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[Paul Bell]

So I had set up this amp earlier in the day, slapped it on it’s ass and went off to do other things.

The setup:

In club Rebel, we have a four stack of front loaded dual 18 boxes, two boxes per channel of a PL6.0II. It works well for live bands, fills the entire space evenly with bass. We had a big name DJ last week and the bass wasn’t body pounding like these guys like. In came the BASSMAXX Trips, two of them. I figured the 6.0 wasn’t enough so I dragged in my 9.0. All worked well for the event.

Here we are today. I simply swapped the cables from the 9.0 to the 380. The 380 was placed directly on top of the 9.0 with a bar rag to prevent scratching it (I don’t own it...) I did have the thought that the cloth might lead to some heating but that would  have to wait and see. The input sensitivity switches easily allowed matching it to the DSP with little tweaking.

I was at another club much of the night. The Rebel tech said all was great and the amp was running real cool. I was skeptical. I was able to stop by around 2AM. It was hard trance/electronica with solid low frequency. I was downstairs in the crowd first, the place was packed. This particular track had sustained low frequency, had to start at 35Hz, then 45, then 60, etc. Each tone was held for a good thirty seconds, the bass output was huge. I was thinking that the amp must be at it’s end and the DSP in limiting and are these woofers going to survive? The bass level was pretty eyeball shaking. Those of you who were at The Sub Shootout heard these subs at this level. I shoved my way upstairs to see the amp only hitting the second lights, one more light to go before the red lights. There was good headroom left. Also surprising me was the lack of heat from the amp. The poor lighting tech was stationed top side and thanked me for not using the other amp as it ran way hotter. He was talking about the 9.0.

Overall, I’m very impressed with the amp. It works as advertised. No issues at all. I can't say it bested the 9.0 but it was pretty good. There was no lack of bass level. One can only tell if you A/B it with the 9.0. It's certainly lighter, cooler and cost less than the 9.0 did when new. One more night at Rebel is in line for this amp. Huge power. 24 pounds. HUH. I don’t know how this amp will compare to the 9.0 with a 2 Ohm load but it’s starting to look good.

[Paul Bell]

The only thing I'd like to add is that the amp was turned on about an hour before any music was played. The unit was more than warm, almost hot. The rack next to it has a bunch of PLX amps and they were powered up too, they stayed cool. I guess when idling, the 380 has something going on. Interestingly, when putting out lots power, it doesn't seem to get much hotter than when idling.

I have no opportunity to test it against the 9.0 with both on a 2 Ohm load.

This is a great amp and I'll be specing it when I can.

-Paul Bell

[Michael 'Bink' Knowles]

Thanks for the report. Seems like a great new amp!

-Bink

[Langston Holland]

Paul - your observations match QSC's PL380 whitepaper quite well:

"Why Does the PL380 Feel Warm at Idle?

Class D circuitry has a different loss curve than conventional linear designs. Linear designs can be adjusted for minimum idle loss, but losses increase rapidly at any significant power. (Figure 5) Therefore, linear designs idle cool but require a lot of fan boosting at high power levels. Class D designs constantly process a certain amount of power, and therefore run somewhat warm at idle. However, you will notice considerably less increase in temperature and fan speed, even at very high power levels."

[John Roberts {JR}]

I will reinforce the observation that class D amps have an overhead power loss at idle that may seem counter intuitive. Some may remember the early Crown amp that stopped switching, and effectively turned itself off below some threshold output level. It ran cold at idle, but was disturbing if you could hear it gate on and off

The very high switching speed that buys you other efficiencies under power has a cost at idle. I have even seen a variant on class AB (Class AD?) where a small residual on current was left circulating in the off device to make it switch faster (this was decades ago and probably not needed with newer faster devices).

There is a similar overhead issue with class AB amps in the "A" part of the "AB" operation.

If we look at efficiency instead of simple loss the curves are not as simple. The class D will exhibit peak efficiency at some middle power level, high enough to negate the fixed overhead and drop again with increasing power output due to simple resistive losses in switching devices. Class AB OTOH will exhibit worst case efficiency at 1/3rd power and generally deliver peak efficiency approaching max output. Class G/H spreads out and reduces this 1/3rd power worst case efficiency but you still have the general characteristic of poor efficiency at modest output power.

Class D efficiency characteristic is a very good match with the dynamic nature of music. Bench efficiency tests don't reveal how much better class D is with real world music. I used to promote class D amps for generator applications because of this increased real world efficiency. I suspect the combination of PFC and class D would be a slam dunk for folks saddled with marginal mains power applications. Unfortunately those same folks are often on a limited budget. Class D will get them part way there and be helpful.

JR

 {edit: added example of cold idle]

[Michael 'Bink' Knowles]

Looks like this graph could be entitled Heat Output with Relative Heat on the vertical axis.

-Bink

[Paul Bell]

I suppose I should've read the white paper? It seemed kinda long and sometimes I have issues paying attentio

[Kent Clasen]

Paul Bell wrote on Sun, 15 July 2007 18:29

I suppose I should've read the white paper? It seemed kinda long and sometimes I have issues paying attentio

Hi Paul,

I even read the white paper, but this point didn't stick.  Thanks Langston for posting the chart. This now makes sense.

I have some PL380s that I have turned on and listened to some stuff in my shop, but not yet used in battle.  I, and others had noticed that just sitting in idle, there is warm air blowing out the front.

So far, I am impressed with the price, size, USA, weight, QSC support, and features (love all the LED indicators for button positions and the ability to lock out the attenuators).  I will powering some SB1000z in place of a MA5002 soon.

[Andy Peters]

John Roberts  {JR} wrote on Sun, 15 July 2007 08:58

The very high switching speed that buys you other efficiencies under power has a cost at idle.

Is the lack of efficiency at idle due to the output device gate drive requirements?

-a

[John Roberts {JR}]

Andy Peters wrote on Sun, 15 July 2007 13:48

Is the lack of efficiency at idle due to the output device gate drive requirements? -a

I can't easily separate out which are the more significant components, but lots of high edge rate signals and large power devices will have capacitance due to gate area, etc. The front end must be fast and low noise too but that's probably not huge current in whole power amp terms.

There are different output stage topologies so I'm guessing a little here, but devices don't switch instantaneously so there will be some VxI product during the switching transition. If there's any overlap there will be mutual conduction, but inductance and/or dead bands designed around transitions could mitigate that.

The idle power isn't huge in power amp terms, but since full power dissipation is so much less than conventional amps, the total heat sink capacity will be sized smaller so idle power may appear more significant than it really is.

I need to repeat I'm not and never was a working designer of class D amps so my knowledge about them is at least one step removed, but I've spent time shoulder to shoulder with folks who have, so I know a little 'bout it.

JR