Pro Audio PA Amps - Bargains to Be Had?

sounds good - any implications regarding what sort of cable I need for the long run - I've not measured it exactly, but by the time its gone round various corners etc, I think we are talking about 8m, so expensive audiophile cable need not apply. Currently of course they are long runs of speaker cable - 2.5mm Van Damme Blue, which sound fine.

losing a couple of db isn't an issue - 12 o'clock is loud, 1 o'clock is "bloody well turn it down" and 2 o'clock is noise abatement soc.time so there's still plenty of leeway

 

I could get you a reel of balanced screened cable when I get the case to make this thing. There is one I know is pretty good as I use it myself, 25meters for £15.

 

that'll do the trick - is it nice and flexible? - its got to go round corners and fit through 1 inch dia holes in the skirting.

 

I'd be careful not to read too much into that test.

Just because the warning light doesn't come on, it doesn't mean you are out of the danger zone. If the output stage is running close to the thermal limits of the components, i.e. "just before" the warning indicator comes on, I'd be worried about thermal failure in the long term (a matter of months). In typical racked operation, the thermal warning lights are normally triggered when something absurd happens like the amp is running hard into a 2ohms load or if the fans actually fail. In reality, normal fan operation keeps the transistors well out of the danger zone for long term relability. I'd be worried that any non-fan operation may well have the output stage running near the "danger zone" (only guessing)

Good luck with it anyway, interesting thread.

 

Yeah its not super thick insulated stuff.

 

Point taken, one of the reasons I'd best speak to MC2 about it I think. I'll need to know what margin they have builtinto these things when the warning light kicks in.

I don't doubt that it could be made to run cool enough and much more quietly though - its more a question of how best to approach it.

It might simply be a case of using new and quieter fans (God knows how many hours these have put in), especially once its in a cupboard (I'll have to think about ventilation for that too I guess). Or it may take something more inventive - the world of PC cooling, overclocking and silent PCs looks to be a fruitful source of ideas.

 

Okay, sound like a good way forward. I'll PM you when I have a better idea what my options are around cooling and quiet operation. Thanks for the help so far.

 

and you would be right ... I did some more digging and actually read the manual .

There are two levels of "overheating" where it reduces the voltage to the output devices before the red light starts flashing for an imminent close down ie. where the light stays green:

"Each power module has two fans to assist in temperature control. These are set to run at their slowest speed at switch on, and will remain in the slow idle speed even when the amplifier is cold. As the amplifier warms up the speed will increase, being controlled by the temperature of the power module heat sinks.

If the heat sinks get excessively hot, the controller will automatically reduce the supply voltages to the power devices. This is totally inaudible, it does not effect the levels, but merely reduces the amplifier headroom by about 1.5dBs. The heat generated by the heat sinks is reduced by 30%. If the amplifier is still getting too hot there will be a further reduction. If the temperature still continues to rise the controller will disconnect the outputs. Just before this “shutdown†point, the temperature LED will start flashing, and it will stay on permanently whilst the amplifier is “shutdownâ€Â."

So when I tested it last night for 5 hours and the light stayed green it could have been in this state, but never got so hot it red lighted.

Here's the interesting bit:

"The operation of the temperature LED can be changed (see Sw1, internal configuration switches below). In the alternative position, the LED will come on as soon as the first headroom reduction point is reached. In certain applications it is important for the user to know this. All other functions remain the same."

So I can test to see if it at least gets to the first stage where the voltage is initially dropped, so I did ... lid off, fans off ... red light comes on in about 20 minutes.

Looks like I won't get away with NO fans even with better ventilation. Might get away with slower less noisy ones though.

 

Remember that bigger fans are quieter, and that less turbulent air is both better at cooling and less noisy.

 

Yes ... except bigger fans won't fit. I'll see what they say.

 

You need turbulent air for effective cooling-lamilar flow is not good you need turbulence for max mixing and max heat exchange-lamilar flow creates a boundary layer that is insulating.

 

Makes sense. It was just something I remember being told, obviously incorrectly.

 

The amp may well last for years at that state. I am continually amazed how this type of gear can take sustained abuse.

I use a Lab Gruppen ip2100 to run our sound insulation test rig. It feeds pink noise into a 3-4ohm load ,i.e. pair of EAW 15" boxes bridged (a real torture test). I have abused this rig for over a year now and it's never given up. We run it so hard that the fans ramp up to max. speed within about 60sec of turning it on. The heat rejection at the rear of the amp is similar to small electric radiator . And the amp was bought second hand ex-install from a nightclub where it ran 24/7 for years.

 

I hope you have good ear-plugs!

 

Oh yes....we normally run gear at about 110dBA plus.

Cinema sound insulation tests are really fun. We basically hire in the a 20k band touring PA and play pink noise at 115dBA+. It's an experience I'd recommend to anyone.

 

Its hard to say. Lid off and fans off its getting to at least the first headroom reduction point but not getting as far as the shutdown point, its stabilising somewhere in between. What I don't know is where its stabilising. It could be anywhere between just inside the first headroom reduction stage up to close to the top of the second stage and near to the point of shutdown.

I'll see if the MC2 people can shed any light. If they think it may be closer to the bottom end of the range, it strikes me that taking the fans out and using the space for some much bigger heat sinks might be enough to bring it back to below the first point, so long as there's plenty of ventilation in the lid and its in a ventilated space.

Does anyone here know anything about heatsinks and what sort of design/materials are most efficient?

 

I think you would be best off putting some very slow and quiet fans on. A heatsink without fan will have to be far bigger to do the same job as one with a fan, even at slow speed.

 

Yes, but the heatsinks in there now with the fans attached but with the fans off, are doing a good enough job with the lid off that its at least not getting as far as switching down ... the current heatsinks are relatively flat and have a lot less surface area than you could fit in there if the fans weren't taking the space .... I think you are probably right though

EDIT, I'm wrong, I just had a closer look, they are a lot deeper than I thought.

 

So I decided to delve:

This won't be helping the noise any:



and this won't be helping keep it cool either:



I now have a table full of bits. Oops

Mmm. a concentrated crust of "gig" I think. I rather wish the previous owner hadn't lost the filter at the front.

Oh well - an hour to take it apart and ten days to get it back together again

 

Hey Uncle, That looks nasty;-)
FYI the filter is no more than a piece of cooker hood mesh/foam stuff.
I've just picked up an MC650(there's a few on eBay now) that is luckily absolutely spotless doesn't even look to have been cleaned-it's too clean if get my drift not a speck, anyhoo that dust probably isn't gonna be good for you....while you're there check the caps for leakage-I had to have my T1000 recapped.
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