Does Power Amp "heat" = how hard the amp is working?

[mikekars]

I have a tri-amp system using QSC amps and have noticed that the mid-range amps always run a lot hotter than the sub amps or the HF amps.   Is how hot the amp gets, judging by the exhaust air flow temp, an indication of how hard the amp is working?

It is surprising to me because I always thought the amp for the subs had to do more work, in general. 

This hot mid-range amp has been true in a variety of amp/speaker configurations but here is an example of a simple bi-amp case:

mid/hi (passive) times 2 for left and right stacks

  srx722  1200W continuous 4 Ohm

  PLX 3402 stereo  1100 W continuous per channel into 4 Ohms

Subs times 2 for left and right stacks

  2x srx718 run parallel,  total 1600W continuous at 4 Ohms

  PLX 3420 stereo  1100 W continuous per channel into 4 Ohms

Doing  typical "rock" type shows or Reggae the amp for the 722's always run a lot hotter.  The sub amp by comparison seems to be loafing along.

 

[Dogoth]

Since your amps are of the same manufacturer/model class, yes hotter means working harder (usualy). There are many other reasons an amp runs hot (clogged heatsink fins, failing fan or fan sensor, different "real" load vs what's stated on the box). In an ideal world amplifiers/speaker combinations would take all of the electricity and convert it to mechanical energy and there would be no heat. We live FAR from an ideal world. What really happens is what an amp cannot turn into mechanical energy, gets turned into heat. Once you send a cone outward, it's inertia will make it want to continue in that direction even when the signal has biased it's self in the opposite direction (this is called Back EMF). It's my understanding that an amps ability to comtrol this behaviour is rated as the Damping Factor. Far more importantly though is the simple fact that there is friction & mechanical resistance to the speaker cone moving, this divergence from the speaker acting just as the electrical waveform would predict is also dissipated as heat (both in the voice coil and the output stage of your amplifier). The other thing I mentioned is that a 4 or 8 ohm rating from a speaker is just an agerage nominal impedence across it's rated bandwidth. There are resonance (and anti resonant) points of any given driver/box that makes for huge peaks & valleys of the real impedence if plotted on a frequency graph. In other words some notes are much lower impedence (hence draw more current) than others. Maybe play in a different key (small joke there :-).

If it's always the same amp that runs hot, check the amp (start with the ventilation system). If it's always the same set of speakers that makes their amp run hot, check your speakers. Check the speakers. I'm taking for granted that you are crossing them over correctly (I.E. part of the filter arrangment is a HPF for the 722s). If you were still running full range to the 722s this could be your problem as they are not a super efficient speaker to begin with (sound great though :-).

Just some thoughts

[mikekars]

JR- thank you for the considered reply.  I don't think it is a particular amp or a particular speaker box as I have four of those amps and 4 of those srx722 speakers and have mixed them up.  I also have taken the amps apart and cleaned them a couple of times. 

Also, I do use a DR260 to set the crossovers per JBL recommended tunings so the 722's are not trying to reproduce anything lower than 80Hz.

But that is kind of low in a way and I guess I figured the 722's just had the bulk of the frequency spectrum to deal with compared to the subs.

When tri-amping I put a PLX 1202 on the horns and it only sees 8 Ohm in stereo and does not seem to be working very hard at all. 

In the tri-amp situation  the internal crossover in the 722 is taken out of the equation  and also we take the 1kHz and up  load off.  So  the mid amps only have to work with the dual 12" woofers. 

But they still run a lot hotter than anything else.  I thought it was, again, just a matter that they had most of the spectral content to deal with.  Your suggestion that the real impedance on the mids might actually end up being lower than nominal makes a lot of sense though.  Maybe the real impedance on the subs is actually a bit higher?

 

[dboomer]

mikekars wrote:

 

  Is how hot the amp gets, judging by the exhaust air flow temp, an indication of how hard the amp is working?

 

 

You would think so ... but that's not the case.  To over simplify ... conventional amplifiers always make the same amount of power (with a single load) whether you are delivering a lot of power or a little power.  The power that is not delivered to the speaker must be dissapated back inside the power amp.  So when an amp is delivering 100% of it's power to the speaker then there is virtually none that must be disappated back inside the power amp.

 

Your amps are class H which means they switch back and forth between low power and high power.  So depending on exactly what is happening you could be near the total output but of the low powered amp and therefore not building up as much heat as you would if you only delivered a tiny bit more power.  As always ... it all depends.

 

 

[Dogoth]

Listen to Don.

He understands modern amp topology and makes more sense than any of my theories. My thinking is old school (although not totaly usless - in this case, just not as applicable :-).

Thanks

[agedhorse]

The reason for the additional heat is that the dynamic range of the midrange is generally less (lower peak to average ratio) and the power bandwidth is wider leading to more energy delivered and dissipated (on a class ab/g/h amp)

 

On a linear amp like above, the power dissipated is a function of the delivered power and the load impedance. There are dissipation curves that us engineers "sleep with" when designing amps.

[mikekars]

Wow - ask a seeming simple question that you think you already know the answer to and find out, once again, you really don't know sh*t!

I guess those mid range amps will always be running hotter then.  I have tried basically doubling the power to both subs and mids by bridging the amps and the same thing happens - the mids run hotter.  I switched out the PLX3402 and replaced with the lower powered  PLX2502 in bridge mode and the mids still run hotter than the subs.

When the whole system is at lowish volume all amps run cooler but the mids are hotter.  When the system is cranking outdoors at close to capacity on a hot day all the amps are putting out some heat and mids are very warm.  So I assume it is the same situation with waste heat, dynamic range, or whatever, just at a higher level.

What I was originally after was the thought that the hot amps were doing way more work and maybe there would be some benefit from using a larger amp (with limiting)  for the mids.  Sounds like that is not really the way it works.

[agedhorse]

The average power to the midrange is higher. If you were to compress the sub feed, and increase its bandwidth to say 500hz, the difference would be much smaller or nothing.

 

This is the reason why we generally see mechanical damage on a sub driver and thermal damage on a mid driver when overpowered. It has to do with average versus peak ratios.