. So I'd be questioning just which amp you're planning to run at 2 ohms.

I say, heat is the enemy of SS amps and low loads. So, plan ahead. Rig up some extra fans to move massive air, and you should be good to go. If it's an amp designed to drop to 2 ohms, that means its output transformer is robust enough to take the heat. So go for it, IF you test things out and get some major ventilation in there. Then you should be fine.
 
#19
bassybill said:
Jimmy, there's no difference electrically between plugging the two cabs into two outputs on the head (they're connected in parallel) or daisy-chaining the cabs (they're still connected in parallel). The two 8 ohm cabs will result in a 4 ohm load either way.

The key thing about your original question is how you interpret the specs published by manufacturers. When they say "This SS head is okay with loads down to 4 ohms", I interpret that as them saying, "This amp has been designed in such a way that the current flowing through the output stages shouldn't be enough to cause internal damage when you connect any cab or combination of cabs that has a rated nominal impedance of 4 ohms or more - but any lower than that may mean damagingly high current could flow in some performance situations, so we don't advise risking it and it won't be our fault if you do and damage results."

That would be one mouthful to print in every set of specs! But, if you accept that that is what they're actually saying, then the best policy to me seems to be "better safe than sorry." I personally wouldn't run any head at 2 ohms unless the manufacturer explicitly stated that the amp could work with this. If you halve impedance from say 4 ohms to 2 ohms (nominally), this will mean about 41% more current flowing on average for the same power output (and that's ignoring any increase in power owing to the reduced load). And current generates heat, so that sounds pretty risky to me.
Click to expand...

"Better safe than sorry" is exactly the first thing that came to my mind. Causing a failure because you didn't folllow the amp's recommendations just doesn't make sense to me. Otherwise where do you stop? Three 4 ohm cabs on a 4 ohm minimum SS head?

Even though it may not blow up you can stress some of the components and that's rarely if ever a good thing long term.
 
#20
SPEAKING AS AN ENGINEER - It IS dangerous to run outside of the manufacturer's recommendations. SURE - Some amps can handle it, but there is a wide manufacturing tolerance in all parts, and for every amp that can be abused a little, there is one that will blow up when you exceed the ratings by even a little.

Going UP in impedance will not hurt anything - but you will lose power, particularly with tube amps as the output transformer wants to see a load that matches it's own impedance. Going lower is a big-no-no. The output current can double and the safe operating range of the output devices will be far exceeded.

You CAN operate lower impedances if you keep the volume low. It is not the impedance that kills the amp, it is the total output current. Just keep in mind that even at 1/2 volume, a spike or transient can hit 100% and blow an output or driver transistor, starting a cascade that blows them all in an instant - too fast for the fuse to catch.

Not worth it!!
 

Latest posts

Top Bottom
Back