LM4780 Based Amp - Design Help Needed

[surajram]

Hi,

This is my first attempt at building an amplifier. After reading through the forums I'm pretty keen on building one.

I have planned to base the amplifier on the LM4780. I chose this due to it's ability to produce 60W of Power into an 8Ω load with THD+N at 0.03%. (This is what i gathered from the datasheet.) The fact that the chip has two opamps also is an additional bonus as I wont need two dies to be cooled etc for a Stereo Amplifier.

I didnt want to order any kits for the projects as most of the ones i found on the internet had to be shipped internationally to reach me in India. I liked this kit from chipamp.com and I understand that adding and additional input for the second opamp in lieu of feeding the same input to both opamps will give me my required stereo amp

For my power supply I guess http://www.chipamp.com/supply.shtml supply should do.

Now what transformer should I use? I guess a torroidal transformer with twin secondaries at 25V should be fine. I really need help with the power rating though. For no reason, i feel a 200 or 250 VA unit should be enough to power two 60 W amplifier.

Since I am going to buy every component separately, Is there any component that is in terms of price & quality when compared to the ones mentioned in the schematic? I dont mind sacrificing a bit on quality as I want to keep the costs low on my first project.

Any help is appreciated.

--
Regards,
Suraj

[pacificblue]

Hi,

Good choice. However going for two LM3886 instead also has it advantages. One is that the total amount of power dissipation is the same like from one LM4780, but two chips have more superfice to dissipate. And you live in a country, where temperatures are an issue. Look for AN-1192 for details on possible heat dissipation per chip.
Another advantage is that soldering is easier, because there is more space between the pins.

Transformer size is not only a question of delivering enough power for the nominal output. It is also a question of how you use the amplifier, and not to forget, of personal taste. Some people like to have a transformer with enough power for a steady 100% output, others choose a transformer that is small enough to present the output power limit at peaks, thus hoping to protect the amplifier. So you will find opinions starting from 100 VA unto no upper limit. 200 to 250 VA is a reasonable size.

Enjoy building.

[wakibaki]

Originally posted by surajram
60W of Power into an 8© load with THD+N at 0.03%. (This is what i gathered from the datasheet.)

No. 0.03 @ 30W. 0.5 @ 60W

Originally posted by surajram
I guess a torroidal transformer with twin secondaries at 25V should be fine. I really need help with the power rating though. For no reason, i feel a 200 or 250 VA unit should be enough to power two 60 W amplifier.

This will give you ~ +-35VDC, good for 60W into 8ohms. A VA rating of twice the wattage is a common compromise so 250VA is OK.

Have a look here at Mick Feuerbach's minimalist stereo 3886. This shows how to construct an amp directly on the pins of the chip. You can adapt the technique to the 4780.

You don't need the huge capacitances or the regulators you'll see in his PSUs, IMO.

w

[surajram]

Originally posted by pacificblue

Good choice. However going for two LM3886 instead also has it advantages. One is that the total amount of power dissipation is the same like from one LM4780, but two chips have more superfice to dissipate.

I agree it the LM3886 has it's advantages. I was planning to either order a new Heat sink or use a copper block like the one in Mick Feuerbach's design.

But your post sounds a lot like my brainstorming before swithching to dual core processors on my PCs. This makes me think, will it be efficient if I couple a heat sink from a desktop PC, onto a LM4780 ? I'll have to do some research on that now...

Anyway, I guess buying a couple of 3886s will be more expensive than a single LM4780. (I dont have the local prices yet. Need to check on availability & prices)

Originally posted by wakibaki

Have a look here at Mick Feuerbach's minimalist stereo 3886. This shows how to construct an amp directly on the pins of the chip. You can adapt the technique to the 4780.

Now, this can be very small!! But as pacificblue said, the pins are a lot closer on the 4780 and they also have a crazier pin out. Moreover, i guess it would be more stable for the components to be mounted onto a stable perfboard or stripboard. Moreover, if i need to attach one of those Intel Coolers onto the 4780, it'll be highly inconvenient to have the components soldered directly onto the pins.

[ttan98]

[i]
I agree it the LM3886 has it's advantages. I was planning to either order a new Heat sink or use a copper block like the one in Mick Feuerbach's design.

[/B]

no need to use copper block, besides being expensive, and hard to get. I use computer cpu heatsink and mount 2 lm3886 on one unit, runs quite cool and save space. you can use the fan ie attached to the heatsink if required.

you can buy USED cpu heatsink(for PIII or PIV) at a cheap price at swap meet.

[surajram]

Originally posted by ttan98

you can buy USED cpu heatsink(for PIII or PIV) at a cheap price at swap meet.

That is exactly why I am considering that. Besides, i have a few old computers that are not in working order. I guess one of them will be glad to donate a Heatsink.

[artquake]

Originally posted by surajram

Now, this can be very small!! But as pacificblue said, the pins are a lot closer on the 4780 and they also have a crazier pin out. Moreover, i guess it would be more stable for the components to be mounted onto a stable perfboard or stripboard. Moreover, ..........

Check THIS out

And HERE the PSU ...