Impedance?

[Buck Spunni]

A question for the techies/amp gurus
What does an amp actually do when you load it with less than the recommended impedance...i.e. say, using a 2 ohm speaker load with an amp rated at 4 ohms?
I know you'll stress it as it's trying to produce more power than it's design and could possibly overheat or even blow up but, assuming you have enough headroom that you don't have to drive it hard enough to self-destruct or trigger the protection circuit how is the sound/tone affected? Sloppy low end? Stupid question?

 

[B-string]

You very quickly exceed the amps ability to dissipate heat. 0 ohms is a dead short, 2 ohms is not too far from that. Too much heat = burned out amp. Voltage from power supply goes low and current goes up. The only magic to be had is a lighter wallet?

 

[Joe Louvar]

You very quickly exceed the amps ability to dissipate heat. 0 ohms is a dead short, 2 ohms is not too far from that. Too much heat = burned out amp. Voltage from power supply goes low and current goes up. The only magic to be had is a lighter wallet?

^^^ This ^^^

 

[billfitzmaurice]

What does an amp actually do when you load it with less than the recommended impedance...i.e. say, using a 2 ohm speaker load with an amp rated at 4 ohms?

It overheats and shuts down.

 

[1958Bassman]

A question for the techies/amp gurus
What does an amp actually do when you load it with less than the recommended impedance...i.e. say, using a 2 ohm speaker load with an amp rated at 4 ohms?
I know you'll stress it as it's trying to produce more power than it's design and could possibly overheat or even blow up but, assuming you have enough headroom that you don't have to drive it hard enough to self-destruct or trigger the protection circuit how is the sound/tone affected? Sloppy low end? Stupid question?

You know how too much current blows a fuse? That's why. If the output is rated for a specific power output, decreasing the impedance increases the current that passes through the output devices. I=E/R and if E remains constant, I must increase when R decreases.

Best case, the amp can handle the load. Middle case, it blows the fuse or shuts down when the protection kicks in. Worst case, it lets the smoke out.

 

[Exxcell]

It overheats and shuts down.

So are your saying, if an amplifier is rated for 8ohms, your could run it at 4 if you increase airflow? Without pushing the amp too hard that is...

 

[lowpass]

Exxcell -- Heat is a problem, but it is not the only problem, the amp has to be designed for it.

Sorry for the length of the reply - I hope it gives you a better understanding of what is happening inside the amp when you change speaker impedances.

If I may add to 1958Bassman’s comments, here’s a quick example as to how changing speaker impedance affects your amp. (Some formula’s here but I’m not trying to be smart, sometimes seeing the formula helps you understand the why.)

The amp’s output stage has an internal impedance too just like the speaker, so there is power dissipated inside the amp, which translates to HEAT. The power formula is P = I^2 * R, or “power equals current squared times resistance”. So for example, if you have a 4 ohm speaker attached, to get 100W out of it the amp has to drive 5A through the speaker. This means that the amp is also driving 5A through its own internal impedance, so it is dissipating power internally which produces heat.


Now, if you decrease the speaker impedance to 2 ohms, to get the same 100W out of the speaker, the amp has to drive 7A through the speaker (refer to previous power formula), which means that there is now 7A flowing through the amp’s output stage. Since P = I^2 * R, and the amp’s internal impedance (R) has not changed, but the current (I) through its internal impedance went up, P goes up internally to the amp even though the speaker power stayed the same! This means that there is more power dissipated internally in the amplifier to get the same 100W out of the speaker. If the amp is not designed to dissipate the additional internal power (HEAT) or supply the additional current, you have problems!

Hope this helps...

 

[JimmyM]

Listen, I know you really want to, but don't try to run your stuff out of spec. They spec them that way for a reason, and no amount of wanting it to work is going to make it work, and you seriously risk your gear if you do. Throwing an extra fan on your stuff is not going to prevent it from overheating when you run out of spec...just slightly delays the inevitable for a few seconds.

 

[Buck Spunni]

Thanks for the replies, guys, but you're missing the crux of my question. I already know that heat/current etc are potentially going to be a problem.
IF the amp in question has enough headroom AND the protection circuit doesn't kick in to protect against either overheating or what it might see as a dead short, what does the amp sound like? Is the damping factor negatively changed so the amp just doesn't sound good? Anything else?

This isn't an idle question - I'm looking at buying a new head and can't afford to replace my cabs immediately. My existing amp produces 500 watts RMS into a 2 ohm load, so I use a pair of 4 ohm 210 cabs with it. The head I'm looking at (TC RH750) is rated at 750 watts into 4 ohms and has a published minimum impedance of 2.66 ohms.
The RH750 has waaayyy more power than my existing head so for the volume I need it wouldn't be pushed very hard and I'm wondering if I could get by with using my existing cabinets with it 'til I can scare up the $$$ to replace them. That idea only works if it will sound OK, even though it's technically being run outside the manufacturers spec.

 

[Pilgrim]

It's very simple. Use both 4 ohm cabs and chances are that you will eventually fry the head. Don't buy the head you are looking at.

 

[RickenBoogie]

Headroom is not part of the equation. Forget all that, and listen to the advice you're getting.

 

[Bass_Pounder]

. The head I'm looking at (TC RH750) is rated at 750 watts into 4 ohms and has a published minimum impedance of 2.66 ohms.

......The RH750 has waaayyy more power than my existing head

You may want to do some more research regarding the "true power" of TC amps (plenty of threads here).

Their power ratings are false.

 

[JimmyM]

You're operating on a lot of non-factual info here. Not your fault...manufacturers don't explain it well, especially TC.

The RH750 is, in fact, a 240w head, and they use compression and filtering of lows to achieve levels of volume usually associated with a 750w head. I've played them, and it works to make it go louder, but is in no way a 750w head, and after being browbeaten into it once it got exposed, TC finally puts that info on a chart on the PDF about the APM system. Buried in hopes you can't find it, but at least it's there, whereas they used to flat out lie.

As far as impedance, it's suspected that their cabs also run at a higher impedance than what they tell you so you can use 3 of their 8 ohm cabs with it instead of just two like most 4 ohm heads require. So I wouldn't trust their minimum rating of 2.66 ohms, however, this one's a little harder to prove definitively because of the way impedance fluctuates with notes and volume.

Also, the difference between 500w and 750w is something you would never be able to tell the difference with, other than you might notice slightly more headroom, and even that's debatable. And then you still have to be able to use it, and most 210's are lucky if they can handle 200w. If you want more volume, bigger and more efficient cabs is the only way to go. That's why loud bassists always have big cab rigs. Only so much you can do with a 210. I'll add, though, that headroom is VERY important with micros, more important than it is with solid state and tube. So fi you want to be balls loud, get as much REAL power as you can afford.

I gave you the short version because I have to go to a gig, but you can read about it all on here, and several people will give you a more detailed explanation, I'm sure. TC is a real hot button of controversy, and before you buy, you're entitled to know the facts. And knowing that you're interested in them, I can tell you categorically that running two 4 ohm cabs in one is a recipe for disaster.