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Old 31st July 2014, 04:18 PM   #1
rol is offline rol
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Default Transformer for LM4780

I am planning to build a LM4780 chipamp to use in my dorm room. My main music is instruments and new age, so I often play at moderate volume. The speakers are just some cheap bookshelf from Amazon. Either of these:
Amazon.com: Micca MB42X Bookshelf Speakers with 4-Inch Carbon Fiber Woofer and Silk Dome Tweeter, Black: Electronics

Amazon.com: Monoprice Premium 5.25-inch 2-Way Bookshelf Speakers (Pair), Black Finish: Electronics

Right now I only have a 2x22v 130va transformer to power the amp. My question is whether it is sufficient to drive those speaker, or I should buy a higher ratings transformer?

Thanks
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Old 31st July 2014, 08:06 PM   #2
TP143 is offline TP143  United States
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Hi Rol,

Apologies in advance for not directly answering your question. I do have a completed dual mono LM4780 for sale in the swap meet section. I have a 500va in mine, which is about 20% more than what is recommended but why not have it, right? Anyway, if you haven't purchased any of the parts for your build I could part mine out and you would recieve all the necessary boards (already populated), transformer, power supply, speaker protection and soft start. I would even throw in my gold plated rca and speaker connects. This way you would only have to wire it all together in your own chassis. Otherwise, if you like the look of my chassis, I could sell it as a working amp - just plug it in.

If you decide your own route, good luck. It is not too difficult of a build and sounds excellent!
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Old 31st July 2014, 10:06 PM   #3
AndrewT is online now AndrewT  Scotland
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130VA will supply a pair of pwoer amp chips with a total maximum output of upto 130W.

The dual 4780 will try to deliver ~60W to 68W into a 4ohms speaker and will run hot unless you fit an enormous heatsink.

Either buy 8ohms speakers, or buy a pair of 3886.
or do both !
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Old 1st August 2014, 03:00 AM   #4
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The flip side to your situation is that you'll never listen to more than 5 watts at the very most in your dorm room. Much more and you'll suffer permanent hearing loss before they throw you out, which will be soon. Actually a couple of watts is probably more like it.

So you could hook up the transformer you have and run it, sure. If you turn up the volume toward max something bad will happen, but you'll never have the volume that high anyway--unless you attend the University of Georgia, in which case all bets are off.

But actually, two 60 watt channels is a wee bit more power than your setup seems to call for. I'm going to get all busybody and suggest that a pair of LM1875s would be more appropriate. They go for something like 2 bucks apiece on eBay, and the audio is first class. You'd be interested in the "Typical Applications" circuit diagram in the data sheet, which is here. http://www.ti.com/lit/gpn/lm1875

Your dual 22 volts rectifies to about +/- 30 volts, which pushes the LM1875 up to around 30 watts per channel. This will still shake the walls of your dorm room, and the room next door, and the room next to that. But it seems to be a somewhat better fit for what you're doing than that 120 watt monster amp. So just thought I'd mention.

As for your concerns about whether the amp will drive the speakers, can a 16-wheeler pull a garden tractor? Unless a speaker specifies "RMS power," (true power) then it's very likely rated at "music power," which is generally 1/2 to 1/3 of true power.

So take speaker ratings with a grain of salt. And remember that even if a speaker won't burn out at it's rated power, distortion will be extreme as you approach the upper part of its rating.
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Old 1st August 2014, 04:03 AM   #5
rol is offline rol
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Quote:
Originally Posted by TP143 View Post
Hi Rol,
Hi,

Thanks a lot for your offer, but I already populated the LM4780 board, and about to test it very soon. Would love to try a dual mono, but my room is quite limited in space, so I try to things as small as possible.

My amp is only a single LM4780 and as mentioned, I don't play loud music, only moderately to fall asleep. Never built a speaker amp so I am pretty much clueless regarding the specs. Looks like I will give it a try with this 130va transformer. Seriously, buying and shipping a 300va or 400va traffo internationally is very costly and it would be great to be able to use what I got on hands.
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Old 1st August 2014, 04:17 AM   #6
sangram is offline sangram  India
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If you're not chasing maximum output and just want something that works, you'll do fine. I routinely test all sorts of amplifiers with 25V rails off a 36VA transformer and a very basic power supply (sort of a test mule) and it works fine, plenty loud for plenty people.

Do remember to size your capacitance bank according to the transformer VA and rectifier capacity. My rule of thumb in most cases is power supply = 2.5x power output, which means 25 watts or so out of that transformer, per channel. This would suggest to me that a cap bank of 10,000 to 15,000uF per rail should be sufficient. You can be very happy with 25 watts in a small room, but I would heed the heatsink warnings carefully.

Normally when we want reduced power output we also reduce the supply voltage to keep the dissipation in check. You have a pretty healthy supply voltage, which means the idle dissipation and that at about 20 watts will be quite high, so you will have to deal with a bunch of heat. Class AB amps including chipamps will dissipate maximum power at 1/3 power output, which you can calculate from your supply rails and the National datasheet. These 4780s can run really hot.
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Old 1st August 2014, 07:51 AM   #7
AndrewT is online now AndrewT  Scotland
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I reported a couple of weeks ago on the heatsink temperatures of a newly built 2channel amplifier.

Two days ago I did a similar test on a lower power version.
Two 3886 fitted to a common 0.9C/W heatsink.
15-0-15Vac transformer giving a quiescent voltage at the chipamps of +-23.7Vdc
Attached two 4r0 dummy loads to the outputs and sent a 1kHz signal to the input.
Room Ta=22C
with each chipamp sending W to the 4r0, the heatsink reached about 52C and the chips were too hot to hold my finger on either of them.
The sink backplate was just too hot for a prolonged finger test.

BTW, the 15-0-15Vac allows 24W into 8ohms/8r0, that leaves W @ ~ -17dB ref maximum.
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Old 1st August 2014, 12:06 PM   #8
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<< I already populated the LM4780 board, and about to test it very soon >>

To jump in with another 2 cents, you're fine. Watts = volts x amps, so your 130VA transformer is essentially a 130 watt transformer. You're using it to power a 120 watt amplifier. You'll be fine as long as you don't crank up the volume.

The trick is that transformers don't work at 100% efficiency. More like 60-80%, with emphasis toward 60. This is why power supplies are usually made oversize. My opinion is 150% oversize, or thereabouts.

Additionally, when the drummer or bass player go to work--or the cellos or tympani--there's need for additional power. The capacitor bank (the smoothing capacitors) helps with this, but the capacitors can't supply what's not there in the first place.

But all of this comes into play at higher volume levels. Normal room listening level is perhaps 2-5 watts, so 2-5 watts is all the amp is putting out, so 2-5 watts is all the power supply has to provide. You'll be perfectly OK in an environment like a dorm room.

And again, don't worry about being able to drive the speakers you mentioned. The amp you're talking about can melt them.

<< buying and shipping...internationally is very costly >>

Have you looked at Mouser (mouser.com), or Jameco (jameco.com), just to mention two? Jameco is more DIYer oriented, neither has a minimum order. Disclosure: I have no association with either, just mentioning.
.
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Old 1st August 2014, 05:52 PM   #9
rol is offline rol
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Thanks all.

My amp has 15,000uf per rail so quite sufficient for the task. I will surely watch out for the heat. I don't actually use a real heatsink, just a chunk of CNC'ed solid aluminum block.
I am really not looking for a monster amp that puts out hundred watts, just something that fits perfectly in my environment.

I did order many times from Mouser. What I actually meant is the exorbitant cost to ship a huge, heavy transformer to my place. Shipping cost would be easily more than that of the transformer.
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Old 2nd August 2014, 09:55 AM   #10
AndrewT is online now AndrewT  Scotland
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Quote:
Originally Posted by bentsnake View Post
....................The ...... is that transformers don't work at 100% efficiency.
agreed, there are losses in the transformer that prevent 100% efficiency
Quote:
More like 60-80%, with emphasis toward 60. This is why power supplies are usually made oversize. My opinion is 150% oversize, or thereabouts....................
Wrong.
Small transformer say around 1VA to 10VA will have efficiencies around 80% to 90%
Medium transformers from 12VA to 200VA will have efficiencies increasing with size to around 95%.
Large transformers, 225VA to 1kVA. will have an efficiencies around 96%.
It seems to be either expensive, or technically difficult, to get much over 96%.
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Last edited by AndrewT; 2nd August 2014 at 09:57 AM.
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