LM4780 Parallel 4-ohm subwoofer @ +/- 37V...Dangerous?

[TheLaw117]

It's been like 6 months since I started my amp project and I was semi and idiot for buying a 25VAC transformer. It's only 250VA because I don't really plan on cranking it too much.

The transformer powers 2x LM3886 which are driving a ~8ohm load each, and a 1x LM4780 board configured in parallel driving a single 4ohm Tang Band sub.

The problem is...the Avel Lindberg transformer is actually like 2x 28VAC, not 25VAC as advertised. Because of that, I get around 37V after rectification. Now, 37V is pretty much the max it'll go, it will dip under load to around 35.5V-36V, but it sill seems a little high.

The heatsink is a copper heatpipe design with a fan actively cooling it, which keeps it only warm to the touch...But I'm not sure whether running at 3-4V off the 40V limit is such a good idea. I feel like an idiot for buying a 25VAC t-former. I should have purchased 18VAC which would have solved my problems...but I don't really feel like throwing another $50+ into it.

The sub I believe has a nominal handling of around 40W and the L+R speakers around 25W each. So I don't that much power in the first place.

If it dies, then I guess I could just replace it. Then I would know I should get a new transformer or something. It's only like a $5 chip. The power supply section is 4x 4700uF 50V Nichicons if that makes a difference.

The board I am using has the power supply integrated onto the board, but I think I can tweak if it I need. Is there anyway I could possibly drop the voltage on this thing? Like zeners or something?

I should also add that I have a preamp just for the subwoofer, which uses 2 opamps to preamp and then some passives to convert the two channels into 1 mono channel. Would a preamp have any effect on anything? Make the power amp work less? I never really understood preamps too well..

Just looking for some input.

Thank you.

[nigelwright7557]

I wouldnt worry about it, 3-4V isnt a poor margin.

[TheLaw117]

Originally Posted by nigelwright7557

I wouldnt worry about it, 3-4V isnt a poor margin.

Thanks.

What I've deduced over the last few minutes (lol), is that in parallel driving a 4 ohm load, each LM3886 in the LM4780 is really just seeing an 8ohm load...which is what my other LM3886s are doing...

Therfore, I think the only real problem is keeping it cool...

[CharlieLaub]

Originally Posted by TheLaw117

Thanks.

What I've deduced over the last few minutes (lol), is that in parallel driving a 4 ohm load, each LM3886 in the LM4780 is really just seeing an 8ohm load...which is what my other LM3886s are doing...

Therfore, I think the only real problem is keeping it cool...

True, but power dissipation is also proportional to rail voltage, even with no signal applied, IIRC. Make sure you have enough heat sink and use heat transfer paste to better join 4780 and heat sink.

The other option to drop the rail voltage slightly is to use two bridge rectifiers instead of one, which you can do if you have two secondaries (not possibly if the transformer has a center tapped secondary).

-Charlie

[!]

Power dissipation proportional to rail voltage is only a few mA difference.

Higher voltage (typical spread people pick) does not make it run hotter by more than a couple watts, people suggest higher heat at the higher voltage within the context that at the higher voltage you would have higher output power "potential" too, that the higher heat is at a 100% output power higher than the lower 100% output power you are limited to with lower voltage... but to get that you'd have to raise gain anyway or another way to look at it is with the higher voltage you are less likely to get clipping.

The chip is rated for 84V max, GUARANTEED to withstand 84V. You have only 74V, it is not a problem. I'd pick the 2x 25VAC over 2 x 18VAC every time except in a situation where you would integrate the amp into a 4 ohm speaker cabinet so there is no chance it would ever drive something else.

[CharlieLaub]

Originally Posted by !

Power dissipation proportional to rail voltage is only a few mA difference.

Higher voltage (typical spread people pick) does not make it run hotter by more than a couple watts, people suggest higher heat at the higher voltage within the context that at the higher voltage you would have higher output power "potential" too, that the higher heat is at a 100% output power higher than the lower 100% output power you are limited to with lower voltage... but to get that you'd have to raise gain anyway or another way to look at it is with the higher voltage you are less likely to get clipping.

The chip is rated for 84V max, GUARANTEED to withstand 84V. You have only 74V, it is not a problem. I'd pick the 2x 25VAC over 2 x 18VAC every time except in a situation where you would integrate the amp into a 4 ohm speaker cabinet so there is no chance it would ever drive something else.

Dear "!",

I just looked at the datasheet regarding power dissipation. I was wrong - at zero output, there is little to no dissipation, so no heat generation. On the other hand, With an 8 ohm load, the IC will dissipate up to 70W (at 35W output power per channel), and this will be slightly higher with 37V rails. This is a pretty good amount of heat I think, e.g. like an 80W incandescent light bulb. Even with only 5W output power, the LM4780 dissipates almost 50W in heat. Something to think about when choosing the heat sink.

-Charlie

[!]

^ It is true you can approach 70W+, what I wrote might be misleading, was within the context of no output and having a good heatsink otherwise and I should have mentioned it!

TheLaw117 has "heatsink is a copper heatpipe design with a fan actively cooling it, which keeps it only warm to the touch" which seems about right given that even economized (~$20 after rebate in the US) versions of such heatsinks are often capable of cooling 140W+ CPUs.

I should also mention, I am not being "green" about power consumption... would assume someone who is, would build class D instead of A/B.

[TheLaw117]

Thanks guys. I think I'm just going to let it be the way it is. I know it's not a very efficient amp. I started building this when I didn't know any better...which i regret. Within the past 6 months, I've learned so much more about audio and electronics, though you might not be able to tell by some of the stuff that I write. =P

Again, I have never had the amp all closed up and with absolutley everything running, so I don't really know what the real deal on heat will be, but I also have an 80mm exhaust fan also on ~8V. Very quiet between the two of them.

So I guess I'm fine...Though when I play some really boomy songs, it gets pretty hot...however...these chips are rated at 85*C, which is hotter than you think, and I think they can withstand a good deal of "abuse". Maybe not full blast 40Hz for 6 hours straight, but even if it did get a little bit hot here and there, I doubt it would die. It is genuine National silicon...not some Chinese copy.

So thanks for the information about the power and dissapation. I guess I'm really the only one who can really judge what's good etc.