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Old 3rd April 2013, 01:42 PM   #11
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Quote:
Originally Posted by samp78 View Post
Hi Guys, just a quick question can the LM4780 run a 4 ohm load at rail voltages of around 32V?
Yes, pretty much. You are somewhere around the comfortable limit and I'd want to know which side of 4 ohms your load actually is, how much heatsinking you are using and how loud you like your music.

If you are putting both halves of the 4780 in parallel, then it's a breeze and you could go up in Voltage.

But, for the most part, you are almost certainly going to be OK.
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Old 3rd April 2013, 04:07 PM   #12
AndrewT is offline AndrewT  Scotland
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I am trying to work out (or maybe guesstimate would be a better term) how long the life will be.
Could it be measured in weeks, or days, or hours, or minutes?
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Old 3rd April 2013, 04:22 PM   #13
samp78 is offline samp78  United Kingdom
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So would it be ill-advised to run into 4 ohm load at that rail voltage?
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Old 3rd April 2013, 05:19 PM   #14
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Ignore AndrewT. You'll be fine.

The chip has a current limit at 11A - let's call that 7A - and a voltage range of 84V. Both identical to the 3886. You are not going to go near either, realistically. Moreover, with a standard linear supply the voltage will sag significantly if it's called on to produce anything nearby, wiping off a few 10s of theoretical watts available.

What it doesn't have is twice the area to dissipate its own heat - hence my wanting to know that you would properly heatsink it (ie a 10cm x 10cm x 12mm slab of heatsink will not be meaty enough) and wanting to know how hard you are going to push it and into what DCR (which you can measure with any multimeter). Unless the bass unit has a DCR of 3.3 ohms - so getting just inside the IEC definition of 4 ohms - you will be fine.

Besides, I have not heard of these chips failing through 'wearing out'. There are examples of them failing in things like guitar amps (yes they are used in that role) where they have little in the way of heatsinks and are forced to be 100W amps driving extraordinarily uncertain loads - like as many speakers the user can parallel up to get it as loud as the headbanger wants.

Does anyone else disagree - or have a reason to think that they might?
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Old 3rd April 2013, 05:49 PM   #15
samp78 is offline samp78  United Kingdom
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Thanks Christian, I will probably be pushing them pretty hard, but not using them for guitar amplification that's another project all together! And not sure what the DCR is at the moment as I need to check this and the drivers aren't at my house. what sort of size heatsink will it require if I was to use 2 8 ohm drivers in parallel? Thanks
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Old 3rd April 2013, 06:39 PM   #16
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AndrewT has a lot of experience with these chip amps and a rigorous approach. He may be being a bit overconservative in this case, but I doubt it. You probably ignore him at your peril.
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Old 3rd April 2013, 06:48 PM   #17
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what sort of size heatsink will it require if I was to use 2 8 ohm drivers in parallel? Thanks
Two 8 ohm drivers in parallel will have a DCR of about 3.4 ohms almost certainly (ie lower than a single 4 ohm unit is likely to be) so you are again chomping at the boundaries.

In general I would like to see at least twice the area mentioned above, and probably at 16mm or 19mm. On top of that I would like to see the dissipation area of some casework as well. This last part can be really quite important. It doesn't have to be thick but it is worthwhile having decent joints that will conduct heat. At the very least have a panel that is perhaps between the chip and the heatsink and with liberal quantities of thermal paste. Incidentally, vertical mounting of the heatsink (so air can flow upwards through it) is significantly more efficient than horizontal mounting.
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Old 4th April 2013, 06:14 PM   #18
samp78 is offline samp78  United Kingdom
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So instead of bridging the LM4780 if i ran it in stereo config with the 2 8 ohms drivers, and then just run the mono signal in, would this be better? and could I use some kind of ventilation system ie fans? I can get hold of a lot of CPU heatsinks... Thanks for all your help everyone
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Old 4th April 2013, 07:08 PM   #19
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.... and could I use some kind of ventilation system ie fans? I can get hold of a lot of CPU heatsinks... Thanks for all your help everyone
Yes active cooling does help a lot - as long as fans are working without any dust accumulation.
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Old 4th April 2013, 08:52 PM   #20
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So instead of bridging the LM4780 if i ran it in stereo config with the 2 8 ohms drivers, and then just run the mono signal in, would this be better? and could I use some kind of ventilation system ie fans? I can get hold of a lot of CPU heatsinks... Thanks for all your help everyone
Where are you going with this? Bridging is the last thing you want to do at this voltage. Then all of AndrewT's dire predictions will certainly come true!

You can run it with an 8 ohm driver each side, if you like, and that's a breeze. You can run it with 4 ohms drivers each side and it will manage, with some care. That also probably holds true for two 8 ohm drivers in parallel as well, but more care will be required - as outlined in my last response. Active cooling will inevitably help as mentioned above.
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